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.