C4 Photosynthesis: Quantum Requirement, C4 and Overcycling and Q-Cycle Involvement

Abstract

The relationship between overcycling of the C4 acid cycle in C4 photosynthesis (due to CO2 leakage) and the quantum yield of photosynthesis is considered. From a comparison of theoretical and measured quantum yields we suggest that the high efficiency of light utilisation by most C4 plants can only be explained by the mandatory involvement of both the Q-cycle and cyclic or pseudocyclic electron transport in the proton partitioning process. The existence of the Q-cycle mechanism may have been a prerequisite for the evolution of the C4 pathway.