Nitrogen‐Centric Molecular Mechanisms in Crop Plants Under Elevated CO₂ : Interactions, Responses, and Implications

ABSTRACT

Increasing atmospheric carbon dioxide (CO₂) is transforming the climate space in which plants grow, severely affecting crop physiology and crop productivity. Elevated CO₂ enhances photosynthesis and biomass; however, it can nitrogen (N) metabolism, inhibiting the nutritional value and the yield capacity of crops. The most important central N‐regulated protein machineries are: transporters, nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, and urease which control N assimilation, distribution and remobilization in crop plants. With high CO₂, these molecular components exhibit altered expression and activities mostly due to the reduction in N concentration. Complex systemic plant responses under N control like adaptation of photosynthetic capacity, flowering time, reproductive development and seed nutrient profiles further support the complexity of the interactions between C and N signaling. The high CO₂ environment requires a more holistic analysis of the regulatory networks of N metabolism and anticipative crop improvement strategies. Future breeding and crop improvement strategies should focus on enhancing the resilience of N assimilation by optimizing the N transporter function and maintaining C–N stoichiometry, thereby sustaining crop performance and nutritive quality under changing climatic conditions. The present review identifies the molecular processes that regulate N responses in crops grown under elevated CO₂, highlighting the differences between legume versus non‐legume and C₃ versus C₄ plant responses and providing details that can ensure mitigation against negative impacts and outline future perspectives on crop improvement.

1. Introduction

Crop productivity is profoundly influenced by elevated carbon dioxide (eCO₂) levels, a defining feature of global climate change. Atmospheric CO₂ concentrations are rapidly increasing due to both natural and anthropogenic activities. The Intergovernmental Panel on Climate Change (IPCC) projects that the CO₂ concentrations will increase significantly from the current 400 μmol mol⁻¹ to 550–700 μmol mol⁻¹ by 2050 and to 670–940 μmol mol⁻¹ by 2100 (IPCC ²⁰²³). This unprecedented increase poses serious challenges to global food security by significantly impacting the physiology, growth, and productivity of crop plants. In the short term, eCO₂ enhances photosynthesis in many crop plants by increasing the carboxylation efficiency of RUBISCO while simultaneously reducing photorespiration. This physiological shift allows more efficient utilization of ATP and NADPH produced during light reactions, thereby optimizing energy channeling toward carbon assimilation (Kumar et al. 2017). Additionally, eCO₂ induces stomatal closure, a primary plant response aimed at regulating water loss and CO₂ uptake. Stomatal conductance (gs), which is proportional to the assimilation rate and humidity but inversely proportional to CO₂ concentration, underscores the complex interplay between photosynthesis and water‐use efficiency in crops exposed to eCO₂ conditions (Sreeharsha et al. 2021).

In addition to central physiological responses, eCO₂ induces significant changes in anatomical, morphological, and phenological features. Adaptations are mainly linked to changes in leaf area, thickness, number of branches, stem and root length, dry biomass, flowering patterns and timing, and seed quantity and quality (Sekhar et al. 2015). Carbon‐Nitrogen (C–N) interactions play a crucial role in the systemic responses of crop plants, as eCO₂ enhances carbon assimilation, leading to a gradual decrease in the leaf N content particularly on a mass basis (Sekhar et al. 2014; Unnikrishnan et al. ²⁰²¹). Thus, prolonged exposure under no external N supply leads to photosynthetic acclimation characterized by reduced carboxylation capacity (Vc _max ) and ribulose—1,5—bisphosphate (RuBP) regeneration (Jmax; Qian et al. ²⁰²¹). Apart from N limitation, the feedback inhibition associated with excessive accumulation of carbohydrates, increased stomatal resistance, and chlorophyll degradation are important factors dealing with the changes in photosynthesis with increasing CO₂ levels (Wang et al. 2014). Hence, the N‐mediated or dependent molecular mechanisms are central to plant responses under eCO₂, ultimately determining the C pools in plant biomass.

Plants employ fine metabolic coordination between photochemical activity and other downstream processes. The photosystem II (PSII) efficiency which can be measured through chlorophyll a fluorescence is the reliable diagnostic indicator of the photosynthetic apparatus under any climatic conditions (Sreeharsha et al. 2015). High CO₂ affects the opening of PSII reaction centers, and consequently alters the light harvesting, energy transduction and the assimilation of CO₂. Changes in CO₂ assimilation interrupt the energy balance of the photochemical quenching (qE) of light, resulting in diminished heat dissipation or non‐photochemical quenching (NPQ) (Murchie and Ruban 2020). Photosystem dynamics under eCO₂ are also influenced by stomatal and non‐stomatal limitations, including reductions in stomatal aperture and diminished ATP turnover, respectively (Sakoda et al. 2021). Furthermore, enzymes involved in the Krebs cycle and the Calvin cycle play a crucial role in utilizing the reducing equivalents and regulating the C assimilation under eCO₂, ultimately influencing plant dry matter production (Dusenge et al. 2019). Interestingly, legumes and non‐legumes significantly differ in their responses to eCO₂. The capacity of legumes to trade C for N with their nitrogen‐fixing symbiotic bacteria provides an advantage over other C₃ plants under eCO₂ environments (Cernusak et al. 2011). In legume nodules, a complex interaction exists between plant and microbial partners, mainly depending on environmental and growth conditions. Once inside the root nodule, the Rhizobium bacteria differentiate into specialized structures called bacteroids, which are responsible for N₂ fixation. These bacteroids obtain C from the host cell in the form of sucrose and return NH₄ ⁺ to the host cell. It was also reported that eCO₂ enhances nodule growth and N₂ fixation in certain legume crops, thus overcoming the photosynthetic acclimation that is commonly seen in several non‐legume crops upon prolonged growth under eCO₂ (Sreeharsha and Reddy 2015). Therefore, an efficient source‐sink balance under eCO₂ and transportation of excess sugars to roots and nodules ultimately influence the photosynthetic response of legume plants.

Elevated CO₂ also influences the way crops respond to abiotic and biotic stresses (Ainsworth and Long 2021). The process of respiration and photosynthesis leads to the production of reactive oxygen species (ROS) by cell wall NADPH oxidases and peroxidases (Tripathy and Oelmüller 2012). This occurs in mitochondria (superoxides, O₂ ^˙―), chloroplasts (hydrogen peroxide, H₂O₂), and in peroxisomes (hydroxyl radicals, OH^˙―; singlet oxygen, O^―; Iqbal et al. ²⁰²⁵). Normally, ROS serve as important secondary messengers in signaling pathways in plants while preserving normal metabolic fluxes as well as other functions within the cell. However, excessive ROS can lead to oxidative damage, including lipid peroxidation, and the oxidation of proteins and nucleic acids, resulting in the production of harmful reactive carbonyls such as: acrolein, malondialdehyde (MDA), 4‐hydroxy‐2‐nonenal (HNE), and methylglyoxal (MG) (AbdElgawad et al. ²⁰¹⁵; Xu et al. ²⁰¹⁵; Mudalkar et al. ²⁰¹⁷). The balance between ROS production and turnover determines whether ROS act as a defensive or destructive molecule. Plant cells contain multiple enzyme systems to operate signal transduction pathways for eliminating ROS using the reducing power of NADPH or ferredoxin, thus shielding the plant from the harmful impacts of oxidative stress (Iqbal et al. 2025). N plays an intriguing role in the alteration of the cell's redox state through enzymatic and non‐enzymatic antioxidants under eCO₂ in the presence of oxidative stress (Sreeharsha et al. 2019). In addition, N availability under eCO₂ influences the metabolism of osmolytes, including glycine betaine, mannitol, trehalose, and proline, which contribute to osmotic adjustment and membrane stability.

The current article provides a comprehensive understanding of the physiological and molecular responses of legume and non‐legume crops to eCO₂ and associated environmental factors. The concepts of photosynthetic acclimation and photosynthetic nitrogen use efficiency (PNUE) with reference to eCO₂ are discussed. Furthermore, the impact of climate change on flowering, seed/grain yield and quality is evaluated based on the existing data.

2. Central Protein Machinery in Crops Regulated by N Levels Under eCO₂

In crop plants, N is one of the vital macronutrients required for development and sustenance. As its requirement is huge, crop plants are often supplied with N in the form of fertilizers. However, excessive use of N fertilizers can lead to faster growth of competitive grasses and weeds, thus perturbing the agronomical diversity of crop plants. Innate soil N can significantly impact plant growth and reproduction by altering phenology, resource partitioning to reproductive tissues, and the quantity and quality of seeds. Different types of soils possess different N forms, based on pH and precipitation levels (Maathuis 2009). The preferred form of N for each plant depends on its adaptation to the soil; for example, plants grown in reducing soil with low pH prefer ammonium or amino acids, while plants grown in aerobic soils with high pH take nitrate as their preferred form (Masclaux‐Daubresse et al. ²⁰¹⁰). Since the rising CO₂ levels pose a concern to soil N at an ecosystem level, this impacts N assimilation enzymes significantly, leading to changes in N metabolism in the plants.

N metabolism in plants involves the uptake, translocation, assimilation, and remobilization of nitrogen (Waqas, Singh, et al. ²⁰²⁵; Waqas, McCouch, et al. ²⁰²⁵). Understanding the molecular mechanisms underlying these sequential events is crucial for enhancing nitrogen use efficiency (NUE) and improving crop production. The N uptake happens at the root epidermis with the help of transporters, and it subsequently gets translocated to different organs through the xylem, including aerial parts. The principal enzymes involved in N metabolism are nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), and urease. Nitrate reductase and nitrite reductase are mainly involved in initiating inorganic N utilization, while the latter two enzymes mediate the organic N formation from inorganic N and complete the N metabolism. The following sections discuss in detail the protein machineries involved in the systemic metabolism that are under the influence of N dynamics under eCO₂. The mechanistic aspects of N assimilation in root tissues and subsequent transportation to above‐ground tissues are illustrated in Figure 1.

FIGURE 1.

Schematic representation of cellular metabolisms and synchronous source‐sink relationships in legume crops under eCO₂: The excess carbon sequestered in leaves is actively translocated to the sink tissues and becomes the carbon source for rhizobium bacteria in root nodules. The inorganic N acquired through N₂ fixation and soil sequestration is assimilated and translocated via the GS‐GOGAT pathway (see Section 2 for mechanistic details). The organic N pool is translocated to above ground tissues wherein the plastid GS is involved in assimilation of NH₄ ⁺ generated during photorespiration and other metabolic processes. Image was created using Biorender.com tools.

2.1. Nitrogen Transporters

In crop systems, N uptake involves two transport systems: the high‐affinity transport system (HATS) and the low‐affinity transport system (LATS); both depend on the energy provided by proton gradients (Wang et al. 2012). LATS helps in transport under high external N concentrations (> 0.5 mM), and HATS is involved when low external N concentrations are present (< 0.5 mM; Noguero and Lacombe ²⁰¹⁶). The major transporters involved in N uptake, translocation and loading into the xylem in crop plants are nitrate transporter 1/peptide transporter (NRT1/NPF), nitrate transporter 2 (NRT2), chloride channel (CLC), and slow anion channel‐associated 1 homolog 3 (SLAC1/SLAH). The majority of NRT1 transporters function as low‐affinity transporters, with the exception of NRT1.1 in Arabidopsis thaliana , which serves as a dual‐affinity transporter (Lin et al. 2008). On the other hand, the NRT2 family of transporters is associated with the high‐affinity transport of nitrates (Kumar et al. 2023). Elevated CO₂ directly affects crop plant nitrate and ammonium transporter systems, thus influencing the acquisition and partitioning of N to belowground and aboveground tissues. The activity and expression of transporters are differentially regulated physiologically, transcriptionally and post‐translationally by water transport, CO₂ availability and transpiration‐dependent mass flow of nutrients through the plant xylem (Halpern et al. 2019). Hence, transporters might play a crucial role in shaping the NUE, C/N balance and crop productivity under future climate conditions.

The altered expression of nitrate transporters, such as NRT1.1 and NRT1.2, under eCO₂ may result from feedback inhibition caused by reduced NR activity and subsequent N assimilation processes. Additionally, the decline in mineral nutrient transport under eCO₂, likely due to reduced transpiration rates, may further influence the expression of these transporters (Jauregui et al. 2015). These findings suggest that hydraulic conductance and molecular regulation of gene expression collectively influence N uptake under eCO₂ conditions. Reduced nitrate uptake associated with decreased NRT1 protein abundance was observed in several crop species including wheat and tomato (Vicente et al. 2016; Chitranashi et al. ²⁰²⁶). eCO₂ is also known to alter ammonium uptake via AMT transporters. AMT1 activity was significantly reduced under ammonium nutrition in tomato plants grown under eCO₂ and warming, leading to decreased overall N uptake (Jayawardena et al. 2021). Moreover, ammonium uptake is closely coupled to plasma membrane H⁺‐ATPase activity; therefore, CO₂‐induced alterations in root metabolism or proton gradients may impair AMT function. Studies in cucumber under stress conditions indicate that eCO₂ can change proton pump activities and root hydraulic conductance, which is known to indirectly affect the nutrient transporter activity (Li et al. 2020). A Free Air CO₂ Enrichment (FACE) study conducted on rice with long term exposure to eCO₂, reduced nitrate and ammonium uptake capacity per unit root mass during the reproductive stages (Kim et al. 2003). Hence, prolonged CO₂ enrichment induces adjustments in intrinsic transporters efficiency to match the N supply and demand.

Aquaporins, including plasma membrane intrinsic proteins (PIPs), play a crucial role in water and CO₂ transport under the influence of eCO₂ in response to N levels (Ding et al. 2015; Groszmann et al. ²⁰¹⁷). Several PIPs facilitate diffusion of CO₂ across membranes, thus affecting mesophyll conductance and photosynthetic capacity, which in turn are governed by N uptake (Groszmann et al. 2017; Gao et al. ²⁰¹⁸). Under eCO₂, PIP isoforms show differential expression to optimize intracellular CO₂ diffusion in response to altered transpiration demands. Further, the altered expression of aquaporins tightly controls the root hydraulic conductivity, influencing the kinetics of root nitrate uptake. Thus, aquaporins act as molecular links between atmospheric CO₂ signals, water transport, mesophyll conductance, and N acquisition. Current studies suggest that eCO₂ modulates N transporter networks as well as water‐ and nutrient‐transport processes (Gao et al. 2018). However, whether the selective upregulation of the transporters can enhance N acquisition under eCO₂ remains unclear, as alterations in root architecture and transpiration‐driven nutrient fluxes under varying soil moisture conditions may constrain transporter efficiency. Resolving these interactions will be critical for developing crops with improved NUE while maintaining grain protein content under future atmospheric CO₂ conditions.

2.2. Nitrate Assimilation Enzymes

Nitrate reductase (NR) catalyzes nitrate reduction to nitrite in the cytosol by using NAD(P)H, which will be subsequently transported into chloroplasts (or plastids in roots) through HPP transporters. Subsequently, nitrite is reduced to ammonium by NADPH‐dependent NiR which is present both in leaves and roots (Krapp 2015). Environmental perturbations, particularly eCO₂, alter NR‐NiR activity, thereby disrupting the tight coordination between C/N assimilation. eCO₂ creates a C‐rich environment in leaves; nevertheless, the nitrate availability becomes a limiting factor due to reduced photorespiration, while the supply of reducing equivalents for nitrate reduction becomes constrained because reduced photorespiration limits malate export from chloroplasts (Scheibe et al. 2005). In contrast, maize conducts C₄ carbon fixation that generates high amounts of malic acid and NADH in the cytoplasm of mesophyll cells, and eCO₂ has little effect on NO₃ ⁻ assimilation in maize (Cousins and Bloom 2003). In such scenarios, NR‐NiR becomes the primary bottleneck controlling growth, yield, and protein synthesis unless sufficient inorganic N is translocated from the roots. As mentioned earlier, the role of transporters becomes crucial under these source‐sink imbalance conditions. NR activity in leaves is regulated not only by nitrate availability but also by the whole‐plant C/N balance, long‐distance N transport, and the cellular energy status (Krapp 2015). Several C₃ species including wheat and Arabidopsis supplied with different forms of inorganic N showed that the type of N plays a crucial role in NO₃ ⁻ assimilation (Rubio‐Asensio and Bloom ²⁰¹⁷). NO₃ ⁻ assimilation requires substantial metabolic energy, consuming approximately 25% of the ATP and reducing power generated through photosynthesis in the expanded leaves, whereas NH₄ ⁺ assimilation requires considerably less energy because NH₄ ⁺ bypasses the NO₃ ⁻ reduction steps (Domiciano et al. 2020). Grain yield of plants receiving NH₄ ⁺ showed a greater enhancement by eCO₂ than those receiving NO₃ ⁻ (Carlisle et al. 2012). Results have shown that eCO₂ did not intrinsically suppress NR activity in field‐grown wheat; instead, nitrate assimilation capacity was maintained or even enhanced at canopy level due to increased growth (Dier et al. 2018). However, experiments with hydroponics have shown a reduction in NR levels in wheat leaves, resulting in less nitrate being reduced and subsequently less N being available for synthesizing amino acids (Adavi and Sathee 2021). These results suggest that the mechanistic bottlenecks of NR‐NiR signaling arise primarily from imbalances in cellular redox homeostasis and reduced availability of C skeletons caused by the disruption of photorespiration‐dependent nitrate assimilation.

eCO₂ drives a shift from shoot‐centric to root‐centric N assimilation in non‐legumes, functionally resembling the intrinsic N assimilation strategy of legumes (Andrews et al. 2020). While eCO₂ alters upstream drivers affecting both nitrate reduction and ammonium assimilation, NR remains functionally buffered in case of legumes, through continuous root‐derived ammonium and amino acid supply resulting from symbiotic N₂ fixation. When shoot NR activity was decreased in wheat, root NR expression balanced the photosynthetic activity to some extent by replenishing the N levels (Vicente et al. 2016). However, there was a general trend of decreased nitrate assimilation under eCO₂ due to reduced NR activities in both C₃ and C₄ plants resulting in excess nitrate accumulation which was more pronounced in the shoot compared to roots (Bloom et al. 2020). However, there is no direct evidence that nitrate accumulation per se inhibits plant growth (Igarashi et al. 2021). Similar to NR, NiR also showed decreased expression under eCO₂ in several non‐legume crops leading to a decrease in NUE (Wang et al. 2022). Significant varietal differences also exist in the expression patterns of N assimilation enzymes under eCO₂ (Padhan et al. 2020). Hence, modulating the root NR—NiR system is presumed to be an effective strategy to improve nitrate utilization, NUE, and ultimately grain yield under future eCO₂ conditions.

2.3. Ammonium Assimilation Enzymes

The classical GS‐GOGAT pathway occurs both in leaves and roots but with different enzymatic isoforms and substrate routes. In plastids/chloroplasts it plays an important role in the assimilation of ammonium into glutamine and then to glutamate, which subsequently enter into the amino acid pool (Lea and Miflin 2010). The carbon skeleton 2‐oxo‐glutarate utilized in this reaction is primarily supplied by the TCA cycle and associated to anaplerotic pathways. However, GDH may also contribute under specific metabolic conditions via the deamination of glutamate (Vega‐Mas, Cukier, et al. ²⁰¹⁹; Tang et al. ²⁰²¹). Although GDH can catalyze both aminating and deaminating reactions in vitro, in vivo studies indicate that it predominantly operates in the deaminating direction under optimal growth conditions, while its aminating role becomes significant under stress or excess ammonium (Skopelitis et al. 2007; Kishorekumar et al. ^2020a; Kishorekumar et al. ^2020b). Among the two isoforms of GS, GS1 is encoded by GLN1 in the cytosol and is majorly involved in ammonium recycling during leaf senescence and in the synthesis of glutamine in roots which will be transported into the phloem sap (Lee et al. 2023). GS2 is encoded by GLN2 in plastids and plays a role in the primary assimilation of ammonium derived from nitrate reduction in both C₃ and C₄ leaves as well as in the re‐assimilation of ammonium generated during photorespiration in C₃ plants (Masclaux‐Daubresse et al. ²⁰¹⁰). GOGAT also exists in two forms in higher plants, Fd‐GOGAT and NADH‐GOGAT, depending on the type of corresponding electron donor (Yoneyama and Suzuki 2019). Fd‐GOGAT is specific to leaf chloroplasts, whereas NADH‐GOGAT is mainly present in plastids of non‐photosynthetic tissues, including roots, etiolated leaf tissues, and companion cells.

Under eCO₂, though nitrate reduction is maintained through elevated NR‐NiR, the GS–GOGAT cycle becomes one of the crucial control points limiting N assimilation. Both these enzymes play a critical role in maintaining N flux, which is crucial for amino acid biosynthesis and overall NUE in plants (Fortunato et al. 2023). GDH primarily contributes to maintaining the N balance under stress or low‐N scenarios, thereby supporting plant resilience. While the well‐controlled GS‐GOGAT pathway does not allow much flexibility, GDH provides adaptable activity when both energy and carbon supplies fluctuate depending on the cellular redox state, carbon availability and NH₄ ⁺ concentration (Vega‐Mas, Rossi, et al. ²⁰¹⁹). The flexibility is very useful under high CO₂, because GDH may provide metabolic flexibility through alternative routes for N and C metabolism, helping to overcome limitations of the GS‐GOGAT cycle.

Experiments on several legume and non‐legume crop species indicate that GS has been the rate limiting factor for N assimilation under eCO₂ as evidenced from both pot and field studies. Under the FACE environment, wheat has shown lowered leaf N; nevertheless, GS1 expression was enhanced, helping to sustain NUE and protein content in grains (Pandey et al. 2017). However, unlike GS1, the GS2 plastid expression behaves differently under eCO₂ concentrations, showing a reduced expression likely due to the feedback inhibition from nitrate levels, thus affecting N assimilation. In contrast to GS, the response of GOGAT to eCO₂ is more closely associated with its integration into photorespiratory and reductant‐generating pathways, especially in leaf tissues. In wheat, although research is limited, the activity of NADH‐GOGAT has been associated with improved NUE under N‐deficient conditions, indicating its potential role in countering a CO₂‐induced N dilution through support of N assimilation and remobilization (Dellero 2020). Studies in rice have noted that GDH expression was strongly increased when CO₂ levels are raised and N is scarce. Therefore, GDH may contribute to maintaining ammonium homeostasis and glutamate supply under N limiting conditions when GS‐GOGAT is limited due to its central role in integrating C/N and energy metabolism. Enhanced OsGDH ₂ in rice turned out to be beneficial for low‐N stress and coping with oxidative damage because it lowers ammonium and maintains a constant supply of glutamate that is needed for dealing with stress and N metabolism (Zhou et al. 2015). Greater root growth and more efficient N remobilization were seen in maize under eCO₂, which were linked to improved GDH activity (De La Mata et al. 2013).

It has been postulated that the enhanced GS activity through overexpression could remove the bottleneck and restore C–N balance (Thomsen et al. 2014). Transgenic wheat overexpressing GS1 exhibited enhanced NUE, maintaining stable yields despite reduced tissue N levels (Habash et al. 2007; Bernard and Habash ²⁰⁰⁹). In addition, transgenic lines with extra GS1:1 or GS1:2 had enhanced root development, densely spaced tillers and a higher grain yield when grown in CO₂‐rich air, suggesting that GS1 plays a key role in helping the plant to synchronize source‐sink tissues (Hu et al. 2018). Similarly, genotypes with strong overexpression of Fd‐dependent GOGAT were able to sustain glutamate levels, which are critical for chlorophyll biosynthesis and maintaining photosynthetic efficiency, thereby supporting N remobilization and overall NUE (Zeng et al. 2017). GS, especially its cytosolic isoforms, is the primary enzyme responsible for maintaining C/N balance and metabolic homeostasis. While, under eCO₂ and nutrient‐limited conditions, GDH complements GS activity by providing adaptive metabolic flexibility, together enhancing crop resilience, grain yield, and productivity.

2.4. Urease

Urease is a nickel‐dependent metalloenzyme that plays a central role in N assimilation by catalyzing the hydrolysis of urea into ammonia which is subsequently assimilated into amino acids through the action of glutamine synthetase. This reaction is vital, as ammonia is readily incorporated into organic compounds, contributing to the N economy of plants. Urea is currently the predominant N fertilizer used in agriculture, accounting for more than 50% of total N fertilizer consumption (Glibert et al. 2006). Efficient utilization of urea by plants depends on the presence and activity of urease. In addition to exogenous sources, urea is also produced endogenously within plants as a byproduct of arginine catabolism via the action of the enzyme arginase (Myrach et al. 2017). Urease ensures efficient incorporation of both exogenous and endogenous urea‐derived N into organic molecules essential for plant growth and development. Thus, urease is a pivotal enzyme in plant N metabolism, bridging the gap between inorganic urea and organic N assimilation.

In soybean, studies have shown that urease activity is enhanced under eCO₂ which facilitates more efficient urea utilization and contributes to improved NUE (Alam et al. 2023). Moreover, the response of urease activity in Brassica napus and Oryza sativa to soil pH and microbe changes linked to CO₂ shows its influence on urea hydrolysis and N uptake. Soil factors affect the peak performance of the enzyme which was highest between pH 6.0 and 6.5 and related to ureolytic bacteria in the soil (Tabatabai and Bremner 1972; Wang and Xiong ²⁰²²; Mekonnen et al. ²⁰²¹). Rising urea levels and higher soil pH allow urease enzymes to split urea which increases the chance of N being lost. To reduce such losses, spreading sulfur and zinc‐coated urea is helpful, as it slows down the reaction and makes nutrients more easily taken up by plants (Mustafa et al. 2022). This method helps control N emissions, lowers any negative impact on the environment, and boosts crops' productivity by releasing nutrients at the right time for each growth stage (Mustafa et al. 2022). The enzyme also plays a role in mitigating potential urea toxicity by regulating ammonium release in synchrony with plant demand (Gupta et al. 2023). These findings highlight urease as an important player in N management under eCO₂, particularly in cropping systems dependent on urea fertilization, and underscore its potential for genetic or agronomic enhancement to improve NUE in future climate scenarios.

3. Plant Systemic Responses That Are Under N Control

Differences exist between C₃ and C₄ as well as legume and non‐legume crop plants in terms of growth, photosynthetic physiology, development, yield and nutritional quality when grown under eCO₂ (Figure 2). Comprehensive analyses of physiological and molecular elements in different crop plants reveal the role of N in regulating these molecular mechanisms. The following sections outline the N mediated regulation of three key mechanisms of the plant life cycle including photosynthetic physiology, reproductive development and yield and nutritional quality under eCO₂.

FIGURE 2.

Role of C‐N dynamics in regulating the physiological, developmental and yield characteristics of crop plants under eCO₂. Elevated CO₂ increases carbon assimilation, generating a greater N demand in both above‐ and belowground tissues. In legumes, this triggers an increase in the nodule mass ratio (NMR). The photochemical quenching and carbon assimilation are sustained through superior electron transport rates and gas exchange physiology. The accumulated sugars subsequently change the C/N balance at the shoot apical meristem (SAM), thereby changing the flowering patterns. Increased vegetative growth subsequently translates into enhanced crop yield. However, in the absence of external N supply, non‐legume crops may experience N dilution, ultimately compromising seed nutritional quality. Abbreviations: Fv/Fm, photochemical efficiency of PSII; gs, stomatal conductance; LMR, leaf mass ratio; Pn, photosynthetic rate; qE, energy‐dependent quenching; SGR, specific growth rate. Image was created using Biorender.com tools.

3.1. Photosynthetic Physiology

The impact of N availability on photosynthesis is huge as the majority of the leaf N per unit area accounts for photosynthetic proteins. In general, N within C₃ leaves is distributed between chloroplasts (75%), mitochondria (5%), peroxisomes (2.5%), cytosol (7.5%), and cell walls (10%; Evans and Clarke ²⁰¹⁹). Within the chloroplast, the N is utilized for light capture and the bioenergetic reactions of thylakoids in the proportions of 37.3 mol nmol⁻¹ Chl and 10.86 mol nmol⁻¹ Cyt f respectively (Evans and Clarke 2019). Nevertheless, for optimum developmental programming, crop plant systems demand a balanced C and N stoichiometry rather than an absolute baseline N level which usually gets disarrayed under a eCO₂ atmosphere even on external supply of N, causing impairments in physiological functions (Ghannoum et al. 2011). Interestingly, differential responses of C3 and C4 plants as well as legume and non‐legume crops towards photosynthetic physiology indicate the regulatory role of N in photosynthesis (Table 1).

TABLE 1.

Physiological and molecular responses of C3 and C4 crop plants to eCO₂ growth conditions.

Plant species	Growth and photosynthesis	Flowering	Yield and productivity	Grain quality	References
C3 legumes
Phaseolus vulgaris (Common bean)	↑ shoot and stem biomass	Not reported	Yield increased via ↑ pod and seed number	Leaf N ↓; seed protein unchanged	Bunce 2008
Arachis hypogaea (Groundnut)	Elevated CO2 mitigated O3 −induced growth suppression	Not reported	Elevated CO2 restored pod and seed yield	Protein and oil unchanged; fatty acid composition altered	Burkey et al. 2007
Glycine max (Soybean)	↑ Above‐ground biomass/NPP (~15%–22%)	Flowering/maturity delayed (~2–7 days)	Seed yield ↑ (~9%–31%) mainly via ↑ pods plant−1 and seeds pod−1; strong genotypic variation (0%–24%); harvest index slightly ↓ (~2%–3%)	↓ Seed protein (~1.4%–3.3%); ↓ amino acids (~7.5%); ↓ Fe & Zn; ↑ oil (~2.8%); ↑ oleic, linoleic & palmitic acids; ↑ Mg, Ca, S & K; ↑ total isoflavones	Morgan et al. 2005; Hao et al. 2014; Li et al. 2018; Bishop et al. 2015a; Bishop et al. 2015b; Dietterich et al. 2015a; Dietterich et al. 2015b; Taub et al. 2008
Pisum sativum (Garden Pea)	Biomass increased under elevated CO2 (rotation system)	Not reported	Grain yield increased under elevated CO2	↓ grain N (−6%), P (−5%) and Zn (−10%); ↓ grain Zn, Fe and protein; ↑ phytate:Zn ratio (nutrient dilution stronger in wheat & canola within rotation system)	Jin et al. 2019; Dietterich et al. 2015a; Dietterich et al. 2015b
Cajanus cajan (Pigeonpea cajan )	No Photosynthetic acclimation, Increased nodule mass ratio and nitrogen fixation, g s ↑	Delayed flowering	Higher productivity	No decrease in seed N	Sreeharsha et al. 2015; Unnikrishnan et al. 2021
Lens culinaris (Lentil s culinaris )	Increased photosynthesis with increased RUBISCO activity	—	—	—	Bourgault et al. 2017
Vigna radiata (Mung bean)	Enhanced photosynthesis and stress resilience	—	—	—	Awasthi et al. 2018
C3 non‐legumes
Oryza sativa (Rice)	Asat ↑; ↑ biomass and tiller number; ↑ carbohydrate accumulation; ↑ biomass at optimal temperature; cultivar‐dependent growth response; urease, GS, GDH, Fd‐GOGAT responses under eCO2 (700 μmol mol−1)	Flowering advanced; flowering temperature‐sensitive; reproductive phase longer	Grain yield ↑ (~10%–71%) via increased tillering; genotype‐ and temperature‐dependent responses	↑ amylose; ↓ protein; ↓ S, Mn, Zn; ↓ grain Zn & Fe; ↓ tissue N and minerals; ↓ leaf N	Alam et al. 2023; Seneweera and Conroy 1997; Ujiie et al. 2019; Baker 2004; Zhu et al. 2014; Dakora et al. 2025; Dietterich et al. 2015a; Dietterich et al. 2015b; Taub et al. 2008
Triticum spp. (Wheat)	↑ shoot biomass; ↑ biomass in rotation system; general C3 biomass response ↑; ↑ photosynthesis; ↑ carbohydrate accumulation; Asat ↑; GS, GDH, NiR, GOGAT responses under eCO2 (700 μmol mol−1)	Flowering accelerated	Yield ↑ (~5%–78%) via increased grain number; rotation‐system effects observed	↓ protein; ↓ lysine; ↑ starch; ↓ N, P, Zn; ↓ tissue minerals; ↓ grain Zn & Fe	Wu et al. 2004; Jin et al. 2019; Dakora et al. 2025; Dietterich et al. 2015a; Dietterich et al. 2015b; Taub et al. 2008; Ekele et al. 2025; Jauregui et al. 2015; Dellero 2020
Hordeum vulgare (Barley)	Asat ↑; gs ↑; WUEi ↑ under eCO2 (800 μmol mol−1) with N supply (NH4NO3 or urea)	Not reported	Not reported	↓ protein (~10%–15%)	Wang et al. 2013; Chen et al. 2022; Taub et al. 2008
Solanum tuberosum (Potato)	Not primary focus	Not reported	Not reported	↓ protein (~14%)	Taub et al. 2008
Solanum lycopersicum (Tomato)	↑ root and shoot biomass; Asat increased under elevated CO2 (800 ppm) with urea and NH4NO3 nutrition; gs and WUEᵢ responses influenced by nitrogen form	Not reported	Not reported	Not applicable	Chen et al. 2022; Yang et al. 2022; Cohen et al. 2018
Arabidopsis thaliana (Thale cress)	↑ shoot and root biomass	Not reported	Not reported	↑ P uptake (NO3 − nutrition)	Niu et al. 2013
Picea abies (European spruce)	↑ carbohydrate accumulation	Not reported	Not reported	↓ needle N, P, K, Mg	Roberntz and Stockfors 1998
Horticultural crops	↑ biomass and productivity	Not reported	Yield increased	↓ protein; ↓ Fe; ↓ Zn	Dong et al. 2018
Herbaceous communities	↑ above‐ground biomass	Harvest stage dependent	Community structure altered	Not reported	He et al. 2002
Pasture species	Total biomass mostly unchanged; species‐specific biomass increased under elevated CO2	Increased flowering/inflorescence density	Increased seed production via more inflorescences and/or seeds per inflorescence; increased seedling recruitment	Not reported	Edwards et al. 2001
C4 non‐legumes
Zea mays (Maize)	↑ starch accumulation under elevated CO2 with higher N; elevated temperature (eT) ↓ biomass, whereas eT + eCO2 partially restored leaf, shoot and total biomass (hybrid‐dependent response); biomass responses variable with N, water and temperature interactions	eT shortened phenophases and ↑ anthesis–silking interval; eCO2 reduced ASI under heat stress	Yield ↑ (~10%–18%) but variable with heat, N and water availability; eT reduced cob weight and seed yield, whereas eT + eCO2 mitigated yield loss	N maintained with N fertilization; TNC:N unchanged; ↓ grain Zn, Fe and protein (smaller response than C3 crops); ↑ phytate:Zn ratio; ↓ tissue N (~14%); ↓ grain Fe, Zn, P, S, Cu, Mn	Xu et al. 2019; Dietterich et al. 2015a; Dietterich et al. 2015b; Dakora et al. 2025; Vanaja et al. 2024

Non‐legume C₃ plants grown under eCO₂ enhance light‐saturated net photosynthetic rate (A _sat) by increasing the intercellular CO₂ and enhancing RUBISCO carboxylation efficiency (V_Cmax ) (Ainsworth and Rogers 2007; Way et al. ²⁰¹⁴). However, long‐term growth under eCO₂ results in a gradual decline in leaf N, leading to photosynthetic acclimation as observed in several forest, grassland, and desert species studied under open top chambers and FACE environment (Jablonski et al. 2002; Ellsworth et al. ²⁰⁰⁴; Song and Wang ²⁰²³; Sekhar et al. ²⁰¹⁵). One of the common findings in most of the studied species was the statistically significant down regulation of V _Cmax , suggesting the plants' inability to maintain internal leaf N for long‐term under eCO₂. The nonlegume perennial ryegrass ( Lolium perenne ) showed a lower V _Cmax and a lower ribulose‐1,5‐bisP regeneration (J _max ), causing photosynthetic acclimation under eCO₂. Increasing CO₂ has decreased leaf N and Rubisco content in rice (Wang et al. 2020). In case of clover, V _Cmax and J _max were constant and no photosynthetic acclimation was observed under eCO₂ (Irigoyen et al. 2014). Pigeonpea grown under eCO₂ showed 13% and 11% enhancement in V _Cmax and J _max respectively than their ambient air grown counterparts (Sreeharsha and Reddy 2015; Sreeharsha et al. ²⁰¹⁵). An increased nodulation was observed in the case of pigeonpea grown under eCO₂, which confirms the enhanced C is transported towards the roots, thus increasing N and overcoming photosynthetic acclimation. This clearly indicates that the difference in responses of legumes and non‐legumes is because legumes have the advantage of fixing atmospheric N using rhizobium, which helps maintain a balance between C and N under eCO₂ growth conditions. The enhanced C fixed during eCO₂ conditions is allocated toward the roots, wherein the rhizobium is stimulated to fix more N₂, thus overcoming the photosynthetic acclimation (Figure 1). Furthermore, increased N acquisition is presumed to trigger the expression and activity of N uptake transporters and GS‐GOGAT signaling cascades in roots under eCO₂, ensuring that each plant organ is provided with optimum N. The increased C from enhanced photosynthesis increased nodulation along with the expression of N metabolizing enzymes, altogether leading to an effective C/N balance, hence overcoming photosynthetic acclimation. In some cases, even legumes beyond a certain level of CO₂ exposure were shown to down regulate photosynthesis due to decreased V _Cmax , J _max , and reduced stomatal conductance which could indicate varietal differences (Zheng et al. 2019).

Interestingly, eCO₂ ameliorates oxidative stress due to abiotic stress including drought (Sreeharsha et al. 2015; Sreeharsha et al. ²⁰¹⁹). This could be attributed mainly to decreased plant stomatal conductance and transpiration due to partial stomatal closure, thereby enhancing the water use efficiency. This reduction in transpiration affects the soil‐to‐root nitrate delivery and xylem mediated N allocations to shoots. Reduced hydraulic conductance under eCO₂ restricts N delivery to source tissues, indicating that transpiration is a key mediator linking eCO₂ with N transport dynamics (Jauregui et al. 2015). Compared to C₃ plants, C₄ plants generally exhibit a weaker physiological response and experience smaller changes in N assimilation under eCO₂ because their carbon‐concentrating mechanism minimizes photorespiration and reduces the dependence of nitrate assimilation on photorespiratory metabolism (Poorter and Navas 2003). C₄ plants possess a unique cellular mechanism to enhance CO₂ levels in the bundle sheath cells at the vicinity of RUBISCO (Adireddy et al. 2024; Ainsworth and Long ²⁰⁰⁵), and several C₄ crops have positively responded to eCO₂ under drought conditions (Leakey et al. 2009; Allen Jr et al. ²⁰¹¹). C₄ plants are able to reduce their stomatal conductance to a minimum, especially when water loss is high, due to cellular carbon concentrating mechanisms. This leads to a higher tolerance to drought of C₄ over C₃ species and to different CO₂‐response curves of photosynthesis (Alfonso and Brüggemann 2012). Hence, the C/N balance between source and sink tissues is one of the crucial determining elements for plant photosynthetic efficiency and growth under eCO₂.

3.2. Flowering and Reproductive Development

Plant development depends on several factors, including macro‐ and micronutrients, hormonal crosstalk, and sugar signaling. The transition from vegetative to reproductive growth is strongly regulated by C and N availability and their coordinated signaling, which together determine flowering time and reproductive fitness. In crop plants, the influence of N on flowering is mediated through hormonal signaling pathways, and its effects vary depending on the chemical form of N, photoperiod, and sugar availability (Lin and Tsay 2017; Weber and Burow ²⁰¹⁸). Accordingly, N‐mediated regulation of flowering may occur through either photoperiod‐dependent or photoperiod‐independent pathways, depending on the plant species. Under eCO₂, optimal N levels promote flowering while limiting N or N starvation delay flowering in major crop plants (Lin and Tsay 2017). Elucidating the precise mechanisms through which N regulates flowering under eCO₂ across diverse crop species remains an important area for future research.

High N availability increases sink capacity, promoting carbon sequestration, assimilate translocation, and vegetative growth. This sustained investment in vegetative tissues can delay the onset of flowering, as demonstrated in legume crops grown under eCO₂ with surplus N conditions (Sreeharsha et al. 2015). High N levels usually delay flowering via glutamine‐mediated regulation of Nhd1 (N‐mediated heading date 1), which controls the expression of Hd3a, a key gene in floral transition (Zhang et al. ^{2019a, 2019b}; Ye et al. ²⁰¹⁹). On the other hand, low N concentrations repress FLOWERING LOCUS C (FLC), a flowering repressor, while inducing positive regulators such as FLOWERING LOCUS T (FT), LEAFY (LFY), APETALA1 (AP1), and CONSTANS (CO) and advancing the flowering. However, there is currently no direct evidence of varying N levels affecting floral gene expression under eCO₂ concentrations. In Arabidopsis, changes in N levels influence transcription factors like NIN LIKE PROTEINS (NLP6 and NLP7), which bind to nitrate‐responsive elements (NREs) in target genes and regulate floral pathways, including SPL3 and SPL5 (Cheng et al. 2023). This regulation modulates the expression of SOC1, a crucial regulatory gene for flowering, with different effects under short‐day and long‐day photoperiods. N is sensed in SAM, and further, an indirect effect of low N was observed on SOC1 through SPL with the help of the NLP transcription factor (Olas et al. 2019). It was also found that N activates SMZ and SNZ transcription factors which act as floral repressor genes via the gibberellin pathway, repressing flowering in Arabidopsis thaliana (Gras et al. 2018).

Furthermore, increasing evidence suggests that miR156‐mediated regulation of flowering under eCO₂ is closely linked to trehalose signaling. Trehalose‐mediated sugar signals, together with N availability, converge at the shoot apical meristem (SAM) to regulate the expression of floral pathway genes (Unnikrishnan et al. 2024). Elevated CO₂ influences flowering and reproductive development primarily by increasing photosynthetic carbon assimilation, which enhances photosynthate production (sugars and starch) in source leaves, leading to greater C availability for growth and development. This increased photosynthate supply provides the energy and structural C required for floral initiation, organ formation, and seed development. In addition, sugars such as sucrose and sugar‐signaling metabolite such as trehalose‐6‐phosphate (T6P) regulate the expression of flowering‐related genes, including FT and SOC1, and interact with hormonal pathways such as gibberellins and auxins to promote the transition from vegetative to reproductive growth (Wahl et al. 2013). The post‐translational activation of phosphoenolpyruvate carboxylase (PEPC) and NR, coupled with the increased C and N demand of sink tissues, enhances metabolic flux during plant development. Elevated T6P further coordinates this response by redirecting assimilated C from sucrose accumulation toward the synthesis of organic acids and amino acids, thereby supporting plant growth and reproductive development (Fichtner and Lunn 2021).

3.3. Seed Yield and Its Nutritional Value

Seed productivity and its nutritional quality are the most sensitive parameters to climate change, determining the agronomical potential of a crop plant. The change or imbalance in C and N dynamics under the influence of environmental stresses including eCO₂ leads to a decreased quantity and quality of grain yields. Meta‐analysis of crop plants analyzing the effect of eCO₂ on vegetative and reproductive tissues showed a divergent response wherein vegetative biomass increase (31%) was higher compared to the fruit and seed yield increase (12 and 25% respectively; Jablonski et al. ²⁰⁰²; Hikosaka et al. ²⁰¹¹). At physiological levels, changes in photosynthetic rates and variations in C and N uptake dynamics could be the major factors influencing yield patterns and the quality of the fruits/seeds under eCO₂. Most of the non‐legume crops have shown a decrease in seed N levels by an average of 14%, leading to poor seed quality under eCO₂ (Hikosaka et al. 2011; Hampton et al. ²⁰¹³). One of the reasons for this decreased N is photosynthetic acclimation due to N limitation and source‐sink imbalances (Sekhar et al. 2014; Sreeharsha et al. ²⁰¹⁵; Gojon et al. ²⁰²³). Other reasons could be N dilution because of C accumulation, reduced soil mass flow of water and nutrients due to reduced stomatal conductance, and decreased uptake and assimilation of nitrate (Bourgault et al. 2016; Bourgault et al. ²⁰¹⁷; Pleijel et al. ²⁰¹⁹).

Under eCO₂, strengthened source–sink relationships improve assimilate allocation to reproductive organs, supporting flower initiation, pollen viability, ovule development, and seed set, often resulting in enhanced reproductive biomass and yield, particularly in C₃ plants. However, the extent and direction of these responses are highly species‐ and genotype‐dependent and can be constrained by sink capacity, nutrient availability (especially N), and environmental stresses such as drought or heat. These constraints may limit the effective utilization of additional photosynthates or shift carbon allocation towards vegetative growth, highlighting that the relationship between eCO₂, photosynthate production, and reproductive development is governed by an integrated balance of C supply, signaling, hormonal regulation, and environmental context. The seed yield got increased when grown under eCO₂ in crop plants like wheat, soybean, pea, and lentils (Bunce 2016; Li et al. ²⁰¹⁸). However, even in legumes, like peas and lentils, when exposed to eCO₂ under semi‐arid conditions, the N of seeds was reduced (Bourgault et al. 2016; Bourgault et al. ²⁰¹⁷). This reduction may result from the N dilution effect, wherein the increased plant biomass leads to lower N concentration on a per area basis. These results indicate that the comparative advantage of legumes because of their N fixation ability has not fully translated into improved yield and nutritional aspects under eCO₂. In wheat grown under eCO₂, external N supplementation failed to offset declines in grain protein content, indicating that additional N was insufficient to overcome reduced N levels (Pleijel et al. 2019). Moreover, wheat exposed to varying concentrations of eCO₂ exhibited decreased N content and lower levels of essential amino acids, such as glutamine (Soba et al. 2019). These studies highlight how factors such as plant variety, seasonal changes, and ecological conditions significantly influence the yield and nutritional composition of seeds under eCO₂. In rice, long‐term exposure to eCO₂ reduced N uptake capacity, with the response varying markedly across growth stages, ultimately influencing grain filling and yield (Yoshinaga et al. 2019; Shimono et al. ²⁰⁰⁹; Shimono and Bunce ²⁰⁰⁹). Also, a combination of eCO₂ and warming obstructs the nutrient uptake and N assimilation in tomato plants, resulting in decreased plant growth and food quality (Jayawardena et al. 2021). Grain yield of plants receiving NH₄ ⁺ showed a greater enhancement by eCO₂ than those receiving NO₃ ⁻ (Carlisle et al. 2012). Studies with wheat showed that there was a decrease in protein concentrations of shoot, grain, and grain protein yield by an average of ~10% (Taub et al. 2008; Wang et al. ²⁰¹³; Pleijel and Uddling ²⁰¹²). Altogether, N plays a crucial role in seed development, nutrition, and protein content, wherein eCO₂ causes changes in physiological, molecular, and biochemical processes, including C and N dynamics. More comprehensive studies are required to better understand plant responses to eCO₂, particularly concerning seed nutrition and yield.

4. Perspectives on Crop Improvement for the Future Climate

Sustaining crop yields to meet the rising global food demand is a pressing challenge, further exacerbated by climate change and increasing CO₂ levels. Studies have shown that food production needs to be increased by 50% by 2030 and doubled by 2050 to maintain the world's demand (Pastor et al. 2019; Waqas et al. ²⁰²³). Rising CO₂ will bring on many other problems like increased global temperatures, drought, etc., leading to poor crop yields. A decrease in yields of crops like rice, wheat, and maize by 10%–25% was observed due to climate change‐related ailments (Deutsch et al. 2018). In such context, N is essential for the various plant developmental processes discussed above, making it an important factor to consider to improve crop productivity.

There needs to be a balance between N applied and N harvested by crops to improve crop yield and minimize the N loss leading to enhance NUE. To improve crop productivity and increase food security, it is necessary to improve the NUE, which can be done by either increasing N application or enhancing the plant's ability to utilize the N. The former step can cause environmental problems; hence, the plant's NUE must be improved. It has been estimated that the global N fertilizer consumption was 112.5 million tons in 2015, increased to 118.2 million tons by 2019, and is expected to increase to 7.9–10.5 billion by 2050 (Javed et al. 2022). To avoid further environmental issues due to excessive N fertilizer utilization, scientists are pursuing ways to improve NUE, a defining parameter to indicate balance between N uptake and utilization (Javed et al. 2022). Usually, to improve the NUE, the application of N fertilizer is increased as there is a considerable loss of N fertilizer due to surface runoff, leaching, and volatilization. However, root N absorption capacity is overlooked in the traditional methods of calculation of NUE, that is, as a factor of crop yield and fertilizer input (Ciampitti et al. 2022). New techniques are to be adapted to minimize the N fertilizer and improve NUE. One effective technique involves identifying and utilizing Quantitative Trait Loci (QTLs), genes, and transporters associated with N metabolism through advanced molecular tools. Furthermore, to combat the consequences of fertilizers on the environment, new types of fertilizers are formulated, such as customized fertilizers, nano fertilizers, biofertilizers, biochar, and zeolites (Waqas et al. 2023). Customized fertilizers (CF) are multi‐nutrient carriers containing macro and micronutrients required for selected crops in specific regions. Research findings indicate that the application of CFs with specific formulations in wheat, tailored to growth locations, have consistently enhanced both yield and NUE. Wheat grown in selective locations in India was supplied with CF (16% N, 24% P, 9% K, 5% S, and 0.7% Zn), while wheat cultivated in Raipur, Chhattisgarh was supplied with CF (11% N, 18% P, 9% K, 5.3% S and 0.7% Zn), ultimately increasing the yield in respective cultivations (Dwivedi and Meshram 2014). Nanofertilizers (NFs) help reduce excessive fertilizer application by releasing nutrients slowly and continuously to meet the nutritional requirements of plants, thereby enhancing grain yield and NUE (Jin et al. 2017). Notably, an alternative pathway to enhance NUE lies in re‐envisioning crop wild relatives as living repositories of climate‐adapted genetic, molecular, and physiological innovations shaped by their prolonged exposure to rising atmospheric CO₂. Their native in situ microbiomes can further be harnessed as co‐evolved partners in N acquisition, mobilization, assimilation, and recycling. This strategy advocates the strategic co‐deployment of plant traits and microbial functions to redesign modern crops with reduced fertilizer dependence, improved N responsiveness, and strengthened resilience to future climate uncertainty and environmental changes (Waqas, McCouch, et al. ²⁰²⁵).

Molecular approaches such as omics (proteomics, transcriptomics, and metabolomics), genome‐assisted breeding, and pan‐genome analysis revolutionize crop improvement. Identifying key genes, QTLs, and transporters associated with N metabolism enables targeted genetic modifications to boost crop productivity. The integration of genome‐assisted breeding techniques facilitates targeted genetic enhancements and comprehensive genomic evaluations. Techniques like CRISPR‐Cas are advancing crop yield improvement and stress resistance. Integrating these technologies with innovative fertilizers and breeding methods will ensure the development of climate‐resilient crop varieties for future food security.

5. Concluding Remarks

Elevated CO₂ levels increase photosynthesis; nevertheless, the impact of this phenomenon is not restricted to growth and physiology alone. It is capable of altering nutrient acquisition, development, and even reproductive performance. As nitrogen is deeply involved in plant productivity and is a key element of metabolic processes, a clear understanding of its uptake, transport, and utilization under eCO₂ is essential. The role of nitrogen transporters, nutrient movement through xylem and phloem tissues, and regulatory mechanisms in root tissues governing nutrient sensing and acquisition deserve special attention. These elements are fundamental to plant growth and adaptation to changing environmental conditions. Future research will have to focus on identifying and engineering N‐responsive targets such as nitrate and ammonium transporters. In addition, root architectural traits involved in nutrient acquisition could be modified to improve the nitrogen‐use efficiency of plants grown under eCO₂ conditions. Although considerable progress has been made, the integration of nitrogen metabolism with plant growth and physiological changes under eCO₂ has not yet been comprehensively understood. Research combining molecular, physiological, and genetic engineering approaches will be required in the future to resolve these complex interactions and help in the development of climate‐resilient crop varieties with improved nitrogen‐use efficiency, productivity, and adaptability to future atmospheric environments.

Author Contributions

R.V.S.: writing – original draft, editing, conceptualization, project administration, funding acquisition; D.K.U.: writing – original draft, editing, visualization; S.M.: writing – original draft; S.K.: writing – original draft; A.K.K.: writing – original draft; G.P.D.: writing – original draft, editing; A.R.R.: writing – original draft, editing, conceptualization.

Funding

This work was supported by a Minor R&D grant from Uttar Pradesh State Council of Higher Education (UPSCHE) (Sattar‐4‐2025‐003‐4(33)/2023) sanctioned to R.V.S. S.K. acknowledges a senior research fellowship (SRF) from UGC, New Delhi. R.V.S. and S.K. acknowledge the Sophisticated Instrumentation Facility at CSJM University developed through the FIST grant from DST, New Delhi.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

No data was used for the research described in the article.