Abstract
Effects of water stress and exogenous abscisic acid (ABA) treatments on thermotolerance of photosynthetic apparatus (PA) in cucumber (Cucumis sativus L.) leaves were investigated. Evaluation of thermotolerance was based on the heat sensitivity of photosystem II (PSII) based on chlorophyll fluorescence emission. Plants were water-stressed by either withholding water to intact plants or placing excised shoots in an open atmosphere in a glasshouse. The extent of water stress was monitored by measuring the water potential of leaves. At intervals during water stress treatment, the ratio of variable to maximum fluorescence (Fv/Fm) of leaves was determined. Immediately after that, the leaves were dipped in a water bath held at 45°C for 10 min and Fv/Fm values were determined again. Percentages of Fv/Fm values remaining after the heat treatment were used as a measure of thermotolerance of PA. With intact plants, thermotolerance of PA increased significantly when the leaf water potential was decreased to -1.0 MPa; it attained maximum at -1.5 to -2.0 MPa. However, the thermotolerance decreased when water potential was further lowered to -4.0 MPa. Leaves of excised shoots showed similar changes in thermotolerance of PA in response to the decrease in water potential, except that the maximum tolerance was obtained at -0.8 MPa. When intact plants were reirrigated after 3 days of water stress treatment, the thermotolerance of PA increased temporarily during the first 2 hr, and then decreased sharply within 24 hr to the level of the irrigated control plant leaves. Exogenous ABA applied to leaves of irrigated plants caused an increase in thermotolerance of PA depending on its concentrations. Leaves tre-ated with 1.00 mM ABA showed nearly complete tolerance of PA within a day; this high tolerance level was maintained for at least 6 days. Leaves treated with 0.01 to 0.25 mM ABA reached a peak tolerance after a day and then lost much of the ABA-induced thermotolerance after 3 days. The results indicate that the enhanced thermotolerance of PA in water-stressed cucumber leaves was probably mediated through a water-stress-induced increase in endogenous levels of ABA.
