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Attached twigs of young Pseudotsuga menziesii (Mirb.) Franco plants were subjected to variations in irradaince. Stomatal responsiveness to irradiance, measured in an open type gas exchange system, varied seasonally. During the autumn and winter, stomatal conductance was relatively unresponsive to changes in irradiance, but during the summer stomatal conductance decreased in response to reduced irradiance. The summer stomatal response to irradiance was such that a nearly constant ratio of stomatal conductance to net photosynthesis was maintained as irradiance was varied. This caused intercellular CO 2 concentration (c i ) and water use efficiency (net CO 2 uptake/transpiration) to also remain relatively constant. At constant irradiance, stomatal conductance was relatively insensitive to experimentally-induced changes in c i . This, and the observation that c i remained relatively constant as irradiance was varied, suggest that changes in c i played a minor role in mediating the stomatal response to light.The ecological significance of the seasonal changes in stomatal response to light is discussed.

Three-dimensional empirical models were constructed, depicting the response surface of water use efficiency (WUE) of Pseudotsuga menziesii saplings in relation to different levels of both irradiance and leaf-to-air vapor pressure difference (VPD). The two models developed depict responses of (1) previous season needles during autumn, winter, and early spring and (2) current year needles during the summer. The steady-state stomatal and gas exchange responses to irradiance and VPD suggest that factors determining adaptive stomatal performance in Douglas fir are complex and may differ according to needle age, developmental stage, and season. Stomatal response to light varied seasonally, with the stomata being responsive during the summer and unresponsive during the autumn, winter, and early spring. Previous season needles exhibit higher maximum WUE, but can be less conservative in their total use of water than the more VPD-sensitive maturing needles.Observations of dynamic stomatal responses to step changes in VPD and irradiance were used to propose a simple model depicting a combined stomatal response to sudden changes in both VPD and irradiance similar to those that would occur with the passage of sunflecks in a forest canopy. Step changes in VPD caused transient stomatal movements opposite in direction to that of the final response, while step changes in irradiance resulted in movements only in the direction of the expected final response. On the basis of the model, it was hypothesized that the dynamic response to changes in VPD may serve to enhance the speed of stomatal opening and closing when changes in irradiance are rapid.

At sites in the United States, creosote bushes (Larrea tridentata (DC.) Cov.) orient foliage clusters predominantly toward the southeast. Foliage of bushes at the southernmost distribution extreme in Mexico shows no predominant orientation. Clusters at all sites are inclined between 33° and 71° from the horizontal. Inclinations are steeper in the drier and hotter Mojave Desert than in the Chihuahuan Desert. Individual leaflets, though not measured, appear more randomly oriented than foliage clusters. In several populations studied, branches were shorter in the southeastern sectors of the crown, reducing self-shading early in the morning. Measurements of direct beam radiation interception by detached branches, using digital image processing, indicated that foliage clusters oriented toward the southeast exhibited less self-shading during spring mornings than clusters oriented northeast. This effect was not apparent at the summer solstice. This type of canopy architecture may tend to minimize self-shading during the morning hours when conditions are more favorable for photosynthesis, resulting in an improved daily water use efficiency.

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 (Ph N ). In maturing needles the ratio of the sensitivities of transpiration and net photosynthesis to changes in stomatal conductance, (∂q w /∂g s )/∂Ph N /∂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 CO 2 uptake with respect to water loss.