Differences in the growth and physiological response of eight Myriophyllum species to carbon dioxide depletion

The growth and photosynthesis of submerged aquatic plants is often limited by the CO2 availability in their habitats, but about 50% of all submerged plants are able to use HCO3− as an additional carbon source. This ability to use HCO3− provides a competitive advantage over non-CO2-users under CO2 limiting conditions. Here, we studied the growth and physiological response of eight Myriophyllum species (M. spicatum, M. triphyllum, M. heterophyllum, M. papillosum, M. variifolium, M. tetrandrum, M. tuberculatum, M. verticillatum) to CO2 and HCO3− use conditions. Physiologically, plants acclimated to HCO3− use showed higher net photosynthetic rates under both CO2 and HCO3− use conditions than plants grown under high CO2 conditions. Furthermore, we found significant differences in the HCO3− use capacity between the Myriophyllum species. The long-term exposure to high CO2 conditions during growth caused an accumulation of starch within the leaves, while the chlorophyll content decreased. Moreover, plants allocated more biomass into roots and reduced the leaf biomass under CO2 enrichment. The growth rates illustrate that M. spicatum is the most efficient HCO3− user out of the tested Myriophyllum species, followed by M. triphyllum and M. heterophyllum. The other five studied Myriophyllum species showed only a minor or no HCO3− use capacity. We conclude, that the HCO3− use capacity varies greatly even within a single genus, and that the HCO3− use capacity, among others, is an important trait of strong competitive submerged plants.