The influence of light quality on C4 photosynthesis under steady-state conditions in Zea mays and Miscanthus giganteus: changes in rates of photosynthesis but not the efficiency of the CO2 concentrating mechanism

Differences in light quality penetration within a leaf and absorption by the photosystems alter rates of CO2 assimilation in C3 plants. It is also expected that light quality will have a profound impact on C4 photosynthesis due to disrupted coordination of the C4 and C3 cycles. To test this hypothesis, we measured leaf gas exchange, 13CO2 discrimination (?13C), photosynthetic metabolite pools and Rubisco activation state in Zea mays and Miscanthus x giganteus under steady-state red, green, blue and white light. Photosynthetic rates, quantum yield of CO2 assimilation, and maximum phosphoenolpyruvate carboxylase activity were significantly lower under blue light than white, red and green light in both species. However, similar leakiness under all light treatments suggests the C4 and C3 cycles were coordinated to maintain the photosynthetic efficiency. Measurements of photosynthetic metabolite pools also suggest coordination of C4 and C3 cycles across light treatments. The energy limitation under blue light affected both C4 and C3 cycles, as we observed a reduction in C4 pumping of CO2 into bundle-sheath cells and a limitation in the conversion of C3 metabolite phosphoglycerate to triose phosphate. Overall, light quality affects rates of CO2 assimilation, but not the efficiency of CO2 concentrating mechanism.