Diurnal change in the relationship between stomatal conductance and abscisic acid in the xylem sap of field-grown peach trees

To evaluate whether abscisic acid (ABA) in the xylem sap plays an important role in controlling stomatal aperture of field-grown Prunus persica trees under drought conditions, stomatal conductance (g) and xylem ABA concentrations were monitored both in irrigated and non-irrigated trees, on two consecutive summer days (three times a day). Stomata1 conductance of non-irrigated trees had a morning maximum and declined afterwards. The changes in g during the day, rather than resulting from variations in the concentrations of ABA in the xylem sap or the delivery rate of this compound to the leaves, were associated with changes in the relationship between g and xylem ABA. The stomata of water-stressed trees opened during the first hours of the day, despite the occurrence of a high concentration of ABA in the xylem sap. However, stomatal responsiveness to ABA in the xylem was enhanced throughout the day. As a result, a tight inverse relationship between g and the logarithm of xylem ABA concentration was found both at midday and in the afternoon. A similar relationship between g and ABA was found when exogenous ABA was fed to leaves detached from well-watered trees. These results indicate that ABA derived from the xylem may account for the differences in g observed between field-grown peach trees growing with different soil water availabilities. Several possible explanations for the apparent low stomatal sensitivity to xylem ABA in the morning, are discussed, such as high leaf water potential, low temperature and high cytokinin activity.