The Influence of Humic Substances and Auxin-Producing Bacteria on

Silver maple is a fast-growing, adaptable tree that often frequents wet places and thus can play an important ecological role in replanting schemes. For this, robust, high-quality seedlings are essential. In other tree species, improved seedling quality has been achieved by treating with a combination of humic substances (HSs) and bacterial strains capable of synthesizing auxin phytohormone; the benefit being attributed, without clear supporting evidence, to changes in phytohormone concentrations in the plant. To clarify the uncertainty, we conducted assays of hormones in silver maple seedlings treated with HSs and appropriate bacteria. We hypothesized that any positive additive effects between HSs and bacteria may be due to the ability of HSs to bind phytohormones. This hypothesis was tested and confirmed by using optical absorption spectra of auxins, humic acids, and their combination, as well as by modeling their interactions. The combination of humic substances and bacteria resulted in an approximately 1.5-fold increase in auxin content in roots, accompanied by a marked increase in root weight and length. We suggest this is likely the outcome of HSs binding to bacterial auxins and delivering them to plant roots. Concentrations of cytokinins and abscisic acid also changed under these treatments, which may help explain observed increases in photosynthesis and improved water balance.