Seasonal and Diurnal Variations in Abscisic Acid, Water Potential, and Diffusive Resistance in Leaves from Irrigated and Non-irrigated Peach Trees1

Abstract

Abstract Water potential, diffusive resistance, and abscisic acid (ABA) were measured at 10-12 day intervals from May to October in leaves from irrigated and non-irrigated peach (Prunus persica L. cv. Fay Elberta) trees, and measurements were taken at intervals from sunrise to sunset on September 8. Leaf water potential, before sunrise, was between −5 and −8 bars in irrigated trees during the entire season whether drip irrigated at 100% evapotranspiration (ET) or 50% ET. Non-irrigated trees showed a decrease in pre-dawn leaf water potential with time, following a pattern similar to that of decreasing soil moisture. Leaf water potential values taken during the afternoon were not associated with soil moisture and did not reflect the stressed condition of the trees. In non-irrigated trees stomatal resistance at mid-day increased rapidly after mid-summer as leaf water potential decreased. ABA concentration in leaves from irrigated trees ranged from 30 to 80 ng/g fresh wt during the entire season. In non-irrigated trees the ABA concentration increased sharply after mid-summer; this was associated with an increase in leaf diffusive resistance and a decrease in leaf water potential. Diurnal variations in leaf water potential were associated with changes in soil moisture, air temperature, relative humidity, and stomatal resistance. Leaf diffusive resistances were similar for all treatments until 1100 hr after which a notable increase occurred with increasing stress, ultimately leading to stomatal closure. ABA concentrations in leaves from irrigated and non-irrigated trees increased as leaf diffusive resistance increased; however in stressed trees, high levels of ABA in the morning were not associated with closed stomata.