Photosensitized inhibitor formation in isolated, aging chloroplasts.
The possible mechanism which leads to inactivation phenomena at high intensities of blue light during the color-sensitive phase in the time course of the Hill reaction was investigated in more detail. The following results were obtained: a) The inhibition of the Hill reaction is also observed during but not prior to the color-sensitive state if the supernatant of a blue-illuminated chloroplast suspension is used as a medium for a fresh assay in red light. b) Addition of linolenic acid, which leads to a stimulation of electron transport in intact chloroplasts, intensifies the inhibition at the blue-light-sensitive stage. c) The content of malondialdehyde, an indicator for free-radical-type breakdown of unsaturated fatty acids, increases selectively after blue-light exposure during the color-sensitive phase of the Hill reaction. This result was obtained with both the thiobarbituric-acid test and fluorescence measurements of Schiff-base type conjugates. d) The gas chromatograms of total chloroplast lipids show a different spectrum for plastids exposed to blue light when compared to a non-illuminated control sample. The changes imply alterations in the composition of long-chain fatty acids. These results warrant the conclusion that the blue-light inhibition is closely linked to fatty-acid breakdown which is sensitized by light absorbed by pigments detached from the photosynthetic centers during the process of chloroplast aging in vitro.
Institute of Molecular Biophysics, Fels Fund, Florida State University, Tallahassee, Florida, USA.
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Harnischfeger