EFFECTS OF ABSCISIC-ACID ON HIGH-TEMPERATURE STRESS INJURY IN CUCUMBER

Exogenous ABA application was evaluated as a countermeasure for high-temperature stress injury in cucumber (Cucumis sativus L.). Seedlings were sprayed with 0, 0.01, 0.1 or 1 mM ABA and then treated at 29-degrees-C (normal temperature) and 41-degrees-C (high temperature) for 3 hr. Leaf temperature, transpiration rate and chlorophyll fluorescence emission of ABA-treated and untreated leaves were determined during the temperature treatment. In addition, the residual effects of ABA and high-temperature treatments on stem elongation rate and the early fruit yield were assessed after the plants were transplanted into the field. Emission of chlorophyll fluorescence from the leaves was inhibited by high temperature and the inhibition was greatly amplified when the leaf was sprayed with 1 mM ABA before the high-temperature treatment. ABA treatment provoked a rise of leaf temperature in proportion to its concentration in both temperature treatments and at 1 mM ABA the leaf temperature attained 45-degrees-C during the high-temperature treatment. This marked rise of leaf temperature was probably due to a great inhibition of transpiration. In the same condition of vapor pressure deficit, transpiration rate was not affected by the high-temperature treatment without ABA, but declined with increasing ABA concentrations. The decrease was particularly great at 1 mM ABA under the high-temperature condition. Subsequently, necrosis, chlorosis, cupping of leaves and depression of stem elongation occurred on the plants treated with 1 mM ABA and high temperature. In the field experiment, early fruit yield was lower, although insignificantly, only when the plants had been treated with both 1 mM ABA and high temperature. It is concluded that leaf temperature critical to injury in cucumber is about 45-degrees-C and ABA is injurious to plant growth under high-temperature stress, i.e., ABA at high concentrations promotes high-temperature injury of cucumber leaves by rising leaf temperature above the critical temperature through decreasing transpiration. Chemical name used: abscisic acid (ABA).