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Degree Name

Doctor of Philosophy




It has been reported that corn following soybeans produces 7 to 10% more grain yield than corn following corn. These results have been observed under optimal conditions where there was no nitrogen fertilizer advantage or moisture superiority of corn following soybeans;This study proposed that one of the reasons for enhancement of corn yield following soybeans may be the effect of stimulatory allelochemicals from soybean residue on corn growth. Chloroform was found to be a better solvent than acetone, benzene or water for extracting stimulatory allelochemicals from soil-soybean residue when using leaf growth of corn seedling as a bioassay. The chloroform extract of soil-soybean residue after filtration and flash evaporation was loaded onto a DEAE-sephadex column. Different elutant solvents such as methanol, isopropanol, chloroform and combinations of them with water or acetic acid were tested. Fractions were bioassayed by applying different concentrations of them on germination papers and growing corn seedlings on the papers. Lengths, fresh weights, and dry weights of radicle and coleoptile of seedlings were measured;The results showed that 100% isopropanol was the best elutant solvent with 5% acetic acid (v/v) added to isopropanol after Fraction 8. The chloroform extract was flash evaporated to dryness and the isopropanol-soluble material was loaded onto a column. The isopropanol-insoluble material was dissolved in chloroform and bioassayed without chromatography. The experiments concentrated on Fractions 2, 3, 6, 9, 12 and 14, and on the chloroform-soluble materials which were insoluble in isopropanol;No significant stimulatory chemicals were observed from the chloroform extract of soil with low amount of residue when coleoptile growth was measured. Results of the bioassay of extract from field samples of soil-soybean residue showed that Fraction 12 increased radicle weight 20% and the 0.4 ml concentration of it stimulated the coleoptile weight by 19%. Fraction 12 of an extract of ground soybean residue increased radicle fresh weight and dry weight 24.7 and 9.2%, respectively. The coleoptile dry weight increased 11.7 and 21% due to effect of Fraction 12 and of the chloroform-soluble material, respectively;Test of different decomposition periods indicate that as soybean residues interact with the soil for progressively longer periods of time, the stimulatory effects of the residue become more pronounced.



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Issa Kalantari



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148 pages