The efficiency of C 4 photosynthesis under low light conditions in Zea mays , Miscanthus x giganteus and Flaveria bidentis

The efficiency of C 4 photosynthesis in Zea mays , Miscanthus x giganteus and Flaveria bidentis in response to light was determined using measurements of gas exchange, 13 CO 2 photosynthetic discrimination, metabolite pools and spectroscopic assays, with models of C 4 photosynthesis and leaf 13 CO 2 discrimination. Spectroscopic and metabolite assays suggested constant energy partitioning between the C 4 and C 3 cycles across photosynthetically active radiation (PAR). Leakiness ( φ ), modelled using C 4 light‐limited photosynthesis equations ( φ mod ), matched values from the isotope method without simplifications ( φ is ) and increased slightly from high to low PAR in all species. However, simplifications of bundle‐sheath [CO 2 ] and respiratory fractionation lead to large overestimations of φ at low PAR with the isotope method. These species used different strategies to maintain similar φ . For example, Z. mays had large rates of the C 4 cycle and low bundle‐sheath cells CO 2 conductance ( g bs ). While F. bidentis had larger g bs but lower respiration rates and M. giganteus had less C 4 cycle capacity but low g bs , which resulted in similar φ . This demonstrates that low g bs is important for efficient C 4 photosynthesis but it is not the only factor determining φ . Additionally, these C 4 species are able to optimize photosynthesis and minimize φ over a range of PARs, including low light.