Estimation of photorespiratory carbon dioxide recycling during photosynthesis

Photorespiration rate can be estimated in vivo but the actual amount of photorespiratory CO2 emitted or recycled by the leaf is largely unknown. We exploited the insensitivity of infrared gas analyzers to (CO2)-C-13 to detect the photorespiratory CO2 emission of leaves exposed to air containing only (CO2)-C-13. Photorespiratory CO2 emission was calculated by subtracting the (CO2)-C-12 emitted under non-photorespiratory (2% O-2) conditions from that emitted under photorespiratory (21% O-2) conditions. Illuminated leaves of herbaceous and tree species emitted a constant amount of (CO2)-C-12 within 30-60 s after switching to (CO2)-C-13. Photorespiratory CO2 emission was less than 20% of the photorespiratory rate simultaneously calculated by combining fluorescence and gas exchange. Thus, the leaves recycled more than 80% of photorespiratory CO2. The highest recycling was associated with high rates of photosynthesis in tomato and spinach. In the dark, mitochondrial respiration measured by the emission of (CO2)-C-12 in air with (CO2)-C-13 was similar to that measured by conventional gas exchange in ambient air, thus confirming the accuracy of our method. In the light, mitochondrial respiration estimated from the emission of (CO2)-C-12 after complete labeling of photorespiration was lower than in the dark, suggesting either light-inhibition or recycling of respiratory carbon.