Photosynthesis and Inorganic Carbon Transport in Isolated Asparagus Mesophyll Cells

The possibility of HCO(3) (-) 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 (14)CO(2) distribution, suggesting that HCO(3) (-) as well as CO(2) crosses the plasmalemma. Intracellular pH values, calculated from the distribution of (14)CO(2) 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-(14)C]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 CO(2) 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 CO(2) arising from the spontaneous dehydration of HCO(3) (-). Furthermore, CO(2) compensation points of the cells in liquid media at 21% O(2) at pH 7.0 and 8.0, and the K((1/2)) CO(2) (CO(2) concentration supporting the half maximal rate of O(2) evolution) at 2% O(2) at pH 7.0 and 8.0 are not consistent with HCO(3) (-) transport. These results indicate that the principal inorganic carbon species crossing the plasmalemma in these cells is CO(2).