Interactions between leaf water potential, stomatal conductance and abscisic acid content of orange trees submitted to drought stress

thirty-month-old 'pêra' orange trees grafted on 'rangpur' lemon trees grown in 100 l pots were submitted to water stress by the suspension of irrigation. co2 assimilation (a), transpiration (e) and stomatal conductance (gs) values declined from the seventh day of stress, although the leaf water potential at 6:00 a.m. (ypd) and at 2:00 p.m. (y2) began to decline from the fifth day of water deficiency. the co2 intercellular concentration (ci) of water-stressed plants increased from the seventh day, reaching a maximum concentration on the day of most severe stress. the carboxylation efficiency, as revealed by the ratio a/ci was low on this day and did not show the same values of non-stressed plants even after ten days of rewatering. after five days of rewatering only y pd and y2 were similar to control plants while a, e and gs were still different. when y2 decreases, there was a trend for increasing abscisic acid (aba) concentration in the leaves. similarly, stomatal conductance was found to decrease as a function of decreasing y2. aba accumulation and stomatal closure occurred when y2 was lower than -1.0 mpa. water stress in 'pera′ orange trees increased abscisic acid content with consequent stomatal closure and decreased y2 values.