The role of ABA and the transpiration stream in the regulation of the osmotic water permeability of leaf cells

The transpiration stream that passes through a plant may follow an apoplast ic route, with low resistance to flow, or a cell-to-cel I route, in which c ellular membranes impede water flow. However, passage of water through memb ranes can be facilitated by aquaporins thereby decreasing resistance. We in vestigated the relationship between transpiration, which can be down-regula ted by abscisic acid (ABA)or by high humidity, and the osmotic water permea bility (P-os) of protoplasts. By using leaf protoplasts of wild-type (wt) A rabidopsis thaliana plants and of mutants that are low in ABA (aba1) or ins ensitive to ABA (abi1 and abi2), we found that protoplasts from aba1 and ab i mutants have very low P-os values compared with those from wt plants when the plants are grown at 45% relative humidity. High values of P-os were fo und 3 h after the addition of ABA to the culture medium of aba1 plants; add ition of ABA to abi plants did not restore the P-os to wt levels. There was no such increase in P-os when excised leaves of aba1 plants were treated w ith ABA. When the transpiration stream was attenuated by growing the plants at 85% relative humidity, the P-os of protoplasts from all plants (wt and mutants) was higher. We suggest that attenuation of the transpiration strea m in whole plants is required for the up-regulation of the P-os of the memb ranes, and that this up-regulation, which does not require ABA, is mediated by the activation of aquaporins in the plasma membrane.