Effect of Leaf Age on Potassium Efflux and Net Flux in Tomato Leaf Slices

Abstract

The effects on leaf age on K (86Rb) efflux, influx and net flux into lamina slices from leaf 7 on a tomato plant (Lycopersicon esculentum Mill.) were determined. The ontogenetic trend in K efflux was dependent on the external K concentration. At external KCI concentrations between 0.5 and 10.0 mM, K efflux rates increased during leaf elongation. Only a small increase in efflux occurred in mature leaves with increasing age. It is suggested that the tonoplast retains its structural integrity through the initial stages of leaf senescence. In fully expanded leaves, a zero net K flux (a balance between influx and efflux) was achieved at external KCI concentrations between 1.0 and 3.5 mM. The K content of lamina slices from leaves 5 and 13 remained constant when bathed in a solution containing 2 to 3 mM K. It is suggested that the decline in K concentration in mature tomato leaf tissue is due to a decline in leaf free space K concentrations below 1 to 3 mM which would result in a net efflux out of leaf cells.