Decoding Plant Metabolomic Response to Potassium and Nutrient Stresses in Controlled Environments

ABSTRACT

Potassium (K) is an essential macronutrient, affecting numerous physiological, biochemical, and metabolic functions in plants. In protected cropping systems (PCS), which are controlled environments for intensive agriculture, optimizing K management is essential for attaining sustainable productivity and resilience under stressful growth conditions. Understanding plant responses to these conditions requires advanced analytical approaches, and metabolomics is emerging as a key tool, though there is still a lack of PCS‐focused metabolomic studies. This review synthesizes the recent knowledge on plant metabolomic responses to K as a nutrient, emphasizing the central role of metabolomics in uncovering intricate biochemical pathways associated with K uptake, transport, and utilization. We investigate essential metabolic alterations in response to K deficiency, encompassing modifications in glucose metabolism, antioxidant synthesis, osmolyte accumulation, and hormonal regulation. The review also explores the relationships between potassium and other nutrients, specifically nitrogen, phosphorus, and essential micronutrients, and their impact on total plant metabolic networks. Furthermore, we discuss cutting‐edge omics integration, precision fertigation, real‐time sensor technologies, and machine learning applications that together promise to transform K fertilizer management in PCS. Future directions highlight the advancement of K‐efficient cultivars, integrating metabolomic biomarkers in breeding, and overcoming challenges in data interpretation, scalability, and the high cost of metabolomic analyses and phenotyping technologies. Collectively, these insights provide a framework for improving crop health, production, and nutrient utilization efficiency for a more sustainable future in protected cropping.

1. Introduction

One of the main objectives of the sustainable development goals (SDGs) of the United Nations is to achieve zero hunger through sustainable agricultural production. However, challenges such as climate change, land degradation, biodiversity loss, urbanization, and high dependence on agrochemicals are making it more difficult to maintain agricultural productivity and leading to destabilizing global food systems. By 2050, the world's population is predicted to reach nearly 10 billion (United Nations ²⁰¹⁹). If agricultural practices continue as they are, there will probably be widespread food insecurity and chronic hunger by the middle of the century (Long et al. 2015). The Green Revolution significantly enhanced agricultural productivity by the application of intensive inputs, including irrigation and fertilizers, alongside the use of high‐yielding crop varieties. However, the excessive application of fertilizers has resulted in nutrient leaching, soil acidification, and ecological imbalances, whereas insufficient usage in resource‐limited environments persists in reducing yields (Shin 2014). Attaining food security will rely not only on enhancing productivity but also on diminishing environmental impacts, optimizing input utilization (e.g., fertilizer use efficiency), and fostering resilience throughout food systems (Nüsslein and Dhankher 2016).

Protected cropping systems (PCS) have arisen as one of the adaptive resolutions to the challenges of food security and sustainability. Incorporating technologies like greenhouses, polytunnels, and controlled‐environment agriculture (CEA) enables producers to regulate temperature, humidity, light, and fertilizer supply to enhance crop yields (Bambara and Athienitis 2019; Geilfus ²⁰¹⁹; Stanghellini et al. ²⁰¹⁹). These systems enhance resource efficiency, especially regarding water and fertilizer utilization and crop protection, while extending growing seasons and facilitating year‐round cultivation of high‐value crops (Castilla and Montero 2008; Torrellas et al. ²⁰¹²; Barbosa et al. ²⁰¹⁵; Rabbi et al. ²⁰¹⁹). PCS also facilitates the cultivation of pest‐ and disease‐free, nutrient‐dense fruits, vegetables, flowers, and other cash crops, hence enhancing the quality of the produce. However, when compared to field crops, PCS still has certain significant drawbacks. Field crops usually have lower operating and capital expenses, are more suitable for large‐scale grain and cereal production, and require less technology. Nonetheless, field‐cultivated crops are significantly susceptible to unfavorable weather, pest infestations, and fluctuating yields. Conversely, PCS is particularly beneficial for high‐value, perishable, or specialized crops, such as tomatoes, cucumbers, leafy greens, strawberries, and herbs, which thrive in controlled environments and warrant higher investments. Extensive field crops such as wheat, rice, and maize are hardly cultivated in PCS due to their spatial demands and reduced market value per unit area. Consequently, although PCS plays a crucial role in diversifying and maintaining fresh food supplies, it serves to complement rather than replace traditional field cultivation, with crop selection predominantly influenced by economic viability, crop physiology, and market demand. Therefore, the global adoption of PCS has surged, and the greenhouse horticulture industry reached around US $30 billion in 2019 at a compound annual growth rate of 9% (Koukounaras 2020). Their expansion is especially evident in areas experiencing land scarcity or climatic instability. In Australia, PCS is a rapidly expanding section of food production, encompassing systems from advanced greenhouses (17%) to hydroponics and substrate‐based cultivation, which accounts for more than 50% of the industry (O'Sullivan et al. ²⁰¹⁹). These patterns indicate an increasing dependence on PCS to satisfy escalating food demand while reducing resource inputs and impacts on the environment.

PCS necessitate strict monitoring of nutrition and irrigation management to maintain optimal crop performance (Bar‐Yosef ¹⁹⁹⁹; Islam et al. ²⁰²⁴). Fertigation, the method of supplying nutrients through irrigation water, has been a conventional practice within PCS owing to its capacity for consistent fertilizer distribution, minimized input waste, allowing adjustments throughout different growth stages, and increasing nutrient uptake efficiency. Fertigation systems, ebb and flow designs, and trough bench patterns enable accurate fertilizer distribution to active root zones (Sun et al. 2023). However, controlling the ionic equilibrium of nutritional solutions continues to pose difficulties in fertigation. Variables including water pH, salinity, solubility, and mineral interactions hinder the formulation of appropriate nutrition solutions (Savvas et al. 2008). The accumulation of some ions, including ammonium (NH₄ ⁺) and sodium (Na⁺), can lead to toxicity symptoms, compromising membrane integrity and interfering with essential metabolic processes in crops (Fernández‐Crespo et al. ²⁰¹⁵; Lu and Fricke ²⁰²³). Conversely, shortages in macronutrients such as potassium or phosphorus affect the photosynthetic ability and root structure of crops (Canales et al. 2017; Liu et al. ²⁰¹⁹).

Among macronutrients, K is particularly important due to its functions in osmoregulation, enzyme activation, ion transport, and photosynthesis (Amtmann and Blatt 2009; Fageria ²⁰¹⁶). K deficit results in compromised physiological activities, including inadequate stomatal control, diminished biomass, and decreased fruit yield (Pettigrew 2008; Gattward et al. ²⁰¹²), posing a substantial limitation inside PCS. Strategic K fertigation has demonstrated enhancements in productivity, resource‐use efficiency, and fruit quality in PCS (Asaduzzaman and Asao 2018; Sonali et al. ²⁰²³). Protected cropping conditions modify the plant's exposure to environmental stressors, nutrient mobility, and evapotranspiration rates, thereby affecting nutrient uptake kinetics and metabolic responses (Voogt 2013; Tittatelli et al. ²⁰¹⁶). Numerous vegetables and fruit crops grown in PCS have expedited growth cycles and increased nutrient requirements, rendering them particularly susceptible to K imbalances. Furthermore, PCS often exposes crops to various concurrent stressors, including elevated light intensity and fluctuating humidity, hence complicating nutrient physiology. Information about the impact of nutrient stress on the regulation of K transporters, energy metabolism, or stress‐responsive pathways in PCS crops is relatively scarce. Moreover, there is an absence of comprehensive studies investigating the interaction of potassium with other nutrients or its impact on whole‐plant homeostasis in practical growth environments. Thus, the optimization of fertigation protocols for potassium remains empirical and inadequate, especially for high‐value horticultural crops.

To bridge this gap, metabolomics presents a viable way to address this disparity. Metabolomics, as an omics technology, facilitates the profiling of molecules, including amino acids, sugars, organic acids, and secondary metabolites, which are pivotal to plant stress responses (Obata and Fernie 2012). Metabolomic investigations of K deprivation have shown significant markers, including elevated proline, modified sugar metabolism, variations in antioxidant component levels, and the accumulation of osmoprotectants (Cuijpers et al. 2008; Zhang et al. ²⁰¹⁹). These modifications offer an essential understanding of how plants adjust to nutrient stress at a systemic level. However, most of the metabolomic studies on K nutrition have been conducted in open‐field conditions or simple hydroponics systems. Limited research has examined how plants cultivated in PCS influence their metabolome under varying environmental conditions. Furthermore, the conversion of metabolomic insights into practical applications, like early stress detection sensors, fertigation decision‐support systems, or customized nutrient solutions, remains challenging, thereby limiting their implementation and broader adoption in PCS management.

This review synthesizes current knowledge on plant metabolomic responses to K deficiency under controlled environment conditions. It emphasizes recent advances in transport mechanisms, stress adaptation, nutrient interactions, and omics‐guided management strategies. Special attention is given to how these processes manifest under controlled environments and how such insights can inform future directions in nutrient management, with the broader goal of advancing sustainable crop production within PCS.

2. Physiological and Metabolic Roles of Potassium in Plants

Potassium is an essential macronutrient for plants, significantly contributing to numerous physiological and metabolic pathways critical for growth and development. Its importance is attributed to its role in enzyme activation, protein synthesis, osmoregulation, photosynthesis, carbohydrate metabolism, water movement, and ion equilibrium (Wang and Wu 2010, 2013, 2015; Hasanuzzaman et al. ²⁰¹⁸) (Figure 1).

FIGURE 1.

Roles of Potassium (K) across root, shoot, and plant levels. Potassium is involved in multiple physiological and biochemical processes that support plant growth and stress tolerance. At the whole‐plant level, K regulates osmoregulation, nutrient balance, morphogenesis, signal transduction, enzyme activation, and protein synthesis. At the leaf level, K plays key roles in photosynthesis, stomatal regulation, transpiration, turgor maintenance, transport of photosynthates from source to sink, and leaf development. At the root level, K contributes to stress adaptation, root development, microbial diversity, and efficient acquisition of nutrients and water (Created in https://BioRender.com).

2.1. Stomatal Regulation and Potassium

The principal role of K in stomatal control is to sustain a delicate equilibrium between CO₂ intake and water vapor release from intercellular spaces. K in the plant directly affects stomatal aperture and, thus, the effectiveness of photosynthesis (Marschner 2011; Israel et al. ²⁰²¹). Guard cell turgor pressure controls the opening and closing of stomata and is primarily governed by K⁺ concentration, which varies by species and ranges from 100 to 800 mM (Taiz and Zeiger 2006; Marschner ²⁰¹²). Light increases osmotic potential and causes stomatal opening by activating H⁺‐ATPase in guard cells, which promotes K⁺ absorption together with Cl⁻, NO₃ ⁻, and malate (de Bang et al. ²⁰²¹; Sardans and Peñuelas ²⁰²¹). In contrast, abscisic acid (ABA) induces Ca²⁺ influx, leading to plasma membrane depolarization, inhibition of K⁺ channels for a decrease in turgor pressure, and stomatal closure (Wang et al. 2017). During drought conditions, K⁺ moves out of guard cells, thereby triggering stomatal closure and reducing water loss (Wang and Wu 2013). K shortage in Eucalypts seedlings resulted in partial stomatal closure during drought, as evidenced by elevated δ¹³C values. However, this was also associated with diminished water use efficiency (WUE), indicating that potassium is required for sustaining an adequate water–carbon balance under stress (Mateus et al. 2021). Similarly, sunflowers exhibited reduced water use efficiency under potassium deprivation (Jákli et al. 2017). The stomatal response to K⁺ has been reported in many field crops such as wheat and cocoa (Abdalla et al. 2021), but there are limited reports on crops grown in controlled environments.

2.2. Roles of K in Photosynthesis and Energy Metabolism

Photosynthesis and its related metabolic pathways are essential for plant development and energy efficiency, where K is an essential regulator of these processes. By enabling electrochemical gradients across the thylakoid membranes, K plays a crucial role in the production of ATP during the light‐dependent processes of photosynthesis (Checchetto et al. 2012). Optimal K⁺ concentrations augment ATP synthase activity, hence facilitating energy‐dependent metabolic processes in plants (Tränkner et al. 2018). It not only contributes to energy generation but also modulates the activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), a crucial enzyme in the Calvin cycle that facilitates CO₂ fixation (Jákli et al. 2017; Du et al. ²⁰¹⁹; Li et al. ²⁰²²). This is frequently associated with impaired chloroplast pH homeostasis and restrictions in CO₂ diffusion resulting from modified mesophyll conductance (Tränkner et al. 2018). For instance, K deficit hinders the production and activity of Rubisco, as demonstrated in alfalfa and soybeans, hence diminishing photosynthetic efficiency (Wang and Wu 2015; Li et al. ²⁰²²). While these effects are well documented in field crops, similar mechanisms in PCS‐grown crops may be modified due to controlled microclimates and fertigation regimes.

Potassium modifies enzyme activity via activating H⁺‐ATPase, which facilitates ion transport and affects internal pH and membrane potential, hence directly influencing photosynthetic rates (Anschütz et al. 2014; Chen, Cao, et al. ²⁰¹⁶). During drought stress, decreased leaf K⁺ is associated with lower H⁺‐ATPase activity in tomato seedlings; exogenous K⁺ reinstates both enzyme activity and endogenous K⁺ concentrations (Siddiqui et al. 2021). Metabolomic studies underscore the significance of K in the regulation of carbohydrate metabolism and transport. For example, sufficient K availability facilitated the photoassimilate transfer from leaves to roots and enhanced nutrient use efficiency in apples (Xu et al. 2020). Potassium deficit in cassava and soybean diminished the net photosynthetic rate and intercellular CO₂ concentration, hence further impairing carbon fixation (Singh and Reddy 2017; Omondi et al. ²⁰²⁰). In addition to gas exchange, K deficit impacts structural and biochemical characteristics like stomatal conductance, mesophyll architecture, and chlorophyll concentration (Lu et al. 2016; Martineau et al. ²⁰¹⁷). However, several plants preserve the equilibrium of reactive oxygen species (ROS) in low potassium conditions by enhancing antioxidant activity, hence reducing oxidative damage and partially sustaining photosynthetic performance (Du et al. 2019). Last but not least, K⁺ availability can affect the expression of genes linked to photosynthesis, including those that encode ATP synthase, Rubisco, and Rubisco activase, especially in stressful situations (Favreau et al. 2019). This highlights the significant roles of K in plant metabolic responses and energy production.

2.3. Linking Carbohydrate Metabolism and Translocation With K Nutrition

Potassium strongly influences metabolite formation, modification, and transport, hence directly or indirectly enhancing crop productivity. A primary role of K is its capacity to enable the transport and distribution of carbohydrates generated during photosynthesis. This is accomplished by modulating many essential enzymes involved in sugar and starch metabolism, such as sucrose synthase, acid invertase, and starch phosphorylase (Luo et al. 2021). By supporting the movement of photosynthates from source tissues, like leaves, to sink tissues, including roots, fruits, and developing seeds, K ensures efficient energy allocation throughout the plant for supporting growth, biomass accumulation, and yield. In rapeseed ( Brassica napus L.), potassium use has been demonstrated to improve carbohydrate utilization and augment leaf area index, resulting in increased dry matter accumulation and enhanced yield. Raza et al. (2014) indicated that potassium administration in wheat enhanced spike length, spikelet quantity, number of grains, and total grain production, especially under drought stress. Çolpan et al. (2013) stated potassium's key role in boosting both productivity and quality of tomato fruits in greenhouse conditions. The foliar application of potassium in maize markedly enhanced grain weight, attributable to better photosynthetic efficiency and effective translocation of photosynthates from source to sink (Amanullah et al. 2016). It was shown that a sufficient supply of potassium considerably improves photosynthetic rates by promoting the utilization and export of photoassimilates, especially sucrose. The content of sucrose in leaves significantly rises when plants receive adequate potassium levels, underscoring its importance in enhancing carbon metabolism and transport (Zhao et al. 2001; Koch et al. ²⁰¹⁹). In contrast, K deficiency adversely impacts CO₂ assimilation and the subsequent consumption of photoassimilates. This disruption results in the buildup of ROS, which exacerbates photooxidative damage and hinders plant growth and production (Waraich et al. 2012; Johnson et al. ²⁰²²).

2.4. Potassium Is Vital for Stress Physiology and Antioxidant Defense

The beneficial effects of potassium also encompass stress alleviation. Under salt stress, the utilization of potassium sulphate (K₂SO₄) enhanced photosynthesis and sugar buildup in salt‐tolerant soybean cultivars, surpassing salt‐sensitive varieties (Parveen et al. 2021). The significance of K in improving sugar transport and photosynthetic activity under cold stress was further supported by a recent study on Plantago major . Ho et al. (2020) revealed that potassium‐supplemented plants displayed elevated sorbitol concentrations, enhanced sorbitol/sucrose ratios in phloem sap and foliage, increased photosynthetic efficiency, and less non‐photochemical quenching (NPQ) after stress acclimatization. Consequently, these underscore the necessity for sufficient potassium nutrition to bolster plant resilience under challenging climatic conditions.

These findings collectively highlight multiple functions of K in enhancing plant productivity through the maintenance of photosynthetic capacity, enhancement of source–sink dynamics, and alleviation of the detrimental impacts of abiotic stress. These aspects are relevant to both field and PCS‐grown crops. However, the distinctive conditions of PCS, such as plant densities, limited root volumes, regulated but sometimes suboptimal fertigation, and extended growth cycles, can alter nutrient dynamics and increase potassium demand. Therefore, potassium fertilization is a key component of nutrient management in PCS for maintaining high yield potential and fostering climate‐resilient production in these intensive and resource‐efficient systems.

3. Potassium Acquisition and Transport Pathways in Plants

Potassium is the predominant essential cation in plant cells, generally found in the cytoplasm at approximately 100 mM and varying from 10 to 200 mM in the vacuole (Voelker et al. 2006; Chen et al. ²⁰¹²; Hills et al. ²⁰¹²; Wang and Wu ²⁰¹³). K channels and transporters govern the selective and non‐selective transport of K⁺ across membranes and are essential for preserving cellular ion homeostasis. High‐affinity K⁺ transporters function effectively at low external K⁺ concentrations, but low‐affinity K⁺ channels activate when external K⁺ values surpass around 0.3 mM (Wang and Wu 2013). The synchronized functions of these components govern the absorption, outflow, and intracellular distribution of K, as depicted in Figure 2.

FIGURE 2.

Membrane transporters for potassium uptake, efflux and distribution in plants. In root cells, AKT1, HAK5, and CNGC channels mediate K⁺ uptake, while the GORK channel facilitates K⁺ efflux to fine‐tune plasma membrane potential. In the root stele, SKOR releases K⁺ into the xylem, and NRT1.5 coordinates K⁺ and NO₃ ⁻ loading for root‐to‐shoot translocation. In leaf cells, KAT1/2 and GORK regulate K⁺ influx and efflux across the plasma membrane, respectively. NHX1/2 transport K⁺ into the vacuole, while tandem‐pore channels (TPKs) and TPC1 channels mediate K⁺ efflux from the vacuole into the cytoplasm. The plasma membrane outward K⁺ channel AKT2 releases K⁺ into the phloem for root return and photosynthate loading. In chloroplasts, KEA1/2 localize to the inner envelope, and KEA3 to the thylakoid membrane, mediating K⁺ transport. KEA4/5/6 regulate K⁺ movement into the Golgi, trans‐Golgi network, and pre‐vacuolar compartments (Created in https://BioRender.com).

3.1. Functional Classification of Potassium Transporters and Channels

Plant K⁺ channels predominantly derive from three gene families: Shaker type, tandem‐pore K⁺ (TPK), and two‐pore channel (TPC) (Ragel et al. 2019; Wang et al. ²⁰²¹). Moreover, multiple gene families that encode K⁺ transporters, including K⁺ uptake permease (KT/HAK/KUP), high‐affinity K⁺ transporter (HKT), and cation‐proton antiporter (CPA), which collectively play central roles in K⁺ uptake, transport, and homeostasis in plants (Li et al. 2018; Sze and Chanroj ²⁰¹⁸; Ali et al. ²⁰²¹). In Arabidopsis thaliana , 75 genes have been identified that encode proteins involved in the uptake, translocation, and homeostasis of K⁺, highlighting the complexity and importance of K⁺ transport in plant physiology. According to their transport modes and structural characteristics, these genes can be generally divided into seven functional categories (Anschütz et al. 2014). These categories include Shaker‐type K⁺ channels (nine genes), two‐pore K⁺ channels (six genes), putative K⁺/H⁺ antiporters (six genes), KUP/HAK/KT transporters (13 genes), high‐affinity K⁺ transporters (HKT; one gene), cyclic nucleotide‐gated channels (CNGCs; 20 genes), and glutamate receptor‐like channels (GLRs; 20 genes). To keep the osmotic balance, turgor pressure, enzyme activity, and membrane potential in plant cells under control, these transport systems work across cellular membranes to enable both high‐ and low‐affinity K⁺ homeostasis. Many studies have focused on Shaker‐type K⁺ channels, which are further divided into three categories according to electrophysiological features and expression patterns: inward‐rectifying channels, outward‐rectifying channels, and weakly‐rectifying or silent channels (Adem et al. 2020; Feng, Liu, et al. ²⁰²⁰; Wang et al. ²⁰²²). Inward‐rectifying channels are triggered when the membrane potential becomes more hyperpolarized, promoting K⁺ absorption into the cell. Outward‐rectifying channels facilitate the efflux of K⁺ and are activated by membrane depolarization, serving a crucial function in regulating K⁺ release during stress or stomatal closure. Weakly rectifying channels can facilitate both the inflow and outflow of K⁺ and are generally activated during membrane hyperpolarization (Wang and Wu 2013). Consequently, while K⁺ transport systems have been thoroughly characterized, their dynamic regulation in response to nutrient limitations and abiotic stresses remains largely unexamined, presenting a significant opportunity to elucidate their integrative roles in augmenting plant stress resilience.

Potassium absorption in plant roots is predominantly facilitated by two principal transport mechanisms: the AKT1 channel, which performs at moderate external K⁺ concentrations, and the high‐affinity K⁺ transporter 5 (HAK5), which operates effectively in conditions of low K⁺ availability. Both are controlled by interactions with calcineurin B‐like proteins (CBLs) and CBL‐interacting protein kinases (CIPKs), constituting an essential signaling module for K⁺ sensing and transport activation (Pettigrew 2008). For long‐distance K⁺ transport, specific outward K⁺ channels such as the Stellar K⁺ Outward Rectifier (SKOR) and the Guard Cell Outward Rectifying K⁺ channel (GORK) play critical roles. SKOR is localized in the root stele, where it promotes the release of K⁺ into the xylem sap, enabling upward transport from the roots to the shoot (Luan et al. 2016). The function of SKOR is affected by external K⁺ concentrations, with an elevated level enhancing the probability of channel activation (Chen et al. 2021). Concurrently, GORK channels in guard cells facilitate K⁺ efflux during stomatal closure, a vital mechanism for regulating transpiration and conserving water under stressful circumstances (Wang et al. 2016). Upon entering the cell, a substantial amount of K⁺ is sequestered in the vacuole, serving as a reservoir to regulate cytosolic K⁺ concentrations.

In instances of K⁺ deprivation, vacuolar reserves are utilized to preserve cytoplasmic K⁺ levels and uphold physiological equilibrium. This dynamic is governed by tonoplast‐localized transporters, including AtTPK1, AtNHX1, and AtNHX2, which facilitate K⁺ efflux from or inflow into the vacuole to sustain homeostasis (Tang et al. 2020; De Luca et al. ²⁰²¹). Moreover, within the cation/H⁺ exchanger (CHX) family, AtCHX17 is the sole member identified to be expressed in the epidermis and cortex of roots, where it facilitates K⁺ absorption under fluctuating environmental conditions (Lu et al. 2011). Additionally, K⁺ exchange antiporters (KEA) belonging to the cation/proton antiporter (CPA) family are also highly selective for K⁺ compared to other cations. These transporters exhibit higher levels of activity during potassium deprivation, salinity, and drought stress, facilitating the regulation of intracellular pH equilibrium and the maintenance of potassium distribution across endomembrane compartments (Aranda‐Sicilia et al. ²⁰¹⁶; Zhu, Pan, et al. ²⁰¹⁸; Wang, Tang, et al. ²⁰¹⁹). Thus, the roles of K⁺ transporters and their regulatory mechanisms in root uptake, long‐distance transport, and intracellular sequestration have been well documented. However, future studies should concentrate on clarifying the co‐regulation of these pathways at the molecular level under dynamic growth settings, especially for vegetables in protected cropping systems.

3.2. Regulatory Signalling Networks Under Potassium Deficiency

Potassium uptake in plants occurs via low‐affinity mechanisms utilizing Shaker‐type K⁺ channels (AKT, KAT, KC) and high‐affinity mechanisms facilitated by HAK/KUP/KT transporters. The function of the plasma membrane H⁺‐ATPase is essential in this situation, producing the electrochemical gradient necessary for K⁺ transport across membranes (Feng, Zhang, et al. ²⁰²⁰). Furthermore, K⁺ transporters play crucial roles that go beyond simple storage and absorption. Three K⁺ efflux antiporters, AtKEA1, AtKEA2, and AtKEA3, are associated with chloroplast formation and photosynthesis in Arabidopsis thaliana . AtKEA1 and AtKEA2 are localized to the inner envelope membrane of chloroplasts, while AtKEA3 is located in the thylakoid membrane (Kunz et al. 2014). Triple mutants deficient in AtKEA1, AtKEA2, and AtKEA3 display significant impairments in chloroplast development and diminished photosynthetic efficacy, leading to markedly retarded growth (Kunz et al. 2014; Dana et al. ²⁰¹⁶). AtKEA1 and AtKEA2 exhibit polar localization in developing plastids, suggesting their involvement in organelle biogenesis (Aranda‐Sicilia et al. ²⁰¹⁶). AtKEA3 is a transporter protein that facilitates the translocation of H⁺ and K⁺ ions via the thylakoid membranes. It facilitates the removal of H⁺ ions from the thylakoid lumen and their transport to the stroma via an exchange mechanism with K⁺ ions. This process aids in the regulation of the proton motive force across the thylakoid membranes. Consequently, AtKEA3 is crucial to regulating photosynthesis and enabling plants to adapt to varying light conditions (Armbruster et al. 2014; Wang et al. ²⁰¹⁷). In potassium‐deficient situations, plants actively augment their ability to redistribute internal K⁺ resources and upregulate transporters to preserve cellular homeostasis and support growth. The cellular response to potassium deprivation entails a complicated signaling cascade that incorporates reactive oxygen species, calcium signaling, and phytohormone interactions to modulate potassium absorption pathways (Figure 3). This intricate network of K⁺ transporters, channels, and regulatory proteins underscores potassium's pivotal function in plant nutrient regulation, energy equilibrium, and stress tolerance.

FIGURE 3.

Potassium signalling cascade under low K⁺ conditions. Low K⁺ triggers multiple signalling pathways in plant cells. The plasma membrane senses K⁺ deficiency, leading to hyperpolarization and activation of Reactive Oxygen Species (ROS) production via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. ROS mediated activation of NSCC (non‐specific cation channel) results in reduced K⁺ uptake, increased leakage, and decreased cytosolic K⁺ levels. phytohormones such as ethylene (ET), auxin, jasmonic acid (JA), cytokinin (CT), and abscisic acid (ABA) play modulatory roles where ET increases under K⁺ deficiency and regulates HAK5 (High‐Affinity K⁺ Transporter 5). Ca²⁺ signals, regulates downstream components like Calcineurin B‐like proteins (CBL1/9) and CBL‐Interacting Protein Kinase 23 (CIPK23) which phosphorylate and activate High‐Affinity K⁺ Transporter 5 (HAK5) and K⁺ uptake channel AKT1. Together with elevated ethylene and ROS levels, this leads to transcription of the HAK5 gene and facilitates its trafficking to the plasma membrane and biochemical activation, which results in increased K⁺ absorption under deficit conditions. K⁺ replenishment depolarizes the membrane and returns the system to homeostatic levels (Created in https://BioRender.com).

4. Plants' Metabolic Responses to Potassium Deficiency

Plant growth and development are fundamentally reliant on the availability of important nutrients, which govern and modulate vital physiological processes including photosynthesis, respiration, and energy metabolism. Plants possess distinct nutritional needs, which are classified into macronutrients, encompassing nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), and sulfur (S), and micronutrients, including iron (Fe), zinc (Zn), copper (Cu), boron (B), manganese (Mn), cobalt (Co), molybdenum (Mo), and nickel (Ni; Podar and Maathuis ²⁰²²). In hydroponic growing systems, the regulation of electrical conductivity (EC) is essential for sustaining optimal nutrient levels (Islam et al. 2024). Alterations in electrical conductivity (EC) directly affect the solubility and accessibility of both macronutrients and micronutrients, thereby influencing nutrient absorption. Nutrient deficiency affects metabolic equilibrium, resulting in physiological changes that may jeopardize both growth and productivity. The progression of high‐throughput analytical technology has facilitated the emergence of metabolomics, encompassing both untargeted and targeted methodologies, as a potent instrument for finding differentially accumulated metabolites (DAMs) in reaction to nutritional deficits (Figure 4). These DAMs function as indicators for identifying nutritional stress and offer insight into the underlying metabolic adaptations. For instance, Gao et al. (2022) demonstrated that low K treated lettuce grown under controlled conditions showed a distinct group of metabolites which act as an early detecting biomarkers for potassium deficiency. Their identification can facilitate targeted breeding (Zhu, Wang, et al. ²⁰¹⁸; Labadie et al. ²⁰²⁰) and agricultural enhancement initiatives focused on improving nutrient‐use efficiency, yield, and quality, as well as deepening our knowledge of plant responses to nutrient stress at the molecular level. A summary of metabolite responses to potassium stress is given in Table 1.

FIGURE 4.

Representative metabolic pathways affected by potassium deficiency in plants. Potassium deficiency induces widespread reprogramming of central and secondary metabolism in plants. Key affected pathways include nitrogen metabolism, the tricarboxylic acid (TCA) cycle, glycolysis, the shikimic acid pathway, and multiple amino acid biosynthetic routes. The disruption of these pathways leads to the differential accumulation of stress‐responsive metabolites such as amino acids, organic acids, and antioxidants, and modulates the biosynthesis of phytohormones, including abscisic acid, salicylic acid, and jasmonic acid. These metabolic adjustments collectively enhance stress tolerance and play a crucial role in plant adaptation under K limited conditions (Created in https://BioRender.com).

TABLE 1.

Summary of metabolite responses to potassium stress.

Metabolite/Group	Category	Response to K stress	Crop and growing condition	References
Glucose, sucrose, fructose	Energy/Carbohydrate Metabolism	Increased under K deficiency	Tomato, Barley (Hydroponic)	(Sung et al. 2015, Zeng et al. 2018)
Proline	Stress/Osmoprotection	Elevated in tolerant cultivars	Barley, Peanut (Hydroponic)	(Zeng et al. 2018, Patel et al. 2022)
Asparagine, Alanine, Ornithine, Histidine	Nitrogen Metabolism	Accumulated under K deficiency	Wheat (Hydroponic and Pot)	(Zhao et al. 2020)
Citric acid, Glutamic acid, GABA	Nitrogen Metabolism	Decreased in roots	Wheat (Hydroponic and Pot)	(Zhao et al. 2020)
L‐Phenylalanine	Secondary Metabolism	Elevated in tolerant barley	Barley (Hydroponic)	(Zeng et al. 2018)
Ascorbic acid	Antioxidant	Increased in roots and leaves	Barley (Hydroponic)	(Zeng et al. 2018)
Glutathione	Antioxidant	Elevated in tolerant wheat	Wheat (Hydroponic and Pot)	(Zhao et al. 2020)
Abscisic acid (ABA)	Phytohormone Signalling	Increased in leaves and roots	Peanut (Hydroponic)	(Patel et al. 2022)
Jasmonic acid (JA)	Phytohormone Signalling	Increased in leaves	Peanut (Hydroponic)	(Patel et al. 2022)
Salicylic acid (SA)	Phytohormone Signalling	Increased in leaves, decreased in roots	Peanut (Hydroponic)	(Patel et al. 2022)
Citric acid, Alpha‐ketoglutaric acid	TCA Cycle (High K supply)	Increased under high K supply	Tomato (Pot)	(Weinert et al. 2021)
Succinic, Malic, Cis‐aconitic acids	Organic Acids (High K supply)	Increased under high K supply	Tomato (Pot)	(Weinert et al. 2021)
Citramalic acid, Dehydroascorbic acid	Organic Acids (High K supply)	Decreased under high K supply	Tomato (Pot)	(Weinert et al. 2021)

4.1. Carbohydrate, Nitrogen, and Secondary Metabolism Under K Deficiency

Potassium is a macronutrient that serves as the primary cation in plant cells and as a crucial cofactor for numerous enzymes engaged in key physiological processes. Recent metabolomic investigations in crops including tomato, lettuce, and citrus have demonstrated that potassium deprivation impairs fundamental metabolic processes, specifically carbohydrate and nitrogen metabolism (Weinert et al. 2021; Gao et al. ²⁰²²; Jiao et al. ²⁰²²). Carbohydrate metabolism is a key metabolic route that is impacted by K stress. It functions as a principal energy source for plant growth and development while supplying vital precursors for protein and lipid production, therefore impacting various aspects of plant metabolism (Rolland et al. 2006). Plants have been known to elevate their sugar content, particularly glucose, sucrose, and fructose, in reaction to various stresses, including potassium deficiency. Increased sugar buildup in both leaves and roots indicates modified source‐sink dynamics (Zeng et al. 2018; Zhao et al. ²⁰²¹). In tomato roots, substantial sucrose buildup was noted under low K circumstances (Sung et al. 2015). Remarkably, barley genotypes that were low‐K tolerant stored more amounts of sugars in both their roots and leaves than their sensitive counterparts, indicating that improved sugar accumulation is essential for K deficit tolerance (Zeng et al. 2018). Sucrose not only serves as an energy source but also acts as a crucial signalling molecule, transported from leaves to roots, influencing root development under nutritional limitation (Schlüter et al. 2012). Potassium is necessary for phloem loading and the long‐distance transport of sucrose; hence, its availability directly affects the efficiency of sucrose translocation from source to sink (Armengaud et al. 2009).

In addition to alterations associated with carbohydrate metabolism, K shortage has a substantial impact on nitrogen metabolism. Amino acids such as asparagine, alanine, histidine, and tryptophan increase in response to low potassium stress, especially in the roots (Zhao et al. 2020). It's interesting to note that barley plants exhibit a drop in negatively charged amino acids and an increase in positively charged ones, indicating ionic compensation in response to disturbed K⁺ homeostasis (Zeng et al. 2018). Furthermore, metabolites from the phenylpropanoid pathway, such as L‐phenylalanine, were increased in low potassium‐tolerant genotypes, indicating an activation of secondary metabolism that facilitates structural reinforcement and stress signaling (Zhang and Liu 2015; Zeng et al. ²⁰¹⁸). Therefore, in K‐deficient environments, sucrose and key amino acids build up in roots, support metabolic activities, and serve as a signal for low‐K stress adaptation, potentially acting as a valuable biomarker for assessing crop tolerance.

4.2. Oxidative Stress Responses and Antioxidant Metabolites

Plants with K deficiency accumulate many ROS, which can cause oxidative stress and damage to the cells (Hernandez et al. 2012). Thus, the overexpression of antioxidant metabolites functions as a vital adaptive mechanism via which plants alleviate oxidative damage and improve their resilience to potassium deficiency‐induced stress. Osmotic adjustment is markedly affected by the accumulation of appropriate solutes, such as proline, soluble sugars, amino acids, and polyols (Khan et al. 2020). Proline is regarded as an invaluable osmoprotectant and antioxidant for stress resilience (Szabados and Savouré 2010). Increasing evidence indicates that potassium deficit results in heightened proline concentrations in several plant tissues (Zeng et al. 2018). Comparable tendencies were seen in peanut ( Arachis hypogaea ), with proline concentrations increased in both leaves and roots due to potassium deficiency (Patel et al. 2022). Furthermore, ascorbic acid is also an antioxidant, protecting the integrity of cell membrane permeability (Shalata and Neumann 2001). Glutathione, a powerful antioxidant, functions as an antioxidant by removing reactive oxygen species (ROS) via the GSH‐ascorbate cycle (Zagorchev et al. 2013). Under K deficient conditions, a metabolomic study showed a considerable rise in glutathione levels in the roots of the low‐K tolerant wheat cultivar KN9204, but the low‐K sensitive cultivar BN207 showed no such increase (Zhao et al. 2020). These findings, together with increased levels of proline and ascorbic acid noted in other K‐tolerant crops, indicate that antioxidant metabolites are crucial in the adaptive response to K deficiency in crops.

4.3. Hormonal Signaling and Responses to Potassium Supply

Metabolites linked to plant hormones, including abscisic acid, salicylic acid, and jasmonic acid, are stimulated by potassium deficiency. ABA is broadly acknowledged as a stress indicator elicited by drought, salinity, and nutrient deprivation (Peuke et al. 2002). It can efficiently regulate water relations to control stomatal conductance and plant metabolism (Kim et al. 2014). Jasmonic acid plays a key role in plant responses to abiotic stress by activating antioxidant defense mechanisms, enhancing the accumulation of amino acids and carbohydrates, and regulating stomatal aperture to optimize water utilization and gas exchange during stress conditions. Salicylic acid is essential for maintaining cell membrane integrity and controlling protein levels associated with secondary metabolites (Patel et al. 2022). The research indicated that potassium deficiency resulted in increased ABA levels in the leaves and roots of peanut plants (Patel et al. 2022). Moreover, the concentration of jasmonic acid in peanut leaves also demonstrated an elevation under low‐K stress circumstances (Patel et al. 2022). Under conditions of limited potassium, the concentration of salicylic acid increased in peanut leaves but decreased in roots, in contrast to abscisic acid and jasmonic acid. Considering the essential functions of phytohormones in modulating plant development and stress responses, it is plausible to assert that abscisic acid, salicylic acid, jasmonic acid, and other signaling molecules are vital in augmenting plant resistance to low potassium stress.

While the metabolic consequences of potassium deficiency are extensively established, a significant vacuum remains in the literature regarding the effects of elevated potassium levels on plant metabolism. However, existing evidence suggests that increased potassium supply can affect critical metabolic pathways. In tomatoes, elevated potassium treatment has been demonstrated to enhance various intermediates of the tricarboxylic acid (TCA) cycle, such as citric acid and alpha‐ketoglutaric acid (Weinert et al. 2021). Moreover, the concentration of other organic acids, including succinic, threonic, malic, cis‐aconitic, and isocitric acid in fruits has been documented at elevated potassium levels. In contrast, concentrations of other metabolites such as citramalic acid, dehydroascorbic acid, and galacturonic acid were observed to decrease with increased potassium input (Weinert et al. 2021).

In summary, metabolomic analyses have elucidated the critical role of potassium in plant physiology, demonstrating that potassium deficiency initiates a series of metabolic alterations, including modifications in carbohydrate metabolism, amino acid accumulation, antioxidant metabolite levels, and phytohormone concentrations, which are important for plant adaptation to low potassium stress. Further research is necessary to clarify the metabolic responses to elevated potassium availability and to ascertain optimal potassium levels that harmonize growth, quality, and stress resilience.

5. Interactions Between Potassium and Other Nutrients

5.1. Potassium‐Nitrogen Interactions on Plant Growth and Metabolism

The interactions between potassium and nitrogen (N) significantly impact plant metabolic pathways, affecting growth, development, and resilience. Understanding these interactions between the two vital macronutrients from a metabolic perspective is essential for improving nutrient management in agriculture. The assimilation and transport of nitrogen are two primary processes influenced by the interactions between potassium and nitrogen. Potassium enhances nitrogen uptake via regulating the activity of nitrate and ammonium transporters at the root plasma membrane (Coskun et al. 2017; Bonfim‐Silva et al. ²⁰¹⁸). Nitrogen, an indispensable part of amino acids and proteins, promotes essential processes in plant growth and development. Abundant potassium availability augments nitrogen absorption and promotes its utilization efficiency (Xu, Wang, et al. ²⁰²²). The utilization of K has been shown to mitigate nitrogen deficiency symptoms, enhance nitrogen metabolism, and augment plant biomass and yield (Zhang et al. 2010; Duncan et al. ²⁰¹⁸). In greenhouse‐grown Hippeastrum, a synergistic relationship between potassium and nitrogen markedly improved bulb growth and nutrient utilization efficiency, especially under increased carbon dioxide levels, exhibiting a curvilinear response (Silberbush et al. 2003). A further investigation on nitrogen and potassium interactions in strawberries cultivated in soilless greenhouse environments revealed that elevated nitrogen levels boosted productivity, but increased potassium concentrations enhanced fruit quality, specifically regarding soluble solids, phenolic contents, and antioxidant capacity (Preciado‐Rangel et al. ²⁰²⁰). Table 2 summarizes the K and other nutrients' interaction with key metabolic effects on plants.

TABLE 2.

Summary of potassium and other nutrients interactions.

Interaction	Key metabolic effects	Outcome	References
Potassium‐Nitrogen (K‐N)	Enhances nitrate/ammonium uptake, nitrate reductase activity, GS & GDH function, and auxin/cytokinin signalling	Improved N‐use efficiency, growth, and yield	(Coskun et al. 2017, Bonfim‐Silva et al. 2018, Xu, Tian, et al. 2022)
Potassium‐Phosphorus (K‐P)	Regulates photosynthesis, glycolysis, and secondary metabolism (e.g., phenolic compounds); influences P transporter gene expression	Better P‐use efficiency, carbohydrate metabolism, and stress resilience	(Wang, Tang, et al. 2019, Courbet et al. 2021, Lu et al. 2023)
Potassium‐Magnesium (K‐Mg)	Synergistic in enzyme activation (Rubisco), antagonistic in uptake under high K conditions	Balanced metabolic activity; risk of Mg deficiency with high K	(Xie et al. 2021)
Potassium‐Manganese (K‐Mn)	Activates photosynthetic enzymes (PEPC, Rubisco), improves carbon fixation and nitrogen metabolism	Enhanced stress tolerance and productivity	(Kabata‐Pendias 2011, Hafsi et al. 2014)
Potassium‐Iron (K‐Fe)	Supports chlorophyll synthesis, redox reactions, and enzyme cofactor roles in N metabolism (NR, GS, GDH)	Improved N and carbon metabolism; oxidative stress mitigation	(Armengaud et al. 2009)
Potassium‐Zinc (K‐Zn)	Stabilizes sugar metabolism; supports flavonoid and phenolic compound biosynthesis; modulates hormone pathways	Increased stress defence, root development, and metabolic stability	(Cakmak 2008, Hafsi et al. 2014)
Potassium‐Boron (K‐B)	Enhances K permeability, supports pollen germination, cell division, and hormone transport; complements K function	Improved reproductive function and vascular integrity	(Cakmak 2008, Marschner 2011)
Potassium‐Copper (K‐Cu)	Improves antioxidant enzyme activity (SOD, APX), redox balance, and stress resilience	Enhanced antioxidant defence and abiotic stress tolerance	(Hafsi et al. 2014)

The influence of K⁺ on nitrogen metabolism has been well examined. Elevated external K⁺ concentrations have been demonstrated to augment nitrate translocation from roots to shoots, thus enhancing nitrate reductase (NR) activity and facilitating nitrate buildup in the leaves (Armengaud et al. 2009; Chen, Wang, et al. ²⁰¹⁶; Xu et al. ²⁰²⁰). In contrast, K⁺ deficit reduces nitrate translocation, leading to enhanced nitrogen uptake in roots but diminished NR activity, as evidenced in Arabidopsis thaliana (Armengaud et al. 2009; Fang et al. ²⁰²⁰). This response relates to reduced carbon‐skeleton synthesis caused by low cytoplasmic K⁺ concentrations, impacting enzymes like glutamine synthetase (GS) and glutamate dehydrogenase (GDH) (Armengaud et al. 2009). In rice, increased external K⁺ significantly enhances the activity of glutamine synthetase (GS) and phosphoenolpyruvate carboxylase (PEPC), along with total protein content and biomass, leading to considerable improvements compared to nitrate‐cultivated plants (Balkos et al. 2010). The results highlight the importance of K⁺ in the restructuring of carbon and nitrogen metabolism, corroborated by transcriptomic evidence of metabolic changes during nitrogen deficiency (Scheible et al. 2004).

Moreover, the interaction between potassium and nitrogen profoundly affects phytohormone signalling pathways, especially those related to auxins and cytokinins. Deficiencies in potassium or nitrogen have been linked to diminished cytokinin levels, which subsequently lead to the accumulation of ROS, the stimulation of root hair formation, and the upregulation of nutrient transporter gene expression (Takei et al. 2002; Nam et al. ²⁰¹²). These modifications underscore the reorganization of metabolic and signalling networks affected by K and N availability, influencing nutrient utilization efficiency and plant stress responses. Integrating these concepts lays the groundwork for subsequent research focused on optimizing nutrient management strategies, hence improving the yield and resilience of greenhouse crops.

5.2. Potassium‐Phosphorus Interactions in Energy and Carbon Metabolism

The primary metabolic pathways influenced by the interactions of potassium and phosphorus pertain to the regulation of photosynthesis. Phosphorus is vital for the synthesis of ATP and nucleic acids, which are crucial for energy transfer and genetic information processing in plants (Wang, Qin, et al. ²⁰¹⁹). The interactions between K and P influence carbohydrate synthesis and metabolism. K is acknowledged for enhancing the activity of enzymes related to carbohydrate metabolism, especially those inside the glycolytic pathway (Figure 4; Lu et al. ²⁰²³). Carbohydrates are essential as they serve as the primary energy source for various metabolic processes, including the creation of nucleic acids and proteins, which depend on phosphorus (Wang et al. 2018). Maintaining the integrity of metabolic pathways requires the balance of K and P since deficiencies in either mineral can affect glucose metabolism, leading to stunted development and decreased agricultural production (Du et al. 2022).

The interaction between K and P affects the synthesis of secondary metabolites, essential for plant defense and stress responses. For instance, adequate potassium levels may enhance the production of phenolic compounds and other secondary metabolites that aid in plant defense (Arbačauskas et al. 2023). Phosphorus influences the synthesis of these metabolites by engaging in energy transfer and signaling pathways (Bielecka et al. 2015). The combined effect of potassium and phosphorus on secondary metabolism highlights the importance of these nutrients in enhancing plant resistance to biotic and abiotic stresses. Furthermore, the signaling pathways regulating nutrient intake and utilization are influenced by interactions between K and P. K is acknowledged for its function in regulating the expression of genes associated with P absorption and transport, therefore enhancing phosphorus usage efficiency in plants (Courbet et al. 2021). This interaction is especially crucial in nutrient‐deficient soils, where potassium availability can markedly influence the plant's capacity to absorb and utilize phosphorus efficiently (Du et al. 2022). Apart from the molecular and physiological mechanisms, in greenhouse‐cultivated cucumbers, K‐P interactions markedly affected yield, with optimal phosphorus concentrations augmenting productivity and elevated potassium levels exerting a sustained positive impact on yield under adequate nitrogen availability (Al‐Jaloud et al. ²⁰⁰⁴). The confluence of K and P signaling pathways underscores an important aspect of nutrient crosstalk, which is essential for increasing nutrient usage efficiency and improving overall crop development and resilience.

5.3. Potassium‐Micronutrient Interactions Balance Cofactors and Stress Resilience

Potassium is not only crucial for plant development and stress resilience but also affects the absorption, distribution, and physiological activities of other micronutrients. The interactions between potassium and other micronutrients, including magnesium (Mg²⁺), iron (Fe²⁺/Fe³⁺), manganese (Mn²⁺), zinc (Zn²⁺), copper (Cu²⁺), and boron (B), may be synergistic or antagonistic, contingent upon nutrient availability, plant species, and environmental conditions (Römheld and Kirkby 2010; Marschner ²⁰¹¹). These interactions influence essential metabolic activities like as photosynthesis, enzymatic activation, redox equilibrium, and nutrient signaling pathways.

The interplay between K⁺ and Mg²⁺ in plants demonstrates both antagonistic and synergistic effects, essential for nutritional equilibrium, physiological function, and crop yield (Xie et al. 2021). Excessive potassium (K⁺) input can impede magnesium (Mg²⁺) absorption by competing for nonspecific transport channels at the root–soil interface, frequently resulting in K‐induced Mg shortage, especially in crops with high K requirements or under acidic soil conditions (Rhodes et al. 2018). Recent research indicates that this relationship transpires not just at the root absorption level but also encompasses root‐to‐shoot translocation, internal distribution, and nutrient utilization inside the plant (Ding et al. 2008; Senbayram et al. ²⁰¹⁵; Huang et al. ²⁰¹⁹). At the molecular level, this antagonistic relationship is evident as K⁺ and Mg²⁺ affect the expression of each other's transporter genes, including OsHAK1 and OsMGT, suggesting a sophisticated regulatory system under nutritional stress (Cai et al. 2012). Despite this antagonistic relationship, K and Mg work together in several important physiological functions, such as phloem loading, protein synthesis, and photosynthesis (Zörb et al. 2014; Jákli et al. ²⁰¹⁷; Tränkner et al. ²⁰¹⁸). Both nutrients are necessary cofactors for enzymes like nitrate reductase and Rubisco, and a balanced supply of them improves assimilate translocation, turgor regulation, and carbon and nitrogen metabolism. To maximize production and preserve plant health, it is crucial to maintain an ideal K:Mg ratio during fertigation.

Potassium demonstrates direct synergistic interactions with the micronutrient manganese, which is vital for numerous physiological activities in plants. Manganese is important in photosynthesis, nitrogen metabolism, and nitrogen absorption. It additionally functions as an activator for other essential enzymes, such as decarboxylases, dehydrogenases, and oxidases (Alejandro et al. 2020). K blends with Mn to regulate photosynthetic enzymes, including Rubisco and phosphoenolpyruvate carboxylase (PEPC), thereby improving carbon fixation and directing metabolites into growth‐enhancing pathways (Kabata‐Pendias ²⁰¹¹). The interaction of these nutrients also influences secondary metabolism by regulating the production of flavonoids and phenolic compounds, which increases the plant's resistance to abiotic stress (Reshi et al. 2023).

Iron (Fe) is required for chlorophyll production and is integral to numerous redox reactions within the plant system. Iron, as an essential element of ferredoxin, enables vital oxidation–reduction reactions, such as the reduction of nitrate and sulfate, in addition to biological nitrogen fixation (Zanetti and Pandini 2013). Moreover, iron serves as a structural element in crucial defensive enzymes like peroxidase and catalase, which assist in alleviating oxidative stress in plants. K enhances the efficiency of nitrogen metabolism by improving nitrate uptake and assimilation, principally through the upregulation of enzyme activities such as nitrate reductase (NR), glutamine synthetase (GS), and glutamate dehydrogenase (GDH). These enzymes necessitate cofactors such as molybdenum and iron for optimal functionality, highlighting the synergistic interaction between potassium and micronutrients in facilitating nutrient digestion and plant defense. These activities integrate nitrogen and carbon metabolism, enabling the production of amino acids and proteins.

The relation between potassium and zinc stabilizes sugar metabolism by modulating enzymes related to carbohydrate transport and storage (Cakmak 2008). Potassium, in combination with Mn and Zn, facilitates the manufacture of phenolic compounds and flavonoids, therefore augmenting plant defense systems and enhancing resilience to both biotic and abiotic challenges (Hafsi et al. 2014). It also modulates phytohormone pathways, including auxins and cytokinins, influencing root growth and nutrient uptake (Cakmak 2008; Marschner ²⁰¹¹).

Boron and potassium have complementary roles in plant physiology, demonstrating synergy. Like K, B serves a complex function in essential reproductive and physiological processes, encompassing pollen germination, cell division, nitrogen and carbohydrate metabolism, active salt absorption, hormone transport and action, as well as water management and overall plant hydration dynamics (Ahmed et al. 2024). K and B function as buffering agents essential for the preservation of vascular tissue functionality, while also regulating multiple metabolic processes. Research indicates that sufficient B levels improve K permeability across cell membranes, underscoring a functional synergy between these two nutrients (Atique ur et al. ²⁰¹⁸).

Besides its physiological functions, K is essential in regulating antioxidative metabolism and alleviating oxidative stress. The combination of micronutrients like Zn, Cu, and Mn considerably improves the activity of essential antioxidant enzymes, including superoxide dismutase (SOD) and ascorbate peroxidase (APX; Hafsi et al. ²⁰¹⁴). These interactions promote redox equilibrium and improve stress tolerance. These enzymes are crucial for scavenging ROS; therefore, preserving redox equilibrium and enhancing plant resilience under stress circumstances. The interacting effects of K with B, Zn, Cu, and Mn highlight the significance of nutritional synergy in enhancing plant defence and improving overall stress tolerance. In summary, K interacts both synergistically and antagonistically with many nutrients, influencing plant metabolism, nutrient utilisation efficiency, and stress resilience.

6. Conclusions and Future Directions

Metabolomics has greatly enhanced our understanding of plant responses to K and other nutrients. However, most of the existing knowledge is based on research conducted on field crops or a simple hydroponic setup. As we already discussed, crops cultivated in PCS, including greenhouses and polytunnels, experience distinctly different growing conditions, defined by meticulously regulated temperature, humidity, light, and fertigation than the field crops. These regulated settings can markedly modify physiological and biochemical responses, encompassing nutrition absorption kinetics, stress signaling, and metabolic regulation, rendering it unsuitable to directly apply findings from field‐derived data to high‐input, high‐efficiency PCS systems. Therefore, by revealing detailed metabolic reprogramming and stress adaptation pathways, metabolomics offers an invaluable tool for improving nutrient use efficiency, stress resilience, and overall crop productivity, particularly in PCS, where precise management of inputs and environmental conditions facilitates the efficient application of these discoveries (Obata and Fernie 2012). Moving forward, the future of understanding plant metabolomic responses to K stress depends on the integration of modern omics methodologies, including genomics, transcriptomics, proteomics, metabolomics, epigenomics, and ionomics, which collectively offer a systems‐level perspective on plant responses under controlled atmospheres (Zhang et al. 2020). This integration can facilitate the discovery of crucial metabolic indicators and gene targets for the creation of K‐efficient cultivars via standard breeding methods and novel gene‐editing technologies like Clustered Regularly Interspaced Short Palindromic Repeats‐associated protein 9 (CRISPR‐Cas9).

Beyond discovery, the vision for nutrient management in PCS is transitioning into a dynamic, closed‐loop system that integrates environmental control, plant monitoring, and genetic advancement work. We propose that three interlinked pillars propel the future of PCS: the growing environment, crop modeling and monitoring, and breeding integrated with multi‐omics data (Figure 5). In contrast to field systems, PCS enables narrower root zones, more precise phenotyping, and real‐time sensor‐based monitoring, which greatly enhances the actionability of feedback loops between plant response, environmental management, and genetic features. In these systems, environmental factors like light, temperature, humidity, carbon dioxide, irrigation, and fertilizer supply will be dynamically regulated by precision climate‐control technologies (Kaya 2025). These will be backed by high‐resolution sensor networks that can record metabolic and physiological reactions in real time. Data obtained from sensors will be analyzed using advanced modeling frameworks, including artificial intelligence and machine learning techniques (Zhang et al. 2022) to provide ongoing optimization of environmental conditions and nutrient management, particularly potassium delivery. Concurrently, metabolomic insights will contribute to multi‐omics pipelines to expedite trait identification and the breeding of stress‐resilient cultivars adapted to PCS circumstances. This ongoing feedback mechanism among crop monitoring, environmental regulation, and selective breeding would provide real‐time enhancement of crop management, resource utilization efficiency, and quality characteristics. To fully realize this potential, several critical research gaps must be addressed, particularly the necessity for PCS‐specific metabolomic studies under varying potassium regimes, the development of potassium‐efficient cultivars suitable for controlled environments, and the integration of multi‐omics data into real‐time fertigation control systems.

FIGURE 5.

A systems‐level framework for next‐generation protected cropping system nutrient management. (1) Breeding and multiomics integration focus on incorporating advanced breeding techniques and multiomics approaches (genomics, transcriptomics, proteomics, metabolomics, epigenomics, and ionomics) to develop crops optimized for controlled environments. (2) Growing environment highlights the management of environmental factors such as light, temperature, humidity, CO₂ levels, water, and nutrients, ensuring optimal conditions for crop growth and sustainability. (3) Crop modeling and monitoring emphasizes the use of precision technologies, including pest monitoring, disease detection, and digital tools, to enhance crop management and yield prediction in protected environments (Created in https://BioRender.com).

However, realizing this vision comes with challenges. The analysis of extensive metabolomic data is intricate, and existing analytical systems are frequently expensive and technically challenging (Qiu et al. 2023). Furthermore, metabolomic investigations specific to PCS are scarce, especially under varying potassium regimes and coupled stress situations. The interplay between potassium and other nutrients complicates nutrient management tactics, and the transfer of laboratory discoveries to commercial precision crop systems remains challenging despite the good progress. Furthermore, maintaining economic and environmental sustainability is still a major problem, particularly in smallholder or low‐resource systems. Dealing with these obstacles necessitates robust interdisciplinary collaboration among plant scientists, modellers, engineers, breeders, and data scientists to jointly develop scalable, cost‐efficient, and sustainable solutions. Integrating metabolomics with technological innovation and biological insight in protected cropping system might enhance climate‐smart agriculture, fostering resilient and productive systems that address the needs of a growing, resource‐limited global population.

Author Contributions

M.M.I. contributed to the conceptualization, preparation of the original draft, writing, review, editing, and visualization. L.L. participated in writing, review, and editing and was involved in funding acquisition. J.H. contributed to writing, review, and editing. A.N. was responsible for drafting the original manuscript and preparing visualizations. S.H. contributed to writing, review, and editing, as well as funding acquisition. T.A. participated in writing, review, and editing and supported funding acquisition. D.T. contributed to writing, review, and editing and assisted with funding acquisition. Z.‐H.C. was involved in conceptualization, preparation of the original draft, writing, review, and editing, and secured funding acquisition.

Conflicts of Interest

The authors declare no conflicts of interest.

Islam, M. M. , Li L., He J., et al. 2025. “Decoding Plant Metabolomic Response to Potassium and Nutrient Stresses in Controlled Environments.” Physiologia Plantarum 177, no. 5: e70547. 10.1111/ppl.70547.

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.