Characterization of photorespiration in intact leaves using carbon dioxide labeling

The ¹³C nuclear magnetic resonance spectra of sucrose extracted from intact soybean and corn leaves labeled with ¹³CO₂ for 5 to 180 minutes have been obtained at 22.6 megahertz. The spectra provide a direct determination of the relative concentrations of ¹²C-¹³C and ¹³C-¹³C pairs of carbons in the labeled triose components of sucrose. This distribution of label is strongly dependent on the O₂ 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.