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.