Abstract
Regulation of photosynthetic electron transport was studied using leaves of Arabidopsis mutants (pgr5; proton gradient regulation, crr2-2; chlororespiratory reduction) defective in activity of cyclic electron flow around photosystem I (PSI). The quantum yield of photosystem II (PSII), as determined from measurements of chlorophyll fluorescence, was higher in the wild type than in the mutant pgr5, lacking the ferredoxin-dependent cyclic pathway. Measurements of the redox state of P700 (PSI reaction center) revealed that in the wild type the PSI quantum yield was higher than that of PSII, but not in pgr5. Those experiments in crr2-2, in which NAD(P)H-dependent cyclic PSI is impaired, resembled closely the results obtained from the wild type. The results indicate that quantum yield of PSI becomes higher than that of PSII as a result of occurrence of PSI cyclic, and this serves to increase PSII quantum yield and thus rate of linear electron flow.
