Shoot-Dependent Regulation of Sodium and Potassium Fluxes in Roots of Whole Barley Seedlings

Unidirectional fluxes and the cytoplasmic and vacuolar contents of potassium and sodium in root cells of intact barley seedlings (Hordeum vulgare L., cv. Villa) were determined by use of compartmental analysis. In addition, the net vacuolar accumulation Jcv and the xylem transport øcx of K+ and Na+ were measured. Both of these data were needed for the evaluation of the efflux data. Fluxes and compartmental contents of K+ and Na+ were comparable to data obtained with excised roots. The effect of the shoot-to-root ratio—as varied by partial excision of the seedlings seminal roots—on the fluxes and contents was investigated. Higher shoot-to-root ratios induced an increase in xylem transport, in plasmalemma influx, and also in the cytoplasmic content of K+ and Na+. With potassium the plasmalemma efflux was almost unaltered while the tonoplast fluxes and vacuolar content were decreased (in presence of Na+). With sodium, on the other hand, the plasmalemma efflux and the tonoplast fluxes were also increased in the plants having one root and a high shoot-to-root ratio. These changes occurred even under conditions of low humidity, when transpiration was low and guttation occurred. The latter was also increased at the high shoot-to-root ratio. The observed changes could be due to a relieved feedback control of ion fluxes by the shoot and mediated in part by a relatively higher supply of photosynthates in the plants having one root In addition, hormonal signals were suggested to participate. In particular a possibly decreased level of cytokinins in the plants having only one root could contribute to the signal. The observed changes appear to be responses of the plant to an alteration that can occur under natural conditions when the root system is damaged.