MATHEMATICAL SIMULATION OF C4 GRASS PHOTOSYNTHESIS IN AMBIENT AND ELEVATED CO2

A mechanistic leaf photosynthesis model was developed for C4 grasses b ased on a general simplified scheme of C4 plant carbon metabolism. In the model, the PEPcase-dependent C4-cycle was described in terms of CO 2 concentration in the mesophyll space using Michaelis-Menten kinetics , and the activity of PEPcase was related to the incident PAR to take account of the influence of light on the activty of C4-cycle processes . The CO2 refixation by Rubisco in the bundle sheath was described usi ng a widely accepted C3 photosynthesis model. The model assumes a stea dy state balance among CO2 diffusion from surrounding atmosphere into the mesophyll space, CO2 transport into the bundle sheath by the C4-cy cle, CO2 refixation by the C3-cycle in the bundle sheath, and CO2 leak age from the bundle sheath. The response to temperature of photosynthe sis was incorporated via the temperature dependence of model parameter s. The photosynthesis model was coupled with a stomatal conductance mo del in order to predict leaf photosynthesis rates at different atmosph eric conditions. The empirical model of Ball et al. (1987) was adopted and slightly modified to describe responses in stomatal conductance. The coupled model was parameterized for the C4 grass Andropogon gerard ii grown in both ambient (350 ppm) and elevated (700 PPM) CO2 atmosphe res. The key parameters of the model were estimated by fitting the mod el to the measured data using non-linear regression. The model was val idated by comparison the predicted photosynthetic response to PAR in b oth CO2-pretreatments with the measured data from an independent gas e xchange experiment. The predicted photosynthesis and stomatal conducta nce matched the measured data quite well for both atmospheric CO2-pret reatments. At 25-degrees-C, the estimated maximum carboxylation rate o f Rubisco V(cm,25), potential electron transport rate J(m,25) and quan tum efficiency alpha were increased by CO2 enrichment. The maximum PEP case activity V(pm,25) was lower in elevated CO2. The model predicted that the light-saturated leaf photosynthesis will increase by about 10 % with the rising of atmospheric CO2 from 350 to 700 ppm at 30-degrees -C, and that the optimal temperature of photosynthesis will shift from 37 to 38.5-degrees-C. The estimated slope of the stomatal conductance model was increased by atmospheric CO2 enrichment. Stomatal conductan ce was significantly reduced by increasing atmospheric CO2 concentrati on.