Chiang Mai Journal of Science

The interactive effects of carbon dioxide (CO₂) and biocarbonate (HCO₃^-) on a tropical submerged species: Egeria densa Planchon (Hydrocharitaceae) was studied. A factorial combination of two concentrations of CO₂ (370ppm and 2,000 ppm) and three concentrations of HCO₃^- (0.1mM, 0.85 mM and 1.7 mM) were set at the same condition of light intensities (300 µmol m^-2s^-1) and temperature (23 °C) the relative growth rate (RGR) and some morphological characteristics were measured after 14 days. A significant difference in RGR, shoot growth rate based on shoot length (SGR), number of new roots, and new shoots between plants treated by low and high CO₂ concentration were observed. The results indicate that the concentrations of both CO₂ and HCO₃^- affected growth. At low concentrations of CO₂ and HCO₃^- there was the lowest RGR, growth rate, and production of new shoots and roots. At the high CO₂ concentration, the results showed the growth of plants will decrease when the HCO₃^- concentration was low. The ability of E. densa to use HCO₃^- as an inorganic carbon source for photosynthesis was studied by a pH-drift experiment. Small E.densa shoots (1-1.5 cm long) were incubated in 30 ml glass-stoppered bottles with a growth medium containing 0.8 mmol l^-1 of HCO₃^- at 23 C and 300  µmol m^-2s^-1 of light for 20 hours. The final pH, which was 10.13-10.78, showed that E.densa can take up HCO₃^-, after an attended treatment at a high CO₂ concentration. Morover, the effect of HCO₃^- on photosynthetic oxygen production was studied at low (50µmol m^-2s^-1) and high (300 µmol m^-2s^-1)  light in tensiites. The rate of photosynthesis increased when the concentration of HCO₃^- was increased. Furthermore, the photosynthetic rate at the high light intensity condition was 10 times greater than at the low intensity condition.