Kiliu Xie , Ismil Ckmk , Shiyu Wng , Fusuo Zhng , Shiwei Guo ,d,

a Jiangsu Provincial Key Lab of Solid Organic Waste Utilization, Jiangsu Collaborative Innovation Center of Solid Organic Wastes, Educational Ministry Engineering Center of Resource-saving Fertilizers, Nanjing Agricultural University, Nanjing 210095, Jiangsu, China

b School of Land Resources and Environment, Jiangxi Agricultural University, Nanchang 330045, Jiangxi, China

c Faculty of Engineering and Natural Sciences, Sabanci University, 34956 Istanbul, Turkey

d International Magnesium Institute, College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, Fujian, China

e College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China

Keywords:

ABSTRACT Magnesium(Mg)affects various critical physiological and biochemical processes in higher plants,and its deficiency impedes plant growth and development.Although potassium (K)-induced Mg deficiency in agricultural production is widespread, the specific relationship of K with Mg and especially its competitive nature is poorly understood.This review summarizes current knowledge on the interactions between K and Mg with respect to their root uptake, root-to-shoot translocation and distribution in plants.Their synergistic effects on certain physiological functions are also described.The antagonistic effect of K on Mg is stronger than that of Mg on K in root absorption and transport within plants, indicating that the balanced use of K and Mg fertilizers is necessary for sustaining high plant-available Mg and alleviating K-induced Mg deficiency, especially in plant species with high K demand or in highavailable-K soil.The relationship between Mg and K in plant tissues may be antagonistic or synergistic depending on plant species, cell type, leaf age, source- and sink organs.There are synergistic effects of K and Mg on photosynthesis,carbohydrate transport and allocation,nitrogen metabolism,and turgor regulation.Definition of optimal K/Mg ratios for soils and plant tissues is desirable for maintaining proper nutritional status in plants, leading to a physiological state supporting crop production.Future research should concentrate on identifying the physiological and molecular mechanisms underlying the interactions between K and Mg in a given physiological function.

1.Introduction

Magnesium ion (Mg2+) is one of the most abundant cations in living plant cells, which usually require 1.5–3.5 g kg-1for optimal plant growth [1].Magnesium is vital for several plant physiological and biochemical processes, including photosynthesis, carbohydrate partitioning, and protein synthesis, that affect plant growth and yield formation [2–5].In addition to being the centrally bound ion of the chlorophyll molecule, Mg is an activator of more than 300 enzymes (Fig.1) [1,6].The early response of plants to Mg deficiency is an impaired partitioning of photoassimilates between source and sink organs, resulting in accumulation of carbohydrates in leaves (source organs) and inhibition of growth of sink organs such as roots, nodules, and seeds [4,5,7,8].Symptoms of Mg deficiency appear as leaf interveinal chlorosis, with the development of chlorotic and necrotic lesions in later stages, particularly under high light intensity [9,10].

Magnesium deficiency can be simply a consequence of very low amounts of plant-available Mg in soils, resulting, for example, from the loss of soil Mg via leaching, especially in acidic and sandy soils.Because of its large hydrated ionic radius and its correspondingly weak adsorption to soil colloids, Mg2+is highly prone to leaching [11,12].Leaching losses of Mg2+below the root zone can be up to 90 kg Mg ha-1, as shown in a sugarcane-cultivated soil in Brazil [13].An absolute Mg deficiency m ight a lso b e a ssociated with long-term unbalanced crop fertilization neglecting input of magnesium fertilizer, and intensive production systems leading to severe Mg depletion in soil caused by Mg removal by crops [11,12,14].Removal of Mg with the harvested part of plants may reach up to 100 kg MgO ha-1[15].

Remarkably, because of the complex competition between Mg2+and other soil cations, high soluble concentrations of Mg2+in the soil solution do not necessarily mean that it is plant-available.Acidic soils are usually highly saturated with cations such as H+, Al3+, and Mn2+, which induce Mg deficiency in plants by interfering with root Mg2+uptake [11,16,17].Addition of high Mg2+alleviates the inhibition of plant growth and development by many toxic heavy metals, such as copper and cadmium [18,19].An antagonistic relationship between iron and Mg has also been described [19].Application of liming materials to acidic soils is common practice to correct the pH; however, in the case of calcitic limestone, this agronomic practice may alter the availability of K+and Mg2+to plants owing to several chemical processes occurring after liming,such as increased soil adsorption and reduction in root uptake owing to competition from Ca2+[20,21].Continuous excessive application of K+andfertilizers in cropping systems can increase the risk of Mg deficiency, resulting from antagonistic interference with plant Mg2+uptake [11,22].As the best-known nutrient interaction in plant mineral nutrition, the antagonistic interaction between K and Mg has been widely studied and is considered the main cause of Mg deficiency.Accordingly, this review focuses on this interaction.

The locations commonly associated with K-induced Mg deficiency are usually found in tropical and subtropical regions with high rainfall and soil acidity as well as high soil concentrations of cationic nutrients such as K and Ca[11,12,23].K-induced Mg deficiency often occurs in crops with high demand for K for the formation of yield and quality, such as banana, potato, and sugarcane,especially at their critical developmental stages with rapid carbohydrate accumulation[5].For example,up to 1.6 t of K application per ha was required for banana [24], and imbalanced nutrition with Mg in banana due to excessive application of K fertilizers may impair the nutritional composition and value of banana fruits.In South Africa, use of high amounts of K fertilizers induces Mg deficiency by reducing root Mg2+uptake [25].In some instances,growers and crop advisors fail to recognize K-induced Mgdeficiency symptoms in leaves,especially in the case of greenhouse crops in North China,where high amounts of K fertilizers are commonly applied[26].Although calcareous soil is usually considered to be rich in available Mg and K in North China,K-induced Mg deficiency frequently occurs because excessive K fertilizer increases soil-available K to over 600 mg kg-1, higher than its optimum soil concentration (240–300 mg kg-1) [26,27].Insufficient or unbalanced fertilization with K and Mg is widespread in agricultural production, resulting in imbalances in their distribution in plants and thereby limiting plant growth as well as the nutritional quality of edible parts of plants [28].

Fig.1.The major physiological functions of K and Mg in plant cells.(1) Free Mg2+ and K+ together regulate cation–anion balance and serve as osmotically active ions in turgor regulation of cells.(2)Both K and Mg play critical roles in photosynthesis,and functional synergy or functional substitution between K and Mg in photosynthesis has been found in plants.(3)Both K and Mg are activators or cofactors of key enzymes controlling plant physiological and biochemical processes.(4)Mg plays a vital regulatory role in cellular energy balance,interacting with the pyrophosphate moiety of nucleotide tri-and diphosphates.The energy-rich compounds Mg-ADP and Mg-ATP,which is in equilibrium with the free Mg2+ pool and is required for K+ absorption across root cell membranes, represent the primary complexed Mg pools in the cytosol.(5) and (6) As cations, Mg2+ and K+ play important roles in maintaining cellular pH balance, and K+, a major osmolyte in vacuoles, is essential for stomatal opening.

Elucidating K–Mg interactions in plants may help to clarify their physiological consequences to plant growth and yield improvement, and provide a theoretical basis for balancing fertilization with K and Mg.The competition between Mg2+and K+is generally considered as unidirectional and governed only by K+, an increase in K+concentration reduces Mg2+uptake, and this inhibition occurs mainly in plant roots [29].Evidence is now mounting that their interaction occurs not only during root uptake but probably also during root-to-shoot translocation, distribution, and utilization [12,30,31].

Although K–Mg interaction in plants has been extensively studied, the synergistic and antagonistic relationships between K and Mg and its agronomic and physiologic relevance in crop nutrition have not been well characterized and reviewed.The objectives of this review are (i) to summarize and analyze the current knowledge of K-Mg interactions with respect to the root absorption, translocation, and distribution of these cations in plants; and (ii) to highlight important future research areas including the physiological and molecular mechanism underlying K–Mg interaction, and functional synergism between Mg and K.

2.Interaction between Mg and K during root uptake

Fig.2.Possible competitive effects and mechanisms of Mg2+ with K+ in plants.(1) Mg2+ uptake is facilitated by two systems: a non-selective ion channel (NSCC)and an H+/Mg2+exchanger(MHX).NSCC is also capable of K+transport in leaves and roots cells[42].(2)and(3)The two members of class II transporters(OsHKT2;4 and TaHKT2;1)in rice and wheat were originally described to transport K+,and evidence[38,39]suggests that they also transport Mg2+ in plants.(4) and (5) A high-affinity K+ transporter gene (OsHAK1) was up-regulated under Mg starvation in rice roots,and expression of a putative Mg transporter gene(OsMGT)was up-regulated under K starvation [46], suggesting that an interaction between Mg and K regulates the expression of genes controlling their uptake.(AKT, a K+ selective channel).

Root absorption of Mg2+may involve two mechanisms, which are either highly specific and insensitive to K+or highly sensitive to K+[12,32].In previous studies [33,34], Mg2+uptake at the root surface was inhibited at K+concentrations greater than 20 μmol L-1.The generally accepted competition mechanism of Mg2+with K+is a lack of specificity of the individual uptake systems for the two cations [1,35].The inhibitory effect of K+on Mg2+uptake may result from competition for metabolically produced binding compounds or active sites present on the plasma membrane [36].In Arabidopsis, the Shaker K+channel AKT1 and KUP/HAK/KT transporter HAK5, which are expressed primarily in roots and function in K+uptake from the external environment, mediate almost all K+absorption in roots [37].Under most conditions, the majority of K+is taken up by AKT1, not HAK5, because AKT1 functions in both the low-affinity and high-affinity ranges.The highaffinity plant K+transporter (HKT) family is divided into two subgroups (HKT1 and HKT2).Most HKT2 members function as Na+/K+transporters with a role in maintaining Na+/K+homeostasis in plants, but HKT2;4 seems to be an exception, as it shows permeability to a wide range of cations.The two members of class II transporters responsible for K transport, OsHKT2;4 in rice and TaHKT2;1 in wheat, have been suggested to mediate Mg2+transport in plants (Fig.2) [38,39], and it can be limited and depend on competing K+concentrations in plants [38].In plants, Mg2+uptake by roots is mediated mainly by MGT family members,and OsMGT1 in rice and AtMGT6 in Arabidopsis have been identified as plasma membrane-localized transporters mediating root Mg uptake [40,41].Mg2+uptake is also facilitated by a nonselective ion channel (NSCC), which is also permeable to K+(Fig.2)[42].Guo[43]reported that AtCNGC10(Arabidopsis thalianacyclic nucleotide-gated channel 10), located in the plasma membrane, is involved in Mg2+uptake and long-distance transport.In contrast to MGTs, AtCNGC10 is not specific for Mg2+transport,given that it also regulates K+transport (Fig.3) [43–45].These molecular mechanisms provide direct evidence for K–Mg antagonism in the process of root absorption.A high-affinity K transporter gene (OsHAK1) was prominently up-regulated in Mg-starved rice root, and expression of a putative Mg transporter gene (OsMGT)was markedly up-regulated under K starvation [46], suggesting that K-Mg interaction might regulate expression of the genes,which thus control K+or Mg2+uptake (Fig.2).In response to deficiency of a given nutrient, plants may indirectly increase the uptake of other nutrients by overexpressing or up-regulating nonspecific transporters with the effect of improving the uptake of the deficient nutrient [47].The study of K or Mg transporter genes under corresponding deficiency conditions contributes to understanding K–Mg interaction at the molecular level.The abovementioned mechanism, such as competition for active sites or transporters, could partly explain the synergism or antagonism between K+and Mg2+in root uptake or transport in plants.The molecular basis of K–Mg interaction during root uptake awaits further study.

Indirect results or effects in K–Mg interaction during root uptake are also reported from many studies.Mg2+uptake was stimulated at low K+concentration and inhibited at high K+concentration [29].In rice, Mg2+concentration in shoot and root was significantly reduced with increasing K+concentrations in growth medium [48], and a similar negative effect of high K supply on Mg2+uptake was found in safflower [49].Reduction in root Mg2+uptake by high K+concentration may involve blocking of nonspecific Mg2+transporters by K+[12].In contrast, the effects of increasing Mg supply on K+uptake in plants are inconsistent,and usually,no or a slight inhibitory effect of high Mg2+on K+uptake has been reported[11,12].At low K levels in the growth medium,K+concentration in rice leaves showed no change with increasing Mg2+supply; however, an increase in Mg2+concentration in the medium significantly reduced K+uptake at high K levels [30,48].At adequate K levels, K+concentration in plants under Mg deficiency was increased or not affected.For example, K+uptake was significantly increased under insufficient Mg supply in sugar beet, sunflower, onion, banana, andCitrus sinensisseedlings [4,50–53],whereas in perennial ryegrass and safflower, Mg supply had no effect on K absorption [49,54].Plant K+concentrations were decreased with supply or resupply of Mg in coffee and maize[55,56], whereas increased levels of Mg showed a minimal effect on leaf K levels in sugarcane [25].These conflicting observations suggest that there is a moderate synergistic, an antagonistic, or even no effect of Mg2+on K+uptake, and the inconsistency among research groups likely depends on the ratio of K to Mg used in the growth medium and plant species used in the experiments.Mg2+exerts little effect on K+absorption, in part because very specific K+transporters belonging to the high-affinity transport systems in root cells are not competitively blocked (antagonized) by Mg2+[38].The effects of Mg2+on root K+uptake might be explained by effects on the active absorption of K+across root cell membranes, given that this process depends on ATPase activity, which is disrupted under Mg deficiency [2,55].In brief, the antagonistic effect of Mg on K absorption is not as strong as the inhibitory effect of K on root Mg uptake, and the magnitude of K and Mg antagonism seems to be greatly affected by the K/Mg ratio in the growth medium.

Fig.3.Interactions between Mg and K in plants.(A)Uptake,translocation,and distribution.K+has a strongly antagonistic effect on Mg2+uptake,but there is no consistently verifiable influence of Mg2+ on K+ uptake, owing to the nonspecificity of Mg2+ uptake systems.In general, K+ inhibits Mg2+ translocation from roots to shoots, but the mechanism remains unclear.Guo [43]reported that AtCNGC10, localized in the plasma membrane, is involved in Mg2+ uptake and long-distance transport, and it also regulates K+transport in Arabidopsis.However,because not all transporters responsible for root-to-shoot translocation of Mg2+have been identified,the effects of Mg2+on K+translocation are not well characterized.The antagonistic effect on ionic distribution was most significant in older leaves.(B)Synergism or substitution in function.K and Mg nutrition greatly affects photosynthetic electron transport, formation of photo-assimilates, phloem sucrose loading, and nitrogen metabolism.(SKOR, the K+ loading transporter in xylem; CNGC10, cyclic nucleotide-gated channel 10, involved in long-distance Mg2+ transport; AA, amino acid; NRT2, nitrate transporter 2; NR, nitrate reductase; NiR, nitrite reductase; GS, glutamine synthetase; Glu, glutamic acid).

3.Interaction between Mg and K during translocation and distribution in shoots

Nutrients are transferred to shoots via xylem loading after uptake from the soil by roots.Synergistic and antagonistic effects between K and Mg also occur during their transport from root to shoot and distribution within plants (Fig.3).Mg2+transport from roots into shoots markedly decreased with increasing K supply in wheat plants [57,58].Similar depressive effects of high K+on Mg2+transport were also found in rice [48], tomato [59], andPinus radiate[60].In tomato, the K/Mg ratio in the shoots increased significantly under high K+and was higher than that in roots, indicating that high K+inhibits Mg2+root–shoot translocation [59].A reduction in the translocation of Mg2+may be an indirect consequence of a reduction of its uptake at the root.In contrast, K+translocation was not affected by Mg2+concentration in wheat forage [58], and there was even an increase in K+translocation with increasing Mg2+[48].Ohno and Grunes [58]reported that both in soil and hydroponics experiments, Mg2+concentration in shoot was depressed with increasing K supply because of the inhibitory effect of K+on Mg2+translocation; however, Mg supply did not affect the total uptake or concentration of K+in plants.It is clear that K exerts a strong antagonistic effect on Mg transport, while Mg exerts either a synergistic or no effect on K transport into shoots.Karley and White[61]showed that K+and Mg2+behave differently during xylem and phloem transport, although both are highly mobile cations.Compared with K+, Mg2+is more easily absorbed by parenchymal cells owing to its higher valence.When the K/Mg ratio becomes imbalanced because of high K+concentrations, the transport rate of K+may be much higher than that of Mg2+[62].The major contributor to K+translocation to the shoot is the SKOR channel, which is involved in xylem K+loading [63].The transcript levels ofSKORin tomato and Arabidopsis were downregulated under N, P and S deprivation, partly contributing to the reduction in K+translocation to shoot[64];however,to date,there is little evidence that K+transport is affected by Mg2+.There is an overall lack of knowledge of the effects of K+on Mg2+transport because the Mg2+transport proteins in plants are yet to be identified [39,65].One candidate, the cation channel AtCNGC10 involved in xylem-mediated long-distance Mg2+transport, might allow K+to inhibit Mg2+translocation [43,45].The physiological and molecular mechanisms mediating the transport of Mg2+and K+, including the transporters involved in root-to-shoot translocation of Mg, should be studied to further elucidate the relationship of K+and Mg2+during root-to-shoot translocation.

Another link between K+and Mg2+is their distribution in plant tissues after root-to-shoot translocation.The distribution of K+among shoot organs was affected by low Mg supply in sugar beet[4].In most cases,when the K/Mg ratio supplied to plants is unbalanced, the relationship between K+and Mg2+is often found to be antagonistic in source organs but synergistic in sinks.For example, in potato, high K application was effective in reducing Mg concentrations in leaves, but not in roots and tubers [66].In pear, an antagonistic effect between K and Mg occurred in leaves, but synergetic effects of K on Mg in fruits were observed [67].The positive relationship in sink organs was attributed partly to the synergistic and positive effects of K and Mg on the export of photoassimilates into sink organs such as fruits [68,69].After root-to-shoot translocation, both K+and Mg2+are preferentially delivered to developing tissues (the sink); moreover, a K/Mg imbalance in tissues can trigger the redistribution of these two mobile elements among leaves (the source), requiring phloem-targeted Mg transporters that also remain to be identified.In the case of typical source organs such as mature or old leaves, strong antagonism was more obvious (Fig.3) [55,70], suggesting that the regulation of K and Mg remobi-lization from source organs might depend on the K/Mg ratio in tis-sues.For example, Diem and Godbold [70]reported that the K/Mg ratio in elongating (sink) leaves ofPopulus trichocarpawas increased from 3.75 at 0 μmol L-1K to 8.31 at 4000 μmol L-1K supply; however, in older (source) leaves, the same ratio was increased more sharply, from 0.25 under low K to 13.35 under high K.Similarly, in Arabidopsis,short-term Mg deficiency reduced the contents of K in mature but not in expanding (young) leaves, a finding attributed to the stimulated nutrient retranslocation [71].

Most minerals are sorted into vacuoles for subcellular compartmentation after transport into cells [72].Both intracellular K and Mg are important for maintaining osmotic balance, enzyme activation, and cellular pH control (Fig.1).Transport of Mg2+or K+into vacuoles has been proposed to act in maintenance of cation–anion balance and osmotic potential, as well as to store excessive Mg2+or K+[6], where they could substitute for each other (Fig.1).For example, under serpentine-rich growth conditions (high Mg:Ca ratios but N, P, and K deficiency), Mg2+as the dominant cation is responsible for turgor regulation [73,74].Diem and Godbold [70]reported that an increase of Mg2+concentrations in vacuoles compensated the reduction of K+at low K levels, but that K+deficit in the cytoplasm was not balanced by Mg2+.This observation suggests that K+in vacuoles responsible for maintaining ionic and osmotic balance can be substituted by Mg2+, but that K+in cytoplasm responsible for physiological and biochemical reactions may not be.Conn and Gilliham [75]compared ion distribution in monocot and dicot plants, showing that Mg2+concentration in mesophyll was greater than in epidermis in monocots, whereas Mg2+in dicots was accumulated mainly in epidermis.However, in Arabidopsis (a dicot) leaves, Mg2+concentration was highest in vacuoles of the mesophyll rather than of epidermal or bundle sheath cells [73], and after Mg supply to leaves was increased, Mg2+concentration increased primarily in the vacuoles of palisade and spongy mesophyll cells [76].These results suggest that the antagonistic or synergistic relationship between Mg and K in different tissues of plants depends on plant species, sink- and source organ, leaf age, cell type, and the roles of these elements in plants.

4.Functional synergism between Mg and K in photosynthesis, phloem loading, and nitrogen metabolism

Focus is also given to the effects of K and Mg on formation, transport, and distribution of carbohydrates and nitrogen (N) metabolism.The productivity of plants is greatly affected by their capacity i) to fix atmospheric carbon into organic carbon by photosynthesis, ii) to translocate the assimilated carbon from source into sink organs such as roots, flowers, and seeds, and iii) to use the assimilated carbon in sink tissues for growth and development [6].It is well known [68,69,77–79]that the K and Mg nutritional status of plants strongly affects these physiological processes in plants.

A deficiency of K or Mg resulted in a substantial decline in photosynthesis [69,80].Given the differing forms and roles of the two elements in plants,their effects on photosynthetic rate may vary in different ways.As a dominant osmolyte,K+accumulating in guard cells regulates stomatal morphology and function[81],thus affecting stomatal conductance.Given that K+plays vital roles in maintaining chloroplast osmoregulation and pH stability, its deficiency may reduce the activity and content of the Rubisco enzyme and lead to chlorophyll degeneration [79,82].In contrast, most Mg2+is bound in or incorporated into cellular compartments [76], with the highest concentrations in chloroplasts [61].Mg2+is probably best known for its central position in the chlorophyll molecule,and facilitates a well-structured organization of grana and stroma lamellae.Accordingly, Mg deficiency exerts adverse effects on the structure and function of chloroplasts, the activity of Rubisco, the capacity of light reactions in the stroma, and the partitioning of carbon among sink organs [31,68,69], and thereby on carbon assimilation.Although K and Mg differ in their physiological roles associated with photosynthesis, their interaction may have synergistic effects on regulation of leaf photosynthetic capacity [48,66],which needs a balanced supply of K and Mg [59].The few studies that have investigated the effects on photosynthesis of the interactions between K and Mg have not identified their mechanisms.

Both K and Mg play critical roles in phloem loading and transport of sugars into sink organs,and their deficiency led to the accumulation of sucrose in source leaves [2,5,83–85].K promotes loading of sucrose into phloem channels and activates H+-ATPase,which resides in the plasma membrane of sieve tube cells and contributes to the maintenance of sucrose loading into phloem channels [69,86].As with K, there are two potential reasons for the impaired phloem loading of sucrose under Mg deficiency: (i) a reduction of Mg-ATP availability, which leads to impairments in the H+-ATPase activity and proton gradient formation [3,68,87],and (ii) a direct effect on sucrose symporter activity (BvSUT1, a sucrose/H+symporter expressed in companion cells in sugar beets)[83].Koch[66]suggested that although both low K and Mg supply increased transcript levels of H+/sucrose symporters, the increase was less pronounced during Mg deficiency,which probably owing to sugar accumulation in different cell compartments.These observations indicate that adequate Mg and K supply is required to guarantee sufficient re-translocation of assimilates into harvest products, such as grain, fruits, and tubers.

Magnesium and K are also required for protein biosynthesis and affect root absorption, utilization, and metabolism of N in plants[88–90].Nitrate uptake in roots can be affected by Mg2+and K+[50,91,92]via regulation of NRT2 (nitrate) transporters in Arabidopsis [92]and soybean [93].K is crucial to the activity of key enzymes required in protein synthesis (such as nitrate reductase)[89,92],ribosome synthesis,and mRNA turnover [94,95].Approximately 75% of leaf Mg appears to be associated either directly or indirectly with protein synthesis, owing to its roles in ribosomal structure and function and N metabolism [33].Mg deficiency reduced the activities of nitrate reductase and glutathione synthase (Fig.3) [48,96], resulting in a decrease in protein concentration and an increase in amounts of amino acids and amines in tea and soybean mature leaves[72,90].However,there are few reports of functional substitution between Mg and K in N metabolism.Ding et al.[48]reported that Mg could partially replace K in nitrate reduction and ammonium assimilation (Fig.3).

These results support the hypothesis that certain non-specific functions in the plant, at least in part, can be performed by one cation in the absence of another.The function of K and Mg in plants is characterized by crosstalk and redundancy.In summary, there are synergistic effects of Mg and K on physiological processes such as photosynthesis, carbohydrate allocation, and nitrogen metabolism (Fig.3), and Mg can partially replace K in some functions.The commonality of these functions appears to be responsible for physiological synergism between K and Mg in plants.

5.Effects of balanced fertilization of K and Mg on crop yield and quality

Because of the antagonistic interactions of K+and Mg2+in the process of their absorption, translocation, and distribution, plants need to maintain a homeostatic balance between K+and Mg2+response to changing nutrient status in the soil for optimal growth and development.In soils, a K/Mg imbalance (such as may be caused by unbalanced fertilization) limits the growth and survival of many plant species.Ertiftik [97]found that certain combinations of K and Mg (80 kg K2O ha-1and 40 kg MgO ha-1) maximized the yield components of maize.Gerendás and Führs [28]concluded that quality parameters decisive for horticultural crops, such as total soluble solid concentrations and acidity, often more closely correlated with K/Mg ratios than with Mg2+concentrations alone.The fertilizer ratio of K/Mg required for best yield may be different from that for quality in the same crop.Zengin [98]reported that maximum potato tuber yields were achieved with a K/Mg fertilizer ratio of 3 (120 kg K2O ha-1with 40 kg MgO ha-1) applied to acidic soils deficient in K and Mg, whereas a K/Mg ratio of 1.6 led to the highest starch concentration in tubers, reflecting good potato quality [99].These findings emphasize the potential of a balanced between K and Mg fertilization to increase crop yield and quality.Suchartgul [100]reported that when the K/Mg ratios in the soil and leaves were 2.0 and 3.0, respectively, the growth of rubber was best.When the K/Mg ratio in the shoot reached 22, dry matter of rice attained its highest value [48].Maximum values were generally obtained with combined K + Mg applications, which adjusted defective balances between soil exchangeable K and Mg cations.The positive effects of balanced K and Mg nutrition are most likely represented by the maintenance of phloem loading and the transportation of sugars from source to sink organs.

The saturation values of K and Mg were respectively 5% and 10%[101], which would be sufficient for plants to absorb K and Mg, and thus the ideal ratio of these two cations in soil would be K: Mg = 0.5.However, the balances between the two elements found in different soils or species discussed above differed from those recommended values.Generally, studies of the recommended values of K/Mg ratio in soil or plants have produced various results, owing probably to the diverse experimental conditions: soil and crop types, form and rate of K and Mg treatments, soil properties, age and position of leaves, and growth stage.To alleviate Kinduced Mg-deficiency risk in plant species with high K demand or in soils with high available K, more attention should be paid to K–Mg interaction.We recommend that overapplication of K fertilizer should be avoided in these situations.This topic represents an interesting scientific question for future study.

6.Conclusions

Root uptake and shoot transport of Mg2+in plants are inhibited by increased K supply, which can exert detrimental effects on productivity and nutritional quality of harvested products.However, there are inconsistent effects of Mg on absorption and translocation of K+, which probably depend on the plant species studied and application rates and ratios of K and Mg in growth medium.There are synergistic effects of Mg and K on photosynthesis, carbohydrate transport and allocation, N metabolism, and osmotic balance of cells.To avoid antagonistic interaction between K and Mg and its adverse consequences on yield and nutritional quality, particular attention should be paid to balanced nutrition with K and Mg.Ranges for optimal K/Mg ratios should be defined for both soils and plant tissues.However, determination of such K/Mg ratios is challenging, because they are influenced by multiple soil chemical and physical conditions as well as plant factors.The physiological and molecular mechanisms proposed in this review do not fully explain the nature of K-Mg interaction, particularly in the context of translocation from roots to shoots.Future research should focus on clarification o f t he p hysiological a nd m olecular mechanisms underlying the antagonistic and synergistic interactions between K and Mg, during their transport within plants and in their physiological functions, with the ultimate goal of increasing nutrientuse efficiency in crops.

CRediT authorship contribution statement

Kailiu Xie:Visualization, Writing - original draft.Ismail Cakmak:Writing—review & editing.Shiyu Wang:Visualization.Fusuo Zhang:Conceptualization.Shiwei Guo:Conceptualization, Funding acquisition, Project Administration, Writing—review & editing.

Declaration of competing interest

The authors declare that they have no known competing finan-cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (2016YFD0200901 and 2016YFD0200305), the Fundamental Research Funds for the Central Universities (KJQN201514 and KYZ201625).We specially appreciate Prof.Chunjian Li of China Agricultural University for helpful comments that helped us to greatly improve the manuscript.