Sigmoidal reduction kinetics of the photosystem II acceptor side in intact photosynthetic materials during fluorescence induction

Illumination of dark-adapted photosynthetic samples leads to fluorescence induction (FI) that can be described by a triphasic O-J-I-P fluorescence rise. Its kinetics follows the accumulation of reduced photosystem II (PSII) acceptors. In isolated thylakoid membranes, FI is often used to study photosynthetic electron transport. A simple quantitative analysis method was recently developed to fit these FI traces and also lead to a better understanding of action sites of artificial electron acceptors. However, a quantitative method was still lacking for FI in intact systems like leaves, where the FI kinetics shows a clear I-peak. Here, we present a new quantitative method to analyze experimental FI traces in leaves and intact chloroplasts. It revealed a sigmoidicity in the reduction kinetics of the PSII acceptor side of intact systems. The results also show that the origin of each phase is independent of the photosynthetic material used. The effects of decyl-plastoquinone on intact chloroplasts retarded predominantly the I-P rise and clearly indicates that this phase is related to the accumulation of a reduce PQ pool, as observed in isolated thylakoid membranes.