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

At elevated atmospheric CO 2 concentrations ([CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] 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 [CO 2 ] a corresponds much more closely to the observations. In general, root fraction increases if the response of photosynthesis to [CO 2 ] a is greater than that of relative growth rate.