Plant Physiol.
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Plant Physiology 58:336-340 (1976)
© 1976 American Society of Plant Biologists

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Articles

Photoreduction of O2 Primes and Replaces CO2 Assimilation 1

Richard J. Radmer and Bessel Kok

a Martin Marietta Laboratories, Baltimore, Maryland 21227

A mass spectrometer with a membrane inlet system was used to monitor directly gaseous components in a suspension of algae. Using labeled oxygen, we observed that during the first 20 seconds of illumination after a dark period, when no net O2 evolution or CO2 uptake was observed, O2 evolution was normal but completely compensated by O2 uptake. Similarly, when CO2 uptake was totally or partially inhibited, O2 evolution proceeded at a high (near maximal) rate. Under all conditions, O2 uptake balanced that fraction of the O2 evolution which could not be accounted for by CO2 uptake.

From these observations we concluded that O2 and CO2 are in direct competition for photosynthetically generated reducing power, with O2 being the main electron acceptor during the induction process and under other conditions in which CO2 reduction cannot keep pace with O2 evolution. The high rate of the O2 uptake reaction observed in the presence of iodoacetamide, KCN, or carbonyl cyanide p-trifluoromethyoxyphenylhydrazone, suggests that a special high capacity oxidase distinct from ribulose diphosphate oxygenase exists in whole cells. The rapid reduction of molecular O2 after a period of darkness probably serves as a priming reaction for the photosynthetic apparatus. The high steady state rate of the O2 cycle in the absence of CO2 fixation suggests that the regulation of photosynthesis does not involve significant changes in the rate of photochemical electron transport.

1 This work was supported in part by the Energy Research and Development Administration Contract E(11-1)-3326, the National Science Foundation Grant PCM74-20736 A01, and Grant AER73-03291. Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the supporting agencies.




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