Osmotic water permeability of vacuolar and plasma membranes isolated from maize roots

The method of stopped flow was used to follow the changes in light scattering by the vesicles of plasmalemma and tonoplast isolated from maize (Zea mays L.) roots and treated by osmotic pressure. In both membrane preparations, the rate of the process depended on the osmotic gradient and was described with the simple exponential function. The rate constants derived from these functions were the following: the coefficient of water permeability in the tonoplast (P = 165 +/- 7 mum/s) exceeded by an order of magnitude the corresponding index for plasmalemma (11 +/- 2 mum/s). The presence of HgCl2 (1.6 nmol/mug membrane protein) decreased the tonoplast water permeability by 80%. Microviscosity studies of the hydrocarbon zone in the isolated membranes by using a fluorescent diphenylhexatriene probe demonstrated that the two membranes do not differ in the phase state of their lipid bilayer. The authors conclude that the observed difference in water permeability does not depend on the state of the lipid phase and probably reflects the dissimilar functional activity of plasmalemma and tonoplast aquaporins.