Condensed tannin (CT) is a compound found in various parts of a plant that helps protect against herbivory, microbial damage, and environmental stresses. In woody plants, fine roots – those with a diameter of less than 2mm – are the most active part of the root system with respect to nutrient and water uptake. The lower order woody roots are primarily devoted to resource acquisition, while higher orders of fine roots are more keyed to water transportation, and differ structurally because of this. CT is present in fine roots, but its relationship to the changing anatomy and function of different orders of fine roots is not well understood.
In a recent article published in Annals of Botany, lead author Izuki Endo and colleagues sampled fine roots from 20 tree species in a cool-temperate Japanese forest and studied their anatomical traits and patterns of CT accumulation. The roots were categorized into first, second, third, fourth, and higher-order, and their stele diameter, cortex thickness, root diameter, and CT accumulation area in cross-section noted.
The authors found that the stele ratio relative to root area within cross-sections increased with root order, while the cortex ratio either stayed the same or decreased. CT showed a strong positive correlation with the stele, but was in some orders negatively correlated with cortex area. Surprisingly, fine root anatomy varied with mycorrhiza type. In all root orders, species forming ectomycorrhizae (EM) had both a higher stele and lower cortex ratio than those forming arbuscular mycorrhizae (AM). EM species also showed greater accumulation of CT with increasing root order than did AM species.
The authors note that their findings are limited to tree species and forest habitats and that the phenomenon should be evaluated in a wider range of species.