Potassium–sodium interactions in soil and plant under saline‐sodic conditions

About 7% of the total land around the globe is salt‐affected causing a great loss to agriculture. Salt stress refers to the excessive amount of soluble salts in the root zone which induce osmotic stress and ion toxicity in the growing plant. Among toxic ions, sodium (Na⁺) has the most adverse effects on plant growth by its detrimental influence on plant metabolism in inhibiting enzyme activities. An optimal potassium (K⁺) : Na⁺ ratio is vital to activate enzymatic reactions in the cytoplasm necessary for maintenance of plant growth and yield development. Although most soils have adequate amounts of K⁺, in many soils available K⁺ has become insufficient because of large amounts of K⁺ removal by high‐yielding crops. This problem is exacerbated under sodic or saline‐sodic soil conditions as a consequence of K⁺‐Na⁺ antagonism. Here K⁺ uptake by plants is severely affected by the presence of Na⁺ in the nutrient medium. Due to its similar physicochemical properties, Na⁺ competes with K⁺ in plant uptake specifically through high‐affinity potassium transporters (HKTs) and nonselective cation channels (NSCCs). Membrane depolarization caused by Na⁺ makes it difficult for K⁺ to be taken up by K⁺ inward‐rectifying channels (KIRs) and increases K⁺ leakage from the cell by activating potassium outward‐rectifying channels (KORs). Minimizing Na⁺ uptake and preventing K⁺ losses from the cell may help to maintain a K⁺ : Na⁺ ratio optimum for plant metabolism in the cytoplasm under salt‐stress conditions. It would seem a reasonable assumption therefore that an increase in the concentration of K⁺ in salt‐affected soils may support enhanced K⁺ uptake and reduce Na⁺ influx via HKTs and NCCSs. Although very useful information is available regarding K⁺‐Na⁺ homeostasis indicating their antagonistic effect in plants, current knowledge in applied research is still inadequate to recommend application of potassium fertilizers to alleviate Na⁺ stress in plants under sodic and saline‐sodic conditions. Nevertheless some encouraging results regarding alleviation of Na⁺ stress by potassium fertilization provide the motivation for conducting further studies to improve our understanding and perspectives for potassium fertilization in sodic and saline‐sodic environments.