Involvement of Abscisic Acid in Regulating Water Status in Phaseolus vulgaris L. during Chilling

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Abstract
During the first hours of chilling, bean (Phaseolus vulgaris L., cv Mondragone) seedlings suffer severe water stress and wilt without any significant increase in leaf abscisic acid (ABA) content (P. Vernieri, A. Pardossi, F. Tognoni [1991] Aust J Plant Physiol 18: 25-35). Plants regain turgor after 30 to 40 h. We hypothesized that inability to rapidly synthesize ABA at low temperatures contributes to chilling-induced water stress and that turgor recovery after 30 to 40 h is mediated by changes in endogenous ABA content. Entire bean seedlings were subjected to long-term (up to 6 d) chilling (3°C, 0.2-0.4 kPa vapor pressure deficit, 100 μmol·m−2·s−1 photosynthetic photon flux density, continuous fluorescent light). During the first 24 h, stomata remained open, and plants rapidly wilted as leaf transpiration exceeded root water absorption. During this phase, ABA did not accumulate in leaves or in roots. After 24 h, ABA content increased in both tissues, leaf diffusion resistance increased, and plants rehydrated and regained turgor. No osmotic adjustment was associated with turgor recovery. Following turgor recovery, stomata remained closed, and ABA levels in both roots and leaves were elevated compared with controls. The application of ABA (0.1 mm) to the root system of the plants throughout exposure to 3°C prevented the chilling-induced water stress. Excised leaves fed 0.1 mm ABA via the transpiration stream had greater leaf diffusion resistance at 20 and 3°C compared with non-ABA fed controls, but the amount of ABA needed to elicit a given degree of stomatal closure was higher at 3°C compared with 20°C. These findings suggest that endogenous ABA may play a role in ameliorating plant water status during chilling.