The effect of vapor pressure on stomatal control of gas exchange in Douglas fir (Pseudotsuga menziesii) saplings.

Increasing leaf to air vapor pressure deficit (VPD) caused reductions in stomatal conductance of both current year and previous season needles of Pseudotsuga menziesii saplings. The stomata of current year needles were found to be more responsive to changes in VPD than those of previous season needles. The reductions in stomatal conductance of current year needles were not associated with decreases in xylem pressure potential. In fact, the reductions in stomatal conductance of current year needles were sometimes sufficient to reduce transpiration and thus raise xylem pressure potential even though VPD was increasing. There was a decline in stomatal responsiveness to VPD in current year needles between early and late summer. Pressure-volume curves determined for different age needles at different times of the year suggested that differences and changes in stomatal responsiveness to VPD may have been caused in part by differences and changes in needle water potential components. Hexane washes of current year needles during the late summer succeeded in partially restoring their VPD sensitivity, suggesting that changes in the water permeability of the external cuticle during needle maturation may also have played a role in causing the summer decline in VPD responsiveness.In both current and previous year needles VPD-induced changes in stomatal conductance had a greater relative effect on transpiration (q w) than on net photosynthesis (PhN). In maturing needles the ratio of the sensitivities of transpiration and net photosynthesis to changes in stomatal conductance, (∂q w/∂g s)/∂PhN/∂g s), remained nearly constant as VPD was varied. This provides experimental support for a recent hypothesis that stomata respond to environmental fluctuations in such a manner as to maintain the above ratio constant, which optimizes CO2 uptake with respect to water loss.