Oxygen Uptake during Photosynthesis of Isolated Pea Chloroplasts

Mass spectrometric analysis of the gas exchange of illuminated leaflets of 10-14 d old pea seedlings revealed not only -liberation from photosynthetic H O-splitting, but also uptake of , applied to the gas phase of the reaction vessel. Isolated intact chloroplasts of such leaflets suspended in a medium containing NaHCO and glycerate 3-phosphate, on irradiation with blue (λ 448 nm) or red (λ 679 nm) light also produced from water oxidation and consumed from the gas phase. The two reactions were saturated at the same quantum fluence rates. Uptake of oxygen was not affected by inhibitors of mito­chondrial respiration (alternative pathway included), such as rotenone (5 x l0 ᴍ), antimycin A (5 x l0 ᴍ), KCN (10 ᴍ), SHAM (10 ᴍ), or propylgallate (10 ᴍ). It was, however, absent, when photosynthetic oxygen evolution was completely inhibited by DCMU (10 ᴍ). DBMIB (10 ᴍ), assumed to prevent electron flow from plastoquinone pool to the cytochrome b /f-complex, suppressed photosynthetic oxygen evolution, but did not impair uptake of . A similar result was obtained at application of 4 x l0 ᴍ antimycin A. The data are interpreted to show a drain off to molecular oxygen of light-excited electrons from the photosynthetic electron transport chain at the site of plastoquinone pool during photosynthesis. This corresponds to chlororespiration, originally described for Chlamydomonas in darkness by Bennoun (1982). It is discussed, whether O -uptake during photosynthesis is an additional means for providing ATP for photosynthetic CO -reduction by increasing the proton gradient across the thylakoid membrane.