Allelopathic potential of leftover materials from sunflower helianthus Annuus L.

Abstract

The allelopathic effects of the extracts of sunflower (Helianthus annuus L.) were evaluated on seed germination and growth inhibition of Mimosa pigra L. Dried leaves, stems and roots of sunflower were separately extracted by different solvents. The inhibition activity using three concentrations of 1.0, 2.5 and 5.0 gE was compared. The CH3OH extract of the leaves inhibited seed germination and the growth of M. pigra. This extract was further evaluated on allelophatic effects of selected weeds and crops. At 1 gE, the extract showed selective inhibition on seed germination of weeds more than crops, but expressed as non-selective inhibition on shoot and root elongation of all tested plants. Further examinations on pot experiments as pre- and post-emergence weed controller were conducted. The use of dried leaves as pre-emergence, and that of the CH3OH extract of the leaves as pre- and post-emergence inhibited seed germination and growth of all weeds, but less than those observed in petri-dish experiments. To control barnyard grass in rice cropping as pre- and post-emergence weed controller, at 1 gE of pre-emergence this extract inhibited barnyard grass germination 33% with no effect on the rice germination and shoot elongation, while inhibited 29% of root elongation. For post-emergence at 1 gE, the extract could completely inhibit the barnyard grass population with no side-effect on the growth of rice seedlings. In addition, all crude extracts were evaluated on other biological activities including antifeedant activity against common cutworm, antibacterial activity against 6 bacteria and 2 fungi. The CH2Cl2 extract of the roots gave the highest inhibition to Rhizoctonia solani (79%). This extract also expressed as selective inhibition on shoot elongation of zinnia and rice by stimulating their root elongation. The chemical composition of this fraction was investigated. Five substances: demethylencecalin, demethoxyencecalin, mokko lactone, a mixture of stigmasta-4,22-dien-3-one and stigmast-4-en-3-one, and a mixture of stigmasterol and β-sitosterol were identified based on spectroscopic evidence.