COS 67-10
The resource-use efficiency of C3-C4 intermediate photosynthesis
Plant species possessing the C4 photosynthetic pathway are characterized by greater photosynthetic resource-use efficiencies than those possessing the C3 pathway. Biochemical models of the C3-C4 intermediate photosynthetic pathway have predicted that it should confer gains in resource-use efficiencies that are qualitatively similar to, though quantitatively smaller than, those associated with the full C4 pathway. Curiously, the expected gains have not been observed consistently in C3-C4 intermediate species. In this study, we sought to understand the basis of the discrepancy between the theoretical predictions and the empirical observations of resource-use efficiency in C3-C4 intermediate photosynthesis. To do so, we compared the photosynthetic performance of six closely-related C3 and C3-C4 intermediate species in the genus Flaveria (Asteraceae). F. cronquistii, F. robusta, F. pringlei, F. sonorensis, F. chloraefolia, and F. linearis were grown under common conditions in a greenhouse and photosynthetic resource-use efficiencies were quantified using gas exchange measurements. Measured resource-use efficiencies were partitioned into components attributed either to C3-C4 intermediate biochemistry or to other aspects of the C3-C4intermediate phenotype.
Results/Conclusions
We found that C3-C4 intermediate biochemistry had effects on measured resource-use efficiencies that were generally of similar or smaller magnitude than other aspects of the C3-C4 intermediate phenotype. In particular, we found that changes in C3-C4 intermediate biochemistry were coincident with changes in photosynthetic capacity, leaf mass per area, operational stomatal conductance and anatomical maximum stomatal conductance, and that these non-biochemical changes were responsible for the majority of the interspecific differences in photosynthetic resource-use efficiencies. These results resolve the discrepancy between the theoretical predictions and the empirical observations of resource-use efficiency in C3-C4 intermediate photosynthesis and raise new questions about the basis of selection for the C3-C4 intermediate pathway and the basis of competitive coexistence between C3, C3-C4 intermediate, and C4 species.