Potassium Substitution by Sodium in Plants

Soil salinity is an ever-increasing constraint to crop productivity worldwide especially in countries with irrigated agriculture. In contrast to all the soil reclamation strategies to decrease salt concentrations in root zone, the use of sodium (Na⁺) in plant nutrition may be an interesting tactic. The roles of potassium (K⁺) and Na⁺ in plant nutrition suggest that K⁺ is the only monovalent cation which is essential for most higher plants and is involved in three important functions, i.e., enzyme activation, charge balance and osmoregulation. Plants need a small amount but high concentration of K⁺ for specific functions in the cytoplasm and a major portion (～90%) of it is localized in vacuoles, where it acts as an osmoticum. Maintenance of osmotic potential in vacuoles, a nonspecific function of K⁺, can be achieved by other cations such as Na⁺. For decades an ample amount of work has been done on the substitution of K⁺ by Na⁺ in plant nutrition. In this regard, Na⁺ has the potential to replace K⁺ for some of its functions. In some plants, supplementation of Na⁺ in reduced amounts can eliminate K⁺ deficiency symptoms under limited K⁺ supply. Thus, the question of K⁺ substitution by Na⁺ in plant physiology is not only of academic interest but has considerable practical implications in relation to fertilizer management and plant growth in salt-affected environments. In this review, we discuss the possibilities of K⁺ substitution by Na⁺ under specific soil and environmental conditions.