Light-dependent net CO-evolution by C3 and C 4 plants.

Light-dependent CO-evolution by the green leaves of C3 and C4 plants depends on the CO2/O2 ratio in the ambient atmosphere. This and other physiological responses suggest that CO-evolution is a byproduct of photorespiration. At CO2/O2 ratios up to 10(-3), the ratio of CO evolved: CO2 fixed in photosynthesis is significantly higher in C3 than in C4 plants. This discrepancy disappears when a correction is made for the CO2-concentrating mechanism in C4 photosynthesis, by which CO2-concentration at the site of ribulose-bis-phosphate carboxylase/oxygenase in the bundle sheaths is raised significantly as compared to the ambient atmosphere. Since the oxygenase function of this enzyme is responsible for glycolate synthesis, i.e., the substrate of photorespiration, this result seems to support the conclusion that CO-evolution is a consequence of photorespiration. CO-evolution may turn out to be a useful and rather straightforward indicator for photorespiration in ecophysiological studies.