Recently it has been hypothesized that light-induced polarity in leaves of the submerged angiosperms Potamogeton and Elodea results in part from a pH-dependent change in the permeability of the plasma membrane for protons. It is assumed that the proton permeability increases at high pH. In this paper we studied the effects on polarity of pH buffers, applied to just one side of the leaf. An experimental set up was used in which the solutions in contact with either side of the leaf surface were separated. The above hypothesis was partly confirmed, in that acidification, normally observed at the morphological lower side, was strongly suppressed by the application of a low-pH buffer at the upper side. Apparently, the low pH at the upper epidermis reduced the proton permeability, and as a result the proton pumps at the lower side were inactivated by a lack of substrate or by an effect on cytoplasmic pH. However, increasing the apoplastic pH at the lower side indicated that the proton permeability of the plasma membrane of these ceils did not change significantly. Hence both cell types seem to behave differently concerning the pH dependent proton permeability of the plasma membrane. This may explain why in the polar leaves of the angiosperms acidification always occurs at the lower and alkalinization always at the upper side. Key words: plasma membrane, proton permeability, pH polarity, Potamogeton, Elodea.
