SIMULATION OF DIURNAL TRANSPIRATION AND PHOTOSYNTHESIS OF A WATER-STRESSED SOYBEAN CROP

The diurnal course of photosynthesis and transpiration of different pl ants can exhibit a dissymmetric behaviour between the morning and the afternoon, a photosynthetic midday depression and a transpiration 'pla teau'. A field experiment conducted in a mediterranean climate allows us to identify these features for a soybean crop undergoing water stre ss, The experimental data also show that, before and after the depress ion and plateau times, photosynthetic and transpiration fluxes are sim ilar from one day to another. The duration of the period of depression and plateau increases as the soil dries out. Two soil-vegetation-atmo sphere transfer models are used to simulate these features. Both are a ble to capture the midday depression and the plateau because they impo se a direct relationship between leaf water potential, stomatal conduc tance and photosynthesis. When the leaf water potential reaches a 'cri tical' value, feedback mechanisms tend to regulate the leaf water pote ntial such that the transpiration becomes nearly constant. The stomata l conductance and the photosynthesis keep decreasing in response to an increase of water vapour deficit. The dissymmetry depends on the diss ymmetry of the diurnal course of saturation deficit in the atmosphere, and on the evolution of the water properties of the soil during the d ay. We conclude that, even if they do not include formulations of phot osynthesis and stomatal conductance based on hormonal regulations of s tomatal conductance and inter-regulations between photosynthesis and c onductance, such models remain useful for the simulation of canopy pro cesses.