Estimation of photorespiratory carbon dioxide recycling during photosynthesis

Abstract

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 analy-zers to 13CO2 to detect the photorespiratory CO2 emission of leaves exposed to air containing only 13CO2. Photorespiratory CO2 emission was calculated by subtracting the 12CO2 emitted under non-photorespiratory (2% O2) conditions from that emitted under photorespiratory (21% O2) conditions. Illuminated leaves of herbaceous and tree species emitted a constant amount of 12CO2 within 30–60 s after switching to 13CO2. 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 12CO2 in air with 13CO2 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 12CO2 after complete labeling of photorespiration was lower than in the dark, suggesting either light-inhibition or recycling of respiratory carbon.