The Interactive Effects of Inorganic Carbon and Light on a Tropical Submerged Macrophyte: Egeria densa Planchon (Hydrocharitaceae)

The interactive effects of carbon dioxide (CO(2)) and bicarbonate (HCO(3)(-)) on a tropical submerged species: Egeria densa Planchon (Hydrocharitaceae) was studied. A factorial combination of two concentrations of CO(2) (370 ppm and 2,000 ppm) and three concentrations of HCO(3)(-) (0.1 mM, 0.85 mM and 1.7 mM) were set at the same condition of light intensities (300 mu mol m(-2)s(-1)) and temperature (23 degrees 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 CO(2) concentration were observed. The results indicate that the concentrations of both CO(2) and HCO(3)(-) affected growth. At low concentrations of CO(2) and HCO(3)(-) there was the lowest RGR, growth rate, and production of new shoots and roots. At the high CO(2) concentration, the results showed the growth of plants will decrease when the HCO(3)(-) concentration was low. The ability of E. densa to use HCO(3)(-) 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 HCO(3)(-) at 23 degrees C and 300 mu mol m(-2)s(-1) of light for 20 hours. The final pH, which was 10.13-10.78, showed that E. densa can take up HCO(3)(-), after an attended treatment at a high CO(2) concentration. Moreover, the effect of HCO(3)(-) on photosynthetic oxygen production was studied at low (50 mu mol m(-2)s(-1)) and high (300 mu mol m(-2)s(-1)) light intensities. The rate of photosynthesis increased when the concentration of HCO(3)(-) was increased. Furthermore, the photosynthetic rate at the high light intensity condition was 10 times greater than at the low light intensity condition.