Abstract
The isotopic CO(2) technique for measuring photorespiration was shown to be a valid technique for measuring the unidirectional inward and outward fluxes of CO(2) from a sunflower (Helianthus annuus L.) leaf in the light. The rate of photorespiration was decreased little as the CO(2) concentration was increased from 20 to 1,150 microliters per liter. This finding contradicts the widely held assumption that photorespiration is suppressed at high CO(2) concentrations. Some discussion regarding this apparent conflict is presented.