THE RELATIVE RATES OF PHOTOSYNTHETIC ASSIMILATION OF ISOTOPIC FORMS OF CARBON DIOXIDE

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Abstract
Metabolic production and consumption of isotopic forms of CO2 were measured using a recording mass spectrometer. Relative assimilation rates of C12O2, C13O2, and C14O2 were measured with suspensions of Chlorella pyrenoidosa and barley leaf tissue. Assuming that light had no effect on metabolic processes leading to production or consumption of CO2, other than photosynthesis, the magnitude of carbon isotope discrimination in photosynthesis was detd. Cl2O2, Cl3O2, and Cl4O2 were utilized at rates which were in the ratios of 1.00:0.96:0.85, respectively. Using tracer oxygen in the gas phase, respiratory oxygen uptake was separated from photosynthetic oxygen production. It was thus shown that light did not affect significantly the rate of oxygen consumption under the conditions employed. The rate of CO2 isotope exchange (no net change in total CO2) was found to be several times the net rate of respiratory CO2 production.