Above- and below-ground environmental influences on leaf conductance of Ceanothus thyrsiflorus growing in a chaparral environment: drought response and the role of abscisic acid
1994
Tenhunen, J.D. (Bayreuth Univ. (Germany). Inst. fuer Terrestrische Oekosystemforschung. Lehrstuhl fuer Pflanzenoekologie) | Hanano, R. | Abril, M. | Weiler, E.W. | Hartung, W.
Small shrubs of Ceanothus thyrsiflorus were grown in 19-l pots irrigated under natural conditions in a chaparral region of Southern California and then subjected to soil drying. Characteristics of leaf gas exchange, leaf water potential, and concentrations of the stress hormone abscisic acid in the xylem sap, ABA(xyl), were determined at various stages of drought. Diurnal changes in conductance were strongly correlated with leaf net photosynthesis rate, which provides an effective, integrative predictor of above-ground climate effects on conductance. In drought conditions, ABA(xyl) concentration increased. Increases in the concentration range of 50-500 nmol/l appeared to induce stomatal closure, restricting water loss and carbon dioxide uptake. When the momentary water potential is related to ABA(xyl), ABA appeared to increase significantly only after a threshold of approximately -1.5 MPa was exceeded. At less negative water potentials, large variation in ABA(xyl) in the 50-1000 nmol/l range occurred for all water-potential values, because ABA(xyl) remains relatively constant over diurnal courses as water potentials decrease and then recover. When the water potential became more negative than -1.5 MPa, ABA(xyl) concentrations occurred between approximately 500 and 10000 nmol/l and even greater in isolated cases. An approximately linear relationship is recognizable between ABA(xyl) and momentary water potential in this range because in plants under drought conditions, ABA(xyl) increases during the course of the day as water potential decreases.
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