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The apparent high molecular mass of humic acids (HAs) hardly seems compatible with their direct effects in plant physiology. However, previous evidence has indicated that HAs are non-covalent associations of relatively small molecules, which can be broken down by the action of organic acids. The aim of this work was to evaluate the effects of organic acids on the structure of HAs by spectroscopy and on their bioactivity by following the responses of maize root growth. Changes in the exudation of organic acids from maize seedlings treated with HAs at 50 mg C L 21 were evaluated by high-performance liquid chromatography. The results are in agreement with the concept that HAs are chemical aggregates that acquire characteristics typical of low-molecular-mass humic substances when exposed to organic acids exuded by the roots. Maize seedlings grown in solutions supplemented with HAs plus citric acid at 0.0005, 0.005 and 0.05 mM exhibited significant changes in their root area, primary root length, number of lateral roots and lateral root density and increases in plasma membrane H + -ATPase activity. Furthermore, the root exudation profile of plants treated with HAs exhibited an increase in the efflux of oxalic and citric acids, with a concurrent decrease in malic and succinic acids. These data reveal a crosstalk between HAs and plants where the exudation of organic acids from the roots influences and is influenced by bioactive molecules released from HAs during root development.

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A series of humic matter samples isolated from a soil sequence, different oxisols, size‐fractionated from a vermicompost humic acid and subjected to chemical modifications, were characterized by CPMAS 13C‐NMR spectroscopy. The relative signal areas in chemical shift regions of NMR spectra of the four sets of samples were analysed by principal component analysis (PCA). Hierarchical cluster analysis (HCA) was applied to build a classification model, which allowed the recognition of humic matter according to its origin. The relationship between carbon species and biological activity of humic acids, as promoters of lateral root emergence, was obtained by applying PLS multivariate analysis. This showed that lateral root emergence was mostly related to NMR parameters such as the hydrophobicity index (HB/HI) and the 40–110 and 160–200 ppm chemical shift regions (hydrophilic carbon HI), while the content of hydrophobic (HB) carbon in humic samples was negatively correlated with induction of lateral root hair. Our results represent a step further in the structure‐bioactivity relationship of natural humic substances and confirm their role as plant root growth promoters.