Shoots of intact peanut and sunflower seedlings evolved CO2 in the light which was enriched more than 10 per mille in 13C compared with simultaneous CO2 evolution from the roots. Carbon dioxide collected from tobacco leaves in the light was 10 per mille enriched in 13C compared with that collected in the dark. Anaerobic conditions inhibited photorespiration but did not change isotopic ratios of dark respiration.
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Hsu et al. (1972) studied this question.