Electron fluxes through Photosystem I in cucumber leaf discs probed by far-red light

Cucumber leaf discs were illuminated at room-temperature with far-red light to photo-oxidise P700, the chlorophyll dimer in Photosystem (PS) I. The post-illumination kinetics of P700⁺ re-reduction were studied in the presence of inhibitors or cofactors of photosynthetic electron transport. The re-reduction kinetics of P700⁺ were well fitted as the sum of three exponentials, each with its amplitude and rate coefficient, and an initial flux (at the instant of turning off far-red light) given as the product of the two. Each initial flux is assumed equal to a steady state flux during far-red illumination. The fast phase of re-reduction, with rate coefficient k ₁∼10 s^-1, was completely abolished by a saturating concentration of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); it is attributed to electron flow to P700⁺ from PS II, which was stimulated to some extent by far-red light. The intermediate phase, with rate coefficient k₁∼1 s^-1, was only partly diminished by methyl viologen (MV) which diverts electron flow to oxygen. The intermediate phase is attributed to electron donation from reduced ferredoxin to the intersystem pool; reduced ferredoxin could be formed: (1) directly by electron donation on the acceptor of PS I; and/or (2) indirectly by stromal reductants, in line with only a partial inhibition of the intermediate phase by MV. Duroquinol enhanced the intermediate phase in the presence of DCMU, presumably through its interaction with thylakoid membrane components leading to the partial reduction of plastoquinone. The slow phase of P700⁺ re-reduction, with rate coefficient k₁∼0.1 s^-1, was unaffected by DCMU and only slightly affected by MV; it could be associated with electron donation to either: (1) the intersystem chain by stromal reductants catalysed by NAD(P)H dehydrogenase slowly; or (2) plastocyanin/P700⁺ by ascorbate diffusing across the thylakoid membrane to the lumen. It is concluded that a post-illumination analysis of the fluxes to P700⁺ can be used to probe the pathways of electron flow to PS I in steady state illumination.