Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C 3 - and C 4 -cycle co-function

Carbon-isotope ratios were examined as δ13C values in several C3, C4, and C3—C4 Flaveria species, and compared to predicted δ13C values generated from theoretical models. The measured δ13C values were within 4‰ of those predicted from the models. The models were used to identify factors that contribute to C3-like δ13C values in C3—C4 species that exhibit considerable C4-cycle activity. Two of the factors contributing to C3-like δ13C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the relationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the δ13C value. Below 50% C4-cycle assimilation there was no significant relationship between the variables, but above 50% the δ13C values became less negative. These results demonstrate that the level of C4-cycle expression can increase from 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increases above 50%, however, increased integration of C3- and C4-cycle co-function occurs.