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.