Humic-like substances obtained by alkaline hydrolysis of composted organic wastes are known to improve plant productivity. Little is known concerning the effect on plant growth of hydrolysates obtained by alkaline treatment of non-composted vegetal residues.The aim of this study was to prepare, characterize, and apply in horticulture the soluble and the insoluble fractions obtained by alkaline hydrolysis of exhausted tomato plants. The hydrolysates were prepared in a pilot plant from the tomato plants residue in powder form and characterized by solid state 13C NMR spectrometry, elemental analysis and potentiometric titration. The characterization of the soluble hydrolysate gave evidence of lignin and hemicellulose moieties, together with protein, peptide or amino acids while most of the cellulose was found in the insoluble fraction. Plant beans were grown on a peat and sand substrate fertilized with low application rate of the tomato plant powder and of the corresponding hydrolysates. The effect on bean plants was assessed by determination of plant growth, chlorophyll content, nitrate reductase, glutamine synthetase, glutamate synthase activities and soluble proteins.The tomato plant powder had no effect on all parameters measured on the grown bean plants. On the contrary, the insoluble and soluble substances sourced by alkaline hydrolysis of the tomato plant powder exhibit strong effects, mainly the increase of nitrogen assimilation typical of biostimulants.The results suggest that residual plant biomasses are source of efficient biostimolant and propose the hydrolysis of residual biomass as a viable profitable process to contribute important improvements also for waste management practices. © 2014 Elsevier B.V

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