Tebuconazole Remodels 5-n-Alk(en)ylresorcinol Profiles in Barley Coleoptiles: Antifungal Properties of the Major Synthetic Homologues

5-n-Alk(en)ylresorcinols represent a major group of constitutive defense phenolic lipids in cereals; however, the non-target effects of fungicides on their accumulation, homologue profiles, and biological activity remain largely unexplored. This study investigated the effects of tebuconazole, a triazole fungicide widely used for cereal seed treatment, on biomass production, 5-n-alk(en)ylresorcinol accumulation, and homologue composition in five barley cultivars, and evaluated the antifungal activity of the predominant synthetic homologues against Rhizoctonia cerealis. Two-way ANOVA revealed that tebuconazole treatment had no effect on dry biomass but significantly influenced fresh biomass and total 5-n-alk(en)ylresorcinol content in a strongly cultivar-dependent manner. Furthermore, exposure to tebuconazole altered the relative distribution of individual homologues while leaving the qualitative composition largely unchanged, except for minor shifts in the occurrence of trace homologues. Among the individual homologues, C23:0 exhibited the strongest inhibition of Rhizoctonia cerealis growth, while C21:0 most effectively suppressed colony regrowth after transfer to compound-free medium. The mixture of the three predominant synthetic homologues exhibited greater antifungal activity than the individual compounds. These findings highlight the importance of considering individual homologue profiles, rather than the total content of the compound alone, when relating the chemical composition of cereal 5-n-alk(en)ylresorcinols to their biological activity.