The influence of light quality on C 4 photosynthesis under steady‐state conditions in Zea mays and Miscanthus × giganteus : changes in rates of photosynthesis but not the efficiency of the CO 2 concentrating mechanism

Differences in light quality penetration within a leaf and absorption by the photosystems alter rates of CO 2 assimilation in C 3 plants. It is also expected that light quality will have a profound impact on C 4 photosynthesis due to disrupted coordination of the C 4 and C 3 cycles. To test this hypothesis, we measured leaf gas exchange, 13 CO 2 discrimination (Δ 13 C), photosynthetic metabolite pools and Rubisco activation state in Zea mays and Miscanthus  ×  giganteus under steady‐state red, green, blue and white light. Photosynthetic rates, quantum yield of CO 2 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 C 4 and C 3 cycles were coordinated to maintain the photosynthetic efficiency. Measurements of photosynthetic metabolite pools also suggest coordination of C 4 and C 3 cycles across light treatments. The energy limitation under blue light affected both C 4 and C 3 cycles, as we observed a reduction in C 4 pumping of CO 2 into bundle‐sheath cells and a limitation in the conversion of C 3 metabolite phosphoglycerate to triose phosphate. Overall, light quality affects rates of CO 2 assimilation, but not the efficiency of CO 2 concentrating mechanism.