PGEN - AN INTEGRATED MODEL OF LEAF PHOTOSYNTHESIS, TRANSPIRATION, ANDCONDUCTANCE

A detailed model of leaf-scale photosynthesis, respiration, transpirat ion, 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 transpi ration, and associated conductances to CO2 and water. The model is int ended to predict species-specific behaviour with minimal need for empi rical parameterisation. The biochemical model of photosynthesis is der ived from the models of Farquhar and co-workers. This biochemical mode l 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 tra deoff 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 r ate, and the ratio of CO2 concentration in the leaf to that outside th e leaf boundary layer, to key environmental factors, closely match obs erved responses. A sensitivity analysis of PGEN v2.0 shows that predic ted net photosynthesis is most sensitive to the degree of co-limitatio n between carboxylation- and ribulose-1,5-bisphosphate regeneration-li mited photosynthesis, the Rubisco carboxylation kinetic parameters, th e atmospheric concentration of CO2, and leaf nitrogen content. Predict ed stomatal conductance is most sensitive to relative humidity, the cr itical leaf water potential for plant dry matter production, the hydra ulic resistance between the root and the leaf, and the atmospheric con centration of CO2.