Absence of fluorescence quenching in a photosynthetic mutant of Euglena gracilis.

Substantial experimental evidence indicates that the level of chlorophyll fluorescence in algae and higher plants is controlled predominantly by the activity of photosystem II (PS II) of the photosynthetic electron transport chain (2). The overall fluorescence yield seems to depend on the oxidation/reduction state of Q, the presumed primary electron acceptor of PS II (7) (H2O ? Y hv hv -> Q -) A -* ? -> P700 XH). AcPS II PS I cording to Duysens' hypothesis (4), the oxidized form of Q quenches chlorophyll fluorescence, while the reduced form (QH) does not. We report here a photosynthetically defici.ent strain of Eutglena which appears to be lacking the fluorescence quenching normally associated with the {presence of Q. Eiglena. gracilis (strain Z) was cultured as previously described (9). The pale green mutant P4 is unable to carry out photosynthetic electron transport because of a block at or near light reaction II; detailed studies of this strain have been reported elsewhere (10). Chloroplast fragments were prepared according to Katoh and San Pietro (6). Fluorescence was measured with the technique devised by Duysens (3). Whole cells or chloroplasts were illuminated with a modulated monochromatic light beam (X = 435 nm, 270 cps), and the fluorescence emis;sion was measured with a photomultiplier/amplifier system tuned to the chopping frequency. Provision was