In intact leaves, the maximum fluorescence level (FM) is independent of the redox state of the plastoquinone pool: A DCMU-inhibition study

The effects of DCMU (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea) on the fluorescence induction transient (OJIP) in higher plants were re-investigated. We found that the initial (F₀) and maximum (F_M) fluorescence levels of DCMU-treated leaves do not change relative to controls when the treatment is done in complete darkness and DCMU is allowed to diffuse slowly into the leaves either by submersion or by application via the stem. Simultaneous 820 nm transmission measurements (a measure of electron flow through Photosystem I) showed that in the DCMU-treated samples, the plastoquinone pool remained oxidized during the light pulses whereas in uninhibited leaves, the F_M level coincided with a fully reduced electron transport chain. The identical F_M values with and without DCMU indicate that in intact leaves, the F_M value is independent of the redox state of the plastoquinone pool. We also show that (i) the generally observed F₀ increase is probably due to the presence of (even very weak) light during the DCMU treatment, (ii) vacuum infiltration of leaf discs leads to a drastic decrease of the fluorescence yield, and in DCMU-treated samples, the F_M decreases to the I-level of their control (leaves vacuum infiltrated with 1% ethanol), (iii) and in thylakoid membranes, the addition of DCMU lowers the F_M relative to that of a control sample.