|Mathematical simulation of C4 grass photosynthesis in ambient and elevated C02
|Year of Publication
|Chen, DX, Coughenour, MB, Knapp, AK, Owensby, CE
|Andropogon gerardii, C4 grass, CO2 enrichment, photosynthesis, Stomatal Conductance
A mechanistic leaf photosynthesis model was developed for C4 grasses based on a general simplified scheme of C4 plant carbon metabolism. In the model, the PEPcase-dependent C4-cycle was described in terms of CO2 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 activity of C4-cycle processes. The CO2 refixation by Rubisco in the bundle sheath was described using a widely accepted C3 photosynthesis model. The model assumes a steady state balance among CO2 diffusion from surrounding atmosphere into the mesophyll space, CO2 transport into the bundle sheath by the C4-cycle, CO2 refixation by the C3-cycle in the bundle sheath, and CO2 leakage from the bundle sheath. The response to temperature of photosynthesis was incorporated via the temperature dependence of model parameters. The photosynthesis model was coupled with a stomatal conductance model in order to predict leaf photosynthesis rates at different atmospheric 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 gerardii grown in both ambient (350 ppm) and elevated (700 ppm) CO2 atmospheres. The key parameters of the model were estimated by fitting the model to the measured data using non-linear regression. The model was validated by comparison the predicted photosynthetic response to PAR in both CO2-pretreatments with the measured data from an independent gas exchange experiment. The predicted photosynthesis and stomatal conductance matched the measured data quite well for both atmospheric CO2-pretreatments. At 25°C, the estimated maximum carboxylation rate of Rubisco Vcm,25, potential electron transport rate Jm,25 and quantum efficiency α were increased by CO2 enrichment. The maximum PEPcase activity Vpm,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°C, and that the optimal temperature of photosynthesis will shift from 37 to 38.5°C. The estimated slope of the stomatal conductance model was increased by atmospheric CO2 enrichment. Stomatal conductance was significantly reduced by increasing atmospheric CO2 concentration.