CYTOSOLIC PH GRADIENTS ASSOCIATED WITH TIP GROWTH

The presence of a cytosolic pH gradient and its relation to polar tip growth was investigated in rhizoid cells of Pelvetia embryos with the use of pH-sensitive microelectrodes and by ratio imaging. Growing rhiz oid cells generated a longitudinal pH gradient in which the apical cyt osol was 0.3 to 0.5 units more acidic than the cytosol at the base of the cell. Treatment with a membrane-permeant weak acid, propionic acid , dissipated the cytosolic pH gradient and inhibited growth. The magni tude of the pH gradient correlated well with the rate of tip elongatio n. The pH gradient spatially superimposed on the cytosolic calcium gra dient, and inhibition of calcium fluxes by treatment with lanthanum ab olished the pH gradient and inhibited growth.