Defense strategies of invasive plant during flowering and growth stages

Studying the defense mechanisms of invasive plants is crucial for understanding their successful invasion. The Enemy Release Hypothesis suggests that invasions thrive due to a lack of specialized enemies, allowing invasive plants to allocate resources typically reserved for defense toward growth and reproduction, enhancing their competitive advantage. However, this hypothesis does not fully explain how invasive plants cope with threats from generalist enemies. This study focuses on the invasive plant Sphagneticola trilobata, investigating its defensive levels during vegetative and flowering stages through field surveys of pest feeding patterns, seed germination experiments with local plants, and assessments of changes in secondary metabolite concentrations after mechanical leaf damage. Our findings reveal significant differences in defensive strategies between these two phases and insights into trade-offs among growth, reproduction, and chemical defenses. Field survey results demonstrated a negative correlation between flower production and leaf tunnels caused by herbivorous pests, suggesting that S. trilobata increases its defensive investment during flowering to mitigate herbivory. Conversely, a positive correlation was observed between flower heads and pest-inflicted tunnels on nearby native plants. Seed germination experiments indicated that leaves of S. trilobata in the vegetative stage significantly inhibited the germination of local plant seeds compared to those during flowering, demonstrating stronger allelopathic effects in the vegetative phase. Analysis of secondary metabolite concentrations revealed higher levels of defense-related compounds (flavonoids, total phenols) and tolerance indicators (DPPH) during flowering. After mechanical damage, these compounds increased, while soluble sugars decreased. Our research suggests that S. trilobata actively enhances its defenses during flowering without sacrificing growth and reproduction. This indicates that effective defense strategies may support the successful growth and reproduction of invasive plants, challenging the idea of a trade-off between defense and growth/reproduction.