Photosynthesis and transpiration in large forest-grown Douglas-fir: interactions with apical control

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Abstract
Net photosynthesis, transpiration, and stomatal and residual conductances for current-year shoots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) were measured in an open gas exchange system. Terminal shoots of branches and branchlets had larger stomatal and residual conductances, and net photosynthetic and transpiration rates than neighboring lateral shoots under conditions which did not limit gas exchange. The differences between terminal and lateral shoots occurred in both exposed and shaded branches and in trees of different dominance classes. For most of the study, current-year lateral shoots were lighter green than terminal shoots. There were no significant differences in shoot water potential or in the microenvironment between terminal and lateral shoots.Effects of apical control on gas exchange rates were strong in shoots subtending the dominant terminal shoot. These effects were not apparent four whorls from the terminal shoot, in agreement with the hypothesis that apical control can not be exerted at long distances from terminal shoots.

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