title: Study of the chlorophyll fluorescence in chloroplasts and algae with the plastoquinone antagonist dibromothymoquinone.
abstract: The effect of the plastoquinone antagonist dibromothymoquinone on chlorophyll fluorescence in vitro and in vivo was investigated. 1. With chlorophyll a in solution quenching is observed, more efficient than that of p-benzoquinone (the Stern-Volmer constant K equals to 200 M-1); ascorbate removes this effect. 2. With isolated chloroplasts, a dramatic enhancement of quenching occurs (also abolished by ascorbate) in the following order of importance: thermal step, photochemical step (of variable fluorescence), and constant fluorescence; K for the total variable fluorescence approximately equal to 480 000 M-1, for constant approximately equal to 190 000 M-1. Parallel to the quenching effect, an enlargement of the complementary area of the variable fluorescence is observed. Addition of ascorbate suppresses the quenching and this enlargement. If this area in the presence of reduced dibromothymoquinone is similar to that of the control, this means that A, the pool of oxidant next to Q, remains connected to it, a result contradicted by the O2-burst measurement. However, when the excess of unbound dibromothymoquinone is washed out, which removes the quenching effect, a significant decrease of the area is seen, in agreement then with the burst results. Therefore, the meaning of the complementary area and the nature of the controlling factors may be questioned. 3. With whole cells (Chlorella), no quenching is observed; instead, an enhancement of the thermal step is noticed, together with an even larger increase of the complementary area. Dibromothymoquinone being in an oxidized form at the thylakoid level, it is proposed that in intact membranes, the chlorophyll molecules are not readily accessible to it: thus, no quenching is observable, and the predominant effect is on the redox chain. The specific effect of dibromothymoquinone on the thermal step supports the hypothesis that is normally controlled by a secondary quencher, related to plastoquinone.
mesh_terms: D002708:Chlorella / Q000187:drug effects / Q000378:metabolism*; D002734:Chlorophyll / Q000378:metabolism*; D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D010084:Oxidation-Reduction; D011809:Quinones / Q000378:metabolism / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence
chemical_list: D011809:Quinones; D002734:Chlorophyll