Electron Transport in Leaves: A Physiological Perspective

Light absorbed by photosystems I and II is used to drive linear electron transport, and associated proton transport, in the thylakoid membranes of leaves. In healthy leaves operating under non-stressful conditions and in which photorespiration is inhibited, photosynthetic electron transport is used primarily to reduce NADP+ to NADPH, which is then used to drive the assimilation of CO2 into carbohydrates with ca. 88% of electrons being consumed in this process. However, such a high quantum efficiency of CO2 assimilation is frequently not observed in leaves. We examine the intrinsic physiological, metabolic and environmental factors that can modify photosynthetic electron transport in leaves. Electron transport is also required for the reduction and activation of key enzymes involved in photosynthetic metabolism and driving other metabolic processes, such as nitrogen and sulfur metabolism. Oxygen can act as an electron acceptor, being reduced by electrons from photosystem I via a Mehler reaction or by electrons from photosystem II via the plastid terminal oxidase. Although such photoreductions of oxygen do not appear to have a significant role in healthy, non-stressed leaves, there is evidence to support the contention that these processes can be important for photoprotection of photosystem II in leaves under light stress. Cyclic electron transport can occur around photosystem I; however, this process would also appear to only be of physiological importance when the ability of the leaf to assimilate CO2 is severely restricted. It is concluded that leaves exhibit a high degree of plasticity in their ability to modify the pathways of photosynthetic electron transport in order to deal with fluctuations in metabolic demands and environmental stresses.