H Cotransports in Corn Roots as Related to the Surface pH Shift Induced by Active H Excretion.

The surface pH shift induced by active H(+) excretion in corn (Zea mays L.) roots was estimated using acetic acid influx as a pH probe (H Sentenac, C Grignon 1987 Plant Physiol 84: 1367-1372). At constant bulk pH, buffering the medium strongly reduced the magnitude of the surface pH shift. This was used to study the effect of surface pH shift on H(+) cotransports. In the absence of buffers, the surface pH shift increased with the bulk pH. Buffers decreased (32)Pi influx and this effect was stronger at pH 7.2 than at pH 5.8, and stronger in the absence than in the presence of an inhibitor of the proton pump (vanadate). Buffers exerted a similar depressive and pH-dependent effect on net NO(3) (-) uptake. They hyperpolarized the cell membrane, and stimulated (86)Rb(+) influx, K(+):H(+) net exchange, and malate accumulation. These results are consistent with the hypothesis that H(+) accumulation at the cell surface is effective in driving H(+) reentry. We concluded that the surface pH shift due to proton pump activity is involved in the energetic coupling of H(+) cotransports.