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Plant Physiology 65:780-784 (1980)
© 1980 American Society of Plant Biologists

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Articles

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa1

Barry J. Shelp and David T. Canvin

Department of Biology, Queen's University, Kingston, Ontario K7L 3N6 Canada

The inhibition of photosynthesis by O2 in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O2 electrode) to measure carbon assimilation. CO2 response curves were determined under different O2, pH, and temperature conditions. Regardless of the experimental technique and condition, O2 inhibition was not evident until a concentration of 50% was reached; Vmax values were reduced whereas Km (CO2) values were unaffected by the increasing O2 concentration. The response of photosynthesis to O2 was independent of CO2 and HCO3– concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O2, a 12% inhibition in 50% O2, and a 24% inhibition in 100% O2. The inhibition by 50% O2 was still reversible after 20 minutes exposure whereas 100% O2 caused irreversible inhibition after only 4 minutes.

The O2 inhibition is discussed in terms of the oxygenase reaction and a Mehler reaction supporting pseudocyclic electron flow. The results are inconsistent with the proposals that photorespiration exists in these algae and that a CO2-concentrating mechanism suppresses the O2 inhibition of photosynthesis.

1 This investigation was supported by a research grant to D. T. C. and a postgraduate fellowship to B. J. S. from the National Science and Engineering Research Council of Canada.






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