Modulation of the molecular size of humic substances by organic acids of the root exudates

Abstract

A water solution of citric, malic, oxalacetic, succinic, fumaric acids at the concentration found in the root exudates of maize seedlings affected humic substances above 10 000 ds (HMS) by shifting their molecular size towards values below 10 000ds (LMS). The percent of LMS fraction obtained was significantly correlated with the release of Ca, Mg, Fe, and Zn. Addition of HC1 was ineffective on the molecular size, while the metal release occurred to a lesser extent. K salts of the organic acids were ineffective on both molecular size and metal content of humic substances. When the organic acids added to the humic solution were neutralized by 0.1 M KOH, the original distribution of molecular size and the metal content were restored to a large extent. The above behaviour suggests that the degree of association and the metal content of humic substances in the rhizosphere might be modulated by the organic acids released by plant roots accordingly with the resultant pH. Fe, Mg and Zn proved to be more involved in the association of humic substances than Ca.

Organic acids in the rhizosphere of Banksia integrifolia L.f.

“…The results presented here add to mounting evidence of an active role of plants in altering the root environment as a means of increasing the availability of nutrients, especially phosphorus, for absorption. Albuzio and Ferrari (1989) suggested that organic acids commonly found in root exudates can compete effectively with humic acids as ligands for metal, particularly at low pH. Among the peculiarities of rhizosphere soils, differences in nutrient availability with respect to the bulk soil have been emphasised (Marschner et al, 1987).…”

“…Among the peculiarities of rhizosphere soils, differences in nutrient availability with respect to the bulk soil have been emphasised (Marschner et al, 1987). In addition to altering nutrient availability, changes in the humus molecular size, as an effect of organic acid release by roots, might influence regulatory signals between soil and plant by affecting the efficiency of transport systems in roots and stimulating or depressing enzyme activities involved in nutrient assimilation (Albuzio and Ferrari, 1989).…”

Soil humic substances affect transport properties of tonoplast vesicles isolated from oat roots

“…Since the plant root grows in contact with humic substances, some components of which can enter the cell, we should assume that cytoplasmic concentration of humic molecules may be limited, in order to avoid the detrimental effect at membrane level. This assumption seems to be reasonable in the light of previous results showing the capacity of the root to modulate the molecular size of humic substances by means of root exudates (Albuzio and Ferrari, 1989) and also considering the presence of the cell wall, which can act as a biological molecular filter toward humic molecules.…”

Role of Humic Substances in the Rhizosphere

“…Attempts have been made to verify whether organic acids released by plant roots may change the structure of humic molecules (Albuzio and Ferrari, 1989 ). Analyses by size exclusion chromatography (SEC) have revealed that high -molecular -weight fractions treated with acidic solutions or organic acids release humic molecules of lower molecular weight with a higher biological activities than that of the former molecules.…”