Photorespiratory Properties of Mesophyll Protoplasts of Nicotiana plumbaginifolia.

The photorespiratory activity of mesophyll protoplasts of Nicotiana plumbaginifolia has been clearly demonstrated by the presence of a Warburg-effect, the occurrence of an important CO(2)-sensitive O(2) uptake and the effect of some photorespiratory inhibitors on photosynthetic activity. At a nonsaturating dissolved inorganic carbon (DIC) concentration (0.1 millimolar), we observed that the rate of CO(2) fixation was 60% lower at 50% O(2) compared to that measured at 2% O(2). Using (18)O(2) and mass spectrometry, we measured O(2) exchange as a function of light intensity and of DIC concentration. Oxygen uptake measured at the CO(2) compensation point (47.4 micromoles O(2) per hour per milligram chlorophyll) was three-fold higher than that measured at a saturating CO(2) concentration. Cyanide or iodoacetamide, inhibitors of the Calvin cycle, were found to reduce the O(2) uptake to the same extent as CO(2) saturation. We conclude from these results that the major part of the CO(2)-sensitive O(2) uptake is due to photorespiration. Further, we investigated the effect on net photosynthesis of some inhibitors of the glycolate pathway. At CO(2) saturation (10 millimolar DIC), 5 millimolar aminoacetonitrile (AAN), and 1 millimolar aminooxyacetate (AOA) did not cause any significant decrease in net photosynthesis. However, when these two inhibitors were added under a period of active photorespiration (10 minutes at the CO(2) compensation point at 20% O(2)), we observed a decrease in the rate of net photosynthesis at 10 millimolar DIC measured afterward (respectively, 18 and 29%). This inhibition did not appear at 2% O(2), but was stronger at 50% O(2) (40% for AAN and 47% for AOA). With 0.05 millimolar butyl 2-hydroxy-3-butynoate (BHB) or 0.5 millimolar l-methionine-dl-sulfoximine (l-MSO), rates of net photosynthesis at 10 millimolar DIC were decreased by 10 to 15%. Additional decreases were observed after a period at the CO(2) compensation point at 20% O(2) (30% for BHB and 20% for l-MSO). From the sites of action of the four inhibitors tested, we suggest the inhibition of photosynthesis occurring after a period of active photorespiration to be due to the toxic accumulation of nonmetabolized phosphoglycolate.