From morphology to metabolites: diversity and functional traits of flax (Linum spp.)

Flax (Linum spp.) is an ancient crop of enduring relevance for its use as a source of fibre, oil, and bioactive compounds. Renewed interest in flax is driven by its potential applications in sustainable agriculture, bio-based industries, and functional food systems. Considerable morphological and biochemical diversity exists among flax varieties and related Linum species, influencing agronomic performance, processing efficiency, and end-use quality. This review synthesizes current knowledge on variation in plant architecture, stem and fibre traits, seed characteristics, oil composition, and secondary metabolites. Key findings include: (i) fibre-type flax accumulates higher lignan and protein levels than oilseed types, suggesting co-selection for stem architecture and seed metabolites; (ii) flower colour (blue vs. yellow) serves as a reliable predictor of lignan chemotaxonomy, with blue-flowered species accumulating arylnaphthalene type lignans (e.g., justicidin B) and yellow-flowered species accumulating aryltetralin type lignans (e.g., podophyllotoxin); (iii) cultivated flax (L. usitatissimum) exhibits an atypical lignan profile, with secoisolariciresinol diglycoside in seeds and dibenzylbutyrolactone type lignans in aerial parts; and (iv) the low conversion efficiency of α-linolenic acid (ALA) to eicosapentaenoic acid/docosahexaenoic acid in humans indicates that flaxseed’s health benefits may mediated by lignans and other bioactive compounds rather than ALA alone. The evidence highlights diversity as a defining feature of flax and a critical resource for expanding its functional, industrial, and health-related applications. Key knowledge gaps are identified, underscoring the need for integrated morphological and biochemical approaches to support targeted selection, breeding, and value-added utilization strategies.