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 investigated, P(os) values were in the range 200-1000 &mgr;m s(-1). The change in membrane surface area induced by an osmotic gradient was smaller than 2-6%. The vacuolar P(os) values for red beet and onion were reduced by 1 mM HgCl(2), to 14% and 30% of the control values, respectively, but were partially restored to 51% and 76% by 5 mM beta-mercaptoethanol. These results suggest that aquaporins were present in all the vacuoles tested. In HgCl(2)-treated onion vacuoles, the reduced P(os) (56 &mgr;m s(-1)) had a low activation energy (approx. 6 kJ mol(-1)), indicating that water permeation was still occurring mainly via aquaporins, and that the water permeability of the lipid part of the vacuolar membrane is probably very low.