Performance of active Photosystem II centers in photoinhibited pea leaves

Effects of photoinhibition on photosynthesis in pea (Pisum sativum L.) leav es were investigated by studying the relationship between the severity of a photoinhibitory treatment (measured as F-v/F-m) and several photoacoustic and chlorophyll a fluorescence parameters. Because of the observed linear r elationship between the decline of F-v/F-m and the potential oxygen evoluti on rate determined by the photoacoustic method, the parameter F-v/F-m was u sed as an indicator for the severity of photoinhibition. Our analysis revea led that part of the Photosystem II (PS II) reaction centers is inactive in oxygen evolution and is also less sensitive to photoinhibition. Correcting the parameter q(P) (fraction of open PS II reaction centers) for inactive PS II centers unveiled a strong increase of q(P) in severely inhibited pea leaves, indicating that the inactivated active centers do no longer contrib ute to q(P) and that photoinhibition has an all or none effect on PS II cen ters. Analysis of q(E) (energy quenching) demonstrated its initial increase possibly associated with dephosphorylation of LHC II. Analysis of q(I) (ph otoinhibition dependent quenching) showed that the half-time of recovery of q(I) increases steeply below an F-v/F-m of 0.65. This increase of the rela xation half-time corresponds with a decrease of the electron transport rate J and tentatively indicates that the supply of ATP, needed for the recover y, starts to decrease. The data indicate the necessity of correcting for in active centers in order to make valuable conclusions about effects of photo inhibition on photosynthetic parameters.