Theoretical studies suggest that partitioning leaf photosynthetic responses to CO2 partial pressures into two components, sensitivity and acclimation, facilitates both scaling-up photosynthetic responses and predicting global terrestrial carbon influx. Here, we experimentally examine these two components by growing soybean (Glycine max) in two CO2 partial pressures, 35 and 70 Pa, and making a suite of ecophysiological measurements on expanding and fully expanded leaves. These CO2 treatments resulted in a variety of acclimation responses, including changes in net photosynthetic rate and capacity, stomatal conductance, transpiration, and respiration. These responses were strongly dependent on leaf age. Despite the wide variety of acclimation responses, the experimentally derived photosynthetic sensitivity did not vary with CO2 treatments or leaf age. In addition, the photosynthetic sensitivity to ambient CO2 partial pressure was consistent with the sensitivity to intercellular CO2 partial pressure, indicating little effect of stomatal conductance on photosynthetic sensitivity. This study supports the theoretical conclusion that photosynthetic sensitivity is independent of growth environment and leaf age, as well as photosynthetic acclimation, even though the latter varies with both environmental and developmental factors. Accordingly, photosynthetic sensitivity may be directly extrapolated from leaf to globe to predict the increment in terrestrial carbon influx stimulated by the yearly increase in atmospheric CO2, whereas the acclimation component must be used to adjust the overall global estimate. (C) 1999 Elsevier Science B.V. All rights reserved.
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