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Abstract :
[en] Arise from the chemical and biological degradation of plant and animal residues and from the synthetic activities of microorganisms in the soil, humic substances (HS) are natural heterogeneous aromatic and organic compounds. These substances are chemically complex with no clearly defined chemical structure, although generalized models have been proposed. Present everywhere in the nature; they take part in basic functionalities in any ecosystems involving soils, sediments, water and landfills. They have long been recognized as plant growth promoting substances, particularly with regard to influencing nutrient uptake, root growth and architecture. They influence plant productivity directly by the stimulation of biochemical and metabolic processes or indirectly through the modification of soil characteristics and microflora activities. All together, these properties mainly affect root architecture by inducing root hairs proliferation, differentiating root cells and enhancing lateral root emergence. Experiments targeting the rooting stages in absence of interferences were conducted in vitro using HS extracted from landfill leachate and a stable commercial formulation (“Humifirst” from TRADECORP company: 12% humic acid and fulvic acid 3%) issued from leonardite. Shoots and leaves explants of silver birch (Betula pendula Roth) and black alder (Alnus glutinosa L. Gaertn) explants were treated with 10 ppm of leachate and leonardite HS for 5 days during the rooting induction/initiation phase or during rooting elongation phase. The results obtained show that treatment with a low concentration (10 ppm) during induction/initiation phase may be slightly unfavorable to the formation of roots in alder but not in birch. While, in root elongation phase, there is an increase in the number of roots per shoot only in birch. The direct effects of leachate and leonardite HS on root development vary from one species to another.
