The interactive effects of inorganic carbon and light on a tropical submerged macrophyte: Egeria densa planchon (hydrocharitaceae)

Abstract

The interactive effects of carbon dioxide (CO2) and bicarbonate (HCO3-) on a tropical submerged species: Egeria densa Planchon (Hydrocharitaceae) was studied. A factorial combination of two concentrations of CO2 (370 ppm and 2,000 ppm) and three concentrations of HCO3- (0.1 mM, 0.85 mM and 1.7 mM) were set at the same condition of light intensities (300 μmol m-2s-1) and temperature (23oC). 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 oC 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. Moreover, the effect of HCO3- on photosynthetic oxygen production was studied at low (50 μmol m-2s-1) and high (300 μmol m- 2s-1) light intensities. 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 light intensity condition.