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.