Influence of abscisic acid on unidirectional fluxes and intracellular compartmentation of K⁺ and Na⁺ in excised barley root segments

Using compartmental analysis, unidirectional fluxes of K + and Na + and their intracellular compartmentation in excised barley ( Hordeum distichon L. cv. Kocher‐perle) root segments have been measured during a steady state in the presence or absence of ABA. Almost all flux rates were altered in the presence of external ABA, in particular the xylem transport R’ and the plasmalemma influx Ø oc (see below) were strongly inhibited in the steady state. At the same time the presence of ABA induced a strong increase in the vacuolar K + and Na + content Q v and a decrease in the cytoplasmic one (Q c ). Since the fluxes of an ion and its vacuolar or, in particular, cytoplasmic concentrations are interrelated, the ratios of fluxes originating from the cytoplasm and the cytoplasmic ion content were taken into account. On this basis ABA had the following effects: a) the secretion of K + or Na + to the xylem vessels was drastically inhibited; b) the plasmalemma K + or Na + efflux Ø co was moderately stimulated and c) the tonoplast influx Ø cv of Na + was stimulated, while the tonoplast influx of K + appeared to be unchanged (the decrease in Ø cv being due to the decreased cytoplasmic K + content). By a similar argument, also the apparent inhibition of the plasmalemma influx Ø oc of K + and Na + in the steady state merely is an indirect effect of ABA. It only reflects the strong ABA‐induced decrease in the xylem transport, that governs the magnitude of Ø oc in the steady state. The results are discussed with reference to possible regulatory functions of ABA. In this respect it is suggested that – in particular under conditions of stress – ABA might regulate cellular metabolic processes by changing the cytoplasmic K + level.