Abstract
Nonlinear responses of photosynthesis to the CO 2 concentration at which plants were grown (C g ) have been often reported in the literature. 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 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 rate measured at C g was highest in plants from the 700 ppm CO 2 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 C g . Measurements of CO 2 ‐induced 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 that the nonlinearity of photosynthetic responses to C g 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 CO 2 ‐induced changes in n m and h. These compiled data indicate that 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 variables (n m and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to C g . Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to C g reported in the literature.