The application of the resource availability hypothesis (Coley et al. 1985; Room et al.
1989, see above) on invasive plant species led to the proposal of the evolution of increased competitive ability (EICA) hypothesis (Blossey and Nötzold 1995). It predicts that the increased vigour of invasive plants might be due to the competitive ability at the expense of defences against natural enemies. The EICA hypothesis is doubted (Willis et al. 1999), and my results on the chemical defences in Elodea as well as the defences found in troublesome invasive species Myriophyllum spicatum (Gross et al. 1996; Choi et al. 2002) clearly contradict this hypothesis. I showed that invasive Elodea spp. are defended against competing primary producers as well as against insect herbivores.
Allelopathy directly influences the availability of essential resources at the advantage of the donor plant. The negative impact on the important herbivore Acentria very likely prevents the establishment of a stable Acentria population on Elodea. Both traits correspond to the low epiphyte and herbivore densities observed in the field, indicating the ecological relevance of chemical defences in these plants.
It is often assumed that the constitutive production of defensive plant secondary metabolites – allelochemicals as well as antiherbivores – as in Elodea is cost-intensive.
Since the nutrient rich water bodies inhabited by Elodea always accommodate potentially light limiting algae and cyanobacteria, a constitutive production might be adequate. However, flavonoid antiherbivore defences might be expensive when herbivores are absent, which might be detrimental at less favourable growth conditions.
On the other hand, if flavonoids really have UV protective function, the constitutive defence might be an advantageous “by-product” with reduced energetic costs.
The example of E. canadensis and E. nuttallii demonstrates that fast growth and chemical defences may be apparent at the same time, and do not necessarily exclude each other. It is true that polyphenolic compounds, which are regarded as defences, are lower concentrated in Elodea than in other plants. However, quality can be more important than quantity. The multiple functionality of flavonoids might offer the advantage to minimize the energetic costs of constitutive defences, which seem to complement the inherently good adaptation of Elodea spp. to a heterogeneous environment. Chemical defences might thus contribute to the invasiveness of waterweeds.
Summary
The invasive waterweeds Elodea nuttallii and E. canadensis contain 7-O-diglucuronides of the flavones luteolin, apigenin and chrysoeriol as well as a phenolic acid similar to caffeic acid. Both species often resemble each other morphologically and may be difficult to distinguish. Consequently, there is a great demand of correct species identification. I present a chemotaxonomic method based on the species-specific flavonoid pattern in Elodea that clearly identified even ambiguous specimens. These had primarily been determined as E. canadensis, but turned out to be E. nuttallii.
Especially the ratio between the apigenin- and the chrysoeriol-derivative is characteristic, and my results are in full agreement with molecular taxonomy based on distinct length and base pair polymorphism in the internal transcribed spacer (ITS) region. None of the methods supports evidence for the existence of hybrids between the species, but rather indicate that the distribution of E. nuttallii in Europe is underestimated.
The qualitative pattern of flavonoids in different Elodea species is very stable, but their concentrations vary intraspecifically. Flavonoid biosynthesis is in part regulated by external abiotic signals such as light, temperature or resource availability.
In first outdoor and laboratory experiments, high light intensities led to a weak accumulation of the phenolic acid and luteolin-7-O-diglucuronide, but in a second experiment no light effect was detectable. However, plants exposed to natural sun light had higher concentrations of either luteolin- or apigenin- and chrysoeriol-glucuronides, indicating that flavonoids exhibit UV protective properties. Temperature might be negatively correlated with the phenolic acid, apigenin- and chrysoeriol-diglucuronide, but additional CO2 supply had no influence at all.
Elodea species are often associated with low phytoplankton densities or epiphytic covers. In growth assays with extracts from E. canadensis and E. nuttallii, I demonstrated that allelochemicals can suppress growth of different cyanobacterial culture strains and epiphytic autotrophs isolated from submersed macrophytes. Only Scenedesmus brevispina was stimulated. Bioassay guided fractionation yielded hydrophilic and slightly lipophilic active compounds. Among them are phenolic
substances, but neither the phenolic acid nor flavonoids were active. Since growth declined also in a moderately lipophilic fraction of culture filtrate of E. nuttallii, I assume that active compounds are exuded in the water. Allelopathy might thus be relevant in situ and suppress phytoplankton and epiphytes. The differences in the susceptibility of target organisms could (1) at least partly arise from adaptation to the respective host plants and (2) indicate that allelopathic interference might reduce the abundance of some species, especially cyanobacteria, in epiphytic biofilms.
Herbivorous larvae of the aquatic moth Acentria ephemerella can severely damage plants like Potamogeton spp. and Myriophyllum spicatum. In contrast, larvae are never found on Elodea nuttallii and avoid feeding on E. canadensis. Using no-choice assays, I showed that E. nuttallii strongly reduces larval growth, independent of culture conditions under which plants were grown. Elodea exposed to high light intensities further increases larval mortality. These results indicate that a negative impact on the fitness of Acentria might be the ultimate reason for the avoidance of Elodea in the field. Neither morphological defences nor nitrogen or phosphorus content of food plants explained these effects, suggesting that allelochemicals – probably flavonoids present in E. nuttallii, but absent in other common host plants of Acentria – are responsible for the reduced growth.
Coating Potamogeton leaf disks with Elodea extracts or flavonoids deterred larvae from feeding of this otherwise preferred food source, and yet unknown compounds present in the extracts reduced growth and survival of Acentria. My study is the first, in which larvae were successfully fed with plant secondary metabolites. The concentrations of flavonoids in the assays were at the lower range of concentrations found in the field, indicating that chemical defence in E. nuttallii might be ecologically relevant. The related E. canadensis produces the same flavonoids and might be similarly defended.
My studies on the chemical ecology of E. canadensis and E. nuttallii revealed that both species produce allelochemicals, which can assure the access to limiting resources and prevent severe loss of biomass. In contrast to the predictions of the evolution of increased competitive ability (EICA) hypothesis, Elodea spp. thus demonstrate that invasive species might well be chemically defended. This trait can complement their tolerance towards environmental heterogeneity and strengthen the competitive ability of Elodea.
Zusammenfassung
Die invasiven Wasserpest-Arten Elodea nuttallii und E. canadensis enthalten die 7-O-diglucuronide der Flavone Luteolin, Apigenin und Chrysoeriol sowie eine phenolische Verbindung mit Ähnlichkeit zu Kaffeesäure. Morphologisch ähneln sich beide Arten und können häufig nur schwer unterschieden werden. Daher gibt es eine große Nachfrage nach eindeutigen Bestimmungsmerkmalen beider Arten. Ich stelle eine chemotaxonomische Methode vor, die auf artspezifischen Flavonoid-Mustern basiert und sogar morphologisch nicht eindeutig bestimmbare Pflanzenexemplare sicher identifiziert. Diese waren ursprünglich als E. canadensis bestimmt worden, stellten sich jedoch als E. nuttallii heraus. Insbesondere das Verhältnis zwischen der Apigenin- und Chrysoeriolverbindung charakterisiert die Arten, und meine Ergebnisse stimmen voll mit denen einer molekularbiologischen Methode überein, die auf Längen- und Basenpaarunterschieden der internal transcribed spacers (ITS) basiert. Keine der Methoden liefert Hinweise auf mögliche Hybride zwischen den Elodea-Arten. Sie zeigen vielmehr, dass die Verbreitung von E. nuttallii eher unterschätzt wird.
Das qualitative Auftreten der Flavonoide in den verschiedenen Elodea-Arten ist sehr stabil, aber die Konzentrationen schwanken intraspezifisch. Die Flavonoid-Biosynthese wird teilweise durch externe Faktoren wie Licht, Temperatur oder Nährstoffverfügbarkeit reguliert. In ersten Freiland- und Laborversuchen führten hohe Lichtintensitäten zu einer leichten Anreicherung der phenolischen Säure und Luteolin-7-O-diglucuronid, wohingegen in einem zweiten Versuch kein Licht-Effekt festgestellt werden konnte. Allerdings enthielten Pflanzen, die Sonnenlicht ausgesetzt waren, entweder höhere Konzentrationen an Luteolin- oder an Apigenin- und Chrysoeriol-Diglucuroniden. Dies weist auf einen möglichen UV-Schutz durch diese Verbindungen hin. Die Temperatur scheint negativ mit der phenolischen Säure sowie Apigenin- und Chrysoeriol-Diglucuronid zu korrelieren, während CO2-Düngung überhaupt keinen Einfluss hatte.
Elodea-Arten werden häufig mit niedrigen Phytoplanktondichten und schwachem Epiphytenbewuchs in Verbindung gebracht. In Versuchen mit Extrakten aus E. canadensis und E. nuttallii zeigte ich, dass Allelochemikalien das Wachstum
verschiedener Kultur-Cyanobakterien und epiphytischer autotropher Organismen, die von submersen Makrophyten isoliert wurden, hemmen können. Lediglich das Wachstum von Scenedesmus brevispina wurde gesteigert. Die weitere Auftrennung von Extrakten lieferte hydrophile und schwach lipophile aktive Komponenten, u.a. auch phenolischen Verbindungen. Jedoch waren weder die phenolische Säure noch die Flavonoide aus Elodea aktiv. Da auch die schwach lipophilen Substanzen in angereichertem Umgebungswasser von E. nuttallii zu schwächerem Wachstum führten, vermute ich, dass aktive Substanzen ins Wasser ausgeschieden werden. Von daher sollte Allelopathie auch in situ relevant sein und das Wachstum von Phytoplankton und Epiphyten hemmen. Die unterschiedliche Sensibilität der Testorganismen könnte (1) zumindest teilweise durch Anpassung an die jeweilige Wirtspflanze bedingt sein, und (2) darauf hinweisen, dass Allelopathie das Auftreten einiger Arten, insbesondere von Cyanobakterien, in epiphytischen Biofilmen verhindern kann.
Herbivore Larven der aquatischen Motte Acentria ephemerella können immensen Schaden an Pflanzen wie Potamogeton spp. und Myriophyllum spicatum anrichten. Im Gegensatz dazu werden Larven nie auf E. nuttallii beobachtet, und sie meiden E. canadensis als Futter. Ich konnte mit Hilfe von no-choice Testdesigns zeigen, dass E. nuttallii das Wachstum dieser Larven stark vermindert, und zwar unabhängig von den Wachstumsbedingungen, unter denen die Pflanze gewachsen war. Weiterhin erhöhten Elodea-Pflanzen, die hohen Lichtintensitäten ausgesetzt waren, die Mortalität der Raupen. Diese Ergebnisse weisen darauf hin, dass der schädliche Einfluss auf die Fitness von Acentria der ultimative Grund für das Meiden von Elodea-Beständen im Freiland ist. Weder morphologische Verteidigungsstrukturen noch Stickstoff- oder Phosphatgehalt der Pflanzen erklären die negativen Effekte. Dies lässt vermuten, dass Allelochemikalien – möglicherweise die Flavonoide, welche zwar in E. nuttallii, nicht aber in den üblichen Futterpflanzen von Acentria, vorhanden sind – das reduzierte Wachstum bedingen.
Beschichtet man Blattscheibchen von Potamogeton mit Elodea-Extrakt oder Flavonoiden, verhindert dies den Fraß an diesem sonst bevorzugten Futter. Bislang unbekannte Substanzen aus dem Extrakt reduzieren außerdem Wachstum und Überlebenswahrscheinlichkeit von Acentria. In meiner Arbeit ist es zum ersten Mal gelungen, Acentria-Larven erfolgreich mit pflanzlichen Sekundärmetaboliten zu füttern.
Die Flavonoidkonzentrationen in meinen Versuchen lagen dabei im unteren Bereich natürlich vorkommender Konzentrationen, weshalb chemische Verteidigung in Elodea
ökologisch relevant sein kann. Da die verwandte Art E. canadensis die gleichen Flavonoide produziert, ist eine vergleichbare Verteidigung anzunehmen.
Meine Arbeiten zur chemischen Ökologie von E. canadensis und E. nuttallii ergaben, dass beide Arten Allelochemikalien produzieren, die einen sicheren Nährstoffzugang ermöglichen und erheblichen Biomasseverlust durch Fraß verhindern.
Im Gegensatz zu den Vorhersagen der evolution of increased competitive ability (EICA) Hypothese, verdeutlicht das Beispiel der Wasserpest, dass invasive Arten sehr wohl chemisch verteidigt sein können. Diese Eigenschaft kann die Toleranz gegenüber schwankenden Umweltbedingungen ergänzen und so die Konkurrenzstärke von Elodea erhöhen.
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