Gas Exchange, Chlorophyll and Growth Response of Three Orange Genotypes (Citrus sinensis [L.] Osbeck) to Abscisic Acid under Progressive Water Deficit

A pot experiment was carried out to determine the changes in gas exchange, growth and chlorophyll contents of Washington Navel , Red Blood and Shamouti oranges (Citrus sinensis [L.] Osbeck) treated with 0.1 and 1 mM abscisic acid (ABA) under water deficit conditions in a partially controlled greenhouse for 150 days. The drought treatments were imposed by the depletion of 25, 50 and 75% of the available water (DAW). The results have shown the decline in growth of oranges grown under severe water deficit stress, with the reduction in leaf CO2 assimilation rate (Pn), transpiration (T) and chlorophyll content. The growth and gas exchange were most impacted by imposing the irrigation at 75%DAW. Considerable genotypic variation in drought tolerance was observed. Based on gas exchange response, Washington Navel was considered a drought sensitive cultivar, while Shamouti was considered drought tolerant under the conditions of this study. Red Blood was also affected but to a lesser extent. Exogenous application of ABA had no effect on growth, chlorophyll content and gas exchange parameters under well-watered conditions (25%DAW). Application of ABA at 1mM supported a considerably lower leaf Pn, T and chlorophyll, in addition to taller plants for Washington Navel , especially under severe drought stress