Photosynthesis and Inorganic Carbon Transport in Isolated Asparagus Mesophyll Cells

Abstract

The possibility of HCO₃⁻ transport into isolated leaf mesophyll cells of Asparagus sprengeri Regel has been investigated. Measurement of the inorganic carbon pool in these cells over an external pH range 6.2 to 8.0, using the silicone-fluid filtration technique, indicated that the pool was larger than predicted by passive ¹⁴CO₂ distribution, suggesting that HCO₃⁻ as well as CO₂ crosses the plasmalemma. Intracellular pH values, calculated from the distribution of ¹⁴CO₂ between the cells and the medium, were found to be higher (except at pH 8.0) than those previously determined by 5,5-dimethyl[2-¹⁴C]oxazolidine-2,4-dione distribution. It is suggested that the inorganic carbon accumulated above predicted concentrations may be bound to proteins and membranes and thus may not represent inorganic carbon actively accumulated by the cells, inasmuch as in a closed system at constant CO₂ concentration, the photosynthetic rates at pH 7.0 and 8.0 were 5 to 8 times lower than the maximum rate which could be supported by CO₂ arising from the spontaneous dehydration of HCO₃⁻. Furthermore, CO₂ compensation points of the cells in liquid media at 21% O₂ at pH 7.0 and 8.0, and the K_½ CO₂ (CO₂ concentration supporting the half maximal rate of O₂ evolution) at 2% O₂ at pH 7.0 and 8.0 are not consistent with HCO₃⁻ transport. These results indicate that the principal inorganic carbon species crossing the plasmalemma in these cells is CO₂.