Osmotic water permeability of isolated vacuoles

We measured the osmotic water permeability (P-os) of vacuoles isolated from onion (Allium cepa L.), rape (Brassica napus L.), petunia (Petunia hybrida Hook.) and red beet (Beta vulgaris L.). For all the vacuolar types investi gated, P-os values were in the range 200-1000 mu m s(-1). The change in mem brane surface area induced by an osmotic gradient was smaller than 2-6%. Th e vacuolar P-os values for red beet and onion were reduced by 1 mM HgCl2, t o 14% and 30% of the control values, respectively, but were partially resto red to 51% and 76% by 5 mM beta-mercaptoethanol. These results suggest that aquaporins were present in all the vacuoles tested. In HgCl-treated onion vacuoles, the reduced P-os (56 mu m s(-1)) had a low activation energy (app rox. 6 kJ mol(-1)), indicating that water permeation was still occurring ma inly via aquaporins, and that the water permeability of the lipid part of t he vacuolar membrane is probably very low.