ABOVEGROUND AND BELOWGROUND ENVIRONMENTAL-INFLUENCES ON LEAF CONDUCTANCE OF CEANOTHUS-THYRSIFLORUS GROWING IN A CHAPARRAL ENVIRONMENT - DROUGHT RESPONSE AND THE ROLE OF ABSCISIC-ACID

Small shrubs of Ceanothus thyrsiflorus were grown in 19-1 pots irrigat ed under natural conditions in a chaparral region of Southern Californ ia and then subjected to soil drying. Characteristics of leaf gas exch ange, leaf water potential, and concentrations of the stress hormone a bscisic acid in the xylem sap, ABA(xyl), were determined at various st ages of drought. Diurnal changes in conductance were strongly correlat ed with leaf net photosynthesis rate, which provides an effective, int egrative 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 clos ure, restricting water loss and carbon dioxide uptake. When the moment ary water potential is related to ABA(xyl), ABA appeared to increase s ignificantly only after a threshold of approximately -1.5 MPa was exce eded. At less negative water potential, large variation in ABA(xyl) in the 50-1000 nmol/l range occurred for all water-potential values, bec ause ABA(xyl) remains relatively constant over diurnal courses as wate r potentials decrease and then recover. When the water potential becam e more negative than -1.5 MPa, ABA(xyl) concentrations occurred betwee n approximately 500 and 10 000 nmol/l and even greater in isolated cas es. An approximately linear relationship is recognizable between ABA(x yl) and momentary water potential in this range because rn plants unde r drought conditions, ABA(xyl) increases during the course of the day as water potential decreases. Increases in ABA(xyl) in the high concen tration range were associated with relatively minor additional restric tions 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 specie s. When we examined long-term seasonal change in the response of irrig ated plants, changes in average daily temperature greater than 10 degr ees C occurred (also associated with shifts in relative humidity and r adiation input), which apparently led to small changes in predawn wate r potential in the -0.1 to -0.7 MPa range. Increases in ABA(xyl) occur red that were in turn negatively correlated with daily maximum leaf co nductance. 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 synthesi s of ABA in the shallow roots. The results support the hypothesis that information on photosynthesis rate and predawn water potential may be used as primary variables to predict canopy conductance of Mediterran ean sclerophyll shrub vegetation.