© 2001 Heron Publishing—Victoria, Canada
Shoot water relations of mature black spruce families displaying a genotype × environment interaction in growth rate. III.
Diurnal patterns as influenced by vapor pressure deficit and internal water status
John E. Major (1) and Kurt H. Johnsen (2)
1. Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, Fredericton, NB, Canada E3B 5P7 / 2. USDA Forest Service, Southern Research Station, PO Box 12254, 3041 Cornwallis Road, Research Triangle Park, NC 27709, USA / Received September 13, 2000
Summary
Pressure–volume curves were constructed and shoot water potentials measured for +20-year-old black spruce (Picea mariana (Mill.) BSP) trees from four full-sib families growing on a moist site and a dry site at the Petawawa Research Forest, Ontario,
to determine whether differences in diurnal water relations traits were related to productivity. To assess the basis for the
observed diurnal patterns, we analyzed effects of environmental and internal water stress variables on diurnal water relations
traits. Among the water relations traits examined, turgor pressure was the most sensitive, responding to site, family and
environmental variables and displaying the strongest diurnal responses to varying soil water availability and atmospheric
vapor pressure deficit (VPD). Overall, there was an 84% drop in turgor pressure with increasing VPD: turgor pressure fell
46% in response to the first 0.75 kPa increase in VPD, and 9.7% in response to a second 0.75 kPa increase in VPD. The families
differed in water relations responses to moderate water stress, but not in responses to minor or more extreme water stresses.
Thus, at a VPD of 0.5 kPa, there was an estimated 83% greater family difference in turgor pressure on the dry site compared
with the moist site. Soil and atmospheric water stress appeared to exert effects in tandem to elicit these responses (r2 = 0.728). A comparison of the mechanisms of response to water deficit indicated that osmotic adjustment was more important
than change in cell wall elasticity. We used a conceptual water relations model to illustrate the differences between tolerant
and intolerant families in their mechanisms of water stress response. We conclude that, because genetic responses to site
factors are dynamic, the integrated response over time contributes to the observed genetic × environmental interaction in
growth.
Keywords:
diurnal variation, genetic variation, modeling, Picea mariana, turgor pressure, water stress.
