Hypothetical framework integrating the main mechanisms involved in the promoting action of rhizospheric humic substances on plant root- and shoot- growth

Root ABA and H+‐ATPase are key players in the root and shoot growth‐promoting action of humic acids

“…Many studies have demonstrated the presence of high correlation between soil fertility, crop productivity, and the content of transformed (humified) natural organic matter in the soil (Chen et al, ; Stevenson, ). The beneficial effects of humified organic matter, principally HS, on plant growth are driven by improvements in plant mineral nutrition and rhizosphere physico‐chemical features (Aguirre et al, ; Chen et al, ; Olaetxea et al, ). In some cases, direct effects of HS on plant metabolism and physiology may also be involved (Nardi, Pizzeghello, Muscolo, & Vianello, ; Olaetxea et al, , ; Vaughan & Malcolm, ).…”

“…Humic substances (HS) are principal components of both soil organic matter (SOM) and dissolved organic matter (DOM) (Canellas & Olivares, ; Chen, Nobili, & Aviad, ; Olaetxea et al, ; Stevenson, ; Tipping, ; Trevisan et al, ). Although the structure of HS is still a matter of debate (Baigorri, Fuentes, Gonzalez‐Gaitano, & García‐Mina, ; Baigorri, Fuentes, Gonzalez‐Gaitano, & Garcia‐Mina, ; Clapp & Hayes, ; Piccolo, ; Swift, ), recent studies indicate that they have singular structural features that are not present in natural, well‐characterized, biomolecules, such as proteins, cellulose, lignin as well as microbial metabolites (Cao & Schmidt‐Rohr, ).…”

“…This action results from the HS capacity to improve plant nutrient availability in soil, mainly through the formation of stable complexes with metals (Chen et al, ; Erro et al, ; Garcia‐Mina, ; Gerke, ; Urrutia et al, ). Besides this effect on improving nutrient bioavailability, HS also promotes plant growth through action on plant metabolism and physiology, which is derived from the interaction of HS with plant roots (Canellas, Olivares, Okorokova‐Facanha, & Facanha, ; Muscolo, Sidari, Francioso, Tugnoli, & Nardi, ; Olaetxea et al, , ; Quaggiotti et al, ). However, knowledge about the mechanisms responsible for this direct effect of HS on the promotion of plant growth is scarce and fragmentary, regarding the effects on both root development and shoot development (García et al, ; Mora, Bacaicoa, Baigorri, Zamarreño, & García‐Mina, ; Olaetxea et al, ).…”

“…It is clear that the whole action of HS on crop development and final yield involves other complementary mechanisms that are not studied here. Indeed, in the case of intensive field crops, those mechanisms associated with the improvement of nutrient (mainly micronutrient) availability, resulting from the interaction of HS with soil components (Chen et al, ; Olaetxea et al, ), may be much more relevant than those linked to HS direct action on plant roots. In fact, the main purpose of this research is to investigate some aspects of the action of HS on plant growth as a main component of SOM, which would be directly involved in the role of SOM in soil fertility.…”

Wheat nutrition and growth as affected by humic acid‐phosphate interaction

“…Recent studies have demonstrated that the features of the HA chemical structure are a driving stimulus for hormone activity and root proliferation ( García et al, ; Scaglia et al., ; Olaetxea et al, ). The chemical structure of HA recorded by the ATR‐FTIR spectrum showed aliphatic C and a variety of aromatic C groups, as well as other stretching and bending vibrations (Fig.…”