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 O-16(2)-liberation from photosy nthetic (H2O)-O-16-splitting, but also uptake of O-18(2), applied to the ga s phase of the reaction vessel. Isolated intact chloroplasts of such leafle ts suspended in a medium containing NaHCO3 and glycerate 3-phosphate, on ir radiation with blue (lambda 448 nm) or red (lambda 679 nm) light also produ ced oxidation and consumed O-18(2) from the gas phase. The two reactions we re saturated at the same quantum fluence rates. Uptake of (18)oxygen was no t 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 photosynth etic oxygen evolution, but did not impair uptake of O-18(2). A Similar resu lt 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-e xcited electrons from the photosynthetic electron transport chain at the si te of plastoquinone pool during photosynthesis. This corresponds to chloror espiration, originally described for Chlamydomonas in darkness by Bennoun ( 1982). It is discussed, whether O-2-uptake during photosynthesis is an addi tional means for providing ATP for photosynthetic CO2-reduction by increasi ng the proton gradient across the thylakoid membrane.