Endophytic Bacterial Exopolysaccharide and Ethylene Hyperactivate Flavonoid Synthesis in Leaves of Fagus sylvatica L. with False Heartwood

The presence of false heartwood in the stems of Fagus sylvatica L. significantly reduces timber quality; to date, its physiological triggers remain understudied. Research on beech stands in the forests of the Bukovinian Pre-Carpathians (Ukraine) was aimed at identifying functional links between leaf structure, synthesis of polyphenols, microbial elicitors, and false heartwood formation. Using morphometric analysis, epifluorescence microscopy, and spectrophotometry, trees with false heartwood (FH morphotype) were compared with those without it, the healthy control (HC morphotype). Sequencing of the 16S rRNA gene identified the endophytic strain Bacillus halotolerans (BHFHB). The composition of its exopolysaccharides (EPS) was analyzed by gas chromatography. Trees with FH were identified as a distinct morphotype characterized by leaf blade thickness and increased flavonoid content in leaves. It was experimentally demonstrated that this morphotype is characterized by the capacity for flavonoid hypersynthesis. Treatment of leaves with ethephon or bacterial EPS resulted in a 20–30% increase in flavonoid content within four hours. The proactive response, combined with xeromorphic traits, indicates a state of systemic priming. FH formation in this morphotype is proposed to be considered as an individual adaptive strategy, where ethylene-dependent hypersynthesis of flavonoids provides antioxidant protection, biomechanical resistance, and metabolomic adaptation under extreme conditions of mountain ecosystems.