Alteration of O-J-I-P Chlorophyll Induction Kinetics by Dichromate: An Effect on the Water-Splitting System

Abstract

The rapid chlorophyll fluorescence tran-sients related to Photosystem II (PSII) activity were investigated when cells of the alga Chlamydomonas reinhardtii were exposed to dichromate ions. We found that dichromate produced an inhibitory effect seen as quenching of variable fluorescence yield in all rapid fluorescence transients: O-J, J-I and I-P. The decreased fluorescence yield at the O-J transient for dichromate-treated algae in dark or light-adapted samples resulted from alteration of the D1 protein. Decreased activation energy for the O-J step in light-adapted samples treated with dichromate indicated a decrease in the number of active PSII reaction centers having a functional D1 protein. The low activation energy for the J-I step for light-adapted samples, regardless of the dichromate effect, was caused by a large fraction of Q B having already been reduced by continuous illumination. The activation energy for the I-P step was negative for light-adapted samples under dichromate treatment, showing that most of the remaining, active PSII reaction centers were reduced under continuous illumination. However, it cannot be excluded that the change of activation energy for the O-J and I-P steps resulted from a deteriorating effect of dichromate on the PSII water splitting system.