Foliar and Ground Application of Abscisic Acid to Increase Cold Tolerance in Citrus

Freezes have led to tremendous economic losses to the fruit production industry in the United States either by direct death of trees and rotting of fruits or indirect losses caused by decreased fruit quality. In leaves, freezes form ice crystals that draw out the water inside the cells and make water unavailable to plant tissues as well as cause damage by disrupting cell membranes. Abscisic acid (ABA) is a hormone that is involved in the cold tolerance response of many plants. The application of exogenous ABA may alter ABA metabolism and induce cold tolerance. Two experiments were designed to test the efficiency of exogenous ABA applications in citrus: foliar and ground applications of the hormone. For the foliar experiment, our hypothesis was that the interaction between drought stress and foliar application of ABA to trees will result in an increase in cold tolerance since drought stress causes an accumulation of ABA inside the plant. For the ground application of ABA, our hypothesis was that it would optimize the efficiency of the hormone avoiding the use of high ABA concentrations. In the foliar experiment, four treatments were applied to 3-year-old grapefruit trees grown under greenhouse conditions. The treatments consisted in the combination of two irrigation regimes (drought stress, where trees were exposed to three weeks of continuous drought, and normal irrigation), and two concentrations of foliar ABA application (0 and 1 mM ABA). Significant differences in stem water potential were recorded between well-watered and drought-stressed trees, confirming differences in tree water status before the ABA application. Leaf samples were taken 3 days after the application and lethal freezing temperature was determined by exposing leaf samples to temperatures between 0°C and –12°C. Results showed that foliar application of ABA increased cold tolerance in grapefruit trees. Nevertheless, there was an interaction with tree water status since ABA application on drought-stressed trees increased cold tolerance more than on well-watered trees (2.3°C vs. 0.5°C). The ground experiment consisted of four concentrations of ABA: 0 ppm (control), 60 ppm, 125 ppm, and 250 ppm) applied to well-watered trees.  In this experiment, lower concentrations of ABA (60 ppm) resulted in an increase in cold tolerance (1.0°C vs. 0.34°C) when compared to higher concentrations. The ground application proves to be of greater efficiency due to there was no need to have the trees at drought stress status to observe significant differences unlike with the use of foliar applications.