NH sub(4) super(+)-stimulated low-K super(+ uptake is associated with the induction of H) super(+) extrusion by the plasma membrane H super(+-ATPase in sorghum roots under K) super(+) deficiency

The effect of external inorganic nitrogen and K super(+ content on K) super(+) uptake from low-K super(+ solutions and plasma membrane (PM) H) super(+)-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4 mM K super(+ and inorganic nitrogen as NO) sub(3) super(-, NO) sub(3) super(-/NH) sub(4) super(+ or NH) sub(4) super(+ and then starved of K) super(+) for 24, 48 and 72 h. NH sub(4) super(+) in full nutrient solution significantly affected the uptake efficiency and accumulation of K super(+, and this effect was less pronounced at the high K) super(+) concentration. In contrast, the translocation rate of K super(+ to the shoot was not altered. Depletion assays showed that plants grown with NH) sub(4) super(+ more efficiently depleted the external K) super(+) and reached higher initial rates of low-K super(+ uptake than plants grown with NO) sub(3) super(-. One possible influence of K) super(+) content of shoot, but not of roots, on K super(+ uptake was evidenced. Enhanced K) super(+)-uptake capacity was correlated with the induction of H super(+ extrusion by PM H) super(+)-ATPase. In plants grown in high K super(+ solutions, the increase in the active H) super(+) gradient was associated with an increase of the PM H super(+-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2 mM K) super(+), only the initial rate of H super(+-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H) super(+)-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO sub(3) super(-)-grown plants. The results suggest that K super(+ homeostasis in NH) sub(4) super(+-grown sorghum plants may be regulated by a high capacity for K) super(+) uptake, which is dependent upon the H super(+-pumping activity of PM H) super(+)-ATPase.