Factors associated with the content of phenolic compounds in the genus Salix: the role of species, plant organ, season, and soil parameters

Abstract Background The genus Salix is a widely used source of phenolic compounds, yet the quality of willow-derived raw materials varies markedly with species, plant organ, harvest season, and habitat conditions. Bark and leaves were sampled from single individuals of five willow species ( S. cinerea , S. × fragilis , S. purpurea , S. triandra , and S. viminalis ), each growing at a separate natural site in Poland, at four time points during the 2022 growing season; soil at each site was characterised in parallel. The aims were (I) compare the profiles of selected phenolic compounds in bark and leaves by spectrophotometric assays and HPLC-PDA, (II) assess antioxidant activity using ABTS and DPPH radical-scavenging assays, (III) describe seasonal variation in metabolite content across harvest periods in the sampled material, (IV) explore associations between selected soil parameters and metabolite content, and (V) compare freeze-dried and non-freeze-dried preparations to evaluate freeze-drying as a stabilisation method for phenolic compounds. Results Metabolite composition varied between the sampled individuals, organs and harvest periods. S. purpurea bark contained the highest mean salicin (19.88 mg/g extract), peaking before leaf development. S. × fragilis bark was richest in (+)-catechin (autumn maximum). S. triandra leaves were the richest in flavonoids (176.84 mg/g extract). Antioxidant activity was comparable across the sampled individuals, organs and harvest times. Freeze-dried preparations contained more of the targeted compounds than non-freeze-dried ones. Because each species was represented by a single individual at a separate site, the soil–metabolite patterns reported are descriptive, not causal: the highest flavonoid contents co-occurred with the highest soil Na⁺ (Spearman r = 0.86 in bark, 0.76 in leaves), bark salicin correlated positively with Ca²⁺ and CEC, and in leaves, soil Mg²⁺ correlated positively with TPC and (+)-catechin. Conclusions Phenolic composition differed primarily between organs and between the sampled individuals, with season as a secondary, compound-dependent modifier. Freeze-drying increased phenolic recovery in the spectrophotometric assays. Phenolic composition co-varied with selected soil parameters –notably the contents of Mg²⁺, Na⁺, Ca²⁺, and K⁺. Because species and site are confounded in this design, these patterns are descriptive, and provide a basis for replicated, multi-site studies in which species and site effects can be disentangled.