Physiological regulation and threshold behavior of lithium uptake and removal by perennial ryegrass

With the rapid expansion of the lithium battery industry, lithium (Li) has become an emerging environmental contaminant, particularly in industrial wastewater where concentrations can reach hundreds to thousands of mg/dm³. However, the phytoremediation potential and physiological regulatory mechanisms for Li remain insufficiently understood. In this study, perennial ryegrass (Lolium perenne L.) was evaluated through hydroponic (0-700 mg/dm³ Li, 42 d) and soil-based (0-150 mg/kg Li, 42 d) experiments to assess its growth responses, antioxidant regulation, photosynthetic performance, and Li accumulation. Structural equation modeling and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were applied to elucidate the underlying mechanisms. Low to moderate Li concentrations had limited effects on germination and growth, with antioxidant enzymes effectively maintaining redox homeostasis and supporting growth-driven Li uptake. With increasing Li concentration, internal Li load became the dominant driver of oxidative damage and growth inhibition. Structural equation modeling identified internal Li load as a central mediator linking external stress to physiological responses, with approximately 280 mg/dm³ representing a transition threshold from tolerance-regulated to damage-driven accumulation. At this level, root and shoot Li concentrations reached 14015.6 and 3805.1 mg/kg (dry weight), respectively. TOF-SIMS revealed that Li was preferentially enriched in root cortical microdomains associated with oxygen-containing organic groups and phosphate-rich regions. Soil experiments confirmed Li accumulation up to 1477.0 mg/kg (DW) at 150 mg/kg treatment, exceeding values reported for other species. These findings define the functional boundary of Li phytoremediation by perennial ryegrass and provide a basis for plant-based remediation of Li-contaminated environments.