Potassium: A Vital Regulator of Plant Responses and Tolerance to Abiotic Stresses

: Among the plant nutrients, potassium (K) is one of the vital elements required for plant growth and physiology. Potassium is not only a constituent of the plant structure but it also has a regulatory function in several biochemical processes related to protein synthesis, carbohydrate metabolism, and enzyme activation. Several physiological processes depend on K, such as stomatal regulation and photosynthesis. In recent decades, K was found to provide abiotic stress tolerance. Under salt stress, K helps to maintain ion homeostasis and to regulate the osmotic balance. Under drought stress conditions, K regulates stomatal opening and helps plants adapt to water deﬁcits. Many reports support the notion that K enhances antioxidant defense in plants and therefore protects them from oxidative stress under various environmental adversities. In addition, this element provides some cellular signaling alone or in association with other signaling molecules and phytohormones. Although considerable progress has been made in understanding K-induced abiotic stress tolerance in plants, the exact molecular mechanisms of these protections are still under investigation. In this review, we summarized the recent literature on the biological functions of K, its uptake, its translocation, and its role in plant abiotic stress tolerance.


Introduction
Most cultivable crops experience one or more abiotic stress(es) of some type throughout their growth stages. Thus, increasing plant resilience in response to abiotic stress is a great challenge in the effort to improve food production by 70% to feed the increasing population by the year 2050 [1]. Abiotic stress hampers plant productivity by altering plant growth patterns and physiological responses [2,3]. The combination of different stresses that affect crops has become more common; for example, the occurrence of drought and high temperature is the most common [4], while in arid and semi-arid regions, salinity and high temperature stresses are imposed at the same time. High light and high temperature stress, drought and salinity, or high temperature, occur simultaneously under current closure or opening depends on the K + concentration in the guard cell, where inward channel KAT1 and KAT2 mediate K + uptake into the cell and outward rectifying K + channel, and the guard cell's outward rectifying K + channel (GORK channel) mediates K + release to close the stomata [33]. In the case of K + , voltage-dependent K + channel (TPK1TPK2, TPK3, and TPK5) and vacuolar Na + , K + /H + antiporters such as NHX1 and NHX2 are present in the tonoplast to facilitate K + influx and efflux in the vacuole [34][35][36].  [37]). AKT: Arabidopsis (Shaker-type) K + channel: CNGC: cyclic nucleotide-gated channel; GORK: guard cell outward rectifying K + channel; HAK/KUP: high affinity K + transporters; KAT: Arabidopsis (Shaker type) K + channel; NCC: non selective cation channels; NHX: Na + proton exchanger; SKOR: stelar outward rectifying K + channel; TPK: tonoplast two-pore K + channel.

Potassium and Plant Responses
Potassium plays significant regulatory roles in numerous plant physiological processes viz. seed germination and emergence, stomatal regulation, phloem transport, cation-anion balance, protein synthesis, photosynthesis, energy transfer, osmoregulation, enzyme activation, nutrient balance, and stress resistance [38].

Seed Germination and Emergence
Potassium assists in seed germination by initiating the rapid imbibition of water, and it also facilitates other physiological processes [39]. Potassium salts have been thoroughly studied as good catalysts for improving seed germination and the emergence rate. The most common forms of K salts used in seed priming are potassium nitrate (KNO3), potassium chloride (KCl) and dipotassium hydrogen phosphate (K2HPO4) [40,41]. By using KNO3 as a priming agent, a good germination rate was obtained in cotton genotypes [42]. Osmopriming with KNO3 in rice suggested that KNO3 performed best for the maximum germination percentage and germination rate as well as other morphological attributes [43]. Therefore, in light of the above experimental findings, it can be concluded that K has a promising regulatory role in seed germination and emergence.

Growth
Among the essential plant nutrients, K is an indispensable mineral constituent, intrinsically playing a key role in plant growth and development processes [44]. Maintaining the K level and its ratio with other essential plant nutrients (especially with sodium, Na) in plant growth medium or at  [37]). AKT: Arabidopsis (Shaker-type) K + channel: CNGC: cyclic nucleotide-gated channel; GORK: guard cell outward rectifying K + channel; HAK/KUP: high affinity K + transporters; KAT: Arabidopsis (Shaker type) K + channel; NCC: non selective cation channels; NHX: Na + proton exchanger; SKOR: stelar outward rectifying K + channel; TPK: tonoplast two-pore K + channel.

Potassium and Plant Responses
Potassium plays significant regulatory roles in numerous plant physiological processes viz. seed germination and emergence, stomatal regulation, phloem transport, cation-anion balance, protein synthesis, photosynthesis, energy transfer, osmoregulation, enzyme activation, nutrient balance, and stress resistance [38].

Seed Germination and Emergence
Potassium assists in seed germination by initiating the rapid imbibition of water, and it also facilitates other physiological processes [39]. Potassium salts have been thoroughly studied as good catalysts for improving seed germination and the emergence rate. The most common forms of K salts used in seed priming are potassium nitrate (KNO 3 ), potassium chloride (KCl) and dipotassium hydrogen phosphate (K 2 HPO 4 ) [40,41]. By using KNO 3 as a priming agent, a good germination rate was obtained in cotton genotypes [42]. Osmopriming with KNO 3 in rice suggested that KNO 3 performed best for the maximum germination percentage and germination rate as well as other morphological attributes [43]. Therefore, in light of the above experimental findings, it can be concluded that K has a promising regulatory role in seed germination and emergence.

Growth
Among the essential plant nutrients, K is an indispensable mineral constituent, intrinsically playing a key role in plant growth and development processes [44]. Maintaining the K level and its ratio with other essential plant nutrients (especially with sodium, Na) in plant growth medium or at the cellular level is crucial for normal function as well as the growth of the plant [9]. Moreover, excess or deficient K in the growing medium hampers the overall growth of the plant. Thus, managing K fertilizer is advantageous for improving plant growth [45]. Hussain et al. [46] applied different levels of K (0, 30, 60, 90, and 120 kg sulfate of potash (SOP) ha −1 ), and they found a maximum plant height at 90 kg SOP ha −1 and a minimum at 0 kg ha −1 . Zelelew et al. [47] experimented on potato (Solanum tuberosum L.) growth with five K doses (0, 75, 150, 225, and 300 kg K 2 O ha −1 ) and found that the plant height, aerial stem number, and leaf number per plant increased with the increasing K levels from 0 kg to 150 kg ha −1 . Gerardeaux et al. [48] observed that the K deficiency in the vegetative phase of cotton (Gossypium hirsutum L.) reduced the plant dry matter production, leaf area, and internode size, which leads to a reduction in plant growth. Tang et al. [44] cultivated three sweet potato (Ipomoea batatas (L.) Lam.) cultivars in K-deficient soil, and they found a lack of K, notably a cut-off in the total biomass productivity and root yield. Nodulation is a very common phenomenon in legume crops, and it is closely related to plant growth itself. The level of K and its ratio to N and P determine the success of the nodulation process [49].

Stomatal Regulation
Proper stomatal regulation (opening and closing) is necessary for the uninterrupted production of energy during the photosynthesis process, plant cooling, and water and nutrient transport. In the presence of K + , stomatal guard cells are swollen by absorbing water followed by stomatal opening and the allowance of gaseous movement in between plants and the environment. Under water-deficit conditions, K + is pumped out from the guard cell, allowing the pores to close tightly. Thus, K controls the evapotranspiration (ET) of water through pores under a water deficit in the soil environment, and it protects the plant from water stress [50]. Insufficient K supply adversely affects stomata functionality, resulting in delayed stomatal closure and even the unfinished closure of the pores. In addition to these effects, the osmotic gradient produced due to the accumulation of K in the roots helps to draw water into the root cells. Therefore, the reduced K supply in the plant results in decreased water uptake, and the plant faces water stress under drought conditions. Taiz and Zeiger [51] described the stomatal activity in detail, indicating that the following three major events occur while light radiates into the plant cell and stimulates stomatal opening: the activation of the proton pump ATPase, solute uptake, and organic solute synthesis. The electrochemical potential generated by the proton pump ATPase helps to take up K and its associated anions such as Cl − and malate. While the solute and sucrose amounts increase in the guard cell vacuole, they eventually decrease the osmotic potential. Later, the turgor pressure is increased with the increasing amount of water uptake, resulting in stomatal opening. By contrast, the stomatal closure operation is largely maintained by the ABA content. Calcium uptake is stimulated by ABA, which blocks the K + channel and paves the way to anion (Cl − ) entry into the cell apoplast. An increased concentration of Ca 2+ at the intercellular level reduces the proton pump ATPase activity that accelerates cell membrane depolarization, followed by the deportation of cytoplasmic K + to the cell apoplast [52]. Therefore, the stomata close due to reduced turgor pressure.

Water Uptake
Potassium is engaged in nearly all the physiological processes of the plant which require water. These processes include stomatal regulation, the translocation of photoassimilates, enzyme activation, and heliotropic leaf movements. In addition, K assists in water transportation and mineral compound translocation for the entire plant through the xylem. In cases in which the K supply is not at  ), and amino acid uptake is reduced [53]. Optimal K fertilization helped plants to mitigate the effect of the water deficit through better water use efficiency (WUE), which was related to the lower leaf ET. Martineau et al. [54] reported the appearance of pronounced leaf rolling under a water deficit condition after K addition, which prevented water losses, and thus, they concluded that adding K to K-deficient soils can help maize to cope with drought and could be a new management option [54]. The hydraulic conductance of the xylem varies in response to changes in the solute content of the sap, and in particular to the K concentration. This phenomenon, which is known as the ionic effect, is improved in embolized stems, in which it can compensate for the cavitation-induced loss of hydraulic conductance. That report suggests that the potassium concentration in the xylem sap and the hydraulic conductance of well-irrigated laurel plants (Laurus nobilis L.) increased 24 h after KCl fertilization [55].

Photosynthesis
Potassium plays an imperative role in the photosynthesis process and the subsequent carbohydrate translocation and metabolism, which eventually increase the crop yield and improve the grain quality [56][57][58]. Both the leaf number and the leaf size are reduced while the plant is deficient in K. The leaf number and size reduction later hasten the diminished photosynthetic rate per unit leaf area and thus account for an overall reduction in the amount of photosynthetic assimilates available for growth [56]. Furthermore, K controls photosynthesis through sunlight interception. The leaf surface area and sunlight interception were both reduced dramatically when the K was below the level required by the plant [59].
Stomatal regulation during photosynthesis is a vital event that governs the continual photosynthesis operation, and it is significantly moderated by the amount of K retained in the plant [38]. Potassium deficiency resulting in reduced stomatal conductance increased the mesophyll resistance and lowered the ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity in plants, which eventually decreased the total photosynthesis rate [60]. The photosynthesis rate in plants increased with the higher utilization and export of photoassimilates. Evidence showed that the sucrose level in the leaves was increased by several folds when the plants were supplied with a sufficient level of K [60]. In addition, both the photosynthetic CO 2 fixation and the utilization of photoassimilates were reduced due to K deficiency, which accelerated the plant ROS production, ultimately hastening rapid photooxidative damage [6]. Thus, the most significant function K plays in regulating the stomatal aperture is in balancing CO 2 entry and H 2 O vapor removal from intercellular spaces. However, the amount of CO 2 entry into the intercellular spaces represents the amount of photosynthate production in the leaf. Potassium plays an essential role in activating the ATP synthase enzyme, thus assisting in the photosynthesis process. Shingles and McCarty [61] suggested that the ATPase performance is best when the K content in the plant is at an optimum level.
However, the molecular identification of the K + transporters involved in photosynthetic regulation has increased the apparent importance of K in plants. Recent studies have revealed the important roles of three K + efflux antiporters, AtKEA1/2/3 were involved in photosynthesis and primary chloroplast development in Arabidopsis. AtKEA1 and AtKEA2 are located at the inner envelope membrane of the chloroplasts, while AtKEA3 localizes to the thylakoid membrane [62]. Both the chloroplast development and photosynthesis rate were significantly damaged in the AtKEA, 2, 3 triple mutant, resulting in the stunted growth of the mutant plants [62,63]. AtKEA1 and AtKEA2 show polar distribution in small and dividing plastids, which regulate chloroplast development [64]. AtKEA3 is an H + /K + antiporter that mediates H + efflux from the thylakoid lumen to the stroma through H + /K + exchange and regulates the proton motive force (pmf) across the thylakoid membranes; and consequently, it regulates photosynthesis as well as photosynthetic acclimation in fluctuating light environments [65,66].

Nutrient Balance
Under K-deficit condition, protein formation is hampered even when there is an abundant N supply; therefore, NO 3 − , amino acids, and amides accumulate in the cell [16]. Potassium activates nitrate reductase (NR), a starch synthetase, and these two enzymes create a balance by producing protein and carbohydrates, respectively. Therefore, K shortages lead to a breakdown in these processes and the plants suffer, even though other nutrients are available. As previously stated, K has a role in the xylem and phloem transport system. Consequently, Ca 2+ , Mg 2+ , NO 3 − , and PO 4 3− as well as plant hormones and enzymes cannot be translocated, and the source-sink relationship is interrupted [67]. The exogenous application of K at different growth stages decreases the uptake of harmful nutrients, enhancing tolerance to abiotic stress and boosting the yield and yield-related characteristics [68]. Iron (Fe) is another vital micronutrient that also has a synergistic relationship with K [69]. Under Mo/K deficiency, the Fe uptake is decreased, which leads to the sterility of female flower parts at the reproductive phase, while Mo application increases nodulation in the roots, leading to additional seed yields in lentils [70].
Potassium can also be toxic. During charge compensation, K is the dominant cation for counterbalancing immobile anions in the cytoplasm, chloroplasts, vacuoles, xylem and phloem. When plants take up excess K, organic acid anions will then accumulate to counterbalance the excess K + transported into the cytoplasm. Notably, with the K + counterbalance anion of NO 3 − at the time of nitrate metabolism and the reduction of excess NO 3 − in the leaves, the K concentration is increased.
As a result, organic acids such as malate are synthesized in the leaf tissue to support the charge balance. Part of this K-malate is transported to the root cells for NO 3 − uptake [38].

Reproductive Development
Potassium plays roles in flowering and pollen germination as well as in seed development. Fan et al. [71] found that externally applied K helps to increase the pollen germination rate, and K also increases tube growth in Arabidopsis. Choudhury et al. [72] noted that sucrose, boric acid, and different salts have a role in pollen germination, and among the studied compounds, KNO 3 plays an important role in pollen germination and development. Makhdum et al. [73] described that in some cases, the yield depends on the reproductive-vegetative ratio (RVR). If the RVR is lower, then the vegetative growth is higher than the reproductive growth, and in this case, the yield is reduced. Amanullah et al. [45] reported that phenological development such as flowering or physiological maturity was delayed due to the lower application of K. Sadiq and Jan [74] observed that a split application of K delayed flowering and physiological maturity even after a split application of a 60 kg ha −1 K application, but when using the 90 kg ha −1 K application as a basal dose, the numbers of days to tasseling, silking, and physiological maturity were increased. Asif et al. [75] showed that the phenological development of maize increased with the application of 90 kg ha −1 K, and they showed a positive relationship to the number of split K applications. A full dose of K helps to increase the flowering, the number of grains and early physiological maturity. In wheat (Triticum aestivum L.), K has an important role during the grain filling stage. Applying K in an appropriate way significantly increased photosynthesis during the grain filling stage, and it also has a positive role in increasing the grain number [76]. The higher amount of K helps to transfer food material to develop grains, thus decreasing the amount of sterile grain. In O. sativa, when the K was applied at 100 kg ha −1 , the grain sterility was lower compared to the treatment with no K application. At 100 kg K ha −1 , the grain sterility was 22.60%, whereas without K, it was 30.33% [77].

Yield
Potassium regulates the biosynthesis, conversion, and allocation of metabolites that ultimately increases the yield. Many research works strongly supported the notion that K is directly or indirectly responsible for higher yield of crops (Table 1). Islam and Muttaleb [77] experimented on rice with various doses of K fertilizer. They reported that K helps to increase the N uptake as well as N use efficiency that help in increasing the yield of rice. As a result, rice yield increased to 6.86 t ha −1 year −1 with optimum doses of K, whereas without K the yield was 5.19 t ha −1 year −1 . Cheema et al. [78] found that K helps to increase the utilization of carbohydrates and it increases the leaf area index, which helps to increase the dry matter accumulation and ultimately increase the yields in rapeseed (Brassica napus L.). Uddin et al. [79] found that 1000 grain weight, grain yield increased by K. Also, when other nutrients are in optimum condition, K played an important role to increase the yield of NERICA 1 rice. Though other nutrients are available, without K yield increase is not so much significant. Duan et al. [80] showed that yield of wheat increased by about 0 to 17.6% when they applied NPK fertilizer compared to only NP, and in case of rice yield increased by about 1.7 to 9.8% after using NPK fertilizer to only NP. Raza et al. [81] showed that K increased the spike length, number of spikelets per spike, number of grains and grain yield of wheat under drought condition. K increased spike length by 21.8%, number of spikelets spike −1 increased by up to 23.27%, number of grains spike −1 increased by 39.24% and ultimately yield was increased by 30.77% than without K application. Waraich et al. [82] reported that 200 mM KNO 3 application increased the plant branches, plant height, and number of balls plant −1 in G. hirsutum but in case of ball weight, 150 mM foliar application is better. According to Colpan et al. [83], K plays a vital role to increase the yield and yield components in tomato (Lycopersicon esculentum L.). They applied various doses of K as for example 0, 40, 80, 120, and 160 kg K 2 O ha −1 and found the highest yield (195.7 t ha −1 ) with 120 kg K 2 O ha −1 . Fruit size, diameters of fruits, number of fruits plants −1 and fruit weight increased with the application of K. Khan et al. [84] experimented to show the effect of K on O. sativa and T. aestivum. They noticed that K increases the yield and yield contributing characters in both crops. In case of T. aestivum, yield was about 13% higher, while in O. sativa it was about 50% higher with the application of 60 kg ha −1 K, compared to control.

Crop Quality
Potassium is responsible not only for higher production but also for the improved quality of the harvest. Thus, K ensures high-value crops and benefits to growers. K is sometimes called the "quality element" for its association with better crop production, which is supported by many scientists. Yang et al. [106] showed that the protein percentage in maize (Zea mays L.) grain was higher when a balanced N-P-K fertilizer was applied. However, when only N-P fertilizer was applied, the protein percentage as well as the grain quality was reduced. Nevertheless, when manure was applied along with N-P-K to T. aestivum, the protein percentage was higher than it was under normal fertilization. In T. aestivum, as well as in most of the studied cereal crops, K helps to provide better milling and baking qualities, a more efficient use of nutrients, increased disease resistance and many other favorable traits. In G. hirsutum, the important quality control parameters are the fiber length, strength, uniformity, micronaire, color, and many others. Many reports strongly support the idea that the fiber quality depends on having optimum levels of K. When the K supplies are not sufficient, the osmotic potential of the fiber becomes more negative. Thus, the turgor pressure of the fiber decreased and the primary fiber cell wall cannot be elongated, ultimately resulting in shorter fibers [107]. The ginning out turn, fiber uniformity, length, and strength increased when KNO 3 was applied at 200 mM as a foliar spray [82]. According to Ashfaq et al. [108], K deficiency reduces the cotton yield and quality. Mehrandish et al. [109] showed that the total soluble solids, soluble sugar content, refineable sugar, and purity percentage of sugarbeet (Beta vulgaris L.) root juice increased with K application. Economakis and Daskalaki [110] explained that K increased the dry matter, total soluble solids, firmness, and vitamin C content in tomatoes under both normal and saline conditions.

Potassium-Induced Abiotic Stress Tolerance
In the previous sections, we described the role of K in the various growth and physiological parameters of plants under normal conditions. Many plant studies also showed that K functions as a vital protector against abiotic stresses. This function is primarily due to its role in maintaining ion homeostasis, cellular integrity and enzymatic activities (Tables 2-4). In the following sections, we will describe the role of K in conferring tolerance to major abiotic stresses.

Drought
In addition to growth and productivity, K plays a role in cell turgor maintenance, osmotic adjustment [6], and aquaporin function [9] under drought conditions. A close relationship between the K nutritional status and plant drought resistance has been demonstrated ( Table 2).
A sufficient supply of K can improve the plant dry matter better than a lower concentration of K can in soil under drought conditions [111]. Increasing root growth, by applying K, increases the root surface area under drought conditions, which ultimately enhances the water uptake by plant cells [112].
The exogenous application of K not only improved the plant dry matter content and leaf area but also stimulated water uptake when the plants faced drought [113,114]. Under drought conditions, excess ROS production in plants may exaggerate cellular lipid peroxidation, leading to an increase in the cellular membrane permeability, which is evidenced by increases in the electrolyte leakage (EL) and malondialdehyde (MDA) content [115,116]. Soleimanzadeh et al. [117] performed an experiment with sunflower (Helianthus annuus L.) and reported that an adequate supply of K significantly decreased the MDA content under water shortage conditions which clearly indicates the role of K in mitigating oxidative stress.
Kanai et al. [118] showed a relationship between the aquaporin activities and the K-channel/transporter, in which K deficiency markedly changed the K + -channel activity, resulting in an alteration in the root hydraulic conductance and signal transduction with consequent changes in the aquaporin activity. Thus, the reduction in the root hydraulic conductance and water supply for transpiration was suppressed under K deficiency. According to Guo et al. [119], there is a positive correlation between water uptake and K absorption in common bean (Phaseolus vulgaris L.). Potassium mediated the xylem hydraulic conductance and maintained cell turgor, stomatal movement, and sufficient gas exchange as part of the drought adaptation because these events helps to maintain water balance in plants [120].
An adequate supply of K can reportedly increase the organic osmolyte production under drought conditions, especially Pro [114]. When plants are exposed to drought stress, Pro accumulation plays a highly protective role in plants, and it is involved in osmotic adjustments [121]. In several studies, it was reportedly increased through K application in O. sativa [122], B. napus [123], and T. aestivum under normal and drought stress conditions [124]. Zhang et al. [125] observed that exogenous K applications increase the Pro content in Z. mays cultivars under drought conditions. At the same time, Ali et al. [126] reported that under drought stress, K applications increase the shoot Pro content in B. napus in a dose-dependent manner. Similarly, Zahoor et al. [127] found that K application generates the Pro content in G. hirsutum during drought stress.
Therefore, proper K supplementation improves the osmotic adjustment and upgrades the plants' ability to withstand drought stress. Figure 2 illustrates the involvement of K in plant tolerance under drought stress.

Salinity
Potassium is an important macronutrient that plays essential roles related to the osmotic adjustment, to maintaining turgor and to regulating the membrane potential, cytoplasmic homeostasis, protein synthesis, and enzyme activation under salt stress (Table 3) [132]).
Under salinity stress, the osmotic effect and ion toxicity inhibited plant root growth, which decreases nutrient uptake and translocation, especially that of K + [9]. The sodium ion (Na + ) competes with K + for major binding sites during key metabolic processes in the cytoplasm, with these binding sites including both low-affinity (non-selective cation channels, NSCC) and high-affinity (KUP and high-affinity K + transporter, HKT) transporters; this competition disturbs the plant metabolism [9,38]. Salinity induces membrane depolarization and decreases the membrane integrity, which results in K + leakage through depolarization-activated outward-rectifying (KOR) K + channels [133]. Maintaining the cellular K content above a certain threshold and maintaining a low Na + /K + ratio is crucial for plant growth and salt tolerance. Thus, higher applications of K increase the K + content in plant cells and reduce the Na + concentration, which increase the K + /Na + ratio. The HTK (high-affinity K + transporter) mediates Na + -specific transport or Na + -K + co-transport, which have vital roles in plant Na + tolerance [134,135].  Taffouo et al. [144] found that the K + content was decreased in the roots and shoots due to the reduced uptake and translocation of K + from the roots to the shoots while increasing the NaCl concentration in Vigna subterranea (L.) Verdc. However, exogenous K can positively correlate with plant root and shoot growth during salinity stress and the K deficient stage. Saida et al. [140] observed that applying 2.39 mM KH2PO4 against 150 mM NaCl increased the shoot and root fresh and dry weights of L. esculentum under salt stress. Fayez et al. [131] experimented with a H. vulgare crop and found that under 150 mM NaCl stress, the shoot fresh weight and height decreased, but treating with K improved the shoot fresh weight and height of a barley crop. Similarly, Amjad et al. [142] found that applying 9 mM K improved the root and shoot dry weights under 75 mM NaCl  Taffouo et al. [144] found that the K + content was decreased in the roots and shoots due to the reduced uptake and translocation of K + from the roots to the shoots while increasing the NaCl concentration in Vigna subterranea (L.) Verdc. However, exogenous K can positively correlate with plant root and shoot growth during salinity stress and the K deficient stage. Saida et al. [140] observed that applying 2.39 mM KH 2 PO 4 against 150 mM NaCl increased the shoot and root fresh and dry weights of L. esculentum under salt stress. Fayez et al. [131] experimented with a H. vulgare crop and found that under 150 mM NaCl stress, the shoot fresh weight and height decreased, but treating with K improved the shoot fresh weight and height of a barley crop. Similarly, Amjad et al. [142] found that applying 9 mM K improved the root and shoot dry weights under 75 mM NaCl stress in S. lycopersicum. Merwad [143] reported that higher salinity negatively affected the tomato roots, shoots, leaves and WUE, and they recorded that exogenous K can have a positive effect on most of the B. vulgaris cultivars under salt stress.
In a previous section (Section 5.1), the exogenous application of K was shown to improve the organic osmolyte synthesis, especially that the rapid rate of cell recovery under osmotic stress was regulated by the higher accumulation of K + , Cl − , and Na + in the epidermal root cells of Arabidopsis [145]. Thus, higher Na + concentrations are toxic for cell metabolism; that is why it is essential to maintain the cytosolic K + contents at a constant level for plant metabolic processes [146,147]. A constant cytosolic K + concentration is attributed to the consumption of vacuolar K + in K + -deficient situations [9]. Fayez et al. [131] observed that salinity increased the Na + /K + ratio in H. vulgare, which significantly decreased after the application of K and increased the K + /Na + . In addition, Chakraborty et al. [141] suggested that external K applications could have reduced the Na + uptake by adjusting the tissue ionic balance in peanut (Arachis hypogaea L.).

Extreme Temperature
When the temperature is greater than the optimum, plants suffer from high temperature stress. Extreme temperatures disrupted various biochemical reactions and the plant metabolism, which are highly related to the temperature [3]. Nutrient management is one of the best options for extreme temperature stress tolerance, and among all the nutrients, K plays a significant role in coping with temperature stress. Potassium helps to activate the various physiological and metabolic processes such as photosynthesis, respiration, and nutrient homeostasis, and it increases the tissue water potentiality, which assists in extreme temperature stress tolerance. Under high temperature stress, plants accumulate various types of osmolytes to overcome the damage caused by the stress. Potassium may work as an osmolyte, and it helps to maintain stomatal conductance to prevent damage [148]. In K deficient plants, not only photosynthetic electron transport pathways but also the NADPH oxidizing enzyme reaction produces ROS [5]. In this case, K helps plants by supporting protein synthesis, stimulating various enzymatic reactions, assisting in carbohydrate production and increasing the WUE. Under heat stress conditions, the foliar spraying of potassium orthophosphate (KH 2 PO 4 ) helps to increase the heat tolerance of wheat by preventing leaf damage [149]. When a significant amount of K is lost from the chloroplast, the photosynthesis is reduced. In this situation, K application helps the plant to tolerate the heat stress by increasing the photosynthetic ability in wheat. Foliar applications of K also help to increase the translocation and accumulation of photosynthates as well as the dry matter. These factors are related to stress resistance and ultimately help to increase plant yields [150].
When plants face chilling or freezing stress, K-regulated mechanisms such as photosynthesis and carbon assimilation, metabolism, and phloem activity are down-regulated. Seed treatments involving KCl in a cold-sensitive maize variety led to better ROS defense and ultimately greater tolerance [39]. Plants facing freezing stress were found to lose apoplastic water due to freezing, which causes dehydration; and a sufficient K supply adjusts the osmotic potential and decreases freezing-induced dehydration [9]. Several field trials in various crops also show a similar phenomenon in which a sufficient K supply can eliminate frost damage [151].

Toxic Metals/Metalloids
Toxic metal/metalloid contamination in soil is dramatically increasing with time owing to rapid industrialization, which has generated tremendous problems in global agriculture [152]. The most obvious reaction of plants under metal/metalloid toxicity is the inhibited uptake of other essential nutrients and the alteration of almost all the physiological processes, including disturbances in stomatal action, alterations in membrane functions, and the inhibition of photosynthesis, affecting the activities of several key enzymes, the generation of excess ROS, the reduction of the water potential, etc. [153,154]. The continuous reduction in plant growth and development ultimately leads to yield losses as well as food insecurity. Hence, remediating toxic metals in soils or increasing plant tolerance or resistance to stress is a very urgent task for plant scientists. Because K plays a crucial role in the activation of several enzymes, the synthesis of protein, photosynthetic activity, osmoregulation, the movement of stomata, the transfer of energy, phloem transport, cation-anion balance, and stress resistance [9], this element is used as a plant protector against most abiotic stresses, including metal/metalloid toxicity (Table 4). The use of K against Cd toxicity confirmed the positive effect of this element by ameliorating Cd-induced oxidative damage in broad bean (Vicia faba L.; [155]). In this study, 6 mM K was used in combination with 200 µM Cd to inspect the role of the potassium. Cd stress reduced the growth parameters (shoot and root lengths), decreased the chl content, altered the enzyme activity and increased the MDA content of the plants. Song et al. [156] experimented with peach plants using elevated exogenous K (10 mM) against Zn toxicity (2 mM). They observed that Zn damages the plant by altering the physiological process and nutritional balance. However, K mitigated the Zn toxicity by improving the photosynthesis, antioxidant defense systems, and plant K nutritional status. Potassium also upregulated the genes related to K acquisition, transport, and homeostasis.

High Light
Several examples illustrated the effect of high light intensity, which caused rapid leaf chlorosis resulted from impaired photosynthesis and photooxidative damages [158,159]. With the severity of stress, photosynthetic ability, RuBisCO activity, quantum yield, and electron transport disrupted in tomato [160]. When plants face insufficient K supply leaf chlorosis increased at high light [6]. This severity decreases the utilization of photoassimilates within the plants under insufficient K supply [161], and transport of sucrose via the phloem decreased [162]. Thus, K is required in a great quantity to utilize the absorbed high light for CO 2 fixation, and source-sink relation. Hence, plants receiving high light intensity may have greater K demand.

Waterlogging
Among the abiotic stresses waterlogging is an important barrier for crop production and affected at least 10% of the global agricultural land. When the root zone is waterlogged plants face a severe shortage of oxygen supply (hypoxia or anoxia), which in terms disrupt the respiration process in roots resulted in energy shortage to the cells. The key mechanism is avoiding K loss, at the time of hypoxia or anoxia, which gives resistance to the plants in waterlogging condition [163,164].
Several researchers have reported the effect of exogenous K application for ameliorating the adverse effects due to waterlogging. Increased plant height, photosynthetic capacity, and chl content was reported in cotton due to K supplementation under waterlogging condition [23]. They also found greater nutrient uptake by the plants as a result of higher K application to soil or foliage [23]. Application of K at a higher dose improves non-structural carbohydrates (NSC) contents, photosynthetic pigments content and higher antioxidative activity as well as lower lipid peroxidation in submerged rice [165].

Role of Potassium in the Detoxification of Reactive Oxygen Species
One of the common consequences of abiotic stress is the overproduction of ROS such as singlet oxygen ( 1 O 2 ), superoxides (O 2 •− ), H 2 O 2 and hydroxyl radicals (OH • ) [166], alkoxy radicals (RO • ), peroxy radicals (ROO • ), and organic hydroperoxides (ROOH) [167,168]. Inside plant cells, low concentrations of ROS act as a signaling molecule to protect plants from stresses, while higher concentrations of ROS enhance the lipid peroxidation, oxidation of proteins, inhibition of enzyme activities, and activation of the programmed cell death (PCD) pathway, ultimately leading to cell death [169]. During photosynthetic electron transport and membrane-bound NADPH oxidase reactions, the formation of ROS in plant cells is increased due to K deficiency. Hence, it was suggested that the exogenous use of K could decrease ROS formation by maintaining the plant photosynthetic electron transport and diminishing the action of NADPH oxidase ( [6]; Table 5; Figure 3).
death [169]. During photosynthetic electron transport and membrane-bound NADPH oxidase reactions, the formation of ROS in plant cells is increased due to K deficiency. Hence, it was suggested that the exogenous use of K could decrease ROS formation by maintaining the plant photosynthetic electron transport and diminishing the action of NADPH oxidase ( [6]; Table 5; Figure  3). Plants that are exposed to environmental stresses such as drought show enhanced K requirements, and furthermore, they show increased oxidative damage to cells by inducing the formation of ROS, especially during photosynthesis [9]. At the time of drought stress, CO2 fixation is limited in plants, and it impacts the stomata regulation, transfer of light into chemical energy and translocation of photosynthates from source to sink (Figure 3; [170,171]). Due to the impairment of photosynthetic CO2 fixation, plant molecular O2 is activated and ROS production is increased within the plant cell [6,9,172], which causes the degradation of the photosynthetic pigment and cellular membranes. Sangakkara et al. [173] found a positive role for K by reducing ROS formation and increasing the net photosynthesis rate under water-stressed conditions in mung bean (V. radiata) and the cowpea (V. unguiculata). Egilla et al. [113] observed that adequate K + availability to China rose (Hibiscus rosa-sinensis L.) under drought stress reduced the inhibition of photosynthesis by mitigating ROS formation. Likewise, Milford and Johnston [174] suggested that K plays a vital role in stomatal opening and closing and in the transpiration and photosynthesis of plant cells. Raza et al. [81] experimented with T. aestivum under drought conditions and found that an application of 1.5% K decreased the ROS formation and improved the transpiration and photosynthesis rates. Thus, it was suggested that an adequate supply of K under drought conditions improved the photosynthetic  Figure 3. Schematic representation of the role of K to detoxify the reactive oxygen species (ROS) formation under stress (Modified from [166] with permission from Springer).
Plants that are exposed to environmental stresses such as drought show enhanced K requirements, and furthermore, they show increased oxidative damage to cells by inducing the formation of ROS, especially during photosynthesis [9]. At the time of drought stress, CO 2 fixation is limited in plants, and it impacts the stomata regulation, transfer of light into chemical energy and translocation of photosynthates from source to sink (Figure 3; [170,171]). Due to the impairment of photosynthetic CO 2 fixation, plant molecular O 2 is activated and ROS production is increased within the plant cell [6,9,172], which causes the degradation of the photosynthetic pigment and cellular membranes. Sangakkara et al. [173] found a positive role for K by reducing ROS formation and increasing the net photosynthesis rate under water-stressed conditions in mung bean (V. radiata) and the cowpea (V. unguiculata). Egilla et al. [113] observed that adequate K + availability to China rose (Hibiscus rosa-sinensis L.) under drought stress reduced the inhibition of photosynthesis by mitigating ROS formation. Likewise, Milford and Johnston [174] suggested that K plays a vital role in stomatal opening and closing and in the transpiration and photosynthesis of plant cells. Raza et al. [81] experimented with T. aestivum under drought conditions and found that an application of 1.5% K decreased the ROS formation and improved the transpiration and photosynthesis rates. Thus, it was suggested that an adequate supply of K under drought conditions improved the photosynthetic CO 2 fixation and the export of photosynthates from source to sink organs and prevented photosynthetic electron transport to O 2 . As a result, the formation of ROS was reduced [6,9].
Under saline conditions, low K increased the toxicity of Na + in plant tissue. Thus, the K + /Na + ratio decreased and led to ROS formation, which affects the stomatal closure and inhibits the plant's photosynthesis activity, and it increases oxidative damage [175]. The higher production of ROS due to severe salinity leads to cellular membrane damage. Programmed cell death occurs as a result of K + leaks from the plant cells due to the activation of K + efflux channels [176]. The external use of K in a saline growing medium was involved in improving salt tolerance through reduced ROS formation in T. aestivum [177], Z. mays [178], and O. sativa [179]. The application of K enhanced the antioxidant enzyme activities such as those of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in zinger (Zingiber officinale Roscoe) [180], which reduced the ROS formation in plant cells. Zheng et al. [181] suggested that applying a suitable amount of KNO 3 detoxified the ROS through increased SOD, CAT, and POD enzyme activities in T. aestivum under salt stress. Jan et al. [182] reported that the SOD, CAT, and ascorbate peroxidase (APX) enzyme activities were enhanced, and applying K under salt stress thus detoxified the ROS.  Figure 3). Moreover, the NADPH oxidase activity was substantially enhanced under K deficiency, increasing the NADPH-dependent O 2 − production. Cakmak [6] found that K deficiency increased the NADPH oxidase activity in the cytosol of P. vulgaris roots, with a corresponding increase in the NADPH-dependent O 2 − generation, but exogenous K decreased the NADPH oxidase activity. Potassium deficiency is most likely the primary reason for the increased NADPH oxidase and NADPH-dependent ROS formation through the generation of ABA. Peuke et al. [184] experimented with castor (Ricinus communis) and found that K deficiency increased the biosynthesis of ABA in roots and increased the translocation of ABA from the roots to the shoots. Moreover, ABA has likewise appeared to be effective at increasing the accumulation of H 2 O 2 and O 2 − in plant roots [185,186], but this point needs to be verified in future studies. It is clear that an improvement in the K status maintains the photosynthetic electron transport and inhibits the NADPH oxidase activities, which reduce ROS formation in plants.

Interaction of Potassium with Other Biomolecules
Potassium is called the policeman nutrient; it plays an important role in the growth, development, and yield as well as the metabolism of plants. It also has some interactive regulatory functions with other biomolecules [5,9]. Therefore, K deficiency leads to dysfunction in numerous physiological and biochemical processes; for example, in the water balance, enzyme activity, and charge balance, in addition to tolerance to biotic and abiotic stresses [187]. Potassium is also essential for the function and performance of many plant enzymes; at least 60 enzymes require K as a cofactor for activation [188]. These enzymes regulate the vital metabolic mechanisms in arable plants [38,189]. To increase substrate attraction, K binds with the specific binding site of inactive enzymes, resulting in their activation, and these enzymes were involved in various metabolic and physiological mechanisms. Several studies have suggested that the activity of nitrate reductase (NR), RuBisCO, starch synthase, sucrose phosphate synthase, β-amylase, invertase, phosphofructokinase, and pyruvate kinase greatly depends on the K sufficiency of plants [190,191].
Among the biomolecules, carbohydrates have a greater interactive relation with K. When plants obtain enough K, they synthesize large biomolecules; for example, cellulose, starch, protein, etc. As a result, the number of small molecules such as free sugars, amino acids, organic acids, and amides are reduced in the cell while the concentration of phenols increases, and these compounds aid in plant resistance [192] and increase the plant response to abiotic stress [193]. Carbohydrates, mostly in terms of hexose content, are decreased in leaves due to a sufficient K supply, which was transported to another plant organ due to better phloem activity. On the other hand, K deficiency resulted in the decreased activity of pyruvate kinase and/or increased invertase activity that reduces the concentration of starch in leaves because of the inhibition of starch synthase [60].
Potassium has a positive relationship to plant hormone synthesis as well [194]. When the K concentration in the cell is low, JA and auxin biosynthesis are upregulated [195], but ethylene synthesis is increased by two-fold in Arabidopsis when the plant is suffering from K-starvation [196], and the other biological functions decreased in the roots and xylem sap, leading to sucrose accumulation. Conversely, the cytokinin concentrations in the leaf and the xylem sap decreased when plants obtained sufficient amounts of K [187,197]. Ethylene, which is another important plant hormone, assists in the progression of root morphology and stimulates ROS biosynthesis to tolerate a low K condition in Arabidopsis [198]. Exogenously applied K and naphthaleneacetic acid (NAA) can interact significantly to increase the growth and yield of V. radiata [199]. A similar result was also obtained by applying K and GA to rice [200] and by applying K and SA to olive trees under salinity [201]. Increased levels of JA, hydroxy-12-oxo-octadecadienoic acids (HODs), and 12-oxo-phytodienoic acid (OPDA) were obtained under K-starved conditions along with the upregulation of the 13-lipoxygenase (LOX) pathway, indicating the transcript levels of several biosynthetic enzymes with K interactions [202]. ABA acts as an important signal-mediated factor during the transduction of the sucrose regulation signal. The ABA content in the seed is negatively correlated with the sucrose content, and the ABA/(Indole acetic acid + GA + cytokinin) ratio is influenced by K nutrition in a way that particularly reduces the ABA content, playing a key role in the increase in the sucrose content, which suggested the K interactions with phytohormones [187].
Polyamines also have a role in a wide range of environmental stresses, and they are involved in various physiological processes. Their concentrations at the cellular level increased under K deficit conditions in oat (Avena sativa L.) [203]. When plants face any stress, they accumulate polyamines at a higher concentration. Polyamines have significant interactions with K at the cellular level, and they regulate the plasma membrane K + channel of the guard cells, modulating stomatal regulation [204]. The authors also reported that spermidine, spermine, cadaverine, and putrescine powerfully block the opening and closing of stomata, which provides a link among the stress, the stomatal regulation, and the polyamine level.

Potassium-Induced Abiotic Stress Signaling
In a dynamic environment, the K content of the soil may not remain the same over the growing period of a crop. Interestingly, plant roots can sense fluctuations in the K + availability. When the K deficiency is sensed by plant roots, a series of events occurs in the plant at the molecular level to cope with this condition. Some signaling components are involved from the signal perception to the adaptive responses ( Figure 4). For example, the Ca 2+ signaling, ROS, microRNA, membrane potential and phytohormones are the signaling components [25,52]. Under K-deficient conditions, CIPK23 (a protein kinase) activates the K + transporter AKT1 by phosphorylation. The calcium sensors CBL1 and CBL2 regulate the activation of CIPK23 [205]. Later, low K + induces two distinct Ca 2+ signals that are read by CBL1/9, as observed in A. thaliana [206]. The CBL1/9 then regulates the AKT1 by activating CBL1/9-CIPK23 complexes [206]. The overexpression of the type III peroxidase RCI3 increased the production of ROS as well as the HAK5 expression. However, a mutant lacking this gene exhibited reductions in both ROS production and HAK5 expression, indicating the relationship between the ROS and the low K response [207]. Potassium channels such as NSCC and GORK are very sensitive to ROS. Under saline conditions, the ROS-mediated activation of NSCC and GORK is the primary reason for the K pool reduction in the cytosol [29,208]. A prolonged K deficiency in the cytosol activates different endonucleases and proteases, which in turn causes cell death [29]. regulate the plasma membrane K + channel of the guard cells, modulating stomatal regulation [204]. The authors also reported that spermidine, spermine, cadaverine, and putrescine powerfully block the opening and closing of stomata, which provides a link among the stress, the stomatal regulation, and the polyamine level.

Potassium-Induced Abiotic Stress Signaling
In a dynamic environment, the K content of the soil may not remain the same over the growing period of a crop. Interestingly, plant roots can sense fluctuations in the K + availability. When the K deficiency is sensed by plant roots, a series of events occurs in the plant at the molecular level to cope with this condition. Some signaling components are involved from the signal perception to the adaptive responses (Figure 4). For example, the Ca 2+ signaling, ROS, microRNA, membrane potential and phytohormones are the signaling components [25,52]. Under K-deficient conditions, CIPK23 (a protein kinase) activates the K + transporter AKT1 by phosphorylation. The calcium sensors CBL1 and CBL2 regulate the activation of CIPK23 [205]. Later, low K + induces two distinct Ca 2+ signals that are read by CBL1/9, as observed in A. thaliana [206]. The CBL1/9 then regulates the AKT1 by activating CBL1/9-CIPK23 complexes [206]. The overexpression of the type III peroxidase RCI3 increased the production of ROS as well as the HAK5 expression. However, a mutant lacking this gene exhibited reductions in both ROS production and HAK5 expression, indicating the relationship between the ROS and the low K response [207]. Potassium channels such as NSCC and GORK are very sensitive to ROS. Under saline conditions, the ROS-mediated activation of NSCC and GORK is the primary reason for the K pool reduction in the cytosol [29,208]. A prolonged K deficiency in the cytosol activates different endonucleases and proteases, which in turn causes cell death [29]. Phytohormones such as ethylene, auxin, cytokinin, and JA are also involved in low K-induced signaling processes. Under K-deficient conditions, HAK5 transcription is regulated by the upstream signaling molecule ethylene and ROS [209]. However, the cytokinin content decreases under low K stress to regulate HAK5 by inducing ROS [209]. A K + transporter in rice has been found to be regulated by JA [210]. The involvement of microRNAs in plant nutrient homeostasis has been reported in many studies. For example, the gene chip overexpression of OsmiR399 increased the nutrient contents of the plant, including the contents of K + . Under nutrient starvation, OsmiR399 expression increases [211]. The elucidation of a complex pathway that was induced by K signaling allowed us to engineer the pathway in a way that would ensure the optimum K level in the plant.

Conclusions
Potassium is vital for plant survival under both physiological and stress conditions. It is not only a part of the chemical structure but also plays vital regulatory functions in biochemical and physiological processes that contribute to plant growth and development. Proper use of K with other nutrients helps to attain sustainable productivity and quality of crops and ensure nutritional food security for animals and human beings. As a sessile organism, plant is continuously disturbed by a range of abiotic and biotic stresses. Among the abiotic stresses, drought, salinity, toxic metal, high temperature, chilling, high light intensity, waterlogging, etc., they exert their deleterious effects in crop plants. Important physiological activities including photosynthesis are greatly hampered under most of the abiotic stresses, which lead to increased toxic ROS in the plant cell. But adequate K supply to the plants during stress condition can lessen the production of ROS and improve the plant condition. Potassium also works in plant signaling systems which helps to defend some stresses by activating antioxidant defense systems. Thus, this review assesses K involvement in normal plant growth and development, in addition to increasing tolerance/resistance against different stress conditions.