Determination of Unidirectional Sodium Fluxes in Roots of Intact Sunflower Seedlings

Abstract

The method of compartmental analysis was applied to study sodium fluxes in roots of intact seedlings of Helianthus annuus L. By measuring sodium uptake and transport to the shoots of the seedlings in parallel experiments, transport of tracer sodium to shoots and net accumulation of Na⁺ in the roots during the flux measurements was accounted for. The steady-state sodium fluxes in the intact sunflower roots were similar in size to those in excised roots but in general they were somewhat higher. This indicates more metabolic activity in the intact tissues. Using whole plants it is possible to study the response of ion fluxes in roots to ecophysiological stimuli received by the shoots, and in the present experiments the effect of continuous light versus long-day growth conditions was investigated. Potassium, when continually present, depressed all fluxes and the cytoplasmic content of sodium but tended to increase the vacuolar sodium content, in particular when this was related to the cytoplasmic content. When added to sodium-loaded roots, potassium stimulated the plasmalemma sodium efflux but slightly, suggesting a low efficiency of K⁺-Na⁺ exchange across the plasmalemma in intact as well as excised sunflower roots. Subsequently, however, potassium induced a transient decrease in the ²²Na efflux that was followed by oscillations in tracer efflux. These changes were attributed to potassium-induced transfer of sodium to vacuoles. Moreover, the oscillations seem to indicate the operation of negative feedback control of sodium fluxes.