Incorporation of Oxygen into Glycolate, Glycine, and Serine during Photorespiration in Maize Leaves.

Glycolate, glycine, and serine extracted from excised Zea mays L. leaves which had been allowed to photosynthesize in the presence of (18)O(2) were analyzed by gas chromatography-mass spectrometry. In each case, only one of the oxygen atoms of the carboxyl group had become labeled. The maximum enrichment observed in glycine and serine was attained after 5 minutes and 15 minutes of exposure to (18)O(2) at the CO(2) compensation point; the labeling was very high, reaching 70 to 73% of that in the applied O(2). Thus, it appears that all or nearly all of the glycine and serine are synthesized in maize leaves via fixation of O(2). In the presence of CO(2) (380 or 800 microliters per liter), (18)O-labeling was markedly slower.Glycolate enrichment was variable and much lower than that in glycine and serine. It is possible that there are additional pathways of glycolate synthesis which do not result in the incorporation of (18)O from molecular oxygen. An estimation of the metabolic flow of O(2) through the photorespiratory cycle was made. It appeared that less than 75% of the O(2) taken up by maize leaves is involved in this pathway. Therefore, other processes of O(2) metabolism must occur in the light.

[An attempt to interpret the kinetics of isotope exchange between C(18)O 2 and the water of a leaf: Experiments in the dark]. / Essai d'interprétation des cinétiques d'échange isotopique entre C(18)O 2 et eau d'une feuille: Expériences à l'obscurité

The interpretation of photosynthesis experiments using (18)O as a tracer becomes very difficult on account of the exchange rate between oxygen atoms of carbon dioxide and water. The mathematical analysis of the kinetics of isotope exchange between CO2 highly enriched with (18)O and the water of an aerial leaf kept in darkness allows the determination of two velocity coefficients, k e and k. k e, related to the gaseous CO2 diffusion, makes it possible to evaluate the CO2 flow between the gaseous phase and the leaf, as well as the diffusion resistance of the boundary layer and the leaf stomata. k, related to the CO2 hydration, gives the in situ enzymic activity of carbonic anhydrase. Measurements carried out on a great number of leaves show that k e and k vary in the same direction.In addition, a simple relation exists between the isotopic composition of the gaseous phase and the corresponding values of the C(18)O2 inside the leaf. Thus it is possible to determine the conditions for a leaf that may be favourable for light experiments designed to ascertain the origin of photosynthetic oxygen by use of (18)O.