Carboxyl and phenolic humic fractions alter the root morphology in Arabidopsis thaliana seedlings.

A humic substance isolated from a forest soil was separated into carboxyl and phenolic fractions by affinity chromatography using a weak-base amine resin. Humic substance and its fractions were analyzed for physical and chemical characteristics. Their biological effects were compared on growth and root development of Arabidopsis thaliana seedlings. A greater degree of aromaticity was observed for the unfractionated humus (F0) and the carboxyl fraction (FI) compared to the phenolic fraction (FII). A high amount of carboxyl carbon and total sugars were detected in the FI fraction. The FII fraction mainly contained fatty acids and phenolic acids. The F0 did not affect the growth and the development of Arabidopsis thaliana seedlings, by contrast, the fractions reduced seedling growth inducing a different development of the root system. In particular the FII fraction, at the highest concentration (5 mg C L-1), strongly inhibited the primary and secondary root length representing an acute stress for plants. Conversely, the reduction of lateral root growth induced by the FI fraction was not dose-dependent and it was mainly addressed to the root elongation rather than to the lateral root number. When F0 and its humic fractions were added to the culture medium at the concentration of 5 mg C L-1, a faster root hair differentiation process was observed. The diverse biological effects of the humic fractions utilized may be attributed to the relative content of specific classes of humic components and it may be also explained by the additive or antagonistic interaction of the single chemical compounds.