NONLINEARITY OF PHOTOSYNTHETIC RESPONSES TO GROWTH IN RISING ATMOSPHERIC CO2 - AN EXPERIMENTAL AND MODELING STUDY

Nonlinear responses of photosynthesis to the CO2 concentration at whic h plants were grown (C-g) have been often reported in the literature. This study was designed to develop mechanistic understanding of the no nlinear responses with both experimental and modelling approaches. Soy bean (Glycine max) was grown in five levels of C-g (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 r ate measured at C-g was highest in plants from the 700 ppm CO2 treatme nt. 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 C-g. Measurements of CO2-induc ed changes in mass-based leaf nitrogen concentration (n(m) 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 t hat the nonlinearity of photosynthetic responses to C-g is largely det ermined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further u nderstand the nonlinear responses, we compiled data from the literatur e on CO2-induced changes in n(m) and h. These compiled data indicate t hat h generally increases and n(m) usually decreases with increasing C -g, but that the trajectories and magnitudes of the changes in h and n (m) vary with species and growth environments. Integration of these va riables (n(m) and h) into a biochemically based model of photosynthesi s enabled us to predict diverse responses of photosynthesis to C-g. Th us a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to C-g reported in the literature.