Effect of carbon dioxide on photorespiration

The isotopic CO₂ technique for measuring photorespiration was shown to be a valid technique for measuring the unidirectional inward and outward fluxes of CO₂ from a sunflower (Helianthus annuus L.) leaf in the light. The rate of photorespiration was decreased little as the CO₂ concentration was increased from 20 to 1,150 microliters per liter. This finding contradicts the widely held assumption that photorespiration is suppressed at high CO₂ concentrations. Some discussion regarding this apparent conflict is presented.