text: In the study of plants' photorespiration, the labeling of atmosphere by oxygen-18 allows for the measurement of oxygen uptake by the photorespiration pathway. Labeling by  gives the unidirectional flux of  uptake, while there is a net photosynthetic  evolution. It was demonstrated that, under preindustrial atmosphere, most plants reabsorb, by photorespiration, half of the oxygen produced by photosynthesis. Then, the yield of photosynthesis was halved by the presence of oxygen in atmosphere.