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

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Abstract
The pH gradient and the electric potential across the tonoplast in mechanically isolated beetroot vacuoles has been studied by following the uptake of [14C]methylamine and [14C]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.