pH-regulated leaf cell expansion in droughted plants is abscisic acid dependent

Elongation rates of barley (Hordeum vulgare L. cv Hanna) leaves decreased w ith decreasing soil water content, whereas the pH of xylem sap increased fr om 5.9 to 6.9 over 6 d as the soil dried. The reduction in leaf-elongation rate (LER) was correlated with the increase in sap pH. Artificial sap buffe red to different pH values was fed via the subcrown internode to derooted s eedlings. Although leaves elongated at in planta rates when fed artificial sap at a well-watered pH of 6.0, LER declined with increasing sap pH. This effect persisted in the light and in the dark. pH had no effect on the rela tive water content or the bulk abscisic acid (ABA) concentration of the gro wing zone of these leaves. LERs of the ABA-deficient mutant Az34 were unifo rmly high over the pH range tested, whereas those of its isogenic wild-type cultivar Steptoe were reduced as the artificial sap pH was increased from 6.0 to 7.0. However, supplying a well-watered concentration of AEA (3 x 10( -8) M) in the artificial xylem sap restored the pH response of the Az34 mut ant. The results suggest that increased xylem sap pH acts as a drought sign al to reduce LER via an ABA-dependent mechanism.