Interactions between Carbon Dioxide and Water Deficits Affecting Canopy Photosynthesis: Simulation and Testing
Increases in atmospheric CO2 concentrations (Ca) are likely to increase CO2 fixation and reduce transpiration by crop canopies. Mathematical modeling of the processes controlling gas exchange may be used to estimate the extent to which exchange is likely to be altered by Ca under site-specific conditions. A simulation model was constructed from fundamental equations for CO2 fixation, transpiration and energy exchange in order to estimate canopy gas exchange under different Ca. The model was then tested with data from a growth chamber study. The model reproduced an increase in net photosynthesis from 35 to 65 μmol m−2 s−1, and a decrease in transpiration from 14 000 to 11 000 μmol m−2 s−1, for an irrigated soybean [Glycine max (L.) Merr.] canopy when Ca was increased from 330 to 800 μmol mol−1. The consequent increase in water use efficiency from 2.7 to .5 mmol mol−1 was confirmed from the growth chamber study. In the model, this increase was governed by changes in the partitioning of latent and sensible components of the crop energy balance arising from changes in canopy stomatal resistance. Changes in the ratios of latent and sensible heat fluxes simulated by the model at Ca = 800 vs. 330 μmol mol−1 caused simulated increases in canopy temperatures of ≈ 1.0 °C, similar to recorded increases in other studies. There is evidence from model output that changes in canopy CO2 fixation caused by Ca may be reduced in water stressed canopies. However, our understanding of water deficit effects on photosynthesis remains largely empirical, so that estimates of canopy gas exchange under combined water deficits and changed Ca remain speculative. There is a need to extend experimental studies of Ca effects on photosynthesis and transpiration to include studies under water stress in order to test model hypotheses.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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