Humic molecules

Other than a nutritional role linked to mineralization processes, humic compounds have been hypothesized to directly affect plant nutrition, since it has been suggested that roots may take up low-molecular-weight humic molecules (21). Interestingly, plants have been ob.served to express carriers for amino acids (22) and small peptides (23) at the root level. Certain components of the humic fraction have been found inside root cells and were, moreover, translocated to the shoots (24,25). Recent experiments performed on rice cells in suspension culture seem to suggest that they may use carbon skeletons from humic molecules to synthesize proteins and DNA (26). 

Microorganisms at the rhizosphere are known to produce plant growth regulators using precursors released by the roots (71), which can influence root morphology (3) the degradation of these hormone-like substances could be prevented by their incorporation into humic molecules, which would preserve their chemical properties. However, definite proof is still lacking recent studies have shown the... 

R. Pinton, Z. Varanini, G. Vizzotto, and A. Maggioni, Soil humic molecules affect transport properties of tonoplast vesicles i.solated from oat roots. Plant Soil 142 203 (1992). 

Warwick, P., Hall, A., Pashley, V. and Bryan, N. (2001). Investigation of the permeability of humic molecules using zeta potential measurements, Chemo-sphere, 45, 303-307. 

Enhancement of the solvent recovery of various sorbed hydrophobic organics by a digestion technique, which degraded the DHS. Simple adsorption onto the nonpolar region of humic molecules by van der Waals forces and hydro-phobic interfacial tension would probably hinder solvent recovery of adsorbed hydrophobic organics. 

A high degree of oxidation leads to a high degree of condensation in humic molecules ... 

Functional groups constitute about 20-30% of the humic molecules About 25% of soil humic acids and about 50-60% of a fulvic acid molecule consist of functional groups... 

The enhancement of kerosene dissolution occurs even at low humic acid content in the aqueous solution. In view of the fact that humic substances are relatively high molecular weight species containing nonpolar organic moieties, Chiou et al. (1986) assumed that a partition-like interaction between a solute of very low solubility in aqueous solution and a microscopic organic environment of dissolved humic molecules can explain solute solubility enhancement. 

Ternary adsorption of humic molecules and Ca ions modified the surface charge and the colloidal behaviour of goethite (Tipping and Cooke, 1982). Fe oxides in natural environments often form in the presence of or through the activity of, micro-organisms (see Chap. 17). The adsorption properties of the oxides may, therefore, be modified by adsorbed organic molecules and ternary Fe-OCO-M complexes (CO-org. 

A high polarity and a high dielectric constant to assist the dispersion of charged humic molecules... 

Infrared spectroscopy was widely used in the second half of the 20th century, and this technique has allowed some advances to be made in awareness of functionalities in, and of complexes formed by, humic molecules. However, the greatest advances in determinations of functionalities, in aspects of compositions and structures, and now in aspects of humic interactions have been made since the introduction of solid-state 13C NMR spectroscopy (Wilson, 1987 Malcolm, 1989). Chapter 15 in this book (by Simpson and Simpson) has reviewed in detail the applications of NMR in the solid and liquid states to studies of compositions and interactions of NOM. We now have a good indication of the types of functionalities that compose HS, and combinations of modem NMR technologies and principal component analysis (PCA) techniques allow us to deduce the origins of some of the functionalities (Novotny et al., 2007). 

As described above, chemical, biochemical, and microbiological conditions in the rhizosphere differ widely from those of bulk soil this can lead to changes in the dynamics and structure of humified organic matter. On the other hand, little is known about the molecular structure and degree of aggregation of humic molecules in the rhizosphere. 

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