Environmental conditions and nutritional quality of vegetables in protected cultivation

Abstract

Despite progress in reducing global hunger, micronutrient deficiencies and imbalanced diets linked to urbanization remain pressing health threats. Protected cultivation offers a promising avenue for sustainable intensification of vegetable production. Additionally, indoor and vertical farming have recently emerged as cutting‐edge strategies, particularly in densely populated urban areas and mega‐cities. However, research has focused on maximizing yield, neglecting the impact of pre‐harvest conditions on produce quality. Here, we explore strategies for manipulating environmental factors within protected cultivation systems to enhance vegetable nutritional value. Research suggests moderate stress can positively influence nutrient composition while plants exhibit stage‐specific metabolic responses to environmental factors. For instance, seedlings thrive under a higher blue‐to‐red ratio, while green light benefits leafy vegetables. Additionally, increased blue light or supplemental UV‐A benefits flowering and fruiting vegetables. When other environmental factors are optimal, light intensity significantly impacts vegetable nutritional quality, followed by CO₂ levels, light spectrum, temperature, and humidity. Further research is needed to fully understand the mechanisms, the complex interplay of environmental factors, and their interaction with genetic material and cultural practices on nutritional quality.

1. INTRODUCTION

The Food and Agriculture Organization (FAO) recently released a report on the severity of global hunger, malnutrition, and food security issues. Despite some stabilization in global hunger from 2021 to 2022, persistent glitches such as hidden hunger—marked by deficiencies in vitamins and minerals—and excessive calorie intake due to urbanization continue to pose significant threats. Projections paint a grim picture, suggesting that by 2030, an estimated 600 million individuals may face chronic undernourishment. Hawkes et al. (2017) warned that micronutrient deficiencies pose a significant public health problem. In low‐ and middle‐income countries, hidden hunger directly affects health and economies (Gödecke et al., 2018; Murray, 2020), although it is not confined solely to these regions. Moreover, the rates of child malnutrition are alarming, with 22.3% stunted, 6.8% wasted, and 5.6% (37 million) overweight in 2021. The ongoing trend of urbanization, aiming for a 70% global urban population by 2050 (Eigenbrod & Gruda, 2015), is driving the consumption of processed foods, thereby exacerbating obesity worldwide (FAO et al., 2023). In addition, climate change directly and indirectly deteriorates food nutrition (Dong et al., 2018; Scheelbeek et al., 2018; Toreti et al., 2020).

Amidst these pressing nutritional challenges, vegetables are renowned for their rich content of vitamins and essential minerals, offering numerous health benefits surpassing staple cereals. Unlike cereals, which predominantly provide primary metabolites, such as carbohydrates, vegetables provide diverse nutrients, including bioactive compounds, dietary fibers, minerals, and vitamins crucial for overall health (Jiefen Cui et al., 2019; Yang et al., 2021). Plant bioactive compounds or secondary metabolites, such as flavonoids, carotenoids, sterols, phenolic acids, alkaloids, and glucosinolates, are essential to adapt and defend against environmental stressors. These compounds attract pollinators, deter herbivores, and initiate complex biochemical pathways in response to microbial attacks involving stress perception and signaling (Cisneros‐Zevallos, 2021; Cisneros‐Zevallos et al., 2014; Ngamwonglumlert et al., 2020). While primary metabolism supports growth through photosynthesis, respiration, and synthesis of essential compounds, it is tightly linked to secondary metabolism (Toscano et al., 2019). Usually, a rapid response is triggered, involving protein activation, calcium (Ca) ion transfer, and gene expression changes, producing defensive proteins (Cisneros‐Zevallos, 2021; Jacobo‐Velazquez et al., 2015). For example, the phenylpropanoid pathway, originating from the shikimate pathway, is critical in plant defense and stress response (Toscano et al., 2019). Terpenes, another class, are synthesized through the mevalonic acid and 2‐C‐methylerythritol 4‐phosphate pathways (Khare et al., 2020). Glycolysis and the pentose phosphate pathway also activate the phenylpropanoid pathway, facilitating secondary metabolite production, including polyphenols (Toscano et al., 2019). Similarly, carotenoids are found in photosynthetic and non‐photosynthetic plant tissues, meaning their content can be influenced by all environmental factors (L. Liu et al., 2015). They also serve as precursors for essential phytohormones, such as abscisic acid (ABA) and strigolactones, which regulate plant development and stress responses. Yuan et al. (2015) reviewed carotenoid biosynthesis, highlighting advances in understanding vegetable metabolism regulation.

Beyond their role in plant biology, these metabolites provide significant human health benefits, including antioxidative, anticarcinogenic, and anti‐inflammatory properties (Hong & Gruda, 2020), protecting against cellular damage (Slavin & Lloyd, 2012). Moreover, the high fiber content in vegetables promotes satiety, helps with weight management, and supports digestive health, reducing the risk of obesity and related metabolic disorders (Boeing et al., 2012). Thus, in academic discussions, vegetable consumption is associated with a lower incidence of chronic diseases, indicating their health‐promoting properties. For instance, Miller et al. (2017) conducted a study with 135,335 persons aged 35 to 70 across 613 communities in 18 countries. Elevated intake of fruits, vegetables, and legumes was associated with reduced risks of cardiovascular diseases. Similarly, F. J. He et al. (2006) and Stanaway et al. (2022) reported a reduced risk of stroke. Stanaway et al. (2022) found that transitioning from no vegetable consumption to the theoretical minimum risk exposure level (306–372 g d⁻¹ capita⁻¹) significantly reduced ischemic stroke, ischemic heart disease, hemorrhagic stroke, and esophageal cancer risks.

Hence, the World Health Organization (WHO) advocates consuming 400 g of fruits and vegetables daily. In comparison, the EAT‐Lancet report recommends a higher intake of 500 g daily for improved planetary health (Innovation in fruit and vegetable supply chains, 2022). Incorporating vegetables into the diet is indispensable for a healthy lifestyle (Hounsome et al., 2008), supported by initiatives such as the “five‐a‐day” program endorsed by the WHO. Bioactive compounds from vegetables and fruits can be detected in human blood within hours of consumption. Yet, they are metabolized quickly, highlighting the necessity of regular intake as advocated by the five‐a‐day guideline (Gruda et al., 2018; X. Wang et al., 2014). For example, the Mediterranean diet, which prioritizes the consumption of vegetables, fruits, and nuts, reduces the risk of developing cardiovascular diseases (Maroto‐Rodriguez et al., 2024; Vincente et al., 2014).

Protected cultivation farming is increasingly crucial for sustainable intensification in agriculture. It has demonstrated significant potential in boosting production (Aznar‐Sanchez et al., 2020). By 2018, global protected vegetable cultivation covered approximately 5.6 million hectares (Cuesta Roble Greenhouse Vegetable Consulting, 2019; Qasim et al., 2021), with 83% of this area in China (Fei et al., 2018). The annual growth rate has been nearly 20% (Espí et al., 2016). However, comprehensive statistical data remain scarce, as even the FAO refrains from systematic monitoring. Nonetheless, protected cultivation remains essential for ensuring food security and meeting the rising demand for resource‐intensive diets (Alexandratos & Bruinsma, 2012; Aznar‐Sanchez et al., 2020; Tilman et al., 2011). Besides, indoor and vertical farming have recently emerged as cutting‐edge strategies, particularly in densely populated urban areas and mega‐cities.

Covering shields crops from adverse conditions while regulating macro and micro‐environments. This regulation improves water use, optimizes plant performance, extends production, promotes early maturation, and ultimately enhances yield and product quality (Gruda & Tanny, 2014, 2015). Growers utilize various covered structures depending on factors such as crop type, regional climate, and expected benefits (Gruda & Tanny, 2014), including plastic‐covered greenhouses, walk‐in tunnels, and low‐plastic tunnels (Jiménez‐Lao et al., 2020). In Mediterranean regions, greenhouses are mainly used for tomato and pepper production, while some plastic covers assist vineyards in southern Italy. Low tunnels are standard for melons, berries, lettuce, and broccoli. In China, tomatoes, cucumbers, peppers, herbs, and occasional flowers are grown mainly in plastic tunnels. Plastic is also increasingly used for tropical fruit production (Jiménez‐Lao et al., 2020). This variety of approaches opens up a world of possibilities for the future of protected cultivation farming and improving the produce quality of vegetables.

Further, two factors are instrumental in the prioritization of protected cultivation: (i) the substantial economic contribution of vegetable production within protected cultivation, accounting for 60% of the global vegetable industry's economic value (Gruda et al., 2024; Hu et al., 2020), and (ii) the ability of protected cultivation to provide superior control over environmental conditions that influence produce quality. Protected cultivation, distinguished by its capacity to yield vegetables with superior characteristics, including taste, texture, color, and flavor, ensures the availability of ready‐to‐eat products throughout the year. It also enables off‐season vegetable production with premium prices, where open‐field farming is not feasible. Expensive protected cultivation practices, particularly soilless culture techniques (Gruda, 2022), lay the foundation for advanced indoor cultivation methods, including vertical farming, sprouts, microgreens, and baby leaf vegetables. These methods maximize nutritional value, providing optimal health benefits to consumers.

Research is often focused on post‐harvest and processing factors when assessing vegetable quality, primarily due to the perishability of fresh produce, which requires strict preservation across the supply chain (Davis et al., 2021). However, while packaging and additives extend shelf life, they also contribute to waste and pollution without significantly improving nutritional quality (X. Liu et al., 2022).

Vegetables from protected cultivation are produced for fresh consumption (Gruda, 2019), aligning with consumer expectations for freshness, flavor, appearance, nutritional value, safety, and environmental sustainability (Gao et al., 2022; Toscano et al., 2019). In addition, while maintaining favorable post‐harvest conditions can preserve produce quality for a limited time, it rarely enhances it, particularly regarding nutritional value (Gruda et al., 2018). Historically, environmental manipulation focused on productivity rather than quality. However, the potential of pre‐harvest conditions to improve nutritional, taste, and health attributes is now a promising area of research.

Vegetable quality traits are classified into three main categories: commodity quality or market value, focusing on appearance, freshness, and consistency; sensory value, covering texture, flavor, and taste; and nutritional quality, which includes essential nutrients like dietary fiber, minerals, vitamins, and bioactive compounds, as well as undesired substances like pesticide residues, nitrates, and mycotoxins (Gao et al., 2022; Gruda, 2019). Several studies have examined the impact of greenhouse environmental factors on improving the nutritional quality of vegetables (Rouphael et al., 2012), offering an alternative to combat malnutrition caused by unhealthy diets, urbanization, and/or climatic changes (Dong et al., 2018; Scheelbeek et al., 2018; Toreti et al., 2020; Yu et al., 2023).

The objective of this research is to conduct a thorough examination of existing studies and explore methods to enhance the nutritional content of vegetables grown in protected environments. We aim to identify and address potential obstacles, investigate novel methodologies, and assess the viability of implementing these methods while maintaining optimal yield. Specifically, the study seeks to comprehensively analyze how various environmental factors influence the nutritional quality of vegetables cultivated within these structures and give some recommendations for improving the nutritional quality of vegetables.

2. MATERIAL AND METHODS

Literature was sourced from Web of Science Core Collection, Scopus, ScienceDirect, and Google Scholar, targeting English‐language journal papers from the past 20 years, emphasizing the last 5 years. Keywords were strategically combined to yield comprehensive search results, using phrases such as “environmental conditions” AND “vegetable(s)” AND “protected cultivation”; “eustress” AND “vegetable(s)” AND “(human) nutrition(s)” AND “protected cultivation”; “environmental conditions” AND “vegetable(s)” AND “nutritional quality”; and “environmental conditions” AND “vegetable(s)” AND “nutraceutical quality.” Priority was given to biochemical, physiological, and molecular studies focused on specific genes, proteins, or metabolites.

Vegetables were selected due to their predominant fresh consumption with minimal processing, excluding post‐harvest treatments. Specific crop names (e.g., tomatoes, peppers, cucumbers, eggplants, lettuces, leafy vegetables, and fruit vegetables) were also used, along with terms like “greenhouse(s),” “plastic houses,” “high tunnels,” “controlled environmental conditions,” and “indoor cultivation” in place of “protected cultivation.” Although the primary focus was on controlled environment vegetables, examples from outdoor cultivation and other plant species were included to clarify mechanisms and illustrate how moderate environmental stress can improve nutritional value.

The search focused on peer‐reviewed studies, excluding non‐peer‐reviewed sources. Selection criteria required an in‐depth examination of the effects of environmental conditions on controlled‐system vegetables and studies on enhancing nutritional profiles. Papers not meeting these criteria were excluded, followed by a supplementary search of references from screened literature and additional relevant case studies.

3. THE IMPACT OF ENVIRONMENTAL CONDITIONS ON VEGETABLES’ NUTRITIONAL QUALITY

3.1. Light

3.1.1. How does light affect vegetable nutritional quality?

Light fuels photosynthesis, the lifeblood of plants. It also signals growth, development, and diverse physiological, biochemical, and molecular responses, including dry matter allocation and water content. Studies show that plants’ metabolism and biochemistry depend on light intensity, spectrum, and photoperiod (Borbély et al., 2022; Paradiso & Proietti, 2022). Plant photoreceptors, including phytochromes, cryptochromes, ZEITLUPE (ZTL), FLAVIN‐BINDING, KELCH REPEAT, F‐BOX 1 (FKF1), and LOV KELCH PROTEIN 2 (LKP2) proteins, and UV resistance locus 8 (UVR8), absorb far‐red (700–800 nm), red (600–700 nm), blue (400–500 nm), green (500–600 nm) and ultraviolet (280–400 nm) spectra (Figure 1).

FIGURE 1.

Light spectra influence plants’ nutritional and bioactive properties, which specific photoreceptors mediate. UV‐B and UV‐A promote the accumulation of essential nutrients such as calcium, potassium, and magnesium, while red and far‐red light encourages phosphorus and nitrogen uptake. UV‐A and blue light strongly influence bioactive compounds like carotenoids, flavonoids, and vitamin C, whereas red and far‐red light primarily enhances chlorophyll and phenols. These effects are regulated by photoreceptors, including UVR8 for UV‐B light, phototropins for blue light, and phytochromes for red and far‐red light, orchestrating the plant's response to its light environment. CRY, cryptochromes; FKF1, FLAVIN‐BINDING, KELCH REPEAT, F‐BOX 1; LKP2, LOV KELCH PROTEIN 2; PHOT, phototropins; PHY, phytochromes; UVR8, UV resistance locus 8; ZTL, zeitlupe proteins.

The primary photoreceptors control gene expression and metabolism, affecting the plant's response to light. Different light wavelengths have varying effects on photosynthesis, with red light having the highest efficiency (Hogewoning et al., 2012). Increased light intensity generally leads to higher photosynthetic rates but can also cause photo‐oxidative stress (Poorter et al., 2019). The proper activity of photosystem II controls the balance between antioxidants and oxidants. The activity of photosystem II (PSII) is maintained by HY5, which can transfer light signals from UV to far‐red spectrum to light‐regulated genes (Gommers, 2020). Both light intensity‐ and spectrum‐dependent regulation of plant metabolic responses could contribute to the added nutritional value of greenhouse vegetables. Alrifai et al. (2019) reviewed the effect of light wavelength, intensity, and photoperiod on antioxidant phytochemical biosynthesis, highlighting the modulatory role of photoreceptors on gene expression, enzyme activities, and synthesis of secondary metabolites. However, due to species‐specific plant responses to light, as well as varying unequal environmental factors, key characteristics of lighting source, and natural light conditions depending on the latitude, it is sometimes hard to compare the scientific results related to greenhouse plants’ nutritional value and light effects (Sipos et al., 2020).

3.1.2. Protected cultivation and supplemental lighting

Greenhouse structures and coverings, despite advances, reduce daylight and alter the light spectrum. Roof design and crop layout further impact light distribution and crop quality (Gruda, 2019). Moreover, greenhouses for high‐value vegetable production are scientifically designed to increase light exposure (Ampim et al., 2022). Solar radiation, intensity, and spectrum vary by location, season, and time of the day (Bantis & Koukounaras, 2023). Thus, protected cultivation heavily relies on daily solar radiation, which must meet minimal light requirements for vegetables. Nowadays, technologically advanced glasshouses, which are particularly prevalent in northern regions, demonstrate the ability to manage environmental parameters, such as light intensity, spectrum, and duration, along with functionalities for cooling, heating, and ventilation (Karanisa et al., 2022; Katzin et al., 2021; Palmitessa et al., 2021).

However, the Mediterranean region predominantly utilizes low‐tech plastic greenhouses (Paucek et al., 2020). Similarly, 83% of greenhouses in Australia are classified as low‐tech or medium‐tech (Chavan et al., 2022). In Canada, 96% of vegetables grown in protected cultivation are fruit, mainly tomatoes (LaPlante et al., 2021).

Supplemental lighting in greenhouses is used for several purposes: (i) to extend the vegetation period and obtain higher yields (Appolloni et al., 2021; Brazaitytė et al., 2009), (ii) to regulate plant developmental processes for length control (Dutta Gupta & Agarwal, 2017; Kalaitzoglou et al., 2019), and (iii) to improve the nutritional quality of greenhouse vegetables (H. ‐J. Kim et al., 2020; Miao et al., 2019). It can increase electricity consumption and carbon footprint, but advancements in light‐emitting diode (LED) technology have enabled more control over plant growth and reduced energy consumption (Appolloni et al., 2021; Bantis & Koukounaras, 2023; Katzin et al., 2021). Supplemental lighting optimizes photosynthesis, metabolism, and vegetable quality (Paponov et al., 2019; Paucek et al., 2020; Y. ‐T. Zhang et al., 2019). Knowledge of the specific light requirements for different crops and development stages can improve greenhouse vegetable production schedules, crop yield, and quality (Paradiso & Proietti, 2022).

3.1.3. Enhanced nutritional quality in vegetables through daily light integral (DLI), light intensity, and photoperiod

The DLI (mol m⁻² day⁻¹) reflects the total daily light usable for photosynthesis by plants. This depends on photoperiod and light intensity, specifically the photosynthetically active photon flux density. Several studies reveal that DLI has substantial effects on plant growth, biomass accumulation (Jiawei Cui et al., 2021; Dou et al., 2018), and nutritional quality (Carvalho et al., 2010; Eghbal et al., 2024; J. Song et al., 2020; Yan et al., 2019).

DLI requirements vary significantly among greenhouse plants. For instance, microgreens, including mustard, arugula, radish, and amaranth, thrive within 6–12 mol m⁻² day⁻¹ (Jiawei Cui et al., 2021; Yan et al., 2019). Leafy greens, such as lettuce, pak choi, herbs like mint and basil, and also strawberry plants prefer DLIs of 12–20 mol m⁻² day⁻¹ (Dou et al., 2018; Eghbal et al., 2024; Gavhane et al., 2023; Kelly et al., 2020). Fruit vegetables, such as tomatoes, cucumber, bell pepper, and chili, perform best within 15 – 30 mol m⁻² day⁻¹ (Garcia & Lopez, 2020; Moe et al., 2006; Morgan, 2013). However, too much light might cause photoinhibition and photodamage in plants (Viršilė, Brazaitytė, Vaštakaitė‐Kairienė, Miliauskienė, et al., 2019). For instance, Gavhane et al. (2023) found that lettuce phenolic content increases with DLI from 8.6 to 11.5 mol m⁻² day⁻¹ but decreases at 14.4 mol m⁻² day⁻¹ due to light saturation. The flavonoids are involved in quenching reactive oxygen species (ROS) and providing photoprotection, especially under high‐light conditions. Their production is upregulated by the expression of flavonoid biosynthetic genes, such as PAL, MYBs, CHS, and so forth (Lingwan et al., 2023).

Light duration, intensity, and quality significantly affect greenhouse vegetables’ phytochemical and mineral content (Yan et al., 2021; Zhen & Bugbee, 2020; Figure 2). When plants are exposed to excessive light, oxidative injury occurs on cellular components such as proteins, lipids, and DNA due to the accumulation of ROS (Sun & Fernie, 2024). Among phytochemicals with antioxidant effects, the content of vitamin C, including ascorbic and dehydroascorbic acid, positively correlates with light intensity in fruit vegetables, and leafy greens (Gavhane et al., 2023; Gruda & Tanny, 2014, 2015; Ntagkas et al., 2019). L‐ascorbic acid is produced via the Smirnoff–Wheeler pathway from D‐glucose‐6‐phosphate, the primary path in photosynthetic tissues (Fenech et al., 2019). Key intermediates, guanosine diphosphate (GDP)‐D‐mannose and GDP‐L‐galactose, also contribute to the non‐cellulosic components of the cell wall. Although light is not essential for ascorbic acid synthesis, it is synthesized from sugars supplied through photosynthesis (S. K. Lee & Kader, 2000). Exposure to light increases its content. For instance, light regulation of ascorbate in tomatoes is locally perceived in the fruit, and leaf irradiance influences photosynthesis and sugar transport to the fruits (Gautier et al., 2008; Ntagkas et al., 2019). Consequently, plants grown in greenhouses generally have lower vitamin C levels than those grown in the field, likely due to reduced light intensity (Paciolla et al., 2019). Thus, enhancing produce quality through ascorbate biosynthesis and regulation requires precise control to balance growth with cell redox stability, avoiding developmental trade‐offs (Fenech et al., 2019; Kathi et al., 2024).

FIGURE 2.

Light intensity, measured as photosynthetically active photon flux density (PPFD), influences plant physiology. As PPFD increases, photosynthesis rises until reaching a genotype‐specific optimum, accumulating sugar. At low PPFD, nitrate levels are higher due to reduced assimilation, while increasing light intensity generates reactive oxygen species (ROS) from excess energy. Antioxidative production and flavonoid accumulation are enhanced to counteract ROS‐induced oxidative stress. This balance between photosynthesis, nitrate assimilation, ROS, and antioxidants reflects the plant's adaptive response to light intensity reflected in nutritional quality.

Colonna et al. (2016) reported a significantly higher total phenolic compound content in lettuce, chicory, chard, and mizuna under higher light intensities than lower ones. Lanoue et al. (2022) demonstrated high DLI and continuous light increased phenolics, anthocyanins, and antioxidants in microgreens. Additionally, the accumulation of sugars increased in lettuce (W. Fu et al., 2012) and tomatoes (Dorais & Gosselin, 2002). Therefore, it is crucial to consider an optimal light intensity for maximizing phytochemical accumulation (Gómez et al., 2019). Adaptation to light intensity includes short‐term adjustments, such as optimizing photosynthetic efficiency through protein function (Voutsinos et al., 2021), an increasing pigment accumulation (Darko et al., 2014; Loconsole et al., 2019; Samuolienė et al., 2013), and long‐term adaptations to improve light interception (Weston et al., 2000).

Light intensity and photoperiod are the primary factors contributing to nitrate levels and are primarily dictated by nitrate reductase activity (Proietti et al., 2004). Proietti et al. (2004) also observed that spinach accumulated higher oxalate and nitrate levels under lower light conditions. Viršilė, Brazaitytė, Vaštakaitė‐Kairienė, Miliauskienė, et al. (2019) found that an increased light intensity provides more energy for photochemistry, enhancing carbohydrate production and accelerating nitrate assimilation into amino acids. However, findings suggest that leaf nitrate concentration can be influenced by both light intensity and quality (Nájera & Urrestarazu, 2019; Figure 2, Table 1).

TABLE 1.

Nutritionally qualitative performance of vegetables under different light quantities.

Lighting treatment	Vegetable species	Nutrient quality	References
Low PAR	Baby leaf vegetables (chicory, green lettuce, lamb's lettuce, mizuna, red chard, red lettuce, rocket, spinach, Swiss chard, and tatsoi)	Increasing protein content, antioxidant activity, P, K, Ca, and Mg. Decreasing total phenolic compounds	Colonna et al. (2016)
Increased PPFD	Lettuce, spinach, tomato fruit	Decrease in nitrates and soluble proteins Increase of total ascorbate, soluble sugar	W. Fu et al. (2012), Nájera & Urrestarazu (2019), Ntagkas et al. (2019), Proietti et al. (2004), Voutsinos et al. (2021)
Shorter photoperiods	Lettuce, red and green amaranth, red beet, red spinach, Swiss chard, sweet potato	Decrease of nitrates Increase vitamin C, total phenolics, antioxidant activity, soluble protein, and sugar	Carvalho et al. (2010), Gavhane et al. (2023), J. Song et al. (2020)

Abbreviations: PAR, photosynthetically active radiation. PPFD, photosynthetic photon flux density.

The photoperiod regulates plants’ transition from vegetative to generative stages, with light absorption during the day and the dark period crucial for flowering, especially in photoperiod‐sensitive fruiting vegetables or breeding purposes (Lambers & Oliveira, 2019; Runkle et al., 2017). Antioxidants mitigate oxidative stress in both plant and human cells. Data indicate that leafy greens like lettuce, red beet, red or green amaranth, Swiss chard, or spinach exhibit peak antioxidant capacity at a 12‐h photoperiod (J. Song et al., 2020). At the same time, the total phenolic content in sweet potato leaves was higher at 16 h (Carvalho et al., 2010; Table 1).

3.1.4. Enhanced nutritional quality in vegetables through lighting quality optimization

Greenhouse vegetables’ physiological response and nutritional quality significantly depend on each light component's spectral composition and dosage, even at low intensities (Gómez et al., 2019). These effects are species‐specific and contingent upon complex environmental conditions (Mitchell et al., 2015; Samuolienė et al., 2017). Through a unique photoreceptor system, plants absorb light and, depending on the receptor type, respond to light intensity and specific light wavelengths (Paik & Huq, 2019). This system regulates photo‐morphogenetic responses and phytochemical accumulation (Azad et al., 2020).

The morphological and metabolic response for light quality is plant growth stage‐specific. For (i) seedlings, the ratio of blue to red is essential; for (ii) biomass accumulation, higher pigments, antioxidant compounds, and minerals, a small amount of green light is beneficial; and for (iii) flowering and fruiting, the increased blue or supplemental UV‐A is significant.

Thus, (i) Gallegos‐Cedillo et al. (2024) noted the extensive utilization of emerging lighting technology in greenhouse vegetable seedlings and transplant production. This enhances efficiency and improves nutritional quality through bio‐compound accumulation during early growth stages. Izzo et al. (2020), Kaiser et al. (2019), and Samuolienė et al. (2021) reported that a combination of red and blue light spectra has a positive impact on enhancing the growth and quality of tomato seedlings.

(ii) The greater total biomass and accumulation of phenolic compounds, organic acids, α‐tocopherol, vitamin C, or soluble sugar was increased in greenhouse vegetables, mainly lettuce and basil, cultivated under various combinations of red, blue, and green LEDs (Bantis et al., 2016; Samuolienė et al., 2013, 2017). Although green light is less efficient for photosynthesis per unit of leaf area, it can still increase growth at the whole‐plant level by altering light distribution, leaf acclimation, and canopy architecture. Horticultural LED lighting commonly utilizes red and blue spectra thanks to their high photosynthetic and photon efficacy. Additionally, a small amount of far‐red light can positively affect basil biomass formation (Bantis et al., 2016).

(iii) Mariz‐Ponte et al. (2021) suggested that supplementing greenhouse‐grown tomato plants with UV‐A during the fruiting/ripening stage activates adaptive mechanisms, such as increased PSII peptides and ribulose bisphosphate carboxylase/oxygenase (Rubisco) transcription. These adaptations mitigate any negative impacts on photosynthesis, ensuring that carbohydrate balances and plant yield remain unaffected, making this approach a valuable tool for precision agriculture. It is established that blue light triggers the CRY/ZTL/FKF1/LKP2 photoreceptors in vegetables by affecting the expression of the flowering time gene and initiating flowering (Hines, 2008; Zoltowski & Imaizumi, 2014). Fan et al. (2021) demonstrated that a red and blue light combination (1:1) significantly accelerated the appearance of the first flower and first tomato fruit while increasing the sugar‐to‐acid ratio, enhancing the tomatoes’ taste.

Generally, LED lighting conception is based on the plant photoreceptor's ability to absorb specific wavelengths. Phytochromes are particularly sensitive to red and far‐red light (Smith, 2000) and affect plant germination, photosynthetic capacity, flower induction, architectural development, and nutrition (Demotes‐Mainard et al., 2016). Red and blue light is absorbed by photo‐morphogenetic receptors, such as phytochrome, cryptochromes, and photosynthetic receptors, including chlorophylls and carotenoids (Hogewoning et al., 2010).

The effects of red and blue light are generally associated with biomass production and photosynthesis (D. Kim & Son, 2022). Despite green light's lower efficiency in driving photosynthesis per unit leaf area, compared to red light, its presence can enhance overall plant growth through alterations in vertical light distribution, leaf light acclimation, and canopy architecture. Kaiser et al. (2019) demonstrated that partially replacing red light with green light enhanced tomato plant growth in dense canopies. The findings suggest that the combined effect of the light spectrum on plant growth surpasses the sum of individual wavelength effects on photosynthetic efficiency.

Cryptochromes and UVR8 absorb UV, which is generally considered a damaging factor for photosynthesis, triggering photoprotective responses. Plant responses to UV‐A on biomass accumulation and morphology can include stimulatory and inhibitory effects (Verdaguer et al., 2017). Light regulates phenolic compounds or carotenoid biosynthesis by affecting transcription factors. HY5 is the primary regulator for light‐induced anthocyanin accumulation in various plants and is activated by many photoreceptors like PHY, CRY, and UVR8 (Lingwan et al., 2023; Y. Liu et al., 2023; Figure 1; Ma et al., 2021). White light has recently been used in protected horticulture because it is characterized by broad‐spectrum features (Nájera et al., 2023). However, white LEDs lack UV‐A wavelengths, influence plant metabolism, especially for phenolic and antioxidant compound accumulation, and serve as a tool to biofortify plants with nutritionally valuable components (Jacobo‐Velázquez et al., 2022; Laužikė et al., 2023).

For leafy and fruit vegetables in the greenhouse, an optimized light spectrum promotes canopy growth and enhances leaf carbon export, improving yield and quality (Hao et al., 2018). Table 2 presents vegetables’ nutritional and qualitative performance under different light spectra. For instance, microgreens showed higher nutritional quality under standard blue, red, and far‐red LED lighting (Brazaitytė et al., 2016). While supplemental green light efficiently reduced nitrate levels in microgreens, it resulted in a decreased antioxidant capacity. High pressure sodium (HPS) lighting supplemented with blue LEDs had the most pronounced effect on lettuce leaves antioxidant response, primarily due to the increased anthocyanin, ascorbic acid, and tocopherol content (Samuolienė et al., 2013).

TABLE 2.

Nutritionally qualitative performance of vegetables under different light quality.

Lighting treatment	Vegetable species	Nutrient quality	References
HPS with supplemental red HPS with supplemental blue	Microgreens (lettuce, mustard, red pak choi, tatsoi, basil, beet, parsley) Lettuce	Increase of secondary metabolites, total phenolics, antioxidants, essential elements	Brazaitytė et al. (2021, 2016), Samuolienė et al. (2013)
Blue LEDs	Cherry tomato seedlings, red leaf lettuce, Brassica crops	Increase of total phenolics, antioxidants, total flavonoids, sugars	Azad et al. (2020), E.‐Y. Kim et al. (2015), Kopsell et al. (2015)
R and R+FR R and FR	Tomato	Increase of K, Mg, Ca in fruit Increase of lycopene	H.‐J. Kim et al. (2020) Alba et al. (2000)
Supplemental blue and UV	Basil, arugula	Increase of phenolic acids and flavonoids	Taulavuori et al. (2018)
Supplemental RB, B‐UVA, W LEDs	Lettuce	Increase of antioxidant capacity, anthocyanins, carotenoids, and total phenolics	Amoozgar et al. (2017), Hooks et al. (2022), Lycoskoufis et al. (2022), Pola et al. (2020)

Abbreviations: B, blue; FR, far‐red; HPS, high‐pressure sodium lamps; LEDs, light emitting diodes; R, red; W, white.

Current research indicates that exposure to blue wavelengths stimulates the biosynthesis of primary and secondary metabolites by activating a cascade of metabolic responses through specialized plant photoreceptors. Azad et al. (2020) demonstrated that under supplemental blue LEDs, accumulation of phenolic compounds and pigments in red leaf lettuce increased due to enhanced photosynthetic performance. The beneficial effects of blue LEDs on primary and secondary metabolism were demonstrated in microgreen and baby leafy Brassica crops (Kopsell et al., 2015) and tomato seedlings (E. ‐Y. Kim et al., 2015; Table 2).

H. ‐J. Kim et al. (2020) reported that supplemental intracanopy lighting with red and far‐red LEDs improved greenhouse tomato fruit quality, increasing total soluble sugars, titratable acidity, and macroelement levels (Table 2). Incorporating far‐red light could potentially enhance the photosynthetic efficiency. This may arise from elevated carbohydrate accumulation in vegetative tissues and a greater allocation of photosynthates to developing fruit. Consequently, this could increase total soluble sugars and titratable acidity (Zhen & van Iersel, 2017). S. Chen et al. (2024) observed that increased competition for nutrients between sweet pepper shoot apices and flowers, caused by the stimulation of far‐red, leads to flower and fruit abortion. The reduced accumulation of sucrose and lower invertase activity in flowers under additional far‐red may signal hormonal changes, ultimately leading to flower and fruit abortion.

Supplemental lighting from red‐blue, blue‐UVA, or white LEDs enhanced the internal quality traits of hydroponically grown greenhouse lettuce, including antioxidant capacity and the concentrations of phytonutrients such as anthocyanins, carotenoids, and total phenolics, compared to those without supplemental lighting (Hooks et al., 2022). However, no significant differences were observed among mentioned lighting treatments. Lycoskoufis et al. (2022) found that the total phenolic and flavonoid content and the antioxidant potential of red leaf lettuce were significantly higher in a UV‐A‐open greenhouse, compared to lettuce grown in a UV‐A‐block greenhouse (Table 2). Thus, such metabolic response might be related to plant‐developed response systems that can adapt to a broad spectrum of light in natural conditions.

Additionally, light influences the nutrient uptake in roots and subsequent utilization in shoots (Lejay et al., 2003; Sakuraba & Yanagisawa, 2018) and is linked to sugar‐mediated gene expression in roots (Ohashi‐Kaneko et al., 2007). Further, it regulates carbohydrate supply from leaves to roots due to photosynthesis (Lejay et al., 2008). For instance, magnesium (Mg) is involved in protein synthesis and energy metabolism. Furthermore, it assumes a critical function in facilitating the transport of photosynthates between shoots and roots, participating in electron transport to CO₂, and mitigating photooxidative damage induced by ROS under high light conditions (Ouzounis et al., 2015). Compared to other lighting treatments, red LEDs significantly increase magnesium accumulation in tomato fruit and leaves (H. ‐J. Kim et al., 2020; Table 2). This suggests that red light is intensely involved in magnesium uptake and accumulation in tomato fruit. Leaves may absorb more photons under red light, increasing magnesium concentration and acting as a protective mechanism for plants (Figure 1). Zhou et al. (2018) also found a significant correlation between magnesium accumulation in plant leaves and magnesium, sugar, and organic acid concentrations in tomato fruits. However, summer supplemental LED inter‐lighting did not considerably affect sweet pepper quality and marketable yield in greenhouses (Kwon et al., 2023). Brazaitytė et al. (2021) found that an increase in mineral nutrients in kale and mustard microgreens correlated with an increase in blue light percentage (Figure 1, Table 2). Such changes may be related to the activation of blue light‐regulated phototropin‐mediated (Phot1, Phot2) ion channels on cell plasma membranes that promote ion flux transport (Kopsell et al., 2015).

Polyphenols participate in plants’ defense reactions, which environmental factors, including UV radiation might cause. These metabolites absorb UV light as protective molecules, saving mesophyll from photooxidation (Sharma et al., 2019).

The effect of blue, red, far‐red, or white light on changes in total phenol, total flavonoid contents, or variation in composition has been reported by many authors (E. ‐Y. Kim et al., 2015; Taulavuori et al., 2018; P. Wang et al., 2020; Figure 1, Table 2). Maroga et al. (2019) proposed that the increased activity of the key enzymes in the shikimate and phenylpropanoid pathways causes these changes.

Carotenoids represent another group of bioactive metabolites (Ngamwonglumlert et al., 2020; Singh & Goyal, 2008). Carotenes, such as α‐, β‐, γ‐ carotenes, and lycopene, are primarily involved in photosynthesis and photoprotection reactions through participation in light‐harvesting reactions and limiting the damage of cell membranes caused by light exposure. Alba et al. (2000) demonstrated that lycopene accumulation in tomato fruit is regulated by phytochromes, which respond to red and far‐red light (Figure 1, Table 2). Meanwhile, Giliberto et al. (2005) suggested that lycopene accumulation is related to the overexpression of Cry2, which is regulated by cryptochromes that absorb blue light. Increased expression of lycopene epsilon cyclase (LYCε) under amber, blue, and red LEDs (at a ratio of 12.02:26.2:61.4) led to the accumulation of α‐carotene and lutein, while beta‐ring carotenoid hydroxylase promoted lutein accumulation in Brassica microgreens (Alrifai et al., 2021). Xanthophylls, such as β‐cryptoxanthin, lutein, violaxanthin, zeaxanthin, and astaxanthin, are involved in non‐photochemical quenching and participate in photoprotective reactions (Maoka, 2020; Pizarro & Stange, 2009). The biosynthesis of carotenoids can be regulated by light‐responsive genes (Pizarro & Stange, 2009), whose expression depends on red, blue, and green light (C. X. Li et al., 2017; Llorente et al., 2017; Figure 1). The carotenoid levels in lettuce (Amoozgar et al., 2017) and green chili (Pola et al., 2020) increased under red and blue LEDs (Table 2).

In conclusion, it is crucial to consider the relationship between DLI, light intensity, photoperiod, and resource use efficiency when developing an optimal light control algorithm to improve greenhouse vegetable nutritional quality. This has far‐reaching consequences for sustainable agriculture. Moreover, many climate conditions change in greenhouses when the light intensity changes. Thus, lighting in the greenhouse should not be considered a separate growth factor as it forms an integral part of the total greenhouse management.

3.2. Temperature

Temperature influences the rates of all chemical and biochemical reactions, significantly impacting plant growth and development and the nutritional quality of vegetable produce (Gruda, 2005; Jung et al., 2023). Factors such as light intensity and, to a lesser extent, CO₂ concentration influence temperature in protected cultivation. Controlled environmental agriculture optimizes plant growth by shielding crops from heat and cold stress. Conversely, extreme temperatures can stunt plant growth, affecting their primary and secondary metabolites and mineral content (Gruda, 2005; Gruda & Tanny, 2014, 2015).

Temperature affects primary vegetable metabolite accumulation by influencing the balance between photosynthesis and respiration (Turnbull et al., 2002), the transcription of enzymes such as Pyrococcus furiosus superoxide reductase, dehydration‐responsive element binding protein 2, and S‐nitroso glutathione reductase, and genes related to heat shock proteins and phospholipids (Ahuja et al., 2010). Heat stress, for instance, reduces photosynthate production primarily due to decreased Rubisco and ribulose‐1,5 biphosphate activities (Sage et al., 2008).

Similarly, temperature profoundly affects secondary compound biosynthesis, regulation, fluctuations (Jamloki et al., 2021), and mineral dynamics. Sun et al. (2023) found in a meta‐analysis that elevated temperatures significantly increased secondary metabolite concentrations, particularly total flavonoids, in herbs, medicinal, and aromatic plants. Heat stress can elevate certain phenolic compounds, potentially attributable to their function as antioxidants in safeguarding plants from heat stress (Chai & Schachtman, 2022). On the other hand, higher temperatures exponentially enhance mineral diffusion rates in soil or substrate, as per the Arrhenius equation and Fick's first law (Benallou, 2019). Elevated temperatures also increase stomatal conductance (Urban et al., 2017), improving mass flow driven by increased transpiration (D. Li et al., 2023). Warmer root‐zone environments promote root growth and activity, enhancing mineral nutrient interception and absorption (D. Li et al., 2022). Thus, optimizing root‐zone temperature enhances the mineral nutritional quality of vegetables.

Numerous studies have extensively explored the relationship between temperature and vegetable quality, a topic of significant interest and relevance (Giordano et al., 2021; Gruda, 2005). In this study, we examine how the temperatures of the aerial parts and the root zone affect the nutritional value of greenhouse vegetables.

3.2.1. How does the aerial part temperature affect the nutritional quality?

Impact of elevated aerial part temperatures on vegetable nutritional quality

Advanced greenhouse structures are equipped with various heating systems utilizing biomass, fuel, electrothermal, geothermal, or solar energy sources. These systems actively raise the air temperature during cold weather, enhancing vegetable nutritional quality (Table 3). Low temperatures constrain water and nutrient uptake. Cold temperatures also hinder nutrient absorption in warm‐season plants, affecting the sensory properties. For instance, Kano and Goto (2003) observed increased bitterness in cucumber fruits in non‐heated greenhouses.

TABLE 3.

Nutritionally qualitative performance of vegetables under different aerial part temperatures.

Vegetable species	Temp. treatment	Growing conditions	Vegetable nutrient quality	References
Tomato (Solanum lycopersicum L.), cv. Counter and cv. Supersweet	Daily temp. 18.0, 19.9, and 22.0°C in spring, and 15.0, 17.6, and 20.3°C in autumn	Nutrient film cultured	Lycopene increased with higher spring and autumn temp	Krumbein et al. (2006)
Tomato (Solanum lycopersicum L.), cv. Pannovy and cv. Supersweet	Day/night heating set points: 20/18°C, 16/14°C, and 11/9°C	Nutrient film technique	No change in sugar content; titratable acid content slightly decreased; insoluble dry matter decreased under 11/9°C, compared to 20/18°C	Kläring et al. (2015)
Tomato (Solanum lycopersicum), cv. Velasco	Air temp. of 24°C increased to 32°C at six different stages	Coir fiber and perlite (85:15)	Vc, phytoene, phytofluene, lycopene, γ‐carotene, and violaxantin concentrations significantly lower at a short exposure of 32°C	Hernandez et al. (2015)
Tomato (Solanum lycopersicum L.), cv. Laura	Day and night heating set points: 23/18°C, 30/25°C, 28/23°C, 18/13°C, and 16/11°C	Hydroponics	Titratable acidity and soluble solids decreased with decreasing temp.; no changes for Vc content	Fleisher et al. (2015)
Tomato (Solanum lycopersicum L.)	Max. air temp. increased from 25 to 35°C	Hydroponics	Vc and homovanillic acid‐O‐hexoside increased. Hydroxycinnamic acids, flavanones, phytoene, phytofluene, and violaxanthin decreased	Botella et al. (2021)
Tomato (Solanum lycopersicum L.) cv. Moneymaker	Rise of diurnal temp. by 5°C. Rise of diurnal temp. by 2°C and lower the nocturnal temp. by 3°C	Soil cultured	A 5°C rise in temperature increased Vc, hexoses, lycopene, and carotenoids, lowered starch, and reduced sugar/acid ratio. Citric and malic acid levels increased; phenolic levels remained unchanged	Ruiz‐Nieves et al. (2021)
Cherry tomato (Solanum lycopersicum L.) cv. Cervil	The air temp. 21 and 26°C or 27 and 32°C	Soil cultured	Carotene decreased from 21 to 26°C. Vc and lycopene decreased, but rutin, caffeic acid derivatives, and glucoside increased from 27 to 32°C	Gautier et al. (2008)
Eggplant (Solanum melongena L.), cv. Bonica F1	Air temp. decreased by 2.5–3.5°C in the fog‐cooled greenhouse	Rockwool slabs	Ca concentration increased; total‐N concentration decreased in the fruit on 65 DAT. Ca and Mg concentrations increased in the fruit on 112 DAT	Katsoulas et al. (2009)
Eggplant (Solanum melongena L.), cv. Ryoma and sweet pepper (Capsicum annuum L.), cv. Kyo Yutaka	Four different day and night temp. (DIF) set at 15/25°C, 17.5/22.5°C, 20/20°C, 25/15°C	A mixture of mountain soil and bark compost at a ratio of 1:1	DIF of 15/25°C produced the eggplant fruit's highest Ca, K, and Mg content. The highest K content in the sweet pepper fruit under DIF of 20/20°C	Inthichack et al. (2013)
Lettuce (Lactuca sativa L.), cv. Buttercrunch	Cooling of air temp. from 20–26°C to 10–22°C	Hydroponics	Nitrate decreased by 40%; sugar, malic acid and potassium increased under cool air temp	Gent (2016)
Dark red Lollo Rosso lettuce	Air temp. increased from 25 to 33°C.	Hydroponics	Flavonoid (quercetin glucoside, quercetin glucuronide and luteolin) increased; Mg, Ca, K, Mn, and Mo decreased	Sublett et al. (2018)
Lettuce (Lactuca sativa L.), cv. Green Wave	Cooling the air temp. from 25 to 15°C	Hydroponics	Cooling promoted the chlorogenic acid accumulation in lettuce leaves	Endo et al. (2022)
Lemon catmint (Nepeta cataria L. f. citriodora), lemon balm (Melissa officinalis L.), and sage (Salvia officinalis L.)	Air temp. 15, 20, and 25°C	Wood fiber substrate	15 and 25°C: high content of essential oil, while 20 and 25°C maximum recovery of essential oil. 25°C: low flavonoid content	Manukyan & Schnitzler (2006)
Red chicory (Cichorium intybus L.) and Garland chrysanthemum (Chrysanthemum coronarium L.)	Air temp. 20, 25, and 30°C during the day combined with 18°C at night	Hydroponics	Highest Vc, total phenol, and flavonoid contents under 25°C	S. G. Lee et al. (2013)
Beet (Beta vulgaris L.) and ssamchoo (Brassica lee L. ssp. namai)	Day air temp. 20, 25, and 30°C combined with 18°C at night	Hydroponics	Vc content increased; total polyphenol and flavonoid contents decreased as air temp. increasing	S. G. Lee et al. (2015)

Abbreviations: DAT, day after transplanting; Temp, temperature; Vc, vitamin C.

The temperature of the aerial parts significantly impacts bioactive compounds. Elevating the maximum aerial part temperature from 25 to 35°C significantly increases tomato fruits’ vitamin C and homovanillic acid‐O‐hexoside concentrations (Botella et al., 2021). Similarly, an increase from 25 to 33°C enhances flavonoids, including quercetin glucoside, quercetin glucuronide, and luteolin, in dark red Lollo Rosso lettuce; however, it decreases concentrations of Mg, Ca, K, Mn, and Mo (Sublett et al., 2018).

Lower greenhouse temperatures diminish carotene content in fresh tomatoes, with lycopene production potentially inhibited below 12°C, except for β‐carotene (Dumas et al., 2003). Similarly, eggplants grown in unheated greenhouses in cool winter conditions show less intense fruit skin color than in warmer periods (Zipelevish et al., 2000). Nevertheless, exceedingly high temperatures can also elicit a similar response. High temperatures of 30–35°C reduce lycopene content in tomatoes by converting it to β‐carotene (Hamauzu et al., 1998). Still, no effect on β‐carotene concentration was observed.

Conversely, Gautier et al. (2008) reported that raising temperatures from 21 to 26°C reduced total carotene content in tomatoes, while further increases to 32°C lowered ascorbate and lycopene levels but elevated rutin and caffeic acid derivatives. Hernandez et al. (2015) stated that exposing tomatoes to 32°C reduced nutrient concentrations, including vitamin C, phytoene, phytofluene, lycopene, γ‐carotene, and violaxanthin, compared to 24°C. This was attributed to the inhibition of phytoene synthase, a key enzyme in carotenoid biosynthesis. Prolonged exposure to high temperatures mitigated these effects, restoring carotenoid synthesis processes during cooler intervals of the photoperiod. This study underscores the influence of temperature on fruit pigmentation and lycopene, compounds recognized for human health benefits.

Aerial part temperatures significantly affect the raw materials and essential oil contents of lemon catmint, lemon balm, and sage in protected cultivation. Higher temperatures, up to 25°C, reduce the flavonoid content in these plants, while essential oil concentrations peak at 25°C for lemon catmint and lemon balm (Manukyan & Schnitzler, 2006).

Aerial part temperatures of 25/18°C yielded the highest concentrations of ascorbic acid, total phenol, and flavonoid in the leaves of both red chicory and garland chrysanthemum, compared to those observed at 20 or 30°C (S. G. Lee et al., 2013). Raising daytime air temperatures from 20 to 25 and 30°C while maintaining a steady night temperature of 18°C for beet (Beta vulgaris L.) and ssamchoo (Brassica lee L. ssp. namai), increased in ascorbic acid content, however, still decreased total polyphenols and flavonoids (S. G. Lee et al., 2015).

Impact of reduced aerial temperatures on vegetable nutritional quality

Cooling is also essential for greenhouse vegetable production in tropical and subtropical areas, especially in the summer, to prevent them from heating stress. Shading, wet fan pads, fogging, and roof evaporative systems are usually operated to cool the aerial part temperature in greenhouses (Gruda & Tanny, 2014, 2015).

Fog‐cooling of greenhouse temperatures in Greece by 3.3°C decrease minimally affected eggplant fruit soluble solids and acidity. However, it increased Ca and Mg but reduced N concentrations (Katsoulas et al., 2007, 2009). As observed in lettuce, cool air temperatures between 10 and 22°C reduced nitrate concentration by 40%, compared to warmer temperatures from 20 to 26°C. Sugar concentration was approximately 50% higher under the cooler air temperatures. Cool air temperatures also increased the malic acid and K concentrations in lettuce tissues (Gent, 2016).

Daily air temperatures from 20 to 24°C were optimal for lycopene production in cherry tomatoes, cv. Supersweet, and conventional round tomatoes, cv. Counter. In contrast, a decrease to 15°C diminished the lycopene content (Krumbein et al., 2006). Fleisher et al. (2015) reported that titratable acidity and soluble solids decreased with lower temperatures (18/13°C) over 2 weeks, starting 10 days after fruit set. However, they observed no significant differences between moderate and high temperatures in ascorbic acid and lycopene contents.

Impact of diurnal temperature fluctuations (DIF) on vegetable nutritional quality

Day–night temperature fluctuations significantly affect photosynthate distribution and vegetable nutritional quality. Raising daily temperatures by 5°C increased ascorbic acid, hexoses, lycopene, and carotenoids in tomatoes without affecting phenols. On the other hand, increasing daytime by 2°C and reducing nighttime temperatures by 3°C raised citric and malic acid levels, thus lowering the sugar/acid ratio in tomatoes (Ruiz‐Nieves et al., 2021). Compared to a normal DIF of 25/15°C (day/night), a negative DIF of 15/25°C (day/night) significantly decreased the dry weight but increased Ca, K, and Mg content in the fruits of eggplant and tomato (Inthichack et al., 2013).

3.2.2. How does the root zone temperature affect the nutritional quality?

In winter, soil temperature lags behind air temperature increases. Suboptimal root zone temperatures hinder produce quality (Gruda, 2005; D. Li et al., 2022). Therefore, there is a need for greenhouse vegetable production to be focused on optimizing root zone temperature, with increased attention to nutritional quality (Table 4).

TABLE 4.

Nutritionally qualitative performance of vegetables under different root zone temperatures.

Vegetable species	Temp. treatment	Growing conditions	Vegetable nutrient quality	References
Tomato (Solanum lycopersicum L.), cv. ‘heniz1015’	Soil temp. at 20 cm depth increased from 21.35–27.34°C to 24.80–32.48°C	Soil cultured	Vc and soluble sugar concentrations increased; the titratable acid concentrations decreased	Jia et al. (2020)
Tomato (Solanum lycopersicum L.), cv. Daier 1689	Soil temp. increased from 16.3–20.7°C to 18.2–26.2°C	Soil cultured	Vc, soluble sugar, lycopene, soluble solids, and total acid contents increased	Ouyang et al. (2022)
Cocktail tomato (Solanum lycopersicum L.), cv. Amoroso and cv. Delioso	Cooling the root temp. from 16–27°C to 10°C	Rockwool	Cv. Amoroso: Glucose, sugar, and Vc increased in second cluster fruits; N, P, S, Zn, Fe decreased. Cv. Delioso: Sugar and Vc increased in the third cluster fruit; P and Zn decreased	F. He et al. (2019)
Cucumber (Cucumis sativus L.), cv. Albatros and melon (cv. Arava)	Nutrient solution: non‐heating, heating at 12–16°C and 18–22°C	Rockwool and coconut coir dust bags	Plants absorb more water and minerals but have no impact on firmness and sugar content in the warmer nutrient solution	Urrestarazu et al. (2008)
Cucumber (Cucumis sativus L.), cv. Jinmei 3	Soil temp. increased from 15.5 to 20.3°C.	Soil cultured	P and Ca concentrations increased; no change in crude protein, crude fiber, soluble sugar, and starch concentration	D. Li et al. (2024)
Tomato cv. Celebrity and watermelon cv. Sangria	Average soil temp. at 15 cm depth increased from 17.65 to 20.73 without mulch to 19.43–22.10 under clear plastic mulch	Soil cultured	Sugar content increased	Vaddevolu et al. (2021)
Melon cv. Honey Green	Substrate temp. increased to 25.93–26.45°C, was 1.88–2.40°C higher by film covering	Substrate cultured	Total and soluble sugar contents increased	Xuechun Wang et al. (2022)
Corn salad cv. Gala	Temps. of nutrient solution: 15, 20, and 25°C	Hydroponics	20°C: highest NO3 –, SO4 2–, and Fe3+ uptake rate and lowest nitrate concentration	Costa et al. (2011)
Asparagus (Asparagus officinalis L.), cv. Grande	Soil temp. at 10 cm depth covered with plastic film (7.2–22.2°C) was 6.4°C higher than that covered with rice husk (7.0–17.8°C)	Soil cultured	Warm soil temp increased reducing sugar, organic acid, Vc, flavonoids, antioxidant activity. Soluble protein and amino acids decreased	Chen et al. (2022)
Strawberry (Fragaria × ananassa Duch.)	Soil temp. at 15 cm depth covered with polyethylene film (3.7–21.9°C) was 2.5°C higher than that of bare ground (3.6–19.5°C).	Soil cultured	Increasing soil temperature raised the juice pH, lowered titratable acidity; no effect on the concentration of total soluble solids	Xia Wang et al. (2022)
Spinach (Spinacia oleracea L.)	Reduce root zone temp. from 20 to 5°C 1 week before harvesting	Hydroponics	Sugars, Vc, and Fe2+ increased; NO3 − and oxalic acid decreased	Hidaka et al. (2008)
Chinese broccoli (Brassica oleracea var. alboglabra Bailey)	Reduce the root zone temp. from 20 to 10°C 1 week before harvest	Hydroponics	Cooling plant roots increases sugars and glucosinolates. In autumn, N, K, and Mg decreased; P increased. In winter, N, P, Ca, and Mg decreased	F. He et al. (2021)

Abbreviations: Temp., temperature; Vc, vitamin C.

Impact of elevated root zone temperatures on vegetable nutritional quality

Costa et al. (2011) found maintaining a constant root and aerial part temperature of 20°C led to the highest baby leaf yield, NO₃ ^–, SO₄ ^2–, and Fe³⁺ uptake rates, and the lowest leaf nitrate concentration in corn salad, compared to a root temperature of 15°C. Similarly, it resulted in a root zone temperature of 25°C. For fruit vegetables, heating the nutrient solution to 12–16°C or 18–22°C significantly enhances water and nutrient uptake in cucumber and melon, thereby influencing fruit mineral content without changes in firmness or soluble sugar contents (Urrestarazu et al., 2008). A rise in soil temperature of 1.9–4.4°C in autumn/winter and 1.4–4.3°C in spring/summer enhances vitamin C, soluble sugar, lycopene, soluble solids, and total acid content in tomatoes (Ouyang et al., 2022). Similarly, increasing soil temperature from 15.5 to 20.3°C raised P and Ca concentrations in the fruits by 25.7% and 14.9%, respectively (D. Li et al., 2024).

In addition to active warming methods, mulching is an effective strategy to mitigate heat loss in the root zone. Different types and colors of mulches yield varying thermal insulation effects. Jia et al. (2020) found that mulching with polyethylene (PE) film raised soil temperatures by 5.66°C in the 5–20 cm soil layer, compared to bare soil, leading to significant increases in vitamin C and sugar concentrations in tomatoes, alongside a decrease in titratable acid concentrations. Similarly, Vaddevolu et al. (2021) showed that transparent plastic mulch kept soil temperatures about 2°C higher at 15 and 30 cm depths than black plastic mulch, leading to higher sugar contents in tomatoes and watermelons. In asparagus, soluble sugar, ascorbic acid, rutin, flavonoid content, and total antioxidant activity significantly increased under plastic film mulch. In contrast, the contents of soluble protein and free amino acids decreased (L. Chen et al., 2022). Similarly, in strawberries, mulch significantly increased fruit juice pH, compared to bare ground, while titratable acidity decreased slightly, and total soluble solids concentration remained unchanged (Xia Wang et al., 2022). In melon, biodegradable film led to a 1.26% higher total sugar and 0.68% higher soluble sugar contents than PE film (Xuechun Wang et al., 2022).

Impact of reduced root zone temperatures on vegetable nutritional quality

Like aerial temperature, excessively high root zone temperatures can impede vegetable growth, affecting produce quality. The optimal root zone temperature tends to be lower than the optimal aerial temperature for the same vegetable (Gruda, 2005). Short‐term low root zone temperature could induce plants’ osmoregulation and antioxidation responses. When spinach cultivated in a soilless culture system at a root zone temperature of 20°C experienced a low root‐zone temperature of 5°C for 1 week before harvest, sugars, ascorbic acid, and Fe²⁺ in leaves significantly increased. In contrast, NO₃ ⁻ and oxalic acid content markedly decreased (Hidaka et al., 2008).

Cooling tomato roots of cv. Delioso in soilless culture increased glucose by 7.7%–10.3%, vitamin C by 20%–21%, and lycopene by 16.9%–20.5% in winter and summer, while cv. Amoroso saw a 6.9%∼7.8% rise in glucose. However, root cooling decreased N, P, S, and Fe in Delioso by 12.1%–15.7% in winter and P and Zn by 9.1%–22.2% in both cultivars during summer (F. He et al., 2019). Further root cooling varied depending on cluster position and cultivar, increasing sugar and vitamin C in the second cluster of Amoroso and the third cluster of Delioso (F. He et al., 2022). In another study, root cooling of Chinese broccoli led to a significant increase in the accumulation of sugars and glucosinolates. However, the nutritional concentration of N, K, Ca, and Mg in broccoli leaves decreased during summer and autumn (F. He et al., 2021).

3.2.3. The combination of the aerial part and root zone temperature control

In controlled environments, vegetables’ aerial part and root zone temperature could be manipulated simultaneously (Table 5). Bazgaou et al. (2021) reported that optimal thermal conditions during cold periods enhanced tomato fruit quality, increasing sugar content by 0.99 °Brix while the citric acidity by 2.88%. Similarly, compared to the uncovered control, higher temperatures reduced nitrate content in wild rockets by 44% (Cantore et al., 2013). Due to reduced heat stress for vegetables, shading increased N, P, and Mg concentrations in lettuce, Fe, and Zn in basil and N, Ca, Fe, and Cu in arugula (Laur et al., 2021).

TABLE 5.

Nutritionally qualitative performance of vegetable species under different aerial parts and root zone temperatures.

Vegetable species	Temp. treatment	Growing conditions	Vegetable nutrient quality	References
Tomato (Solanum lycopersicum), cv. Zayda	Air temp. and soil temp. 6 and 2.5°C higher by heating system than non‐heating	Soil‐less substrate	Sugar content increased by 0.997 °Brix and citric acidity by 2.88%	Bazgaou et al. (2021)
Wild rocket (Diplotaxis tenuifolia L. DC.)	Air temp. at 10 cm (height) and soil temp. at 5 cm (depth), 0.8 and 1.8°C higher by covering	Soil cultured	Nitrate content reduced by 44%	Cantore et al. (2013)
Basil (Oscimum basilicum L.), arugula (Eruca vesicaria subsp. Sativa L.), and lettuce (Lactuca sativa L.)	Air temp. decreased from 25.0 to 24.1°C during the day. Soil temp. dropped from 24.0 to 22.5°C and from 22.3 to 21.0°C during the day and night	Soil cultured	Cooling increased Fe and Zn in basil; N, Ca, Fe, and Cu in arugula, and N, P, and Mg in lettuce	Laur et al. (2021)

Abbreviation: Temp, temperature.

3.2.4. The optimal temperature ranges for nutritional qualities of greenhouse vegetables

The above results summarized the optimal temperature ranges for each nutrient in greenhouse vegetables (Tables 6 and 7). For fruit vegetables, the optimal aerial part temperature for primary metabolites (carbohydrates, organic, and amino acids) is 20–30°C, while for secondary metabolites (vitamin C, flavonoids, and carotenoids), it is 25–29°C. The optimal root zone temperatures are generally lower and narrower: 18–22°C for primary metabolites and 18–26°C for secondary metabolites (Table 6).

TABLE 6.

Nutritionally qualitative performance of fruit vegetables under different temperature treatments.

Nutrient quality	Optimal temperature range	Vegetable species	References
Sugar or soluble solid	Aerial part 20–30°C, root zone 18–26°C	Tomato, watermelon, melon	Bazgaou et al. (2021), Fleisher et al. (2015), Jia et al. (2020), Kläring et al. (2015), Ouyang et al. (2022), Vaddevolu et al. (2021), Xuechun Wang et al. (2022)
Vitamin C	Aerial part 24–29°C, root zone 18–26°C	Tomato, cherry tomato	Gautier et al. (2008), Hernandez et al. (2015), Jia et al. (2020), Ouyang et al. (2022), Ruiz‐Nieves et al. (2021)
Organic acids or titratable acidity	Aerial part 16–30°C, root zone 18–27°C	Tomato	Bazgaou et al. (2021), Fleisher et al. (2015), Jia et al. (2020), Ouyang et al. (2022), Ruiz‐Nieves et al. (2021)
Protein or amino acids	Root zone 18–22°C	Cucumber	D. Li et al. (2024)
Flavonoids	Aerial part 25–30°C	Tomato, cherry tomato	Botella et al. (2021), Gautier et al. (2008)
Mineral nutrients	Aerial part 15–28°C, root zone 16–27°C	Cocktail tomato, eggplant, sweet pepper, cucumber	F. He et al. (2019), Inthichack et al. (2013), Katsoulas et al. (2007), D. Li et al. (2024), Urrestarazu et al. (2008)
Lycopene or carotenoids	Arial part 17–29°C, root zone 18–26°C	Tomato, cherry tomato	Gautier et al. (2008), Hernandez et al. (2015), Krumbein et al. (2006), Ouyang et al. (2022), Ruiz‐Nieves et al. (2021)

TABLE 7.

Nutritionally qualitative performance of leafy/stem/flower vegetables under different temperature treatments.

Nutrient quality	Optimal temperature range	Vegetable species	References
Sugar or soluble solid	Aerial part 10–22°C, root zone 5–10°C	Lettuce, spinach, Chinese broccoli	Gent (2016), F. He et al. (2021), Hidaka et al. (2008)
Vitamin C	Aerial part 25–30°C, root zone 5∼22°C	Red chicory, garland chrysanthemum, spinach, beet, ssamchoo, asparagus	Chen et al. (2022), Hidaka et al. (2008), S. G. Lee et al. (2013, 2015)
Organic acids or titratable acidity	Aerial part 10–22°C, root zone 4–22°C	Lettuce, spinach, strawberry, asparagus	Chen et al. (2022), Gent (2016), Hidaka et al. (2008), Xia Wang et al. (2022)
Protein or amino acids	Root zone 7–18°C	Asparagus	Chen et al. (2022)
Flavonoids	Aerial part 15–25°C, root zone 7–22°C.	Lemon catmint, lemon balm, sage, red chicory, beet, garland chrysanthemum, ssamchoo, asparagus	Chen et al. (2022), S. G. Lee et al. (2013, 2015), Manukyan & Schnitzler (2006)
Mineral nutrients	Aerial part 10–25°C, root zone 20–22°C.	Basil, arugula, lettuce, Chinese broccoli	Gent (2016), F. He et al. (2021), Laur et al. (2021); Sublett et al. (2018)

Optimal temperature ranges for fruit and leafy/stem/flower vegetables are distinct (Table 7). For leafy/stem/flower vegetables, the optimal temperature ranges for each nutritional compound are lower than those in fruit vegetables, except for vitamin C. The optimal aerial part temperature for primary and secondary metabolites in these vegetables is 10–22°C and 25°C, respectively. The optimal root zone temperatures are 7–10°C for primary and 7–22°C for secondary metabolites.

The differing optimal temperatures between fruit and leafy vegetables in protected cultivation are attributed to their origins, growth patterns, and bio‐physiological demands. Fruit vegetables, undergoing both vegetative and reproductive growth, require higher temperatures (20–30°C) to support energy‐intensive processes like photosynthesis, enzyme activity, and nutrient uptake. In contrast, leafy vegetables thrive at cooler temperatures (around 10°C). Additionally, fruit vegetables' longer growth cycles (3–5 months) and larger biomass necessitate higher resource inputs, such as nutrients, light, irrigation, and temperature, compared to leafy vegetables (1–2 months). The pattern of stems and flower vegetables is similar to leafy vegetables.

3.2.5. The biochemical, physiological, and molecular mechanisms of temperature on nutritional quality of vegetables in protected cultivation

The primary metabolites are generated from photosynthesis, photorespiration, and respiration, providing energy, dietary fiber, and sweet or sour flavor (Rouphael et al., 2018; Toscano et al., 2019). Besides, secondary metabolites widely affect vegetables’ taste, bioactive compounds, and flavor and benefit human health (Qaderi et al., 2023; Sun & Fernie, 2024). For example, carotenoids, precursors of vitamin A, are essential vegetable bioactive compounds. Lycopene is a kind of carotenoid with a high tomato content, which could lower the risk of cancer and cardiovascular disease (L. Liu et al., 2015).

The effects of temperature on primary metabolism of vegetables

Because the enzyme activity is significantly temperature dependent, each vegetable species has a specific range of maximum, optimal, and minimum temperature (Jung et al., 2023). Most fruit vegetables are warm‐weather plant species, with their optimum growth temperature between 20 and 30°C and main growing seasons from late spring to early autumn (Maier et al., 2022). As shown in Table 6, the optimal aerial part temperature for fruit vegetables ranges from 20 to 30°C. Plants perceive environmental temperature signals through various sensors, in which the membrane fluidity is widely mentioned (Ding et al., 2020; Sakamoto & Kimura, 2018). Cold or heat changes the cellular membranes’ fluidity and alters membrane‐localized proteins’ structure and activity, inducing an influx or outflux of ions and triggering the temperature‐responsive gene expression (Ding et al., 2020).

On the other hand, heat stress influences the biochemical and physiological functions of the plant by modulating molecular mechanisms (Kumar, 2020). Sucrose transporter is typically downregulated under heat stress, reducing sucrose transport from leaves to fruits and lowering the sugar content in fruits (Du et al., 2024). Plants also develop adaptive responses to resist chilling, in which C‐REPEAT bINDING fACTOR (CBF) genes play a vital role in cold acclimation (Kidokoro et al., 2022). Proteins encoded by CBF genes activate the expression of COR (COLD REGULATED) genes, resulting in the accumulation of protective substances such as osmolytes and cryoprotective proteins that facilitate cold acclimation and cold tolerance (Kidokoro et al., 2022). Vegetables usually increase osmotic pressure to promote cold tolerance by accumulating soluble sugars, enhancing vegetables’ sweetness and tenderness (Yoon et al., 2017).

The effects of temperature on secondary metabolism of vegetables

Interestingly, plants prioritize primary metabolism for growth under favorable conditions but shift to secondary metabolism under stress to enhance tolerance (Sun & Fernie, 2024). Therefore, vegetables’ optimum temperature for secondary metabolites, such as vitamin C, flavonoids, and carotenoids, differs from that for primary metabolites (Tables 6 and 7). Thus, moderate temperature stress could induce the accumulation of antioxidants and secondary metabolites in vegetables, which have a high level of bioactive compounds and high nutraceutical value (Jamloki et al., 2021; Toscano et al., 2019).

The bioactive compounds in plants are biosynthesized mainly through the phenylpropanoids pathway, in which the shikimate is considered the core compound (Vogt, 2010). Under specific stresses, these compounds are accumulated in plants as defence or signal materials (Dixon & Paiva, 1995). Heat stress could activate the phenylpropanoids pathway, enabling the biosynthesis of secondary metabolites, including vitamin C, proline, glycine betaine, sugar alcohols, and phenolic compounds. These metabolites help protect the cell membrane from breakdown and peroxidation (Rivero et al., 2015). In tomatoes, carotenoids and polyphenols are the primary antioxidants that could dissipate excess energy. So, a high temperature (30–35°C) promoted the conversion from lycopene to β‐carotene (Dumas et al., 2003; Y. Liu et al., 2015) and increased the concentration of polyphenols in tomatoes (Sánchez‐Rodríguez et al., 2012). A series of heat stress‐responsive genes are involved in enhancing plant thermotolerance, which could encode proteins of chaperones and ROS scavengers to eliminate ROS and keep robust cell membranes (Ding et al., 2020).

Cold stress usually induces a large amount of shikimic acid into the shikimate pathway, enhancing the biosynthesis of secondary metabolites (Qaderi et al., 2023). Therefore, vitamin C and phenolic compounds, including flavonol, quercetin, kaempferol, and isorhamnetin, were increased under low temperatures in spinach (Spinacia oleracea L.; Watanabe & Ayugase, 2015; Yoon et al., 2017), pak choi (B. rapa ssp. chinensis L.; Mahmud et al., 1999), lettuce (Lactuca sativa L.; Oh et al., 2009), and kale (B. oleracea var. sabellica; Neugart et al., 2012). Since sucrose acts as an upstream signal molecule that triggers flavonoid biosynthesis under cold stress, the increase of flavonoids has a strong relationship with sucrose (Chaves et al., 2011). Cold stress can activate and express some specific genes, most of which belong to the ethylene signaling, ABA signaling; the AP2/ERF, WRKY, and NAC transcription factor families, and the sugar pathways (Londo et al., 2018).

3.3. CO₂

3.3.1. How does the aerial elevated CO₂ (eCO₂) affect the nutritional quality?

The impact of eCO₂ on crop quality has attracted tremendous attention in recent decades due to the serious issue of human health and nutrition under climate changes, such as “hidden hunger” (Myers et al., 2014; Scheelbeek et al., 2018; Toreti et al., 2020). The potential implications of eCO₂ on human health and nutrition are significant. Gojon et al. (2023) comprehensively reviewed the impact of eCO₂ on physiological and molecular aspects of crop quality for plant mineral nutrition, particularly nitrogen.

In the realm of vegetable studies, Scheelbeek et al. (2018) and Doddrell et al. (2023) reviewed the impact of eCO₂ on vegetable nutritional composition. Scheelbeek et al. (2018) provided a brief general overview, while Doddrell et al. (2023) focused on fruit vegetables in greenhouse settings. Before, Dong et al. (2018) conducted a comprehensive meta‐analysis elucidating the extent of eCO₂ on various quality‐related parameters of vegetables and summarized the underlying mechanisms. It concluded that eCO₂ could decrease protein, Fe, and Zn concentrations of different crops, including vegetables, while improving carbohydrate‐related quality, such as soluble sugars, due to increased photosynthesis (Dong et al., 2018; Gojon et al., 2023; Uddling et al., 2018; Figure 3). However, the knowledge gaps have yet to be entirely narrowed concerning the specialty of vegetable crops, compared, for example, with grain crops, which underscores the importance of our study.

FIGURE 3.

Elevated CO₂ (eCO₂) in protected environments enhances the concentrations of soluble sugars, total antioxidant capacity, phenols, flavonoids, ascorbic acid, and carotenoids in vegetables. However, protein, nitrate, magnesium, iron, and zinc concentrations are typically reduced. Effects on titratable acidity, total chlorophyll, lycopene, and anthocyanins vary depending on specific conditions. The interactions of eCO₂ with environmental factors such as light, temperature, and humidity are complex and demand further detailed investigation (Doddrell et al., 2023; Dong et al., 2018; Uddling et al., 2018).

Vegetable crops are a large plant species with almost all fresh, tender, flavors, and non‐toxic eatable tissues. The roots, stems, leaves, and even seeds of, for example, cilantro and spinach are preferable. Vegetables have been commercially grown under various conditions, such as in greenhouses with light, CO₂, and temperature controls or natural open fields, in soils, hydroponics, or substrates, with chemical fertilizers or organic farming, under various climates with distribution in almost every corner of the world or even in outer space (Collado‐Gonzalez et al., 2022; Mozaffarian et al., 2021; van Delden et al., 2021). It is, therefore, hard to conclude the impact of eCO₂ on the nutritional quality of vegetables, particularly when considering the various quality‐related bioactive and flavor compounds.

On the other hand, maintaining high CO₂ levels in greenhouses can be challenging, significantly when temperatures rise and the requirement to ventilate the greenhouse is due to solar radiation. Modern closed and semi‐closed designs reduce energy consumption by minimizing window ventilation and using active cooling. In addition, storing surplus solar energy for subsequent utilization represents an energy‐conservation strategy that is particularly beneficial during periods of reduced radiation. eCO₂ concentrations, even with high light levels, are helpful in these setups (Gruda & Tanny, 2014, 2015).

Herein, we examine the impact of eCO₂ levels on vegetable quality within protected cultivation systems, with specific attention directed toward two pivotal aspects: the differentiation in nitrogenous compounds between root and shoot tissues and the elicitation of secondary metabolites under the influence of environmental stressors. Following a comprehensive analysis of environmental variables and their implications for nutritional attributes, we delve into the interactive dynamics between eCO₂ concentration and other environmental factors in an additional section.

3.3.2. The root and shoot differences in nitrogenous compounds

eCO₂ was proposed to decrease nitrate assimilation and thus contributed to lower protein concentration in wheat and Arabidopsis (Bloom et al., 2014, 2010). However, this is not the case in some circumstances (Andrews et al., 2019). It appears that eCO₂ promotes the nitrate assimilation of roots while inhibiting that of shoots (Bloom et al., 2020). Dong et al. (2017) found a clue that eCO₂ can promote nitrate assimilation in roots more than in leaves in cucumber plants, confirming this conclusion. Despite being not significantly different, the extents of the decrease in nitrate concentration by eCO₂ are in order of fruit > leaf > stem (Dong et al., 2018), indicating the limitation of nitrate transportation up forward due to lower transpiration or high inhibition of nitrate assimilation aboveground (McGrath & Lobell, 2013; Figure 3). Since vegetables predominantly prefer nitrate and require higher amounts, eCO₂ may improve their quality by reducing nitrate accumulation. This effect is particularly significant in leafy vegetables, which accumulate more harmful nitrate than fruit vegetables. On the other hand, CO₂ enrichment benefits plant propagation and root growth, especially in low‐light conditions, by reducing photorespiration and the light compensation point. These enhanced physiological processes can indirectly influence nitrate metabolism and assimilation, supporting balanced nutrient uptake. This is particularly useful in higher latitudes during winter/spring (Gruda & Tanny, 2014, 2015), where low light availability enhances nitrate content in leafy vegetables. H. Song et al. (2023) also reported a reduced accumulation of nitrate nitrogen in lettuce leaves when eCO₂ (800 ± 50 µmol mol⁻¹) was applied, compared to the control (400 µmol mol⁻¹). The reasons align with the statement above, as the reduction was attributed to an increased light saturation point, net photosynthetic rate of the leaves, and a decreased light compensation point. The authors also noted higher vitamin C and chlorophyll levels in three lettuce varieties with differentially expressed genes primarily associated with ethylene and jasmonic acid signaling, porphyrin and chlorophyll, and starch and sucrose metabolism. Similar results enhancing chlorophyll and soluble sugar content were observed in pak choi. Hou et al. (2021) identified two upregulated genes involved in chlorophyll synthesis, seven in adenosine triphosphate and nicotinamide adenine dinucleotide phosphate synthesis and six in Rubisco activity. Furthermore, genes for starch conversion, auxin synthesis, and cell growth were upregulated under eCO₂. These studies highlight the physiological and molecular mechanisms by which eCO₂ enhances plant growth and quality. However, future studies must urgently confirm this expectation from physiological and molecular perspectives.

3.3.3. Secondary metabolites induced by eCO₂ levels

eCO₂ enhances carbon availability, enabling plants to better resist environmental stress through increased production of secondary metabolites (Foyer & Noctor, 2020). It also significantly mitigates various environmental stressors affecting plant growth and development. Frantz (2011) observed CO₂’s capacity to lessen the adverse effects of low temperatures. Romero‐Aranda et al. (2002) also documented its ability to counteract the deleterious impacts of high salinity levels in irrigation water, particularly crucial in Mediterranean regions (Gruda & Tanny, 2014; Gruda et al., 2024). eCO₂ increased tomato fruits’ ascorbic acid and lycopene under ultraviolet B, compared to the control (F. Li et al., 2007). Thus, the quality of greenhouse vegetables could benefit from applying eCO₂ as it essentially decreases ultraviolet radiation. eCO₂ and mild light stress (400 µmol photons m⁻² s⁻¹) briefly increased lettuce plants’ phenolic compounds and antioxidant capacity (Pérez‐López et al., 2018). In conclusion, eCO₂ could be successfully used as an elicitor with environmental stressors affecting the internal attributes of vegetables in protected cultivation. Doddrell et al. (2023) provide detailed insights into the nutritional content of various crops from different plant families, including vitamins C, E, and pro‐vitamin A. The level of CO₂ enrichment also influenced the observed increases. However, the biosynthesis pathways corresponding to the interaction of eCO₂ and environmental stresses are mainly unknown, requiring further in‐depth studies.

3.4. Humidity

Humidity significantly influences the water status of greenhouse vegetable plants, impacting the soil–plant–atmosphere continuum and key processes such as water balance and transpiration cooling (Gruda, 2005). Consequently, it affects plant water and nutrient transport (Gilliham et al., 2011; Yu et al., 2023). In academic literature, vapor pressure deficit (VPD) is often used to measure humidity. VPD is the difference between the air's saturation and actual moisture content. Air humidity and VPD have an inverse relationship; as air humidity increases, VPD decreases, and vice versa. These findings highlight the importance of humidity regulation for effective crop growth and development.

In protected cultivation, humidity is often overlooked unless diseases and pests arise. Grange and Hand (1987) observed that VPD (0.2–1.0 kPa) had minimal impact on the growth and development of horticultural crops. However, exploring air humidity/VPD on crops’ nutraceutical properties remains relatively underexplored, compared to other environmental factors (Amitrano et al., 2021), while humidity regulation is intertwined with room temperature, effective crop growth, and development necessitate meticulous control over water supply and air humidity. In this study, our focus lies solely on elucidating the impact of air humidity/VPD on nutritional quality.

During daylight hours, most vegetable crops have an optimal VPD range between 0.50 and 1.50 kPa (Yu et al., 2023). Very low VPD reduces evapotranspiration rates, which is the critical mechanism for cooling them. This reduction in sap flow through the phloem can result in lower visual quality and nutrient deficiency symptoms due to the decreased movement of ions within plant tissues (Gruda, 2005).

On the other hand, high VPD levels can increase atmospheric transpiration, hinder stomatal conductance and photosynthesis, and impede nutrient accumulation (Yu et al., 2023). High VPD, coupled with increased temperatures and solar radiation in Mediterranean greenhouses, induces oxidative stress, adversely impacting the marketability of produce and the levels of carotenoids and minerals in cherry tomato fruits (Rosales et al., 2011). However, it promotes the accumulation of phytonutrients like phenolic compounds and ascorbic acid alongside enhancing antioxidant capacity. Moreover, under such stress conditions, sugars also exhibit heightened accumulation, resulting in a sweeter and milder flavor profile in these tomatoes (Ampim et al., 2022; Rosales et al., 2011).

Yu et al. (2022) documented a decrease in water flow from the xylem to the tomato fruit and an elevation in the solute concentration of phloem sap in conditions characterized by an elevated VPD of 2.22 kPa. Consequently, there is a subsequent increase in the levels of soluble solids, soluble sugars (Leonardi et al., 2000; D. Zhang et al., 2015), and acids within the fruit, thus contributing to an enhancement in the fruit's flavor profile and overall quality (J. Chen et al., 2014; Yu et al., 2023). J. Chen et al. (2013) suggested that starch accumulation in developing fruits may be a contributing factor. Furthermore, F. Wang et al. (2011) demonstrate the conversion of starch into hexose in mature fruits, increasing levels of soluble solids and sugars.

As mentioned before, the regulation of air humidity is a complex interplay between air temperature and water supply. Leyva et al. (2014) studied cherry tomato fruit quality in different environments, such as a screenhouse with or without a fogging system and an open field. The fogging system reduced the maximum VPD deficit by 0.7 kPa, compared to open field cultivation, resulting in larger fruit, about 4 g each, and improved nutritional quality, including higher vitamin C and lycopene content and antioxidant capacity.

Lettuces exposed to high VPD at 1.76 kPa consistently increased their phytochemical content, compared to those grown under low VPD of 0.69 kPa (Amitrano et al., 2021). This exposure probably induced a response in the plants, perceiving the surrounding environment as a moderate stressor and impacting various characteristics, including total ascorbic acid, phenols, hydrophilic antioxidant activity, and lipophilic antioxidant activity. Elevated levels of antioxidant molecules, including ascorbate metabolites and phenolic compounds, in high VPD conditions indicate a defense mechanism against oxidative stress in leaves of Salanova cultivars (L. sativa L. var. capitata) of lettuce plants.

Leonardi et al. (2000) demonstrated that moderately increased VPD of 2.22 kPa intensified color in tomato fruits by boosting lycopene content, while Dorais and Gosselin (2002) observed similar effects. Moreover, greenhouse VPD and lighting conditions during growth significantly impact postharvest quality. According to Aliniaeifard and van Meeteren (2018), plants grown in low VPD greenhouse climates struggle to control water loss after harvest, resulting in uncontrolled transpiration and reduced water content. This ultimately leads to diminished quality of leafy vegetables. These findings align with Chowdhury et al. (2021), showing that VPD impacts kale's glucosinolate content by affecting stomatal function and transpiration rates. Glucosinolate levels remained stable within a moderate VPD range, but low VPD combined with high temperatures may reduce their concentration (Chowdhury et al., 2021).

In conclusion, explicit guidelines for utilizing VPD levels to optimize vegetable nutritional quality are lacking. This deficiency arises because both low and high VPD levels could have adverse effects. Moreover, it is essential to recognize that plant responses to environmental stress are multifaceted, with factors beyond VPD playing significant roles in determining nutritional quality. Also, cultivation media and water supply impact soil water retention, affecting crop growth under VPD regulation. Further investigations are necessary to understand the effects of VPD regulation on other environmental conditions and crop root pressure (Yu et al., 2023). A potential approach could involve cultivating plants under optimal conditions and briefly exposing them to moderately high VPD to increase antioxidant levels while maintaining plant photosynthesis and crop yield (Amitrano et al., 2021).

3.5. Interaction between several factors

3.5.1. How does the interaction of environmental factors affect nutritional quality?

Thus far, we have analyzed the impact of environmental factors on the nutritional composition of greenhouse vegetables. Our analysis, supported by studies conducted by Dorais and Gosselin (2002) and Leyva et al. (2014) on tomatoes, underscores the crucial roles of light intensity and temperature as primary environmental factors affecting the nutritional quality of greenhouse vegetables. Further, CO₂ levels, light spectra and duration, and humidity influence nutritional quality. Notably, when any of these factors deviate from their optimal level, they become the limiting factor, adhering to the principles of the law of limiting factors. Considering the intricate interplay of various environmental factors, adjusting all greenhouse microclimatic conditions holds significant promise for enhancing vegetable nutritional traits. Additionally, water (Rouphael et al., 2006) and fertilizer regimes (Dasgan et al., 2022; Dasgan, Kacmaz, et al., 2023; Dasgan, Yilmaz, Zikaria, et al., 2023; Dasgan, Yilmaz, Dere, et al., 2023), salinity (Gruda et al., 2024), and cultural practices—such as cultivation systems (Gruda, 2009), use of biostimulants (Dasgan et al., 2024; İkiz Dasgan, & Gruda, 2024), and harvesting time (Eskandarzade et al., 2024), can influence the vegetable nutritional quality. In some cases, combining factors like moderate salinity with biostimulants helps tailor the outcome (Dasgan et al., 2024; İkiz, Balik, et al., 2024).

Gruda (2005) and Kosma et al. (2013) underline the intricate relationship between light and environmental factors influencing the nutritional quality of greenhouse vegetables. Research by Dong et al. (2018) and Pan et al. (2019) demonstrates the benefits of combining supplementary light and CO₂ enrichment in improving crop growth, yield, and quality. The LED technology enables precise light control and facilitates quality‐targeted interactions with eCO₂, for example, in the plant factory industry (van Delden et al., 2021). Moreover, the increased carbon availability under eCO₂ stimulates the transformation of carbon skeletons, enhancing amino acid accumulation and improving crop quality (Miyagi et al., 2017). Studies by Shimomura et al. (2020) and Endo et al. (2022) further emphasize the synergistic effects of light supplementation and eCO₂ on nutrient accumulation and metabolite changes, such as chlorogenic acid and rutin concentrations in lettuce.

Similarly, compared to cooler day/night temperatures of 15/9°C and a standard photoperiod (12 h), kale exhibited higher levels of soluble sugars while showing reductions in aliphatic and aromatic glucosinolates when subjected to regular day/night temperatures of 21/15°C and an extended photoperiod (24 h; Steindal et al., 2015). In contrast, either low day/night temperatures (20/12°C) or high temperatures (38/30°C) led to decreased carotenoid and total phenolic concentrations in basil leaves, compared to average temperatures (30/22°C) under ambient CO₂ levels (420 µmol mol⁻¹; Barickman et al., 2021). However, eCO₂ levels (720 µmol mol⁻¹) significantly increased total phenolic concentrations, even when carotenoid concentrations were reduced.

Temperature, particularly root‐zone temperature, significantly modulates the impact of eCO₂ on vegetable quality. At a low root‐zone temperature of 15.5°C, eCO₂ reduced crude protein and fiber concentrations in cucumber fruits by 6.66% and 10.10%, respectively, alongside decreased N, K, and Fe concentrations by 6.82%, 5.13%, and 16.4%. Conversely, a warmer root zone (20.0°C) maintained stable concentrations of essential nutrients in cucumber fruits under eCO₂ conditions (D. Li et al., 2024). This is likely due to enhanced root growth, sustained stomatal conductance, and increased mineral ion diffusion, promoting nutrient uptake and mitigating photosynthetic acclimation (D. Li et al., 2022, 2023).

Warm air can hold more moisture than cool air, and the air's capacity to retain water increases as temperature rises. Zheng et al. (2022) conducted experiments with four temperatures, three VPD levels, four durations, and a control. The best tomato quality was found at 32°C and a VPD of about 1.42 kPa. Under high temperatures and low VPD, sucrose‐metabolizing enzyme activity decreased, reducing soluble sugar content. Enzyme activity involved phosphopyruvate carboxylase, mitochondria aconitase, and citrate synthetase increased, boosting organic acids, especially malic acid levels. This led to a significant decrease in vitamin C, total sugar, and the sugar‐acid ratio, while titratable acid increased, reducing the nutritional quality of ripe tomatoes.

Additionally, the relationship between light and nitrate levels is well‐documented. Y. Fu et al. (2017) demonstrated that higher light intensity and low nitrogen levels promoted vitamin C accumulation and reduced nitrate levels in lettuce leaves. In greenhouse conditions, high nitrogen rates reduced total phenolic content under high light and antioxidant activity under low light (Stagnari et al., 2015). Viršilė, Brazaitytė, Vaštakaitė‐Kairienė, Jankauskienė, et al. (2019) emphasized the sensitivity of tatsoi growth and nitrate metabolism to light intensity and quality. Light intensities below 200 µmol m⁻² s⁻¹ led to increased nitrite and nitrate accumulation, whereas higher intensities up to 500 µmol m⁻² s⁻¹, particularly in the blue spectrum, facilitated nitrate assimilation into amino acids, highlighting blue light's role in maintaining the carbon‐to‐nitrogen (C: N) ratio. Additionally, red light elevated soluble sugar concentration while reducing nitrate levels (Hogewoning et al., 2010; Liang et al., 2022).

Many climate conditions change in greenhouses when the intensity or quality of a factor changes. Thus, all greenhouse environmental factors are integral to total greenhouse management. Understanding and optimizing the interactions between light, CO₂, temperature, and other environmental factors are essential for enhancing vegetable quality in greenhouse production systems. Moreover, the unpredictable impacts of climate change on ecosystems and agriculture require plants to adapt metabolically, for example, through plant secondary metabolites in plant–environment interactions. Therefore, understanding how plant secondary metabolites respond to climate change is needed for developing future cultivation and breeding strategies (Sun & Fernie, 2024).

On the other hand, plants’ metabolic responses to environmental factors are specific to species, cultivars, and growth stages. For instance, the ratio of blue to red is essential for seedlings; a small amount of green light is beneficial for biomass accumulation (higher pigment, antioxidant compounds, mineral elements); and increased blue or supplemental UV‐A is significant for flowering and fruiting.

The management of environmental conditions opens new perspectives, but it should be considered alongside other factors, such as genetics (Weiss & Gruda, 2025a, 2025b) and agrotechnology (Gruda et al., 2018). This approach can increase beneficial compounds—like proteins, vitamins, carotenoids, flavonoids, and minerals—while reducing harmful ones, such as nitrates and oxalates (O'Sullivan et al., 2019). In this way, regulated nutritional quality can be achieved. In a review of Australian‐protected cropping, Chavan et al. (2022) suggested developing nutritional quality indexes to define and certify the quality of indoor‐grown produce.

Since yield and quality are distinct yet interconnected issues, the challenge lies in finding an optimal balance between genetic, environmental, and agronomic factors to maximize yield and nutritional quality. This delicate equilibrium, referred to as the “golden mean” (Gruda et al., 2024), necessitates a comprehensive approach to harmonize these variables effectively.

3.5.2. Nutritional quality and the holistic impact of vegetable consumption

Similarly, the interactions of various plant metabolites present in vegetables influence the human body in distinct ways. Previously, nutrition research focused on the isolated impact of individual metabolites or elements on human health, leading to inconsistent findings. However, it is now recognized that every individual mineral, vitamin, and secondary metabolite is consumed as part of a whole diet. There is a significant difference between taking a single mineral element in pill form and getting the necessary amount of that mineral from vegetables. Opting for the latter choice is preferable because it is natural and due to its accompanying array of nutrients. Therefore, the positive impacts on health are derived from the combined presence and variety of bio‐metabolites, including dietary fibers, minerals, vitamins, secondary metabolites, and more, intricately woven within vegetables. This intermingling makes vegetables an excellent food choice.

It is important to mention that adding vegetables to your diet is not restricted to those who follow a vegetarian lifestyle. This is one of the benefits of including vegetables in our meals. Increasing the consumption of vegetables in recommended diets benefits vegetarians, vegans, and even flexitarians with a more adaptable approach to their dietary choices.

While not the focus of this study, it is vital to acknowledge the integral role of physical activity in maintaining a holistic approach to a healthy diet. For instance, the latest DKV report (Froböse & Wallmann‐Sperlich, 2023) underscores that Germans progressively allocate more time to sedentary activities. The average daily sitting duration for each German has escalated to 9.2 h, representing an increase from the 2021 pandemic‐period average of 8.7 h. Particularly, concerning trends are evident among the cohort aged 18 to 29, where daily sitting exceeds 10 h. Simultaneously, there is a concurrent decline in mental well‐being. According to the findings of the DKV report (Froböse & Wallmann‐Sperlich, 2023), a positive correlation emerges between consistent physical activity engagement and subjective well‐being.

In addition to considering various nutritional plans and recommended diets, it is essential to acknowledge the influence of other factors on nutritional quality. For instance, the Mediterranean diet goes beyond what we eat and a meal plan. It promotes the practice of cooking and enjoying food with family and friends, emphasizing artisanal cooking, using high‐quality ingredients, and the balanced consumption of various food groups. The Mediterranean diet distinguishes itself through diverse food choices and emphasizes the social aspects of eating and the synergies between dietary practices, social interactions, and physical activity.

Maroto‐Rodriguez et al. (2024) studied 110,799 individuals aged 40 to 75 in the UK Biobank database, examining how adherence to the Mediterranean lifestyle relates to all‐cause, cancer, and cardiovascular disease mortality. They used the Mediterranean Lifestyle index, based on questionnaires and diet assessments, with three categories: (1) “Mediterranean food consumption,” (2) “Mediterranean dietary habits,” and (3) “physical activity, rest, social habits, and conviviality.” All components were associated with reduced risk of all‐cause and cancer mortality, while the third component also showed reduced cardiovascular disease mortality. A salient conclusion drawn from this investigation is that the observed effects are not exclusively contingent upon Mediterranean residents. The study posits that modifying the Mediterranean lifestyle to align with the distinctive attributes of non‐Mediterranean populations could constitute a viable and advantageous facet of a health‐conscious way of life.

4. CONCLUSION

Optimizing environmental factors can significantly enhance the nutritional quality of greenhouse vegetables. However, plants’ metabolic responses to environmental conditions are specific to genetic material, including species and cultivars, as well as their growth stages. Among environmental factors, light intensity significantly impacts vegetable nutritional quality when other conditions are optimal, followed by light spectrum, temperature, CO₂ levels, and humidity.

While increased light intensity generally enhances photosynthesis and nutritional quality, it leads to photo‐oxidative stress and photodamage, including reduced phenolic content due to light saturation. Therefore, the challenge lies in balancing light intensity, duration, and quality to maximize plant health and nutritional output without exceeding harmful thresholds. Optimizing light quality in greenhouse settings can significantly influence the growth stages, physiological responses, and nutritional content of vegetables. Tailored lighting strategies that consider specific wavelengths such as blue, red, and UV‐A can enhance seedling development, biomass accumulation, and fruit quality, offering a powerful tool for improving both yield and nutritional value.

Temperature critically affects greenhouse vegetable nutritional quality. Leafy, stem, and flower vegetables generally prefer cooler conditions for higher nutritional qualities than fruit vegetables. Fluctuations in aerial temperature exert nuanced effects on nutritional composition, influencing the synthesis of bioactive compounds and essential nutrients. While elevated temperatures can stimulate the production of specific nutrients such as flavonoids, excessive heat leads to undesirable outcomes, such as heightened bitterness in certain crops or diminished carotenoid content. Conversely, cooling strategies in tropical regions alleviate heat stress, enhancing overall nutritional quality.

eCO₂ frequently improves vegetable quality within protected cultivation systems. However, negative impacts like decreased protein, Fe, and Zn content raise a multifaceted issue with significant implications for human health and nutrition. Moderate stress induced by elevated VPD can enhance vegetables’ phytochemical content and antioxidant activity, potentially improving their nutritional quality and flavor profiles.

Further research is needed to fully elucidate the mechanisms underlying the complex interplay among environmental factors and their interaction with other factors, for example, genetic material, salinity, and cultural practices on vegetable quality. While eCO₂ has been shown to influence various quality‐related parameters of vegetables, such as protein, mineral, and carbohydrate concentrations, gaps remain in our understanding of the physiological and molecular mechanisms by which eCO₂ affects some secondary metabolites. The interactions of eCO₂ with light are not yet precisely understood and require further investigation. Additionally, explicit guidelines for using VPD to optimize vegetable quality are lacking, necessitating more research. A balanced approach that considers all relevant environmental factors alongside VPD regulation is crucial for maximizing vegetable nutritional quality in greenhouse cultivation.

AUTHOR CONTRIBUTIONS

Nazim S. Gruda: Conceptualization; methodology; formal analysis; writing—original draft; writing—review and editing; supervision; visualization. Giedrė Samuolienė: Conceptualization; visualization; writing—original draft; writing—review and editing. Jinlong Dong: Conceptualization; writing—original draft; writing—review and editing; visualization; funding acquisition. Xun Li: Conceptualization; writing—original draft; writing—review and editing; funding acquisition; visualization.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

Gruda, N. S. , Samuolienė, G. , Dong, J. , & Li, X. (2025). Environmental conditions and nutritional quality of vegetables in protected cultivation. Comprehensive Reviews in Food Science and Food Safety, 24, e70139. 10.1111/1541-4337.70139

DATA AVAILABILITY STATEMENT

All data are presented in the current study.