Application of endogenous plant hormone salicylic acid (SA) or essential oil component carvacrol (CA) in elms enhances tree resistance to the Dutch elm disease pathogen, although the effect of these compounds on tree metabolism is unknown. The chemical changes induced by SA or CA treatments in Ulmus minor were studied through gas chromatography-mass spectrometry (GC-MS) analysis of xylem tissues. Treatments consisted of fortnightly irrigating seedlings with water, SA or CA at 600 mg L⁻¹. The chemical composition of the xylem tissues sampled from treated trees was significantly altered depending on the treatment type. SA treatment induced an accumulation of the sinapyl alcohol, a precursor of lignin and other phenylpropanoid-derived products. CA treatment induced an accumulation of the methyl esters of palmitic, linoleic and stearic acids. Both treatments resulted in early bud burst and SA significantly reduced sapwood radial growth, possibly as a consequence of a trade-off between tree growth and tree defence. The enhanced resistance provided by these treatments is discussed.
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