Do environmental factors influence composition of potential allelochemicals in the submersed freshwater macrophyte Elodea nuttallii (Hydrocharitaceae)?

Since Elodea nuttallii (Planch.) St. John was introduced to Europe from North America in 1939 it has invaded many water bodies and is widely spread over the continent. Its success is usually related to ease of adaptation to several environmental factors (DENDÈNE et al. 1993, JONES et al. 1993, OZIMEK et al. 1993, SIMPSON 1990). Our research, however, indicates that chemical defenses may play an additional role in this process. E. nuttallii inhibited the growth of algae and cyanobacteria in laboratory bioassays, and also the growth of the aquatic herbivorous larvae of Acentria ephemerella (Lepidoptera) (ERHARD & GROSS unpubl. results). The production of allelochemicals causing growth reduction is often influenced by several environmental factors. The amount of allelopathically active polyphenols in Myriophyllum spicatum decreased under light limitation (GROSS 2003). Both light and nitrogen limitation caused a decline of the allelopathically active fischerellin A in the benthic cyanobacterium Fischerella muscicola (GROSS et al. 1994). Which secondary metabolites in Elodea are causing the observed allelochemical effects is still unclear. During our search for these compounds, we found distinct UV-absorbing secondary metabolites in these plants, among them three flavone diglucuronides and a yet unidentified phenolic acid with similarity to caffeic acid (MUES 1983, ERHARD et al. subm.). Flavonoids may protect plant leaves from UV radiation, herbivory, or might exert allelopathic effects (HARBORNE 1986). Their biosynthesis might also depend on light, nutrient availability or temperature. Usually, light intensity and nutrient deficiency increase flavonoid production, whereas temperature might affect the type of flavonoid produced (MCCLURE 1986). We recently showed that E. nuttallii from various locations as well as the related E. canadensis produce the same flavonoid compounds, but in a distinct, species specific pattern (ERHARD et al. subm.). In accordance to MUES (1983), we assume that the overall flavonoid production in Elodea is not affected by environmental factors. Nevertheless, differences in the total amount of individual compounds might be driven by environmental conditions. Here, we tested whether different environmental factors influence the accumulation of four phenolic compounds in E. nuttalli by cultivating E. nuttallii under different light and temperature regimes. Based on observations that E. nuttallii grown in an aquarium with CO2 fertilization had higher concentrations of the yet unknown phenolic acid than plants collected in the field, we tested also the influence of CO2 availability.