© 1995 Heron Publishing—Victoria, Canada
Mechanisms of drought response in Thuja occidentalis L. I. Water stress conditioning and osmotic adjustment
D. R. Edwards and M. A. Dixon
Department of Horticultural Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada / Received October 25, 1993
Summary
We investigated the extent of osmotic adjustment and changes in transpiration rate that occur in response to repeated cycles
of water deficit stress in 6-year-old Thuja occidentalis L. (eastern white cedar) trees. 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 conditioning) or –1.4
MPa (moderate conditioning).
Both the mild and moderate conditioning 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 well-watered controls. Mildly and moderately conditioned trees exhibited an approximately
35 and 50% reduction in cumulative transpiration, respectively, following at least two stress cycles. Transpiration rates
of 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 adjustment as a mechanism of response to
repeated water stress.
Keywords:
psychrometer, relative water content, transpiration, water potential integral.
