Photosynthetic bicarbonate utilization in the aquatic angiospermsPotamogeton andElodea

Summary

As the CO₂ supply often limits photosynthesis a number of aquatic species use HCO ⁻₃ as carbon source as well. The use of HCO ⁻₃ leads to the production of one OH⁻ for every molecule/CO₂ fixed. The OH⁻ is excreted into the medium. We studied the mechanism of HCO ⁻₃ utilization in the leaves ofElodea andPotamogeton. In the so-called polar leaves of these plants the HCO ⁻₃ uptake takes place at the lower and OH⁻-release at the upper epidermis. This flux of negative charge is balanced by a kation flux in the same direction. The use of HCO ⁻₃ and the influx of kations is accompanied by a pH drop. The release of OH⁻ and kations at the upper epidermis causes a raise of the pH there. The pH changes and the kation concentrations (in the present experiments K⁺) are measured by means of miniature electrodes. From this the CO₂ (including H₂CO₃), HCO ⁻₃ and CO ⁼₃ concentrations were calculated. When the light is turned on, after a dark period, the pH increases simultaneously at both sides for 5–10 minutes. During this so-called a-polar phase there is no K⁺ transport through the leaf. Experiments at different ambient pH's and comparison with other aquatic species shows that this initial pH raise results from CO₂ fixation. After 5–10 minutes the polar phase and HCO ⁻₃ utilization start. At the lower side the pH and [K⁺] drop, at the upper side pH and [K⁺] increase. During the a-polar phase [CO₂] at the lower epidermis decreased, as expected. Whereas in the a-polar phase the CO₂ concentration at this side very markedly increased. This sharp increase of [CO₂] may be explained either by CO₂ diffusion from the leaf cells previously taken up as HCO ⁻₃ or by a proton (H⁺) extrusion at the lower epidermis causing conversion of HCO ⁻₃ into CO₂ in the cell wall. This latter mechanism is discussed in more detail.