PUB - Publikationen an der Universität Bielefeld

Mass spectrometric analysis of the gas exchange of illuminated leaflets of 10-14 d old pea seedlings revealed not only O-16(2)-liberation from photosynthetic (H2O)-O-16-splitting, but also uptake of O-18(2), applied to the gas phase of the reaction vessel. Isolated intact chloroplasts of such leaflets suspended in a medium containing NaHCO3 and glycerate 3-phosphate, on irradiation with blue (lambda 448 nm) or red (lambda 679 nm) light also produced oxidation and consumed O-18(2) from the gas phase. The two reactions were saturated at the same quantum fluence rates. Uptake of (18)oxygen was not affected by inhibitors of mitochondrial respiration (alternative pathway included), such as rotenone (5 x 10(-5) M), antimycin A (5 x 10(-6) M), KCN (10(-3) M), SHAM (10(-3) M), or propylgallate (10(-3) M). It was, however, absent, when photosynthetic (16)oxygen evolution was completely inhibited by DCMU (10(-5) M). DBMIB (10(-5) M), assumed to prevent electron flow from plastoquinone pool to the cytochrome b(6)/f-complex, suppressed photosynthetic oxygen evolution, but did not impair uptake of O-18(2). A Similar result was obtained at application of 4 x 10(-5) M 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-2-uptake during photosynthesis is an additional means for providing ATP for photosynthetic CO2-reduction by increasing the proton gradient across the thylakoid membrane.