Quantifying the response of photosynthesis to changes in leaf nitrogen content and leaf mass per area in plants grown under atmospheric CO2 enrichment
Previous modelling exercises and conceptual arguments have predicted that a reduction in biochemical capacity for photosynthesis (Aarea) at elevated CO2 may be compensated by an increase in mesophyll tissue growth if the total amount of photosynthetic machinery per unit leaf area is maintained (i.e. morphological upregulation). The model prediction was based on modelling photosynthesis as a function of leaf N per unit leaf area (Narea), where Narea = Nmass×LMA. Here, Nmass is percentage leaf N and is used to estimate biochemical capacity and LMA is leaf mass per unit leaf area and is an index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multiple correlations that are known, or that are likely to exist between CO2 concentration, Narea, Nmass, LMA and Aarea. Although this is impractical experimentally, we can control for these correlations statistically using systems of linear multiple-regression equations. We developed a linear model to partition the response of Aarea to elevated CO2 into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO2 limitation of photosynthesis. The model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancement at elevated CO2 due to morphological upregulation was negligible for most species. The response of Aarea in these species was dominated by the reduction in CO2 limitation occurring at higher CO2 concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO2 due to an increase in LMA that was independent of any changes in Narea. This morphologically based inhibition of Aarea combined additively with a reduction in biochemical capacity to significantly offset the direct enhancement of Aarea caused by reduced CO2 limitation in two species. This offset was 100% for Acer rubrum, resulting in no net effect of elevated CO2 on Aarea for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological responses to elevated CO2 can have important effects on photosynthesis.