The humic acid-induced changes in the root concentration of nitric oxide, IAA and ethylene do not explain the changes in root architecture caused by humic acid in cucumber

The increase in root growth is one of the major effects of humic substances, but the mechanisms involved in humic acid-mediated changes in root growth, morphology and architecture are poorly known. Probably, humic substances may act on plant development through an action on the hormonal balance within the plant, either directly or indirectly by affecting the root uptake of some nutrients. In this study we investigate in cucumber plants the effects of a purified sedimentary humic acid (PHA), without detectable concentrations of the main phytoregulators in its structure, on root architecture, and its relationships with the functional action of three phytoregulators, indole-acetic acid, ethylene and nitric oxide, which are also affected by the root application of this humic acid. The results obtained using inhibitors of auxin transport or action, inhibitors of ethylene biosynthesis or action, and a scavenger of nitric oxide indicate that the increase in the root concentration of these phytoregulators caused by the root application of PHA does not play an essential role in the expression of the main changes on root architecture caused by PHA in developed cucumber plants. Other factors, which could act in coordination or independently of those phytoregulators affected by PHA root application, must be involved in the whole action on this humic acid on root architecture in cucumber.

► The purified humic acid (PHA) root application was associated with the expression of macro-morphological changes in roots.
► Nitric oxide is not an essential key regulator of PHA-mediated root changes and other phytohormones could be involved in these effects.
► IAA action is not essential for the expression of PHA-induced macro-morphological root changes. Ethylene inhibitors did not block HA-mediated these changes.
► The increase of IAA, ethylene and NO caused by PHA does not play an essential role in the expression of the PHA-induced macro-morphological changes in cucumber plants.