Effects of interrupted K+ supply on growth and uptake of K+, Ca2+, Mg2+ and Na+ in spring wheat

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Abstract
Effects of interrupted K+ supply on different parameters of growth and mineral cation nutrition were evaluated for spring wheat (Triticum aestivum L. cv. Svenno). K+ (2.0 mM) was supplied to the plants during different periods in an otherwise complete nutrient solution. Shoot growth was reduced before root growth after interruption in K+ supply. Root structure was greatly affected by the length of the period in K+ ‐free nutrient solution. Root length was minimal, and root branching was maximal within a narrow range of K+ status of the roots. This range corresponded to cultivation for the last 1 to 3 days, of 11 in total, in K+ ‐free nutrient solution, or to continuous cultivation in solution containing 0.5 to 2 mM K+. In comparison, both higher and lower internal/external K+ concentrations had inhibitory effects on root branching. However, the differing root morphology probably had no significant influence on the magnitude of Ca2+, Mg2+ and Na+ uptake. Uptake of Ca2+ and especially Mg2+ significantly increased after K+ interruption, while Na+ uptake was constant in the roots and slowly increased in the shoots. The two divalent cations could replace K+ in the cells and maintain electroneutrality down to a certain minimal range of K+ concentrations. This range was significantly higher in the shoot [110 to 140 μmol (g fresh weight)−1] than in the root [20 to 30 μmol (g fresh weight)−1]. It is suggested that the critical K+ values are a measure of the minimal amount of K+ that must be present for physiological activity in the cells. At the critical levels, K+ (86Rb) influx and Ca2+ and Mg2+ concentrations were maximal. Below the critical K+ values, growth was reduced, and Ca2+ and Mg2+ could no longer substitute for K+ for electrostatic balance. In a short‐term experiment, the ability of Ca2+ to compete with K+ in maintaining electroneutrality in the cells was studied in wheat seedlings with different K+ status. The results indicate that K+, which was taken up actively and fastest at the external K+ concentration used (2.0 mM), partly determines the size of Ca2+ influx.