Photosynthetic bicarbonate utilization in the aquatic angiospermsPotamogeton andElodea

Autor: R. J. Helder, J. F. H. Snel, H. B. A. Prins, P. E. Zanstra
Rok vydání: 1979
Předmět:
Zdroj: Hydrobiological Bulletin. 13:106-111
ISSN: 1573-5125
0165-1404
DOI: 10.1007/bf02284741
Popis: As the CO2 supply often limits photosynthesis a number of aquatic species use HCO3− as carbon source as well. The use of HCO3− leads to the production of one OH− for every molecule/CO2 fixed. The OH− is excreted into the medium. We studied the mechanism of HCO3− utilization in the leaves ofElodea andPotamogeton. In the so-called polar leaves of these plants the HCO3− 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 HCO3− 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), HCO3− and CO3= 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 HCO3− 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 HCO3− or by a proton (H+) extrusion at the lower epidermis causing conversion of HCO3− into CO2 in the cell wall. This latter mechanism is discussed in more detail.
Databáze: OpenAIRE
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