MECHANISMS OF DROUGHT RESPONSE IN THUJA-OCCIDENTALIS L .2. POST-CONDITIONING WATER-STRESS AND STRESS RELIEF

We examined the extent of osmotic adjustment and the changes in relati ve water content (RWC) and transpiration rate (i.e., relative stomatal function) that occur in water deficit-conditioned 6-year-old Thuja oc cidentalis L. (eastern white cedar) trees in response to a severe drou ght. Trees conditioned by successive cycles of mild or moderate nonlet hal water stress (conditioning) and nonconditioned trees were exposed to drought (i.e., -2.0 MPa predawn water potential) to determine if wa ter deficit conditioning enhanced tolerance to further drought stress. Following drought, all trees were well watered for 11 days to evaluat e how quickly osmotic potential, RWC and transpiration rate returned t o preconditioning values. Both nonconditioned trees and mildly conditi oned trees exhibited similar responses to drought, whereas moderately conditioned trees maintained higher water potentials and transpiration rates were 38% lower. Both conditioned and nonconditioned trees exhib ited a similar degree of osmotic adjustment (-0.39 MPa) in response to drought relative to the well-watered control trees. The well-watered control trees, nonconditioned trees and mildly conditioned trees had s imilar leaf RWCs that were about 3% lower than those of the moderately conditioned trees. Following the 11-day stress relief, there were no significant differences in osmotic potential between the well-watered control trees and any of the drought-treated trees. Daily transpiratio n rates and water potential integrals (WPI) of all drought-treated tre es approached those of the well-watered control trees during the stres s relief period. However, the relationship between cumulative transpir ation and WPI showed that previous exposure to drought stress reduced transpiration rates. Leaf RWC of the moderately conditioned trees rema ined slightly higher than that of the nonconditioned and mildly condit ioned trees.