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 (A(area)) at elevated CO(2) 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 (N(area)), where N(area) = N(mass) x LMA. Here, N(mass) 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 CO(2) concentration, N(area), N(mass), LMA and A(area). 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 A(area) to elevated CO(2) into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO(2) 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 CO(2) due to morphological upregulation was negligible for most species. The response of A(area) in these species was dominated by the reduction in CO(2) limitation occurring at higher CO(2) concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO(2) due to an increase in LMA that was independent of any changes in N(area). This morphologically based inhibition of A(area) combined additively with a reduction in biochemical capacity to a significantly offset the direct enhancement of A(area) caused by reduced CO(2) limitation in two species. This offset was 100% for Acer rubrum, resulting in no net effect of elevated CO(2) on A(area) for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological responses to elevated CO(2) can have important effects on photosynthesis.