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 impo rtant role in controlling stomatal aperture of field-grown Prunus pers ica 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). Stomatal c onductance of non-irrigated trees had a morning maximum and declined a fterwards. The changes in g during the day, rather than resulting from variations in the concentrations of ABA in the xylem sap or the deliv ery rate of this compound to the leaves, were associated with changes in the relationship between g and xylem ABA. The stomata of water-stre ssed trees opened during the first hours of the day, despite the occur rence of a high concentration of ABA in the xylem sap. However, stomat al 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 aftern oon. 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 differen ces in g observed between field-grown peach trees growing with differe nt soil water availabilities. Several possible explanations for the ap parent low stomatal sensitivity to xylem ABA in the morning, are discu ssed, such as high leaf water potential, low temperature and high cyto kinin activity.