Discriminating the Short-Term Action of Root and Foliar Application of Humic Acids on Plant Growth: Emerging Role of Jasmonic Acid.

Marta Fuentes,Angel M. Zamarreño,Victoria Fernández,Jose M. García-Mina,David De Hita,Maite Olaetxea

doi:10.3389/fpls.2020.00493

Abstract

Humic substances (HS, fulvic and humic acids) are widely used as fertilizers or plant growth stimulants, although their mechanism of action still remains partially unknown. Humic substances may be applied either directly to the soil or as foliar sprays. Despite both kind of application are commonly used in agricultural practices, most of the studies regarding the elicited response in plants induced by HS are based on the root-application of these substances. The present work aimed at discriminating between the mechanisms of action of foliar application versus root application of a sedimentary humic acid (SHA) on plant development. For this purpose, six markers related to plant phenotype, plant morphology, hormonal balance and root-plasma membrane H+-ATPase were selected. Both application strategies improved the shoot and root growth. Foliar applied- and root applied-SHA shared the capacity to increase the concentration of indole-3-acetic acid in roots and cytokinins in shoots. However, foliar application did not lead to short-term increases in either abscisic acid root-concentration or root-plasma membrane H+-ATPase activity which are, however, two crucial effects triggered by SHA root-application. Both application modes increased the root concentrations of jasmonic acid and jasmonoyl-isoleucine. These hormonal changes caused by foliar application could be a stress-related symptom and connected to the loss of leaves trichomes and the diminution of chloroplasts size seen by scanning electron microscopy. These results support the hypothesis that the beneficial effects of SHA applied to roots or leaves may result from plant adaptation to a mild transient stress caused by SHA application.

Highlights

There is a growing interest in the development and implementation of more sustainable land management practices, aiming to stop the progressive degradation of soils while maintaining or enhancing food production in a context of increasing demands
In the first set of experiments, we evaluated the dose-effect on plant growth
This conclusion is supported by the fact that we do not observe any increase in secondary root proliferation linked to the increase in root indole-3-acetic acid (IAA), but we do observe an increase in the global root growth as reflected by the dry matter production despite root abscisic acid (ABA) concentration did not increase

Summary

Introduction

There is a growing interest in the development and implementation of more sustainable land management practices, aiming to stop the progressive degradation of soils while maintaining or enhancing food production in a context of increasing demands. Foliar- vs Root-Applied HAh studies have shown that humic substances (HS) from different origins applied to plant roots can improve plant growth and mineral nutrition (see reviews by Chen et al, 2004; Rose et al, 2014; Olaetxea et al, 2018; and references therein). These positive effects involve various mechanisms, including the action of HS on soil and rhizosphere properties, as well as their interactions with plant roots. It is plausible that the mechanisms underlying the response of plants to foliarapplied HS may involve nutritional, metabolic and physiological differences compared to the response to root-applied HS

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