DROUGHT-INDUCED CLOSURE OF PHASEOLUS-VULGARIS L STOMATA PRECEDES LEAF WATER DEFICIT AND ANY INCREASE IN XYLEM ABA CONCENTRATION

Young seedlings of two cultivars of Phaseolus vulgaris L. (cv. Cacahuate-72 and Michoacan-12A3) were subjected to a soil drying treatment. Stomata started to close before any leaf water defict could be detected. Soil drying promoted a small degree of ABA accumulation in roots, but, at the time stomatal closure was initiated, at least in one cultivar, xylem ABA and bulk leaf ABA concentration were not enhanced. The mechanism of the stomatal response is discussed in terms of an as-yet unidentified regulator of stomatal behaviour, redistribution of existing ABA in plants, and a high sensitivity of stomata to small changes in ABA concentration.