A non-invasive assay of the plastoquinone pool redox state based on the OJIP-transient

The plastoquinone (PQ) pool of the photosynthetic electron transport chain becomes reduced under anaerobic conditions. Here, anaerobiosis was used as a tool to manipulate the PQ-pool redox state in darkness and to study the effects of the PQ-redox state on the Chl-a fluorescence (OJIP) kinetics in pea leaves (Pisum sativum L.). It is shown that the F-J (fluorescence intensity at 3 ms) is linearly related to the area above the OJ-phase (first 3 ms) representing the reduction of the acceptor side of photosystem II (PSII) and F-J is also linearly related to the area above the JI-phase (3-30 ms) that parallels the reduction of the PQ-pool. This means that F-J depends on the availability of oxidized PQ-molecules bound to the Q(B)-site. The linear relationships between F-J and the two areas indicate that F-J is not sensitive to energy transfer between PSII-antennae (connectivity). It is further shown that a similar to 94% reduced PQ-pool is in equilibrium with a similar to 19% reduction of Q(A) (primary quinone acceptor of PSII). The non-linear relationship between the initial fluorescence value (F-20 mu s) and the area above the OJ-phase supports the idea that F-20 mu s is sensitive to connectivity. This is reinforced by the observation that this non-linearity can be overcome by transforming the F-20 mu s-values into [Q(A)(-) ]-values. Based on the F-J-value of the OJIP-transient, a simple method for the quantification of the redox state of the PQ-pool is proposed.