THE CONTROL OF LEAF CONDUCTANCE OF WHITE LUPIN BY XYLEM ABA CONCENTRATION DECREASES WITH THE SEVERITY OF WATER DEFICITS

We studied the effects of drought on leaf conductance (g), leaf water relations and on the concentration of abscisic acid (ABA) in the xylem sap of Lupinus albus L. plants. Drought was imposed by withholding watering until predawn leaf water potential (psi(pd)) decreased below -0.7 MPa. The abaxial stomata of water-stressed plants closed to a greater extent than the adaxial stomata and the same trend was evident after feeding exogenous ABA. Under moderate water deficits (psi(pd) = -0.32 +/- 0.05 MPa) xylem ABA concentration was significantly higher in water-stressed plants (0.1 mmol m(-3)) than in the controls, and the observed depression in g could be mimicked when excised leaves of well-watered plants were fed with exogenous ABA at the same concentration found in the xylem sap of moderately stressed plants. As soil drying progressed xylem ABA concentration increased further, reaching 0.8-0.9 mmol m(-3) after 17 d without watering. However, feeding a similar concentration of exogenous ABA to leaves of well-watered plants failed to reproduce the depression in g observed in severely stressed plants. After rewatering, full recovery of g was not achieved immediately, in spite of xylem ABA concentration and psi(pd) returning to pre-stress levels. Our results indicate that the increase in xylem ABA concentration is quantitatively sufficient to account for the depression in leaf conductance observed in white lupin plants subjected to moderate water deficits, but the role of xylem ABA in controlling g apparently decreases when soil drying and plant water stress becomes severe.