N onlinearity of photosynthetic responses to growth in rising atmospheric CO2: an experimental and modelling study

Nonlinear responses of photosynthesis to the CO2 concentration at which plants were grown (Cg) have been often reported in the literatu~e. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of Cg (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at Cg was highest in plants from the 700 ppm CO2 treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective Cg. Measurements of CO2-induced changes in mass-based leaf nitrogen concentration (nnlf an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to Cg is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO2-induced changes in nm and h. These compiled data indicate that h generally increases and nm usually decreases with increasing Cg, but that the trajectories and magnitudes of the changes in h and nm vary with species and growth environments. Integration of these variables (nm and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to Cg. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to Cg reported in the literature.