Histological and chemical comparison of triterpene and phenolic deterrent contents of juvenile shoots of Betula species

Summary Defence-related epithelial structures of juvenile stems and their phytochemistry were compared between six woody species of Betula from different geographical areas. The shoots of B. pendula, B. papyrifera, B. platypylla and B. ermanii are covered by resin droplets secreted by multicellular peltate glands. The seedlings of B. papyrifera and B. pubescens are covered with long epithelial hairs. Closer topical examination also revealed smaller glands on B. papyrifera and B. pubescens. The glands of B. ermanii are oval while the other species have a round shape. Phenol-specific histochemical staining indicated phenolic compounds in the cells in active glands of the resinous birches, but not in those of the pubescent birches. The epithelial hairs were strongly positive to the phenol stain. Analysis of the triterpenoids (3- and 3-papyriferic acid and pendulic acid) and a phenolic (platyphylloside) deterrent indicated that the morphological differences are accompanied by chemical ones. Pendulic acid was the main triterpene in B. ermanii instead of the 3-papyriferic acid of the other resinous species. Concentrations of the triterpenes ranged from about 0.3 to 12 mg/g, while the platyphylloside level was around 0.5 mg/g in all of them. Some triterpenes were also detected in B. papyrifera, but none in B. pubescens. The former contained only about 0.05 mg/g of platyphylloside, but B. pubescens contained 4 mg/g of this compound.