Above- and below-ground environmental influences on leaf conductance ofCeanothus thyrsiflorus growing in a chaparral environment: drought response and the role of abscisic acid.

Small shrubs ofCeanothus thyrsiflorus were grown in 19-1 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, ABAxyl, 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, ABAxyl 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 ABAxyl, ABA appeared to increase significantly only after a threshold of approximately -1.5 MPa was exceeded. At less negative water potentials, large variation in ABAxyl in the 50-1000 nmol/l range occurred for all water-potential values, because ABAxyl remains relatively constant over diurnal courses as water potentials decrease and then recover. When the water potential became more negative than -1.5 MPa, ABAxyl concentrations occurred between approximately 500 and 10 000 nmol/l and even greater in isolated cases. An approximately linear relationship is recognizable between ABAxyl and momentary water potential in this range because in plants under drought conditions, ABAxyl increases during the course of the day as water potential decreases. Increases in ABAxyl in the high concentration range were associated with relatively minor additional restrictions in gas exchange, but they might contribute to improved water use efficiency and explain diurnal changes in the potential for stomatal opening that have been observed in Mediterranean sclerophyllous species. When we examined long-term seasonal change in the response of irrigated plants, changes in average daily temperature greater than 10°C occurred (also associated with shifts in relative humidity and radiation input), which apparently led to small changes in predawn water potential in the -0.1 to -0.7 MPa range. Increases in ABAxyl occurred that were in turn negatively correlated with daily maximum leaf conductance. Thus, chaparral shrubs under non-drought conditions seem to sense even small changes in environmental conditions, in our opinion most probably due to initial drying of the uppermost soil and synthesis of ABA in the shallow roots. The results support the hypothesis that information of photosynthesis rate and predawn water potential may be used as primary variables to predict canopy conductance of Mediterranean sclerophyll shrub vegetation.