ABA XYLEM CONCENTRATIONS DETERMINE MAXIMUM DAILY LEAF CONDUCTANCE OF FIELD-GROWN VITIS-VINIFERA L PLANTS

Differences in maximum leaf conductance in grapevine plants growing in soils with contrasting water availabilities during mid-summer in Port ugal could be accounted for by differences in the concentration of ABA in xylem sap, This conclusion is reinforced by the observation that t he relationship between leaf conductance and endogenous ABA concentrat ion can be mimicked by the application of exogenous ABA to leaves deta ched from irrigated plants, During the day, leaf conductance decreased after a morning peak, even when the leaves remained in a constant env ironment at a moderate temperature and leaf-to-air vapour pressure dif ference, This decline in leaf conductance was not a consequence of an increase in the xylem ABA concentration or the rate of delivery of thi s compound by the transpiratory stream, The afternoon depression in le af conductance was associated with an apparent limitation in stomatal opening potential, which persisted even when detached leaves were fed with water and rehydrated, The reason for this inhibition has still to be identified.