Predicting responses of photosynthesis and root fraction to elevated [CO2]a: interactions among carbon,nitrogen, and growth*

At elevated atmospheric CO₂ concentrations (CO₂]_a), photosynthetic capacity (A_max) and root fraction (η_R, the ratio of root to plant dry mass) increased in some studies and decreased in others. Here, we have explored possible causes of this, focusing on the relative magnitudes of the effects of elevated CO₂]_a on specific leaf (n_m) and plant (n_p) nitrogen concentrations, leaf mass per unit area (h), and plant nitrogen productivity (α). In our survey of 39 studies with 35 species, we found that elevated CO₂]_a led to decreased n_m and n_p in all the studies and to increased h and α in most of the studies. The magnitudes of these changes varied with species and with experimental conditions. Based on a model that integrated CO₂]_a-induced changes in leaf nitrogen into a biochemically based model of leaf photosynthesis, we predicted that, to a first approximation, photosynthesis will be upregulated (A_max will increase) when growth at increased CO₂]_a leads to increases in h that are larger than decreases in n_m. Photosynthesis will be downregulated (A_max will decrease) when increases in h are smaller than decreases in n_m. The model suggests that photosynthetic capacity increases at elevated CO₂]_a only when additional leaf mesophyll more than compensates the effects of nitrogen dilution. We considered two kinds of regulatory paradigms that could lead to varying responses of η_R to elevated CO₂]_a, and compared the predictions of each with the data. A simple static model based on the functional balance concept predicts that η_R should increase when neither n_p nor h is very responsive to elevated CO₂]_a. The quantitative and qualitative agreement of the predictions with data from the literature, however, is poor. A model that predicts η_R from the relative sensitivities of photosynthesis and relative growth rate to elevated CO₂]_a corresponds much more closely to the observations. In general, root fraction increases if the response of photosynthesis to CO₂]_a is greater than that of relative growth rate.