Divergent physiological strategies of two submerged macrophytes under combined low light and nutrient enrichment stress

The decline of submerged macrophytes due to eutrophication-induced low-light stress is critical in freshwater ecosystems. This study elucidates the contrasting strategies of Vallisneria natans and Potamogeton maackianus through a mesocosm experiment simulating combined low-light and nutrient stress (eutrophic vs. hypereutrophic). V. natans exhibited an energy-efficient strategy, modulating its antioxidant investment over time (e.g. elevated APX early, then lower APX later) and maintaining carbon-nitrogen homeostasis, resulting in higher biomass. In contrast, P. maackianus maintained constitutively high absolute levels of POD and APX, but without significant upregulation under hypereutrophic stress; this high-cost defense was accompanied by free amino acid depletion at the final stage, although protein and sugar metabolism did not differ significantly between nutrient treatments. Only V. natans demonstrated photosynthetic plasticity by maintaining higher Chl-b content under hypereutrophic stress. Our findings show that divergent tolerance is governed by fundamentally different energy allocation strategies. The efficient physiology of V. natans makes it a superior candidate for restoring eutrophic waters, providing a physiological basis for species selection in lake management.