PGEN - AN INTEGRATED MODEL OF LEAF PHOTOSYNTHESIS, TRANSPIRATION, AND CONDUCTANCE

A detailed model of leaf-scale photosynthesis, respiration, transpiration, stomatal conductance, and energy balance is described. The model, PGEN v2.0(1), is designed for use in larger-scale ecosystem, climate and hydrological models concerned with fluxes of CO2, water, and heat. Given a set of environmental and biological (mostly leaf) parameters, PGEN calculates instantaneous rates of net photosynthesis and transpiration, and associated conductances to CO2 and water. The model is intended to predict species-specific behaviour with minimal need for empirical parameterisation. The biochemical model of photosynthesis is derived from the models of Farquhar and co-workers. This biochemical model is embedded in a model of the leaf's energy balance, which is based on the work of Monteith and Jones. Stomatal conductance is calculated using an optimisation concept. In this concept there is an assumed tradeoff between CO2 entering and water leaving the leaf, resulting in a single stomatal conductance for each set of environmental conditions, that maximises a function including the costs and benefits. Predicted responses of stomatal conductance, net photosynthesis, transpiration rate, and the ratio of CO2 concentration in the leaf to that outside the leaf boundary layer, to key environmental factors, closely match observed responses. A sensitivity analysis of PGEN v2.0 shows that predicted net photosynthesis is most sensitive to the degree of co-limitation between carboxylation- and ribulose-1,5-bisphosphate regeneration-limited photosynthesis, the Rubisco carboxylation kinetic parameters, the atmospheric concentration of CO2, and leaf nitrogen content. Predicted stomatal conductance is most sensitive to relative humidity, the critical leaf water potential for plant dry matter production, the hydraulic resistance between the root and the leaf, and the atmospheric concentration of CO2.