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 scatteri ng by the vesicles of plasmalemma and tonoplast isolated from maize (Zea ma ys L.) roots and treated by osmotic pressure. In both membrane preparations , the rate of the process depended on the osmotic gradient and was describe d with the simple exponential function. The rate constants derived from the se functions were the following: the coefficient of water permeability in t he tonoplast (P = 165 +/- 7 mum/s) exceeded by an order of magnitude the co rresponding index for plasmalemma (11 +/- 2 mum/s). The presence of HgCl2 ( 1.6 nmol/mug membrane protein) decreased the tonoplast water permeability b y 80%. Microviscosity studies of the hydrocarbon zone in the isolated membr anes 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 no t depend on the state of the lipid phase and probably reflects the dissimil ar functional activity of plasmalemma and tonoplast aquaporins.