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Abstract

Primary responses of C₃-plants to elevated atmospheric CO₂ concentrations are an increase in the net assimilation rate, leading to greater biomass, and an associated decrease in the transpiration rate per unit leaf area due to CO₂-induced stomatal closure. The question has therefore arisen: does canopy transpiration increase because of the greater biomass, or decrease because of the stomatal closure?

The direct impact of an elevated atmospheric CO₂ concentration of 550 mmol mol^-1 on the seasonal course of canopy transpiration of a spring wheat crop was investigated by means of the simulation model DEMETER for production under unlimited water and nutrient supply, production under limited water but unlimited nutrient supply and the production under unlimited water but limited nitrogen supply. Independent data of the Free-Air Carbon Dioxide Enrichment wheat experiments in Arizona, USA (1993-96) were used to test if the model is able to make reasonable predictions of water use and productivity of the spring wheat crop using only parameters derived from the literature.

A model integrating leaf photosynthesis, stomatal conductance and energy fluxes between the plant and the atmosphere was scaled to a canopy level in order to be used in the wheat crop growth model. Temporal changes of the model parameters were considered by describing them as dependent on the changing leaf nitrogen content. Comparison of the simulation and experimental results showed that the applicability of the model approach was limited after anthesis by asynchronous changes in mesophyll and stomatal conductance. Therefore a new model approach was developed describing the interaction between assimilation rate and stomatal conductance during grain filling.

The simulation results revealed only small differences in the cumulative sum of canopy transpiration and soil evaporation between elevated CO₂ and control conditions. For potential growth conditions the model simulated a slightly lower water use under elevated CO₂, while water limitation lead to a slightly higher water use due to the relative increase in root biomass and hence water availability. Under nitrogen limitation elevated CO₂ lead to a stronger decrease of water use than under adequate nitrogen supply. The simulation results confirm the experimental findings that water limitation increases and nitrogen limitation decreases the CO₂-effect.

The qualitative as well as the quantitative behavior of the wheat crop under elevated CO₂ was successfully described by the model for the three production conditions, which makes it possible to use the model with higher confidence for predictions about the future behavior of the crop.