Water Relations of Populus Clones

Stomatal aperture and water balance in the field of eight Populus clones varying in growth rate were closely related to environmental factors and clonal differences were clearly expressed. Leaf water potential (ψ l ) was influenced by solar radiation, leaf conductance, evaporative demand, and soil moisture content. The effects of soil moisture on ψ l were greatly modified by atmospheric conditions and stomatal conductance. Several slow—growing clones exhibited extended periods of ψ l below that of rapidly growing clones, despite high evaporative demand and the much greater transpiring surfaces of the fast—growing clones. Stomata of all clones responded to changes in light intensity and vapor pressure gradient (VPG). Pronounced stomatal sensitivity to VPG of two rapidly growing clones of common parentage, and the resultant capacity of these clones to moderate water deficits under high evaporative demand, were associated with drought resistance in one of the parents. Seasonal maximum leaf conductance was positively related to growth in several clones, suggesting that rapidly growing clones possess the capacity to carry on higher rates of gas exchange under favorable conditions. Analysis of changes in ψ l with changes in transpirational flux density (TFD) showed that for four clones, ψ l change per unit change in TFD decreased as TFD increased, indicating plant adaptation for prevention of damaging ψ l even at high TFD. More rapidly growing clones exhibited a larger initial role of decline in ψ l with TFD, but reduced the rate of decline more than slow—growing clones as TFD increased.