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.