CO2 and water vapor exchange of Pinustaeda in relation to stomatal behavior: test of an optimization hypothesis

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Abstract
Responses of CO2 and water vapor exchange to absolute humidity deficit (AHD) were measured for seedlings of Pinus taeda L. at high and low irradiance in the laboratory. Diurnal patterns of CO2 and water vapor exchange of P. taeda seedlings and trees were monitored in the field. Stomatal behavior was evaluated in relation to a recent hypothesis of "optimal" stomatal behavior, in which changes in stomatal conductance in response to environmental variation are such that water loss is minimized for a given amount of carbon gain. That is, when stomatal behavior is "optimal," the ratio (gain ratio) of the sensitivities of transpiration and net photosynthesis to changes in stomatal conductance (.vdelta.E/.vdelta.g and .vdelta.A/.vdelta.g, respectively) is constant. Laboratory and field stomatal behavior generally did not conform with this hypothesis. Under controlled conditions, at high irradiance, the gain ratio increased with AHD. In the field, the gain ratio increased diurnally on most days. Increasing gain ratios were associated with increasing .vdelta.E/.vdelta.g values but relatively uniform .vdelta.A/.vdelta.g values. Uniform gain ratios in the field were observed on some days, associated with uniform environment, constant .vdelta.E/.vdelta.g, or varying .vdelta.A/.vdelta.g values.

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