Chemical Structure and Biological Activity of Humic Substances Define Their Role as Plant Growth Promoters

Abstract

Humic substances (HS) are dominant components of soil organic matter and are recognized as natural, effective growth promoters to be used in sustainable agriculture. In recent years, many efforts have been made to get insights on the relationship between HS chemical structure and their biological activity in plants using combinatory approaches. Relevant results highlight the existence of key functional groups in HS that might trigger positive local and systemic physiological responses via a complex network of hormone-like signaling pathways. The biological activity of HS finely relies on their dosage, origin, molecular size, degree of hydrophobicity and aromaticity, and spatial distribution of hydrophilic and hydrophobic domains. The molecular size of HS also impacts their mode of action in plants, as low molecular size HS can enter the root cells and directly elicit intracellular signals, while high molecular size HS bind to external cell receptors to induce molecular responses. Main targets of HS in plants are nutrient transporters, plasma membrane H+-ATPases, hormone routes, genes/enzymes involved in nitrogen assimilation, cell division, and development. This review aims to give a detailed survey of the mechanisms associated to the growth regulatory functions of HS in view of their use in sustainable technologies.

Humic Acid Improves Zn Fertilization in Oxisols Successively Cultivated with Maize–Brachiaria

“…The peak recorded at 3290 cm −1 and related to OH stretching is more pronounced when ZnSO 4 is mixed with HA, compared to the intact HA spectral signature, indicating the increase of hydration and production of Zn aqueous complexes [ 7 ]. Hydroxide ions (OH − ) found in carboxylic and phenolic groups and aliphatic domains of HA play a key role in forming soluble metal complexes [ 15 ], resembling the spectral signature of the Zn--humic acid complex synthesized in this study.…”

“…Sulfur groups present in the mixture of HA+ZnSO 4 are indicative of Zn–HA complexation. This was possibly due to the presence of oxygen-, nitrogen-, and sulfur-containing functional groups in the HA structure [ 15 , 20 ]. Another peak indicating the Zn complexation with HA is recorded at 1075 cm −1 , which is an FTIR band related to the formation of a new spectral signature related to RSO 4 − M + groups, where R is the C-humic functional group and M + is the complexed metal [ 21 ].…”

“…Direct effects occur when HA is in contact with the plant cell membrane and tissues, stimulating several plant chemical, physiological, and biochemical processes [ 11 , 12 , 13 , 14 ]. The action of HS on plant tissues includes the stimulation of root proliferation, enhancement of H + -ATPase activity in the cell membrane, and changes in the magnitude of nutrient uptake, assimilation, and use efficiency [ 8 , 9 , 11 , 12 , 13 , 15 , 16 ].…”

“…Humic acid effects on plants also rely on the properties of HA’s structure-organic functional groups-function [ 15 , 17 ]. The positive effects of humic acid on maize or brachiaria were already reported, as well as the residual effects of humic fertilization on brachiaria after successive cuttings [ 8 , 19 ].…”

“…The direct and indirect effects of HA occur simultaneously [ 9 , 11 ], and are linked to the type and concentration of oxygen-, nitrogen-, and sulfur-containing functional groups in the HA structure [ 14 , 15 , 20 ]. Infrared spectroscopy is a suitable technique to reveal the functional groups and specific bonds formed between the metals and HA [ 6 , 7 , 14 ].…”