Effects of elevated atmospheric CO2 on canopy transpiration in senescent spring wheat

The seasonal course of canopy transpiration and the diurnal courses of latent heat flux of a spring wheat crop were simulated for atmospheric CO2 concentrations of 370 and 550 mu mol mol(-1). The hourly weather data, soil parameters and the irrigation and fertilizer treatments of the Free-Air Carbon Dioxide Enrichment wheat experiment in Arizona (1992-1993) were used to drive the model. The simulation results were tested against field measurements with special emphasis on the period between anthesis and maturity. A model integrating leaf photosynthesis and stomatal conductance was scaled to canopy level in order to be used in the wheat growth model. The simulated intercellular CO2 concentration, C-i, was determined from the ratio of C-i to the CO2 concentration at the leaf surface, C-s, the leaf-to-air specific humidity deficit and a possibly unfulfilled transpiration demand, After anthesis, the measured assimilation rates of the flag leaves decreased more rapidly than their stomatal conductances, leading to a rise in the C-i/C-s ratio. In order to describe this observation, an empirical model approach was developed which took into account the leaf nitrogen content for the calculation of the C-i/C-s ratio. Simulation results obtained with the new model version were in good agreement with the measurements. If changes in the C-i/C-s ratio in accordance with the decrease in leaf nitrogen content during leaf senescence were not considered in the model, simulations revealed an underestimation of the daily canopy transpiration of up to 20% and a decrease in simulated seasonal canopy transpiration by 10%. The measured reduction in the seasonal sum of canopy transpiration and soil evaporation owing to CO2 enrichment, in comparison, was only about 5%. (C) 1999 Elsevier Science B.V. All rights reserved.