© 1992 Heron Publishing—Victoria, Canada
Influence of soil water on the physiological and morphological components of plant water balance in Populus trichocarpa, Populus deltoides and their F1 hybrids
J. H. Braatne, T. M. Hinckley and R. F. Stettler
College of Forest Resources, University of Washington, Seattle, WA 98195, USA / Received March 16, 1992
Summary
Patterns of leaf growth, transpiration and whole-plant water balance in Populus trichocarpa, P. deltoides and their F1 hybrids were studied during a soil drying cycle. Plant responses were analyzed during three distinct stages of dehydration.
In stage I, the transpiration rate of drought-stressed plants remained constant and equal to that of well-watered plants even
though soil water content declined by more than 40%. Stage II began as soil and plant water deficits induced stomatal closure.
When soil water was expressed as a fraction of transpirable soil water, the transition from stage I to stage II occurred at
soil water fractions of 0.35, 0.45 and 0.60 for P. trichocarpa, P. deltoides and their F1 hybrids, respectively. Reductions in leaf growth coincided with the shift from stage I to stage II. As soil water declined
further, decreases in relative transpiration and whole-plant leaf area were significantly greater in parental species than
in F1 hybrids. Inherent feedbacks controlling stomatal water loss and the maintenance and growth of leaf tissue appeared to differ
between F1 and parental genotypes in a pattern characteristic of an overdominant mode of inheritance.
Stage III began once the ability of stomata to compensate for water loss had been exhausted. Substantial differences were
found in plant survival during stage III, with F1 hybrids surviving longer than parental species. Survival was more strongly correlated with the hydraulic conductivity of
xylem tissues than with the dehydration tolerance of leaf tissues. Collectively, these responses suggest that F1 hybrids were more drought resistant than either parental species and highlight the importance of whole-plant studies of functional
relationships between plant growth, water balance and hydraulic conductivity.
