Abstract
In plant cells, two distinct types of aquaporins are accumulated in plasma membrane and tonoplast. The finding has drawn our attention to how plants regulate water transport across each membrane. It is assumed that water permeability of tonoplast (Ps2) should be much higher than that of plasma membrane (Ps1), because cytoplasmic water potential could be stabilized by quick water-exchange with vacuole. However, the hypothesis has not been enough proved. Here, we propose a method for measuring Ps1 and Ps2 at the cellular level. Protoplasts were exposed to osmotic shock, and the time course of changes in the cell volume was observed. Swelling or shrinking rate of the protoplasts depends on both Ps1 and Ps2. We developed the numerical model which simulates these processes. Ps2 was directly obtained from swelling/shrinking rate of isolated vacuoles. Ps1 can be determined from the numerical solution of this model combined with the value of Ps2.
