Abscisic Acid Applied to Orchard Trees of Golden Delicious Apple to Control Water Stress

SummaryWhen 2×10-4 M(±) abscisic acid (ABA) with wetter was sprayed onto orchard trees of Golden Delicious apple it led to Stomatal closure, reduced transpiration and increased leaf water potential by 2 to 5 bar in both unirrigated and trickle-irrigated trees. Maximum effects of the spray occurred about 3 h after application and then its effectiveness declined and became negligible after 48 h. Five sprays were applied at about 10-day intervals in 1975 and 11 sprays at about weekly intervals in 1976 but, in spite of severe water stress in both seasons, neither fruit size nor quality was improved by ABA. Trickle irrigation, however, practically doubled fruit size and reduced russet in 1976. The failure of the ABA to benefit fruit size and quality was probably due to the transient effects of the sprays, and to depression of CO2 assimilation following stomatal closure.