MECHANISMS OF DROUGHT RESPONSE IN THUJA-OCCIDENTALIS L .1. WATER-STRESS CONDITIONING AND OSMOTIC ADJUSTMENT

We investigated the extent of osmotic adjustment and changes in transp iration rate that occur in response to repeated cycles of water defici t stress in 6-year-old Thuja occidentalis L. (eastern white cedar) tre es. Groups of trees were water-stress conditioned by repeated exposure to predetermined thresholds of nonlethal water stress by withholding water until the predawn water potential fell to -0.9 (mild conditionin g) or -1.4 MPa (moderate conditioning). Both the mild and moderate con ditioning treatments resulted in a decrease in osmotic potential of 0. 08 to 0.20 MPa and 0.11 to 0.28 MPa, respectively, relative to the wel l-watered controls. Mildly and moderately conditioned trees exhibited an approximately 35 and 50% reduction in cumulative transpiration, res pectively, following at least two stress cycles. Transpiration rates o f conditioned trees remained below those of the well-watered controls even when water potential integrals were similar We conclude that the decrease in transpiration rate was more important than osmotic adjustm ent as a mechanism of response to repeated water stress.