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Environmental and Stress Physiology

The Effect of pH, O₂, and Temperature on the CO₂ Compensation Point of Isolated Asparagus Mesophyll Cells ¹

Department of Biology, York University, Downsview, Ontario, Canada

The effect of pH, O₂ concentration, and temperature on the CO₂ compensation point ([CO₂]) of isolated Asparagus sprengeri Regel mesophyll cells has been determined in a closed, aqueous environment by a sensitive gas-chromatographic technique. Measured values range between 10 and 100 microliters per liter CO₂ depending upon experimental conditions. The (CO₂) increases with increasing temperature. The rate of increase is dependent upon the O₂ concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O₂ concentrations. The differential effect of temperature on (CO₂) is more pronounced at pH 6.2 than at pH 8.0, but this pH-dependence is not attributable to a direct, differential effect of pH on the relative rates of photosynthesis and photorespiration, as the O₂-sensitive component of (CO₂) remains constant over this range. The (CO₂) of Asparagus cells at 25°C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O₂ concentration. It is suggested that the pH-dependence of (CO₂) is related to the ability of the cell to take up CO₂ from the aqueous environment. The correlation between high HCO₃^– concentrations and low (CO₂) at alkaline pH indicates that extracellular HCO₃^– facilitates the uptake of CO₂, possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O₂– and temperature-dependence of (CO₂) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO₂ to a level sufficient to reduce or suppress photorespiration.

¹ Supported by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC).