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 CO2 may be compensated by an increase in mesophyll tissue growth if the to tal 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 est imate biochemical capacity and LMA is leaf mass per unit leaf area and is a n index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multipl e correlations that are known, or that are likely to exist between CO2 conc entration, N-area, N-mass, LMA and A(area). Although this is impractical ex perimentally, we can control for these correlations statistically using sys tems of linear multiple-regression equations. We developed a linear model t o partition the response of A(area), to elevated CO2 into components repres enting the independent and interactive effects of changes in indexes of bio chemical capacity, leaf morphology and CO2 limitation of photosynthesis. Th e model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancem ent at elevated CO2 due to morphological upregulation was negligible for mo st species. The response of A(area), 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 chan ges in N-area. This morphologically based inhibition of A(area) combined ad ditively with a reduction in biochemical capacity to significantly offset t he direct enhancement of A(area) caused by reduced CO2 limitation in two sp ecies. This offset was 100% for Acer rubrum, resulting in no met effect of elevated CO2 on A(area) for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological resp onses to elevated CO2 can have important effects on photosynthesis.