Cold stress severely limits the overwintering survival and productivity of alfalfa (Medicago sativa L.), yet the metabolic determinants underlying cultivar-specific cold tolerance remain insufficiently understood. In this study, we compared a cold-tolerant cultivar (‘WL343’) and a cold-sensitive cultivar (‘Gannong No. 4’) using an integrated framework combining physiological analyses, non-targeted metabolomics, and functional validation with exogenous melatonin. Cold stress caused pronounced membrane damage, oxidative stress, and growth inhibition in ‘Gannong No. 4’, whereas ‘WL343’ maintained greater membrane integrity, stronger antioxidant capacity, and enhanced osmotic adjustment. Metabolomic profiling revealed extensive cold-induced metabolic reprogramming in both cultivars, but with distinct pathway prioritization. Notably, the tryptophan–melatonin metabolic pathway was specifically and strongly activated in ‘WL343’, leading to significant endogenous melatonin accumulation, while ‘Gannong No. 4’ exhibited a weaker response and preferential regulation of secondary metabolism, particularly isoflavonoid-related pathways. Functional experiments further demonstrated that exogenous melatonin markedly alleviated cold-induced injury in ‘Gannong No. 4’ by reducing reactive oxygen species accumulation, stabilizing membranes, and enhancing antioxidant enzyme activities and osmolyte accumulation. Among the tested concentrations, 100 μM melatonin conferred the greatest protective effect. Collectively, these results identify melatonin as a key functional metabolite underlying cold tolerance in alfalfa and highlight the tryptophan–melatonin pathway as a central metabolic hub conferring cultivar-specific cold resilience. This study provides a metabolic basis for improving alfalfa cold tolerance through breeding and targeted cultivation strategies.

Grassland restoration is important for addressing ecosystem degradation and improving ecosystem services; however, the ultimate goal of grassland restoration, namely restoring degraded grasslands to a “healthy state”, lacks systematic and unified technical indicators. Focusing on the degraded alpine grasslands of the Tibetan Plateau, we critically examine the conceptual and operational dilemmas of the “healthy grassland” standard and argue that, when applied as universal standards, neither fixed-threshold criteria (e.g., vegetation cover, biomass, and diversity) nor reference systems based on pristine meadows can provide scientifically sound and practically operable restoration targets. Drawing on alternative stable state theory and the novel ecosystems concept, we propose a shift from reference-based to goal-oriented restoration, with restoration targets stratified and classified by degradation degree, site conditions, and stakeholder demands. Synthesizing published meta-analyses and representative cases from the Tibetan Plateau, we show that restoration outcomes differ markedly with measures, recovery time, and regional conditions, and we outline a tentative tiered goal system of ecological security, functional recovery, structural optimization, and sustainable management as a testable framework, combined with adaptive management to reconcile multi-objective trade-offs. We conclude that goal-oriented, classified restoration provides more realistic and operable indicators of the ecosystem services that restored grasslands can be expected to deliver.