Carbon isotope discrimination correlates with a range of ratios of phosphoenolpyruvate to total carboxylase activities found in two C3 species

The most widely used model that relates leaf carbon isotopic ratios to p(i)/p(a) ratios in C-3 plants suggests that PEPcase contributes approximately 6% of the carbon pool in these species. Variability in this contribution during leaf development is not only possible, but has been shown in studies where PEPcase activity is highest during the heterotrophic (carbon gained from import) stages of leaf development and declines as the leaf attains greater amounts of autotrophy (carbon gained from photosynthesis). We tested the hypothesis that variation in PEPcase:total carboxylase activities associated with carbon sources for leaf growth could affect carbon isotopic discrimination values on mixotrophic (carbon gained from import and photosynthesis) and autotrophic Nicotiana tabacum and Salix interior plants. A range of mean ratios of PEPcase:total carboxylase activities was achieved in these two C-3 species that correlated negatively with mean leaf carbon isotopic discrimination. These results indicate that the proportion of carbon fixed by PEPcase to total carboxylation could vary between different leaf developmental stages within and among different species and that this contribution can exceed 6%.