Comparative physiological and multi-omics responses reveal divergent drought-response patterns in two Melilotus...

Melilotus albus and M. officinalis are closely related legume species with potential value as stress-resilient resources for dry and marginal environments. However, whether they exhibit similar physiological and multi-omics response organization under continuous drought remains unclear. In this study, physiological, transcriptomic, and metabolomic analyses were integrated to compare drought-tolerant and drought-sensitive accessions of the two species at 0, 4, and 8 d of continuous drought. Both species showed coordinated changes in osmolyte accumulation, antioxidant defense, photosystem adjustment, and secondary metabolic remodeling, but differed in response rhythm and regulatory emphasis. Across the selected accession pairs, M. albus was mainly associated with sustained soluble sugar accumulation and relatively moderate photosystem changes, whereas M. officinalis showed a clearer tendency toward POD-associated defense and photoprotective adjustment. Comparative analyses across physiological and omics layers suggested that drought-response divergence became more clearly concentrated at the middle stage within the sampled period. Integrative analysis further indicated that phenylpropanoid- and flavonoid-related remodeling represented a major shared response direction in both species. However, this shared direction was organized differently, with M. albus showing changes across the phenylpropanoid backbone and multiple flavonoid-related branches, whereas M. officinalis showed a more focused pattern involving lignin-related and selected flavonol-related branches. These findings suggest that closely related Melilotus resources may differ in temporal coordination and secondary metabolic organization under continuous drought, providing useful information for understanding their response characteristics in dry and marginal environments.