Amelioration of Chilling-Induced Water Stress by Abscisic Acid-Induced Changes in Root Hydraulic Conductance

Abstract

ABSTRACr Pretreatment of soybean (Glycine max L. var Ransom) root systems with abscisic acid (ABA) ameliorates the deleterious effect of low temperatures on root hydraulic conductance. ABA treatment of root systems subsequently chilled to 10°C with shoots at 25°C resulted in higher leaf water potentials and lower stomatal resistances. If the root systems are left at 25C, ABA causes stomatal closure. Membrane alterations are suggested as a mechanism for the ABA action in plant response to chilling stress. Abscisic acid has been implicated in plant response to environmental stress. ABA accumulates in plants during water stress (8, 21) and closes stomata when applied to the transpiration stream (15, 16, 19). ABA also plays a role in plant response to low temperature stress (2, 17, 18). Treatment ofcucumber cotyledons with ABA reduced injury during subsequent exposure to 4°C (18). Treatment of potato stem cultures with ABA increased the freezing tolerance of the tissue by 3 to 5°C (2). Despite increased interest in ABA, the mechanism by which ABA acts is unknown. Several investigations have shown significant effects of ABA on water and ion transport in tissue. Treatments of carrot root tissue or whole sunflower root systems with ABA have been reported to increase the hydraulic conductivity of the tissues (6, 7). More recent experiments with Glycine max L. Merr. and Phaseolus vuilgaris L. have documented a decrease in the hydraulic conductivity of root systems at 25°C at high flow rates (4, 12). Of considerable interest is the apparent interaction of temperature and ABA on water flux through soybean root systems (12). The hydraulic conductance of non-ABA-treated root systems subjected to a 5 bar pressure gradient decreased slightly with decreased temperature to 15°C. Below 15°C, however, L2 decreased precipitously. ABA treatment (10 Mm) decreased L at 25°C and eliminated the dramatic change in L at the lower temperatures. When presented as an Arrhenius plot, the natural log of the flux from the ABA-treated root system decreased linearly with reciprocal temperature with a slope in between the two portions of the non-ABA-treated root system. These results suggest that, at low temperatures, L of the ABA-treated root system was greater than the non-ABA-treated root system. The experiments presented here further test the hypothesis 'Published as paper No. 13328 of scientific journal series of the Minnesota Experiment Station on research conducted under Minnesota Experiment Station Project 0302-4821-82. 2Abbreviation: L, hydraulic conductance. 81 that treatment of root systems with ABA reduces the decrease in the hydraulic conductance of root systems at low temperatures. MATERIALS AND METHODS Soybean plants (Glycine max L. var Ransom) were grown in environmental growth chambers under a 12-h photoperiod at 25C. The photosynthetic photon flux density at plant height was 600 gE m-2 s-'. Seeds were sown in plastic pots (7.5 x 30 cm) containing a 1:1 mixture of sand and Turface. The bottom of the pot was cut off and replaced with a single hole stopper. Plants were watered daily with alternate applications of half-strength Hoagland solution and distilled H20. Plants were 21 to 25 d old with one fully expanded trifoliolate leaf at the time of the experiment. A stock solution containing 0.1 M ABA (Sigma ± cis-trans) in 95% ethanol was prepared and stored in the dark at 4°C. Treatment solutions were prepared with distilled H20 the day before the experiment and kept in the dark at 22°C. One and one-half hours prior to the beginning of the photoperiod, the holes in the pots were sealed and 100 ml of 50 AM ABA or 100 ml of distilled H20 (with equivalent amounts of ethanol) were added. After 1 h of treatment, the drainage holes were uncovered and the pots allowed to drain for 5 min. The holes of eight treated and eight nontreated plants were resealed and the pots randomly placed into a water bath cooled to 10°C. Thermocouples placed in the center of selected pots indicated that the roots reached 10°C in 15 min and that there was less than 0.5C variation in temperature among individual pots. One-half hour after cooling, the lights were turned on. The remaining ABA-treated and non-treated plants were left with both roots and shoots at 25°C. Stomatal resistances of the upper and lower leaf surface were measured every 2 h with a diffusion porometer (Delta Instruments). Because of a limitation in the number of plants which would fit into the water bath, plant wiltedness, a nondestructive technique, was used to measure plant water status. Plant wilted-ness was measured every 2 h by placing a protractor behind the stem of the plant. The angle of the apical internode relative to the rest of the stem was recorded. Zero degress meant a completely turgid plant, whereas 1800 meant a completely wilted plant. Four hours after the beginning of the photoperiod, the water potential of representative plants was measured with a pressure bomb. The entire experiment was repeated twice with the chilled root and warm root treatments each done separately two additional times. The data are presented as the means and standard error of four measurements for a typical experiment. Student's t test was used to establish the significance of the differences between means.