The efficiency of C4 photosynthesis under low light conditions in Zea mays, Miscanthus x giganteus and Flaveria bidentis

The efficiency of C4 photosynthesis in Zea mays, Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, 13CO2 photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C4 photosynthesis and leaf 13CO2 discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C4 and C3 cycles across photosynthetically active radiation (PAR). Leakiness (f), modelled using C4 light-limited photosynthesis equations (fmod), matched values from the isotope method without simplifications (fis) and increased slightly from high to low PAR in all species. However, simplifications of bundle-sheath [CO2] and respiratory fractionation lead to large overestimations of f at low PAR with the isotope method. These species used different strategies to maintain similar f. For example, Z. mays had large rates of the C4 cycle and low bundle-sheath cells CO2 conductance (gbs). While F. bidentis had larger gbs but lower respiration rates and M. giganteus had less C4 cycle capacity but low gbs, which resulted in similar f. This demonstrates that low gbs is important for efficient C4 photosynthesis but it is not the only factor determining f. Additionally, these C4 species are able to optimize photosynthesis and minimize f over a range of PARs, including low light.