title: Light-induced absorbance changes due to photosystems 1 and 2 in spinach chloroplasts at minus 50 degrees C.
abstract: Absorbance changes in the region 500-565 nm and at 702 nm, brought about by excitation of Photosystems 1 and 2, respectively, were measured in spinach chloroplasts at minus 50 degrees C. Either dark-adapted chloroplasts were used or chloroplasts preilluminated with a number of short saturating flashes just before cooling. Both photosystems were found to cause a light-induced increase of absorbance at 518 nm (due to "P518"). The System 1-induced change was not affected by pre-illumination. It decayed within 1 s in the dark and showed similar kinetics as P700. Experiments in the presence of external electron acceptors (methylviologen or Fe(CN)6-3-) suggested that P518 was not affected by the redox state of the primary electron acceptor of System 1. The absorbance increase at 518 nm due to System 2 decayed in the dark with a half-time of several min. The kinetics were similar to those of C-550, the presumed indicator of the primary electron acceptor of System 2. After two flashes preillumination the changes due to P518 and C-550 were reduced by about 40%, and a relatively slow, System 2-induced oxidation of cytochrome b559 occurred which proceeded at a similar rate as the increase in yield of chlorophyll a fluorescence. The results indicate that at minus 50 degrees C two different photoreactions of System 2 occur. One consists of a photoreduction of the primary electron acceptor associated with C-550, accompanied by the oxidation of an unknown electron donor; the other is less efficient and results in the photooxidation of cytochrome b559.
mesh_terms: D002736:Chloroplasts / Q000187:drug effects / Q000378:metabolism*; D004237:Diuron / Q000494:pharmacology; D005615:Freezing; D007700:Kinetics; D008027:Light; D010785:Photophosphorylation / Q000187:drug effects; D010944:Plants; D013053:Spectrophotometry
chemical_list: D004237:Diuron