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Articles

Inorganic Carbon Uptake during Photosynthesis ¹

II. Uptake by Isolated Asparagus Mesophyll Cells during Isotopic Disequilibrium

Department of Biology, York University, Downsview, Ontario, CANADA M3J 1P3

The species of inorganic carbon (CO₂ or HCO₃^–) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO₂ or HCO₃^– transport, during steady state photosynthesis, is achieved by monitoring the changes (by ¹⁴C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO₂ or HCO₃^– transport) and experimental time-courses of ¹⁴C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO₂, rather than HCO₃^–, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO₂ is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO₃^– transport, as the incorporation of ¹⁴C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO₂ uptake alone. The contribution of HCO₃^– to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO₃^– concentration. The evidence for direct HCO₃^– transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of ¹⁴C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO₂, which is partially alleviated by a high extracellular concentration of HCO₃^–.

³ Present address: Department of Botany, University of Toronto, Toronto, Canada M5S 1A1.

¹ Supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC). G. S. E. and G. W. O. are the recipients of NSERC Postgraduate Scholarships.