EFFECTS OF XYLEM PH ON TRANSPIRATION FROM WILD-TYPE AND FLACCA TOMATOLEAVES - A VITAL ROLE FOR ABSCISIC-ACID IN PREVENTING EXCESSIVE WATER-LOSS EVEN FROM WELL-WATERED PLANTS

The pH of xylem sap from tomato (Lycopersicon esculentum) plants incre ased from pH 5.0 to 8.0 as the soil dried. Detached wild-type but not flacca leaves exhibited reduced transpiration rates when the artificia l xylem sap (AS) pH was increased. When a well-watered concentration o f abscisic acid (0.03 mu M) was provided in the AS, the wild-type tran spirational response to pH was restored to flacca leaves. Transpiratio n from flacca but not from wild-type leaves actually increased in some cases when the pH of the AS was increased from 6.75 to 7.75, demonstr ating an absolute requirement for abscisic acid in preventing stomatal opening and excessive water loss from plants growing in many differen t environments.