Estimation of the cyclic electron flux around photosystem I in leaf discs

Quantification of the cyclic electron flux (CEF) around photosystem I (PSI) in leaves has been hampered by the absence of net product formation or net substrate consumption. We estimate CEF as the difference between the total electron flux through PSI and the linear electron flux through both photosystems, both fluxes measured in the same leaf tissue in identical conditions. This method can be applied to leaves of plants such as spinach grown in high light and in which the Mehler reaction and charge recombination are minimal. However, leaves of plants such as Arabidopsis grown in low light but tested in high light may have a large electron flux component that cycles within the PSI reaction centre by charge separation followed by charge recombination. In such leaves, CEF can be estimated as the antimycin A-sensitive component of the total electron flux through PSI. However, if CEF is significantly mediated by the nicotinamide adenine dinucleotide dehydrogenase-like complex (NDH), inhibition with a combination of antimycin A and an inhibitor of NDH would be necessary. The method has the potential to be fine-tuned to give a closer estimation of the actual CEF.