The Coordination of C4 Photosynthesis and the CO2-Concentrating Mechanism in Maize and Miscanthus × giganteus in Response to Transient Changes in Light Quality

Unequal absorption of photons between photosystems I and II, and between bundle-sheath and mesophyll cells, are likely to affect the efficiency of the CO 2 -concentrating mechanism in C 4 plants. Under steady-state conditions, it is expected that the biochemical distribution of energy (ATP and NADPH) and photosynthetic metabolite concentrations will adjust to maintain the efficiency of C 4 photosynthesis through the coordination of the C 3 (Calvin-Benson-Bassham) and C 4 (CO 2 pump) cycles. However, under transient conditions, changes in light quality will likely alter the coordination of the C 3 and C 4 cycles, influencing rates of CO 2 assimilation and decreasing the efficiency of the CO 2 -concentrating mechanism. To test these hypotheses, we measured leaf gas exchange, leaf discrimination, chlorophyll fluorescence, electrochromatic shift, photosynthetic metabolite pools, and chloroplast movement in maize ( Zea mays ) and Miscanthus × giganteus following transitional changes in light quality. In both species, the rate of net CO 2 assimilation responded quickly to changes in light treatments, with lower rates of net CO 2 assimilation under blue light compared with red, green, and blue light, red light, and green light. Under steady state, the efficiency of CO 2 -concentrating mechanisms was similar; however, transient changes affected the coordination of C 3 and C 4 cycles in M. giganteus but to a lesser extent in maize. The species differences in the ability to coordinate the activities of C 3 and C 4 cycles appear to be related to differences in the response of cyclic electron flux around photosystem I and potentially chloroplast rearrangement in response to changes in light quality.