Characterization of photorespiration in intact leaves using carbon dioxide labeling.

The (13)C nuclear magnetic resonance spectra of sucrose extracted from intact soybean and corn leaves labeled with (13)CO(2) for 5 to 180 minutes have been obtained at 22.6 megahertz. The spectra provide a direct determination of the relative concentrations of (12)C-(13)C and (13)C-(13)C pairs of carbons in the labeled triose components of sucrose. This distribution of label is strongly dependent on the O(2) concentrations used during labeling for soybean but not for corn. Based on the time dependence of the experimental labeling patterns, photorespiration in air appears to be a sizable fraction of the apparent photosynthetic rate for soybean, but not for corn.