Role of cyclic electron transport in photosynthesis as measured by the photoinduced turnover of P700 in vivo

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Abstract
The light-induced turnover of P700 was measured spectrophotometrically in a wide variety of algae [Porphyridium cruentum, Chlorella pyrenoidosa, Anacystis nidulans, Scenedesmus obliquus, Aphanocapsa, Skeletonema costatum, Cryptochrysis sp., Euglena gracilis, Ulva lactuca, Porphyra umbilicalis, Chlamydomonas reinhardi] and some photosynthetic mutants. Analysis of the postillumination recovery of P700+ revealed that the apparent 1st-order rate constant for reduction via the cyclic pathway was much lower than that via the noncyclic pathway. After activation of photosystems 1 and 2 the half-time [t 1/2] for reduction of P700+ was 5-20 ms, whereas after activation of primarily photosystem 1 a longer t 1/2 of approximately 150 ms was observed. The extent of the photooxidation of P700 was the same in both regimes of illumination. The longer t 1/2 was also noted after inhibition of photosystem 2 by 3-(3,4-dichlorophenyl)-1,1-dimethylurea or mild heat shock and in mutant algae known to lack a functional photosystem 2. No signal was observed in mutants lacking P700 itself but those strains lacking either plastocyanin or cytochrome f were capable of a very slow turnover (reduction t 1/2 > 500 ms at room temperature). This very slow turnover was not affected by carbonyl cyanide m-chlorophenylhydrazone or the plastoquinone antagonist, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, indicating that the pathway for reduction of P700+ in these mutants is not energy linked and does not utilize the intersystem electron transport chain. The slow, 150 ms, reduction of P700+ due to cyclic flow was not observed when cells were engaged in photosynthesis at high-light intensities. The data are interpreted as evidence for the involvement of the total functional pool of P700 in both electron transport pathways, and cyclic electron transport probably does not contribute to photosynthesis in O2-evolving autotrophs.