The function of tonoplast ATPase in intact vacuoles of red beet is governed by direct and indirect ion effects.

The pH gradient and the electric potential across the tonoplast in mechanically isolated beetroot vacuoles has been studied by following the uptake of [(14)C]methylamine and [(14)C]triphenyl-methylphosphoniumchloride. In response to Mg-ATP, the vacuolar interior is acidified by 0.8 units. This strong acidification is accompanied by a slight hyperpolarization of the membrane potential, which is probably caused by a proton diffusion potential. In preparations where only a small acidification (0.4 units) occurred, the membrane potential was depolarized by the addition of Mg-ATP. Different monovalent cations and anions were tested concerning their effect on the pH gradient and ATPase activity in proton-conducting tonoplasts. Chloride stimulation and NO 3 (-) inhibition were clearly present. The observed decline of the pH gradient upon the addition of Na(+) salts is probably caused by an Na(+)/H(+) antiport system.