Morphological Traits, Accumulation and Compartmentalization of Phenolic Compounds and Cadmium (CD2+) in Linum usitatissimum L. Seedlings Under In Vitro Conditions: A Comparative Study of Two Varieties.

Flax (Linum usitatissimum L.) is a multipurpose crop with considerable potential as a biotechnological platform for producing in vitro seedlings that serve both as sources of pharmacologically valuable phenolic compounds (PCs) and as potential biosorbents/chelators of heavy metals (HMs), including cadmium (Cd). The use of an in vitro culture system under strictly controlled conditions enables the identification of variety-specific responses in flax. The aim of this study was to compare the morphophysiological characteristics, the level of lipid peroxidation (LPO), and phenylpropanoid (PP) and flavonoid (FL) contents in in vitro seedlings of two flax varieties-fiber flax (FF) and oilseed flax (OF)-grown under control conditions and in the presence of Cd (60 and 100 μM). The stem mesostructure was also examined, as the stem is the principal organ responsible for metal transport via its vascular tissues. Our results showed that Cd exposure induced only minor changes in the morphophysiological characteristics of both flax varieties. Using a standardized in vitro system, we demonstrate for the first time that OF accumulated approximately 40% more Cd than FF at both metal concentrations. LPO, assessed by malondialdehyde (MDA) content, remained relatively stable across all treatments, except for a significant increase in OF exposed to 100 μM Cd. Analysis of the phenolic profile revealed that, in FF, the total phenolic content (TPC) and PP content increased relative to the control at 100 μM Cd, indicating the induction of PC-mediated antioxidant defense. In contrast, OF exhibited reduced PP and FL contents under both Cd treatments. Cytochemical analysis revealed a similar localization pattern for Cd and PC, with both accumulating predominantly in the stem epidermis of the in vitro-grown seedlings. However, the two flax types differed markedly in their accumulation profiles: OF accumulated higher levels of Cd, indicating its potential use as a biosorbent for this pollutant, whereas FF exhibited higher PC accumulation, suggesting its suitability as a natural source of PCs. These findings demonstrate that flax responses to Cd exposure are variety-specific.