Oxygen Exchange in Leaves in the Light

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Abstract
Photosynthetic O2 production and photorespiratory O2 uptake were measured using isotopic techniques, in the C3 species Hirschfeldia incana Lowe., Helianthus annuus L., and Phaseolus vulgaris L. At high CO2 and normal O2, O2 production increased linearly with light intensity. At low O2 or low CO2, O2 production was suppressed, indicating that increased concentrations of both O2 and CO2 can stimulate O2 production. At the CO2 compensation point, O2 uptake equaled O2 production over a wide range of O2 concentrations. O2 uptake increased with light intensity and O2 concentration. At low light intensities, O2 uptake was suppressed by increased CO2 concentrations so that O2 uptake at 1,000 microliters per liter CO2 was 28 to 35% of the uptake at the CO2 compensation point. At high light intensities, O2 uptake was stimulated by low concentrations of CO2 and suppressed by higher concentrations of CO2. O2 uptake at high light intensity and 1000 microliters per liter CO2 was 75% or more of the rate of O2 uptake at the compensation point. The response of O2 uptake to light intensity extrapolated to zero in darkness, suggesting that O2 uptake via dark respiration may be suppressed in the light. The response of O2 uptake to O2 concentration saturated at about 30% O2 in high light and at a lower O2 concentration in low light. O2 uptake was also observed with the C4 plant Amaranthus edulis; the rate of uptake at the CO2 compensation point was 20% of that observed at the same light intensity with the C3 species, and this rate was not influenced by the CO2 concentration. The results are discussed and interpreted in terms of the ribulose-1,5-bisphosphate oxygenase reaction, the associated metabolism of the photorespiratory pathway, and direct photosynthetic reduction of O2.