Role of macromolecules on the physical properties of root and seed mucilage

Their chemical composition confers root and seed mucilage their specific viscous and surface-active properties, enabling mucilage to create "microhydrological niches" in soil. This study investigated the respective contributions of the concentration and of the intrinsic chemical properties of the macromolecules to the physical properties of mucilage. Macromolecules in root (maize and wheat) and seed (chia and flax) mucilage were characterized with several chemical parameters, including molecular size distribution, carbohydrates, bound cations, relative acidity, proteins and lipids contents. The viscosity, surface tension and wettability were measured at different concentrations of macromolecules in mucilage. Macromolecules in seed mucilage were characterized by a high relative acidity of the polysaccharides and a gel-forming mechanism relying on cross-links between uronic acids and divalent cations. A high symmetry of the polysaccharidic backbone in chia seed mucilage increased its viscosity positively. In contrast, one fifth of macromolecules in root mucilage was proteins and ionic cross-links revealed not to be the driving gel formation mechanism but rather physical interactions forces between proteins and polymers. Surface coverage by proteins governed the wettability of mucilage across all species, as the contact angle followed a Langmuir isotherm with the protein surface concentration. Although small surface-active molecules and lipids bound to macromolecules affected the surface tension of mucilage the most, proteins and polysaccharides could also lower the surface tension of mucilage. In light of the results, we highlight the necessity to consider the role of proteins when studying the effect of mucilage in soil.