ENERGIZATION OF POTASSIUM UPTAKE IN ARABIDOPSIS-THALIANA

Plant roots accumulate K+ from micromolar external concentrations. How ever, the absence of a firm determination of the trans-plasma-membrane electrochemical gradient for K+ in these conditions has precluded an assessment of whether K+-accumulation requires energization in additio n to the driving force provided by the inside-negative membrane electr ical potential (E(m)). To address this question unequivocally, we meas ured E(m), and the cytosolic and external K+-activities in root cells of Arabidopsis thaliana (L.) Heynh. cv. Columbia in conditions in whic h net K+-accumulation occurs at low external K+ (10 muM). In these con ditions, net K+-uptake was about 0.1 mumol . (g FW)-1 . h-1, E. varied between - 153 and - 129 mV and the cytosolic K+-activity, determined with K+-selective electrodes, was 83+/-4 mM. These values yield an out wardly-directed driving force on K+ of at least 6.5 kJ . mol-1. Only i f external potassium is raised to the region of 1 mM does E(m) become sufficient to drive net K+-accumulation. It is therefore concluded tha t at micromolar external K+-activities which prevail in most soils, K-uptake cannot be solely energized by E(m) - as exemplified by a chann el-mediated mechanism. The nature of the energization mechanism is dis cussed in relation to processes operating in fungal and algal cells.