Investigation of the CO2 Dependence of Quantum Yield and Respiration in Eucalyptus pauciflora

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Abstract
In leaves of C3 plants, the rate of nonphotorespiratory respiration appears to be higher in darkness than in the light. This change from a high to a low rate of carbon loss with increasing photon flux density leads to an increase in the apparent quantum yield of photosynthetic CO2 assimilation at low photon flux densities (Kok effect). The mechanism of this suppression of nonphotorespiratory respiration is not understood, but biochemical evidence and the observation that a Kok effect is often not observed under low O2, has led to the suggestion that photorespiration might be involved in some way. This hypothesis was tested with snowgum (Eucalyptus pauciflora Sieb. ex Spreng.) using gas exchange methods. The test was based on the assumption that if photorespiration were involved, then it would be expected that the intercellular partial pressure of CO2 would also have an influence on the Kok effect. Under normal atmospheric levels of CO2 and O2, a Kok effect was found. Changing the intercellular partial pressure of CO2, however, did not affect the estimate of nonphotorespiratory respiraton, and it was concluded that its decrease with increasing photon flux density did not involve photorespiration. Concurrent measurements showed that the quantum yield of net assimilation of CO2 increased with increasing intercellular partial pressure of CO2, and this increase agreed closely with predictions based on recent models of photosynthesis.