Tree water status and exogenous abscisic acid affect freeze tolerance in ‘valencia’ trees

Abstract

Abscisic acid (ABA) plays an important role in cold temperature responses of plants and has been reported to increase during cold acclimation. The application of exogenous ABA may alter ABA metabolism and induce cold tolerance. Drought stress also interacts with ABA metabolism and can increase cold acclimation. In this study, exogenous 1 mM ABA was sprayed during late winter onto 14-year old ‘Valencia’ sweet orange trees that were under three different irrigation regimes: 1. drought stress (no irrigation with a rain shield covering the soil during winter) 2. rain only (no irrigation) and 3. normal irrigation plus rain throughout the winter. Freezing tolerance estimated from electrolyte leakage from leaves at freezing temperature was evaluated one and three weeks after ABA spray applications. Overall, trees under the greatest drought stress had greater freezing tolerance than well irrigated trees. Exogenous ABA did not improve freeze tolerance one week after application but ABA sprays increased freeze tolerance in irrigated trees during the third week, although was not as effective in drought stressed trees. Thus, drought stress and exogenous ABA can both increase freezing tolerance in ‘Valencia’ orange trees although we observed no additive effect in this experiment.