The Effect of pH, O 2 , and Temperature on the CO 2 Compensation Point of Isolated Asparagus Mesophyll Cells

Abstract

The effect of pH, O(2) concentration, and temperature on the CO(2) compensation point (capital GHE, Cyrillic[CO(2)]) 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 CO(2) depending upon experimental conditions. The capital GHE, Cyrillic(CO(2)) increases with increasing temperature. The rate of increase is dependent upon the O(2) concentration and is more rapid at high (250-300 micromolar), than at low (30-60 micromolar), O(2) concentrations. The differential effect of temperature on capital GHE, Cyrillic(CO(2)) 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 O(2)-sensitive component of capital GHE, Cyrillic(CO(2)) remains constant over this range. The capital GHE, Cyrillic(CO(2)) of Asparagus cells at 25 degrees C decreases by 50 microliters per liter when the pH is raised from 6.2 to 8.0, regardless of the prevailing O(2) concentration. It is suggested that the pH-dependence of capital GHE, Cyrillic(CO(2)) is related to the ability of the cell to take up CO(2) from the aqueous environment. The correlation between high HCO(3) (-) concentrations and low capital GHE, Cyrillic(CO(2)) at alkaline pH indicates that extracellular HCO(3) (-) facilitates the uptake of CO(2), possibly by increasing the flux of inorganic carbon from the bulk medium to the cell surface. The strong O(2)- and temperature-dependence of capital GHE, Cyrillic(CO(2)) indicates that isolated Asparagus mesophyll cells lack an efficient means for concentrating intracellular CO(2) to a level sufficient to reduce or suppress photorespiration.