Exogenous Applications of Abscisic Acid Increase Freeze Tolerance in Citrus Trees

The exogenous application of the plant hormone abscisic acid (ABA) may provide a new approach for preventing freeze damage to fruit tree crops. Exogenous ABA (1 mM) was foliarly applied to citrus trees under different irrigation regimes: well-irrigated trees or drought-stressed trees. Foliar applications of ABA increased the endogenous leaf ABA concentrations, and drought-stressed trees sprayed with ABA had a greater freeze tolerance than well-irrigated trees and drought-stressed trees that did not receive ABA. On the contrary, soil applications (zero, 1 nM ABA, 1 μM ABA, 1 mM ABA) did not increase the leaf ABA concentration and freeze tolerance.