Chemical plasticity in the fine root construct of Quercus spp. varies with root order and drought

Fine roots of trees exhibit high plasticity to adapt to environmental stress. Although the morphological and physiological plasticity of roots has been well studied, less explored are the accompanying changes in the chemical plasticity of fine roots, which regulates both root function and soil carbon sequestration. We investigated the changes in quantity, composition and localization of compounds in different fine root orders of Quercus alba and Quercus rubra subjected to drought stress. The total quantity of lignins varied only by root orders where distal (1+2) orders had lower lignin compared to higher orders. However, the 1+2 orders had higher guaiacyl lignin and bound phenolics, which provide greater protection from environmental stress. Unlike lignins, drought altered the quantity and composition of tannins. In Q. alba, the ellagitannins decreased in the 1+2 orders exposed to drought. The lower content of ellagitannins with antimicrobial properties reveals a strategic adaptive response by fine roots to facilitate symbiotic association, as evidenced by the higher colonization of ectomycorrhizal fungi, which help in drought tolerance. Our study revealed that the chemical construct of different heteropolymers varied differently across root orders when exposed to drought, indicating the importance of chemical plasticity of fine roots to adapt to environmental stress.