Anatomical patterns of condensed tannin in fine roots of tree species from a cool-temperate forest

Background and Aims Condensed tannin (CT) is an important compound in plant biological structural defence and for tolerance against herbivory and environmental stress. However, little is known of the role and location of CT within the fine roots of woody plants. To understand the role of CT across diverse species of woody dicot fine roots, we evaluated localization of CT, which accumulated in root tissue, and examined its relationships with the stele and cortex tissue in cross-sections of roots in 20 tree species forming different microbial symbiotic groups (ectomycorrhiza and arbuscular mycorrhiza). Methods In a cool-temperate forest in Japan, cross-sections of the sampled roots in different branching order classes, namely, first-order, second- to third-order, fourth order, and >fourth order (higher-order), were measured in terms of the length-based ratios of stele diameter and cortex thickness to root diameter. All root samples were then stained with a ρ-dimethylaminocinnamaldehyde solution and measured by the localized CT accumulation area to the cross-section area (CT ratio). Key Results Stele ratio tended to increase with increasing root order, whereas cortex ratio either remained unchanged or decreased with increasing order in all species. The CT ratio was significantly positively correlated to the stele and negatively correlated to the cortex in 2 nd–4 th-order roots across species during the shift from primary to secondary root growth. Ectomycorrhiza-associated species mostly had a higher stele ratio and lower cortex ratio than arbuscular mycorrhiza-associated species across root orders. Compared with arbuscular mycorrhiza species, there was a greater accumulation in CT in response to changes in the root order of ectomycorrhiza species. Conclusions The development patterns of the stele, cortex, and CT accumulation with changing from root tip to secondary roots were distinguished between mycorrhizal associations. The CT in tissue on mycorrhizal associations could help with root protective in specific orders during shifts in stele and cortex development before and during cork layer formation.