Effects of Sodium and Potassium on Starch Synthesis in Leaves

Abstract

The growth rate of plants of both spinach (Spinacia oleracea) and sugarbeet (Beta vulgaris) was decreased with low levels of K+ in nutrient solutions. Addition of Na+ at low levels of K+ resulted in growth rates at least as great as those observed with adequate K+. By comparison, bean plants (Phaseolus vulgaris) had only low growth rates when part of the K+ was replaced with Na+. The concentration of starch in spinach leaves was not affected by the K+/Na+ supply, whereas sugarbeet leaves from plants grown on adequate levels of K+ contained nine times more starch than leaves grown on high Na+/low K+ culture solutions. K+, but not Na+, stimulated by about 100% the activity of starch-granule-bound ADPglucose starch synthase from sugarbeet, bean and Atriplex nummularia leaves but not from spinach leaves. Neither ion caused a marked stimulation of the activity of soluble ADPglucose starch synthase, phosphorylase or other enzymes likely to be associated with carbohydrate metabolism. Slices of bean or sugarbeet leaves incubated in 12.5 mM K2SO4, solutions in the light for 8 h contained higher amounts of starch, and slices of bean leaves contained lower amounts of soluble sugars, than similar slices in Na2SO4. There was a considerable efflux of sucrose but not of reducing sugars in the presence of K+ or Na+ ; this efflux was several times higher than in the presence of Ca2+. The results suggest that K+ is essential for the conversion of sugars to starch in some plant species and that ADPglucose starch synthase is the enzyme involved in starch synthesis in the leaves of these species.