RESUMO
Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.