Mathematical simulation of C4 grass photosynthesis in ambient and elevated CO2

Abstract

A mechanistic leaf photosynthesis model was developed for C 4 grasses based on a general simplified scheme of C 4 plant carbon metabolism. In the model, the PEPcase-dependent C 4 -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 activity of C 4 -cycle processes. The CO 2 refixation by Rubisco in the bundle sheath was described using a widely accepted C 3 photosynthesis model. The model assumes a steady state balance among CO 2 diffusion from surrounding atmosphere into the mesophyll space, CO 2 transport into the bundle sheath by the C 4 -cycle, CO 2 refixation by the C 3 -cycle in the bundle sheath, and CO 2 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 C 4 grass Andropogon gerardii grown in both ambient (350 ppm) and elevated (700 ppm) CO 2 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 CO 2 -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 CO 2 -pretreatments. At 25°C, the estimated maximum carboxylation rate of Rubisco V cm,25 , potential electron transport rate J m,25 and quantum efficiency α were increased by CO 2 enrichment. The maximum PEPcase activity V pm,25 was lower in elevated CO 2 . The model predicted that the light-saturated leaf photosynthesis will increase by about 10% with the rising of atmospheric CO 2 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 CO 2 enrichment. Stomatal conductance was significantly reduced by increasing atmospheric CO 2 concentration.