Cells, of Synechococcus sp. PCC 7002 showed a low oxidation level of P700 under a far-red light at 6 W m−2 which induced nearly complete oxidation of P700 in spinach leaves, and a stronger far-red light was required to observe the oxidation of P700. DCMU did not affect the level of P700+2 but 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone induced the oxidation of P700 under far-red light, indicating that the low oxidation level of P700 was due to the donation of electrons to P700+2 from the cytosolic respiratory donors through the intersystem chain at the plastoquinone pool. The electron transfer from the cytosolic donors to the intersystem chain was inhibited by HgCl2 but not by antimycin A. The reduction of P700+ in Synechococcus cells, after illumination by strong far-red light was mostly accounted for by the electron flow to the inter system chain from the respiratory donors (t½=0.5 s). P700+ was reduced also after a 50-ms, saturating pulse light in the presence of DCMU (t½=38 ms), which shows the electron flow to the intersystem chain from the photoreductant of PS I, possibly NAD(P)H.
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Mi et al. (1992) studied this question.