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

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 CO2 concentration (c i) and water use efficiency (net CO2 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.