An Empirical Model of Net Photosynthesis and Leaf Conductance for the Simulation of Diurnal Courses of CO₂ and H₂O Exchange

Abstract

An empirical model of CO₂ uptake and water loss of leaves is established using steady-state responses of gas exchange to climatic factors as input. From the model the response surface of net CO₂ assimilation and leaf conductance to climate can be derived. The model consists of two submodels, one describing the response of CO₂ uptake to light and temperature, the other describing the response of leaf conductance to temperature and humidity. Both submodels are joined via the linear relationship between CO₂ uptake and leaf conductance at short-term (minutes) variation of irradiance. From the humidity response of leaf conductance and the 'demand function' (Raschke 1979) of CO₂ uptake in the mesophyll, the effect of stomata on the diffusion of CO₂ between leaf and air is determined. The model is tested by comparing measured and calculated diurnal courses of gas exchange for two plants of Pinus silvestris, differing in photosynthetic capacity due to different levels of magnesium nutrition. Applications and limitations of the model are discussed.