Stage-dependent herbivory-related defense and allelopathic allocation in the invasive plant Sphagneticola trilobata

The adaptability of invasive plants to introduced environments may depend on how they coordinate competitive and defensive allocation across developmental stages. This study investigated herbivory-related defense and allelopathic allocation in the invasive plant Sphagneticola trilobata during growing and flowering stages. We combined field surveys of leaf damage, seed germination bioassays using receptor plant species, and physiological measurements of defense-related indicators after mechanical leaf damage. Field surveys revealed a negative correlation between flower production and leaf damage on invasive plants, whereas flower number of S. trilobata was positively associated with leaf damage on neighboring native plants. In seed germination bioassays, aqueous leaf extracts inhibited germination of lettuce (Lactuca sativa) and pakchoi (Brassica rapa var. chinensis), with stronger inhibitory effects during the growing stage than during the flowering stage. Physiological analyses further showed higher levels of several defense-related indicators, including flavonoids, total phenols, antioxidant capacity, and jasmonic acid, during the flowering stage, and these indicators generally increased after mechanical damage. Together, these results suggest that S. trilobata may allocate more strongly to allelopathic interference during vegetative expansion, while maintaining enhanced herbivory-related physiological defense during flowering. These findings provide ecological and physiological evidence that stage-dependent allocation may contribute to the invasion success of S. trilobata.