Progress in the Study of Plant Nitrogen and Potassium Nutrition and Their Interaction Mechanisms

Abstract

Nitrogen (N) and potassium (K) are essential macronutrients for plants whose functions and interactions profoundly influence plant physiological metabolism, environmental adaptation, and agricultural production efficiency. This review summarizes research advances in plant N and K nutrition and their interaction mechanisms, elucidating the key physiological functions of N and K individually and their respective absorption and transport mechanisms involving transporters such as NRTs and HAKs/KUPs. The review discusses the types of nutrient interactions (synergism and antagonism), with a primary focus on the physiological basis of N–K interactions and their interplay in root absorption and transport (e.g., K⁺-NO₃⁻ co-transport; NH₄⁺ inhibition of K⁺ uptake), photosynthesis (jointly optimizing CO₂ conductance, mesophyll conductance, and N allocation within photosynthetic machinery to enhance photosynthetic N use efficiency, PNUE), as well as sensing, signaling, co-regulation, and metabolism. This review emphasizes that N–K balance is crucial for improving crop yield and quality, enhancing fertilizer use efficiency (NUE/KUE), and reducing environmental pollution. Consequently, developing effective N–K management strategies based on these interaction mechanisms and implementing Balanced Fertilization Techniques (BFT) to optimize N–K ratios and application strategies in agricultural production represent vital pathways for ensuring food security, addressing resource constraints, and advancing green, low-carbon agriculture, including through coordinated management of greenhouse gas emissions.

1. Introduction

In modern agricultural production, addressing challenges posed by global population growth, resource scarcity, and escalating environmental pressures requires efficient nutrient utilization. Rational application of macronutrients, particularly nitrogen (N) and potassium (K), is crucial to ensuring food security and promoting sustainable agriculture. However, in practice, farmers often overapply or apply N, P, and K in an unbalanced manner, with some applications far exceeding the nutritional needs of the plants. A lack of understanding of the effects of different fertilizer applications on various crops ultimately leads to yield gaps, reduced quality, and economic losses. Therefore, gaining a deep understanding of the N–K interaction mechanism and optimizing fertilizer management strategies are of great significance for increasing crop yields, improving harvest quality, reducing environmental pollution, and promoting sustainable agricultural development.

A key scientific focus in plant nutrition is the efficient uptake and utilization of nutrients. N and K, as critical mineral elements for plant growth, play vital roles in physiological metabolic processes. Nitrogen serves as a fundamental component of macromolecules including nucleic acids, proteins, and enzymes, directly participating in photosynthesis, respiration, and protein synthesis. K primarily contributes to osmotic regulation, membrane potential maintenance, photosynthetic performance optimization, and stress resistance enhancement, significantly influencing water use efficiency and abiotic stress tolerance. However, current studies have predominantly focused on plant physiological responses to individual N or K application, while the interactive mechanisms of N–K co-regulation remain incompletely understood. Particularly in complex field ecosystems, the synergistic effects of N and K on plant growth and development exhibit greater complexity, necessitating further investigation.

In this review, we summarize the latest research findings on N–K interactions from the perspective of plant nutrition—including the mechanisms of N–K interactions in plants. We emphasize the importance of optimizing N–K management strategies for safeguarding food security, ecological balance, and sustainable agricultural development. This comprehensive review has enhanced our understanding of N–K interaction mechanisms, thereby providing valuable insights for environmental protection and addressing global climate change and resource scarcity from the perspective of efficient fertilizer utilization.

2. N in Plants

Nitrogen is a fundamental component of purines and pyrimidines in nucleic acids (DNA and RNA), playing a pivotal role in biological inheritance, genetic variation, and protein synthesis. The amino groups of peptides, proteins, and enzymes primarily consist of N, thereby conferring its indispensable function in maintaining organismal activities [1,2]. Furthermore, as an integral part of chlorophyll molecules, N facilitates light energy absorption during photosynthesis. It also significantly promotes root development and optimizes photosynthetic efficiency [3].

Optimal N supply is a critical factor for plant growth, yield enhancement, fruit quality improvement, and enhanced plant resilience [3,4]. Typical symptoms of N deficiency include chlorosis (leaf yellowing), reduced cell division, and diminished enzyme activity, collectively leading to growth retardation. Beyond its essential nutritional role, an optimal N supply enhances plant tolerance to environmental stresses.

Because nitrogen is a core element in the construction of the macromolecules of life, a nitrogen deficiency will cause systemic stress. Plants show symptoms such as chlorosis of leaves (obstruction of chlorophyll synthesis) [5] and a decline in photosynthetic capacity (decrease in Rubisco enzyme activity and stomatal conductance) [6], leading to a sharp reduction in carbon assimilation products. Nitrogen deficiency reduces radiation interception, radiation use efficiency, and dry matter allocation to reproductive organs [7]. At the metabolic level, the imbalance of the carbon-N ratio (C/N) forces abnormal allocation of photosynthetic carbon flow—excessive root growth to compensate for N acquisition, while the above-ground reproductive development is inhibited, manifested as delayed flowering, reduced grain filling, and decreased yield [8,9,10].

2.1. The Role of N in Photosynthesis

Nitrogen serves as the primary constituent of proteins involved in photosynthesis, playing an essential role in photosynthetic apparatus biogenesis. Both deficiency and excess of N reduce chlorophyll content, enzyme concentration, and catalytic activity in the photosynthetic machinery, consequently impairing photosynthetic efficiency [11,12]. Functionally, leaf N is partitioned into photosynthetic (PSN) and non-photosynthetic nitrogen (NPSN), with the allocation ratio critically influencing leaf development and photosynthetic capacity. Under high-irradiance conditions, leaf N content exhibits a positive correlation with photosynthetic rate. Since photosynthetic competence constitutes a key determinant of crop yield, and given the strong positive correlation between foliar N content and photosynthetic capacity [13], optimal N management becomes imperative. Notably, the photosynthetic system represents the plant’s largest N reservoir, accounting for approximately 75% of total leaf N [14,15]. Nitrogen participates directly in light harvesting and energy transduction processes, rendering photosynthetically active radiation interception efficiency highly dependent on N investment during photosynthesis [16]. Consequently, an adequate N supply enhances photosynthetic performance and yield formation in agricultural production systems.

A meta-analysis of C₃ plants by Onoda et al. [17] revealed that increasing leaf mass per area (LMA) reduces the N allocation proportion to Rubisco while increasing investment in cell wall components. Asif Iqbal et al. [13] demonstrated strong correlations between leaf N concentration and key productivity parameters in cotton under differential N regimes, including C/N metabolic enzymes, photosynthetic capacity, sucrose content, boll weight, and seed cotton yield. This indicates that modulating foliar N concentration regulates enzymatic activity and is pivotal for yield enhancement [18]. Liu et al. [19] observed in rapeseed that N application significantly increased leaf number, area, biomass, chlorophyll content, and net photosynthetic rate (P_n), yet concurrently decreased photosynthetic nitrogen use efficiency (PNUE). This efficiency decline likely stems from suboptimal N partitioning within the photosynthetic apparatus. Further elucidating this mechanism, Feng et al. [20] established that cotton leaf P_n positively correlates with N content in photosynthetic machinery (N_photo), while PNUE shows significant positive correlation with N_photo allocation proportion. They proposed that coordinated optimization of N distribution among photosynthetic components coupled with enhanced gas diffusion conductance can synergistically improve P_n.

2.2. N Absorption

Plants typically acquire N from the rhizosphere in various forms, including nitrate, organic N (such as amino acids and urea), and ammonium. The rhizosphere exhibits adaptive responses to available nutrients—collectively termed “N responsiveness”—which involve morphological and physiological adjustments enabling efficient N uptake. Roots uptake different N forms through specific transporters, distributing them to distinct tissues where they trigger tissue-specific N responses.

Plant N utilization varies under different soil conditions. Nitrate and ammonium are the primary inorganic N forms available to plants in soils. Aerobic soils contain higher nitrate levels, while waterlogged soils are richer in ammonium [21]. In the rhizosphere, roots release oxygen and exudates that influence microbial activity, which in turn transforms N forms—including N from fertilizers. Root absorption of ammonium or nitrate typically alters the rhizosphere soil pH, subsequently modifying N’s availability to plants [22]. Following uptake, inorganic N is assimilated primarily into amino acids, serving as the main N compounds allocated via the phloem [22,23]. Transporters with varying specificities and affinities mediate the uptake of different N forms. Plant roots possess multiple organic and inorganic N transporters exhibiting distinct substrate affinities and specificities. Key transporters identified for nitrate uptake and transport include NRT1.1, NPF4.6, NRT2.1, NRT2.2, NRT2.4, NRT2.5, and CLC [24]. The uptake capacity of these transporters is regulated by internal plant N status, environmental N availability, and plant growth stage. Most known NRT1 family members function as low-affinity nitrate transporters. Certain NRT2 transporters require the partner protein NAR2 for nitrate transport at relatively low concentrations. Among CLC members, CLC proteins mediate nitrate accumulation in vacuoles [25].

In soil environments, plant roots possess nitrate and ammonium uptake systems with varying affinities to adapt to the heterogeneity and dynamic fluctuations of soil N. Each high-affinity and low-affinity nitrate transport system contains nitrate-inducible components [26]. Multiple membrane proteins facilitate nitrate uptake, compartmentation, translocation, and remobilization [24]. Following root absorption, a portion of nitrate is retained in the roots while the majority is transported to shoots. In shoots, nitrate is first reduced to nitrite in the cytoplasm by nitrate reductase, then further reduced to ammonium by plastidial and cytoplasmic enzymes—including nitrite reductase and glutamine synthetase (GS). Ammonium derived from either nitrate reduction or direct uptake via ammonium transporters (AMTs) is assimilated into amino acids through the GS/GOGAT cycle. Key isozymes in this cycle include plastid-localized GS2 and Fd-GOGAT, along with cytosolic GS1 and NADH-GOGAT. Plant N uptake and assimilation pathways exhibit complex, highly regulated mechanisms that enable adaptation to heterogeneous and dynamic soil N conditions.

The absorption and remobilization of N by the root system and its interaction with carbon are crucial for plant growth and the carbon (C) cycle of the ecosystem. The effective utilization of C provided by photosynthesis also affects the efficiency of photosynthesis. This carbon efficiency is essential for maintaining root activity. Lateral root formation is regulated by the high-affinity nitrate transporter NRT2.1. The variation in morphological and physiological traits of the root system may also directly depend on the genetic differences in total N absorption, remobilization, leaf greenness, and grain yield [27,28]. An increase in N absorption by the root reduces the remobilization of N storage in the plant, thereby affecting N fertilizer use efficiency [29]. Therefore, larger root systems have a stronger soil carbon storage capacity, which is an important way for plants to counteract the increase in atmospheric carbon dioxide.

3. K in Plants

Potassium is a key essential nutrient for plant growth and development, playing vital roles in osmotic and membrane potential regulation, sugar transport, photosynthetic performance, enzyme activation, charge balance, stress adaptation, and growth. As the second most abundant plant nutrient after N [30,31,32,33], it ensures normal metabolism and is particularly crucial to water use efficiency and drought resistance [30]. Adequate K fertilization enhances plant tolerance to drought [33,34] and other stresses including diseases, pests, salinity, cold, frost, and waterlogging. Sufficient K supply and utilization are critical to crop yield and quality.

K⁺ is the core ion for enzyme activation, osmotic regulation, and charge balance, and its deficiency will lead to multiple functional disorders. In the early growth stage of the plant, a K⁺ deficiency manifests as the withering of the leaf margins of old leaves (the transfer of K⁺ to new tissues causes the death of edge cells) and wilting (loss of turgor pressure) [35]. At the metabolic level, the obstruction of photosynthetic product transport leads to starch accumulation at the source end and sugar deficiency at the reservoir end [36,37], while the collapse of the reactive oxygen species (ROS) scavenging system (decreased SOD/POD enzyme activity) intensifies membrane lipid peroxidation [38]. More importantly, K deficiency directly weakens the activity of nitrate reductase (NR) and amino acid synthesis, forcing N to accumulate in an ineffective form, ultimately leading to yield loss [39] (insufficient grain filling) and decreased stress resistance [40,41].

3.1. The Role of K in Photosynthesis

During photosynthesis, CO₂ enters the leaf’s internal airspace through stomata from the external atmosphere, subsequently diffusing to carboxylation sites within chloroplasts. Stomatal movement depends on turgor changes in guard cells, driven by substantial bidirectional K⁺ fluxes across both the tonoplast and plasma membrane. This mechanism is essential to minimizing water loss. The accumulation of K⁺ in cellular vacuoles is a prerequisite for stomatal opening and constitutes a critical component of the overall K⁺ homeostasis required for stomatal closure. Vacuolar K⁺ fluxes play decisive roles in: (1) the vacuolar dynamics underlying stomatal movement and (2) the regulation of lumenal pH. As stomatal closure represents the primary limitation to CO₂ assimilation, precise control of the stomatal aperture is crucial for plant productivity [42].

Potassium accumulation in vacuoles constitutes the primary osmotic mechanism for stomatal opening. Potassium deficiency reduces vacuolar K⁺ concentration, consequently decreasing stomatal conductance. As the dominant inorganic osmoticum in plant cells, adequate K is essential to turgor-driven stomatal movement and cell elongation while facilitating CO₂ diffusion from the atmosphere to chloroplasts. Thus, K⁺ plays indispensable and multifaceted roles in photosynthetic activity and plant development [43], critically maintaining stomatal functionality and chloroplast integrity through pH homeostasis and enzyme regulation during CO₂ fixation. The transmembrane pH gradient essential for ATP synthesis drives H⁺/K⁺ exchange, initiating key biochemical processes [44].

Potassium deficiency alters leaf anatomy, reduces chloroplast surface area [45], lowers chlorophyll content and the net photosynthetic rate (Pn) [46], and impairs Rubisco/Rubisco activase activity and abundance [47]. Mesophyll conductance (g_m) is reduced, impacting Pn more significantly than stomatal conductance (g_s), particularly under progressive stress [36,48]. Under K-deficient conditions, the photochemical performance of PSII also declines, with reductions in electron transfer rate, PSII efficiency, and photochemical quenching (non-stomatal limitation of photosynthesis), thereby impairing photosynthesis in apples [37]. A hydroponic study by Ge et al. showed that K deficiency significantly reduces the Pn, g_s, intercellular CO₂ concentration (C_i), and transpiration rate (E) in banana seedlings. Concurrent decreases in photosynthesis-related mineral nutrients further impair photosynthetic function and suppress growth [49].

3.2. Mechanisms of K Absorption and Transport in Plants

Potassium is not a component of organic macromolecules. Unlike N and P, K is not metabolized in plants and exists solely in ionic form (as easily exchangeable weak complexes). The absorption and transport of K⁺ in plants depend on K⁺ channels and transporters [50]. Within cells, K⁺ is present in two main pools: the vacuole and the cytoplasm. In the cytoplasm, K⁺ is the most abundant cation. Compared to the high K⁺ concentration in plant cytoplasm (approximately 100 mmol/L), the available K⁺ concentration in soil is very low (typically ranging from 0.1 to 10 mmol/L). Plant growth requires a large amount of K⁺ to be transported from the soil to growing organs, so plant K uptake is largely determined by the K⁺ concentration in the soil solution, as well as its diffusion and mass flow rates [51].

The absorption and translocation of K ions in plant roots involve multiple K⁺ channels and transporters, classified into high-affinity transport systems (HATS) and low-affinity transport systems (LATS) based on external K⁺ concentration [52]. At higher external K⁺ concentrations (>0.1 mmol/L), K⁺ uptake is primarily mediated by K⁺ channels through LATS, whereas under low external K⁺ concentrations (<0.1 mmol/L), K⁺ transporters predominantly facilitate absorption via HATS. K uptake in higher plants is regulated by internal K⁺ status, where K-deficient conditions activate HATS as the dominant mechanism, upregulating high-affinity K⁺ transporter expression. This process concurrently activates molecular regulators including reactive oxygen species and phytohormones that modulate metabolic processes and root development to enhance K⁺ acquisition, requiring energy-dependent active transport across the plasma membrane. Conversely, under K-sufficient conditions, K⁺ influx occurs through passive transport via inward-rectifying K⁺ channels on the plasma membrane without energy expenditure [53,54].

The K⁺ transporter gene families in higher plants, comprising HAK/KUP/KT, HKT, KEA, NHX, and CHX, feature numerous transporters critical to high-affinity K⁺ uptake in roots [55]. Among these, the HAK (High-Affinity K⁺ Transporter) family represents the largest subgroup, with HAK5 and OsHAK5 serving as key determinants of high-affinity K⁺ acquisition under extremely low K⁺ concentrations (<100 μM) [56]. In contrast, KUP7 mediates K⁺ absorption at low-to-moderate K⁺ concentrations while regulating root-to-shoot K⁺ translocation [57]. These K⁺ channels and carriers exhibit distinct expression patterns, subcellular localizations, transport kinetics, and regulatory mechanisms, collectively enabling specialized functions in plant development and ion homeostasis.

Mature plant cells maintain substantial K⁺ reserves within vacuoles, where K translocation across the tonoplast constitutes a critical mechanism for intracellular K⁺ homeostasis. Roots actively absorb K⁺ against concentration gradients while stabilizing cytosolic K⁺ concentrations through strict regulatory control. Within the phloem vascular system, K⁺ regulates osmotic gradient establishment that drives photoassimilate flow from source leaves to sink tissues. Analogously, in xylem vessels, K⁺-mediated root pressure facilitates sap ascent from roots to shoots under limited transpiration conditions [58].

4. Mechanisms of N–K Nutrient Interactions in Plants

4.1. Types of Nutrient Interactions

Plant growth and development are critically dependent on nutrient availability, with interactions occurring in crop systems when the supply of one nutrient influences the acquisition and utilization of others [59]. Understanding the dynamics of nutrient uptake, translocation, assimilation, and their biological interdependencies is therefore essential to enhancing crop productivity [60]. These nutrient interactions manifest as synergistic, antagonistic, or additive effects—categorized formally as: (1) Synergism, (2) Antagonism, (3) Non-interaction, and (4) Liebig’s Law-driven synergism (

Table 1. Classification of nutrient interaction types.

Interaction Type	Definition	Calculation Formula
Synergism	Cooperative interaction where combined application of two nutrients produces yield exceeding the sum of their individual effects.	${\displaystyle \frac{Yab}{Y0}}>{\displaystyle \frac{Ya}{Y0}}\times {\displaystyle \frac{Yb}{Y0}}$
Antagonism	Suppressive interaction where combined application yields less than the sum of individual effects.	${\displaystyle \frac{Yab}{Y0}}<{\displaystyle \frac{Ya}{Y0}}\times {\displaystyle \frac{Yb}{Y0}}$
Null Interaction	Non-interactive state where combined yield equals the sum of individual effects.	${\displaystyle \frac{Yab}{Y0}}\approx {\displaystyle \frac{Ya}{Y0}}\times {\displaystyle \frac{Yb}{Y0}}$
Liebig’s Synergism	Specialized synergism occurring when: (1) Single nutrient addition (A or B) shows no yield effect under deficiency of the other, (2) Combined application demonstrates significant yield promotion.	${\displaystyle \frac{Yab}{Y0}}>{\displaystyle \frac{Ya}{Y0}}\times {\displaystyle \frac{Yb}{Y0}}$

Note: Yab, yield when two nutrients are applied separately; Ya, yield when nutrient a is applied separately; Yb, yield when nutrient b is applied separately; Y0, yield of the control treatment.

) [61]. Such interactions directly govern nutrient use efficiency (NUE), ultimately determining crop biomass accumulation and yield outcomes.

Nutrient interactions dynamically modulate plant growth and yield responses contingent upon supply regimes, quantifiable through correlative analyses of nutrient supply–tissue concentration relationships and nutrient supply–growth response curves [2,62]. Such effects are governed by multifactorial regulators including environmental variables (temperature, photosynthetically active radiation, soil O₂ diffusion, moisture status, pH), plant-intrinsic factors (root system architecture, transpiration rate, respiratory flux, developmental stage, growth velocity, species specificity, internal nutrient homeostasis), initiating at subcellular compartments with subsequent physiological manifestations in respiratory ATP yield, photosynthetic quantum efficiency, cell cycle progression, nutrient utilization coefficients, and the phloem loading dynamics of photoassimilates and organic acids.

In natural ecosystems, plant growth is influenced by multiple nutrient elements. The effects of combined nutrients on plant growth and development are not simply additive. Among these, the synergistic interactions between N × K and N × P are particularly crucial. These interactions are closely linked to crop yield, quality, root growth, and improvements in NUE.

Studying the N × K interaction serves dual purposes. First, it enables better simulation of actual ecological environments to understand plant growth adaptation mechanisms in complex systems and to reveal their synergistic effects and mutual influences on growth regulation. Second, because imbalances in soil N and K can trigger ecological problems (such as soil acidification and water eutrophication), and because rational regulation of their supply is vital for enhancing crop yield and quality in agricultural production, this research can aid in optimizing soil fertility management strategies. This optimization improves N and K use efficiency, thereby strengthening the sustainability and economic benefits of agricultural production [63].

4.2. Physiological Basis of N–K Interaction

Potassium typically exists in plants as the monovalent cation K⁺, whereas N is present in compound forms such as the cation ammonium (NH₄⁺), anion nitrate (NO₃⁻), or amino acids. Following absorption, K⁺ maintains its ionic state, while N-containing compounds undergo diverse chemical reactions involving covalent bonding. This fundamental distinction determines their respective physiological roles in plants [64]. The transportation and utilization of these elements are interdependent, particularly regarding K’s regulatory effects on N nutrition. Consequently, a close relationship exists between K⁺ supply and N metabolism. One such linkage involves the partitioning of nitrate reductase activity between roots and shoots, modulated by plant species, external nitrate availability, temperature, and light intensity. Alterations in N:K and P:K ratios may enhance the adaptation of stress-tolerant species or the competitiveness of fast-growing species, with outcomes contingent on environmental conditions. For instance, under intense drought, reduced N:K ratios may benefit stress-resistant species by improving water stress adaptation, depending on plant growth rates and K demand.

4.3. Interaction Between N and K in Root Absorption and Transport

Adequate K levels are crucial for efficient N utilization in crops under sufficient NO₃⁻ supply. K contributes to NO₃⁻ absorption—the primary soil N form—through two mechanisms. First, K⁺ acts as a counter-cation co-transported with NO₃⁻ via xylem from roots to aerial parts, subsequently cycling through phloem with malate [65]. Generally, an elevated external K⁺ supply enhances cotransport of K⁺ and NO₃⁻ to shoots while reducing root N retention. Conversely, K⁺ deprivation impairs NO₃⁻ translocation efficiency.

One of the most well-known N and K physiological interactions in plants is the significant inhibitory effect of NH₄⁺ on the high-affinity K⁺ absorption system. In barley seedlings, NH₄⁺ can suppress K⁺ absorption by up to 90%. This antagonism arises partly from the competitive nature of NH₄⁺ and K⁺ due to their similar hydrated diameters, charge, and effects on membrane potential, though non-competitive components also exist. Reduced K⁺ accumulation in NH₄⁺-grown plants (compared to NO₃⁻-grown) is partially attributed to NH₄⁺ inhibiting K⁺ influx via the HATS. However, enhanced K⁺ efflux by NH₄⁺ may also contribute. Evidence from Coskun et al. [66] demonstrates that NH₄⁺-stimulated K⁺ efflux from barley roots, fully blocked by the channel inhibitors TEA⁺ and Cs⁺, likely occurs through a specific K⁺ channel, potentially from the Shaker family. Notably, the co-presence of NH₄⁺ and NO₃⁻ (e.g., NH₄NO₃) stimulated K⁺ efflux, whereas NO₃⁻ alone reduced it.

The influence of NH₄⁺ on K⁺ transport is complex. Under certain conditions, some studies report that NH₄⁺ can stimulate high-affinity K⁺ uptake [67], possibly due to plasma membrane hyperpolarization induced by NH₄⁺. However, this stimulatory effect appears to be less common. In the low-affinity range, NH₄⁺’s impact on K⁺ transport is generally minimal, though research using T-DNA insertion lines indicates that K⁺ transport involves not only HAK5 but also AKT1, the primary low-affinity K⁺ influx channel in roots.

N and K interactions extend beyond direct ion competition. K starvation induces the expression of AtNRT1.5, a gene encoding a nitrate transporter responsible for loading nitrate into the xylem, thereby promoting root-to-shoot nitrate transport [68]. During later N assimilation stages, multilevel metabolic analyses show K deficiency increases the N-carbon ratio and sustains carbon flux into amino acids [69]. Additionally, the kinase CIPK23 is implicated in regulating both K uptake and nitrate uptake [70,71,72]. The transport of K+, NO3- and NH3/NH4+ in plant root cells and their regulatory mechanisms are shown in Figure 1.

4.4. Interaction Between N and K in Photosynthesis

Crop yield is fundamentally linked to photosynthesis—a process converting inorganic CO₂ into organic carbohydrates within chloroplasts, constrained primarily by CO₂ diffusion limitations to carboxylation sites under saturating light and Rubisco-mediated assimilation in stroma. Nitrogen and potassium synergistically enhance photosynthetic efficiency by modulating chloroplast morphology and distribution, thereby increasing the mesophyll cell/chloroplast surface area exposed to intercellular air spaces. This structural adaptation reduces liquid-phase CO₂ diffusion resistance, elevating mesophyll conductance (g_m) [74,75]. Concurrently, K⁺ and NO₃⁻ regulate stomatal aperture via osmotic control, optimizing CO₂ entry. Crucially, NO₃⁻ uptake (an energy-dependent process) relies on photoassimilate transport supported by K⁺. An inadequate K supply impairs N utilization efficiency, whereas optimal K availability enhances both photosynthetic performance and N use efficiency. Wang et al. [76] demonstrated in rice that K fertilization reduces leaf N content per unit area but increases N allocation to photosynthetic apparatus, thereby boosting the net photosynthetic rate (P_n) and photosynthetic nitrogen use efficiency (PNUE). In sweet potatoes, N–K co-application promotes “source establishment” during early growth via elevated P_n, F_v/F_m (PSII efficiency), and sucrose synthase (SS) activity, while during tuber bulking it enhances “sink strength” through improved P_n, ΦPSII (quantum yield), and sucrose-phosphate synthase (SPS), facilitating photoassimilate partitioning to storage roots [77]. Collectively, N–K coordination surpasses singular nutrient effects by concurrently regulating stomatal conductance, chloroplast N allocation, and source-sink dynamics.

4.5. N–K Interactions in Sensing, Signaling, and Cooperative Regulation

To compete for limited soil nutrients and prevent excessive accumulation, the genes encoding transporters/channels responsible for nutrient acquisition and enzymes required for assimilation are stringently regulated in both positive and negative manners by their corresponding nutrients in a specific manner. A major challenge in studying transcriptional responses to nutrient signals is determining whether the signal responsible for the transcriptional response originates from the specific nutrient itself, downstream metabolites, or certain global changes (e.g., pH or redox alterations) induced by nutrient assimilation [78]. Plants have evolved complex sensing, signaling, and regulatory mechanisms to manage ion acquisition in fluctuating environments, with key similarities observable in mechanisms involving K⁺ and N transport and assimilation. These similarities include: (1) Co-regulation of N transporters by K⁺ at the transcriptional level; (2) Post-translational modifications of both K⁺ and N transporters by shared regulatory proteins; and (3) Systemic alterations such as membrane potential dynamics, reactive oxygen species (ROS) accumulation, and phytohormone signaling. Key components of the signaling pathway encompass sensors, kinases, miRNAs, ubiquitin ligases, and transcription factors. These elements collectively mediate nitrate-, ammonium-, and K-induced transcriptional responses, root architecture modifications, and adjustments in uptake activity. The integration of these responses enables plants to compete for limited nutrients while ensuring survival under conditions of nutrient deficiency or toxic nutrient excess.

4.6. Interaction Between N and K in Metabolism

Although K⁺ itself is not metabolized, it plays critical roles in multiple aspects of plant metabolism. K⁺ activates approximately 46 enzymes, functioning as a cofactor for key enzymes such as pyruvate kinase, starch synthase, Rubisco, and nitrate reductase (NR). Beyond enzyme activation, K⁺ is essential for protein synthesis through its involvement in ribosome biogenesis and mRNA turnover. The physiological significance of K⁺ in cellular metabolism is reflected in its homeostatic cytosolic concentration, which can reach up to 100 mM. Numerous studies have demonstrated a close interdependence between K⁺ supply and N metabolism. One key linkage involves the partitioning of NR activity between roots and shoots, which varies depending on plant species, external nitrate availability, temperature, and light intensity. An elevated external K⁺ supply enhances cotransport of K⁺ and NO₃⁻ to shoots, resulting in increased nitrate storage and NR activity in leaves while reducing root-based N assimilation. Conversely, K⁺ deprivation restricts NO₃⁻ translocation, consequently promoting higher N assimilation in roots.

5. Regulation of N and K Nutrient Balance in Plants

The nutritional interplay between N and K extensively influences plant growth, development, and metabolism. Combined N–K fertilizer application significantly enhances crop yield by promoting root/shoot development, increasing fruit sugar accumulation, and improving fruit quality. K and N exhibit synergistic complementarity in CO₂ transport and assimilation. K application improves NUE by facilitating N uptake, thereby boosting fruit yield. An imbalanced N–K supply impedes high productivity. Crucially, crop responsiveness to N fertilization diminishes when the soil exchangeable K falls below optimal levels.

Deficiencies of either N or K significantly reduce plant growth, evidenced by decreased shoot and root dry weight [79]. An imbalance between N and K will restrict plant growth [39]. Field trials demonstrate that N–K combinations significantly affect crops: in garlic, bulb yield increased with rising N application at fixed K levels, with an optimal yield at 438 kg N/ha and 189 kg K/ha [80]. Combined N–K application in sweet potatoes mitigated excessive vine growth, reduced the shoot-to-root ratio, and enhanced dry matter partitioning to storage roots [81]. Hydroponic studies on Lilium lancifolium showed significant (p < 0.05) promotion of bulb development, leaf growth, plant height, leaf number, and chlorophyll content under N–K treatments [82].

The rational application of N fertilizer can partially alleviate water stress in tomatoes by improving leaf water use efficiency, while adequate irrigation can offset the effects of N deficiency. Under water-deficient conditions, K fertilizer supplementation is critical for high tomato yields and fruit quality. K fertilizer application enhances N and K utilization efficiency in sorghum, promoting the early peak of dry matter accumulation and nutrient absorption in organs [83]. The combined application of N and K fertilizers has a significant regulatory effect on crop quality—the protein content increases with increasing N fertilizer application rates. Compared to the application of N fertilizer alone, the combined application of N and K fertilizers can simultaneously improve yield and crop quality [84,85,86,87]. Under balanced application of N and K fertilizers, soil nutrient content is effectively enhanced, directly or indirectly altering the physical and chemical properties of the soil. This, in turn, improves soil productivity and further promotes plant growth [88,89,90,91].

K fundamentally enables N utilization. Without sufficient K, plants cannot effectively transcribe genetic information or translate it into functional proteins and enzymes. Despite abundant available N, K-deficient plants accumulate incomplete N metabolites (e.g., amino acids, amides, nitrate) due to impaired protein synthesis. This occurs because nitrate reductase—the enzyme catalyzing the rate-limiting step in N assimilation—is K-activated [92].

In agricultural ecosystems, primary N sources encompass inorganic fertilizer N, crop-residue-derived organic N, and livestock manure-based organic N. Although exogenous fertilizer application elevates total soil N content, plant-available N often remains insufficient. This spatiotemporal mismatch between crop N demand and soil N supply creates N availability constraints that impede crop growth and yield. Moreover, agricultural N inputs are susceptible to losses via runoff and leaching pathways. These losses not only compromise agricultural productivity but also trigger environmental consequences including eutrophication of surface waters, groundwater N contamination exceeding safety thresholds, and water scarcity induced by quality deterioration. Enhancing NUE in agricultural systems simultaneously increases crop productivity and mitigates N pollution. Consequently, improving global N fertilizer efficiency has gained increasing attention as a critical sustainability imperative.

To achieve effective N utilization and enhanced crop productivity, sufficient K uptake from the soil is essential due to strong synergistic interactions between these nutrients. Suboptimal soil exchangeable K levels diminish crop responsiveness to N fertilization, while excessive N application reduces NUE and exacerbates environmental pollution. Therefore, optimizing the N–K co-supply is critical to enhancing agricultural productivity while mitigating environmental impacts [93]. Over the past decade, overreliance on inorganic N fertilizers has been identified as a major driver of agricultural greenhouse gas (GHG) emissions, with rapid intensification of farming practices substantially increasing inorganic N consumption. Implementing a Balanced Fertilization Technique (BFT) to improve fertilizer efficiency represents a viable solution, where effective nutrient management is crucial for both food security and GHG mitigation. Although K is a vital soil nutrient, its influence on GHG emissions remains poorly characterized. As a core component of BFT, K fertilization is increasingly recognized as a promising strategy to address the dual challenges of food security and climate change in agroecosystems. Empirical evidence [94] demonstrates that combined K–N application increases N₂O and CH₄ emissions by 39.5% and 21.1%, respectively, relative to sole N application, while reducing CO₂ emissions by 8.1%. The N:K input ratio primarily governs N₂O flux variation, whereas K source type critically regulates CH₄ and CO₂ emissions. K addition directly modifies N efficiency through competition with NH₄⁺ for fixation sites, and indirectly modulates efficiency via plant N utilization and microbial N-transformation activities [95,96,97]. Critically, N:K ratios significantly control GHG emissions, with K-mediated mitigation effects stabilizing when ratios exceed 1.97 for N₂O, 4.61 for CH₄, and 3.78 for CO₂ respectively.

6. Conclusions and Future Perspectives

This paper explores the core role of N and K nutrition in plants and their interaction mechanisms. N and K exhibit significant synergistic and antagonistic effects in root absorption and transport (such as synergistic/antagonistic absorption), photosynthesis (synergistically optimizing gas conduction and N allocation, enhancing PNUE), signal transduction, and metabolic regulation. Maintaining N–K balance is crucial for improving crop yield and quality, enhancing NUE/KUE, and reducing environmental pollution (such as lowering greenhouse gas emissions, mitigating soil acidification, and alleviating water eutrophication). Therefore, based on the mechanisms of N–K interactions, developing synergistic management strategies, implementing BFT, and optimizing N–K ratios and application schemes are key pathways to ensuring food security, addressing resource constraints, and promoting green and low-carbon agriculture.

However, current research still has many shortcomings. How to quantify the dynamic balance of N and K in the “soil-fertilizer-plant” system, optimize fertilization strategies under the “dual carbon” strategy, develop green and low-carbon agriculture, and respond to challenges such as global climate change and resource shortages remain difficult issues. Based on the status and challenges of N–K interaction research mentioned above, future work should focus on the following priority areas.

(1)

Elucidating the molecular physiological mechanisms underlying N–K interactions is essential to enhancing crop stress resilience and environmental adaptability. The synergistic N–K uptake machinery serves as a cornerstone for plant responses to environmental stressors. Future research must integrate transcriptomic, metabolomic, and protein-protein interaction analyses to unravel regulatory networks connecting distinct N forms (e.g., urea, ammonium nitrate) with K transporters (e.g., AKT1, HAK5 channels). This approach should delineate how N signaling components—particularly nitrate sensor NRT1.1—modulate K transporter expression to confer drought and salt tolerance. For instance, nitrate-N potentially enhances osmotic adjustment by activating calcium signaling cascades that upregulate root K⁺ uptake, whereas excessive ammonium-N may inhibit K⁺ channel activity and exacerbate ionic imbalance. Deciphering these mechanisms will identify molecular targets for designing stress-tolerant cultivars, thereby securing climate-resilient yield stabilization. Leveraging genetic, genomic, and molecular biological tools to achieve spatiotemporal precision in nutrient partitioning—optimizing synthesis and remobilization across developmental stages and organs—represents a pivotal frontier for future nutrient research. Such precision nutrition strategies promise to minimize fertilizer waste while maximizing nutrient delivery efficiency.

(2)

Optimizing N–K fertilizer formulations and developing efficient, environmentally sound fertilization strategies require systematic investigation into the effects of varied N–K ratios and compound combinations (e.g., urea + K sulfate versus ammonium nitrate + K chloride) on nutrient use efficiency, particularly addressing China’s dual challenges of N overapplication and K deficiency. Chloride ions from KCl fertilizers may suppress nitrification in paddy systems to reduce N losses, yet long-term application risks soil acidification. Conversely, K sulfate leverages sulfur-N metabolic synergy to enhance glutathione (GSH) biosynthesis and bolster antioxidant capacity. Building on these mechanisms, strategic innovations include developing polymer-coated slow-release fertilizers or functional K fertilizers amended with nitrification inhibitors (e.g., DCD, DMPP) to concurrently mitigate nutrient leaching and greenhouse gas (N₂O) emissions. Complementarily, foliar application of chelated K-N nanocomposites enables precise modulation of crop nutritional homeostasis, establishing a multipronged approach for sustainable nutrient management.

(3)

Constructing robust soil-plant system models is imperative for safeguarding global food supply stability. Developing dynamic N–K coupling models that integrate soil available K pools, N mineralization kinetics, and root uptake parameters can quantify nutrient fluxes across the “fertilizer-soil-crop-environment” continuum under diverse farming regimes. In intensive agroecosystems, for instance, high N inputs accelerate K fixation—necessitating model-driven prediction of critical K deficiency thresholds to dynamically adjust basal-to-topdressing fertilizer ratios. Furthermore, coupling remote sensing (e.g., Sentinel-2 multispectral data) with in-situ sensor networks (real-time K⁺/NO₃⁻ ion-selective electrodes) enables intelligent fertilization systems that synchronize nutrient supply with crop demand. Such precision approaches simultaneously reduce resource waste by 30–40% and unlock yield potential ceilings of staple crops (Zea mays, Triticum aestivum), providing scalable technological enablers for global food security.

(4)

Current research on the interaction mechanism between N and K mainly focuses on major crops such as rice, wheat, and corn, while exploration in medicinal plants remains relatively weak. Due to the unique accumulation of secondary metabolites (such as alkaloids, flavonoids, and saponins) in medicinal plants, their N and K requirements and interaction effects may significantly differ from those of food crops. For instance, the synthesis of ginsenosides in ginseng is regulated by the form of N, and K ions may influence the content of active ingredients in medicinal materials by regulating the flux of terpene precursors. In the future, it is urgently necessary to conduct research on the correlation mechanism between N–K ratios and active components in medicinal plants (such as the regulation of key enzymes in secondary metabolism, such as CYP450 and PAL, by N–K signals); define the ecological thresholds of N and K in the soil of regions producing genuine medicinal materials (taking into account both quality and environmental carrying capacity); and develop specialized fertilizers for medicinal plants based on the synergy of “quality-yield-ecology”. This will provide theoretical support for green cultivation in the traditional Chinese medicine industry and simultaneously expand the research dimensions of plant nutrition.

Research on N–K nutrient interactions carries profound practical significance and long-term value for agricultural production and ecological conservation. Advancing this field requires groundbreaking innovations spanning from molecular signaling to field-scale agronomy. Through interdisciplinary convergence and technological integration—synthesizing insights across molecular physiology, precision fertilization, and smart farming systems—such research establishes a transformative scientific paradigm for sustainable global agriculture. This paradigm shift enables dual imperatives: securing crop productivity while minimizing environmental footprints, ultimately delivering scalable solutions aligned with UN Sustainable Development Goals.