The Difficult-to-Quantify Cyclic Electron Flux around Photosystem I in Leaves of Flowering Plants

in oxygenic photosynthesis, two photosystems (PS II and PS I), using light, work in series to extract electrons from water to generate reducing power (in the form of NADPH) in linear electron transport, while the energy of the protons after water photo-oxidation is conserved in the form of ATP. Additionally, a cyclic electron flux (CEF) around PS I is also coupled to the formation of ATP . Together, NADPH and ATP drive the conversion of CO2 intocarbohydrates. CEF is central to the regulation of photosynthesis , but its quantification, six decades after the discovery, has been difficult because of the absence of net formation of a product or consumption of a reactant .

 

An indirect method has been used whereby the steady-state total electron flux through PS I (ETR1) and the linear electron flux through both photosystems (LEF) are determined, with both measurements made in the same leaf tissue under identical conditions. The difference (ΔFlux = ETR1 − LEF) is an upper estimate of CEF because ETR1 in general includes other partial electron fluxes through PS I. ΔFlux in greenhouse-grown spinach leaves increases with irradiance, and does not reach a maximum even when LEF is light-saturated .