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

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RESUMO

The effect of pH, O2 concentration, and temperature on the CO2 compensation point (Г[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 Г(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 Г(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 Г(CO2) remains constant over this range. The Г(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 Г(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 Г(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 Г(CO2) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO2 to a level sufficient to reduce or suppress photorespiration.

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