Residual phytotoxicity of parthenium: Impact on some winter crops, weeds and soil properties.

Phytotoxic effects of parthenium residues incorporation and parthenium-infested rhizospheric soil on emergence and seedling growth of winter crops (wheat and canola) and weed species (wild oat and canary grass) were examined in different pot studies. In first experiment, parthenium whole plant residues were incorporated at 6 and 8 g kg(-1) soil five days prior to sowing. Pots without residues incorporation were maintained as control. In a second study, parthenium-infested rhizospheric soil collected from different depths (15 and 22.5 cm) and collar regions (horizontal distance away from plant trunk, 15 and 22.5 cm), was used as growing medium. Parthenium-free soil was used as control. Parthenium residues amendment as well as its rhizospheric soil was detrimental for emergence and seedling growth of all test species. Incorporation of parthenium residues reduced the final emergence of canola, wild oat and canary grass by 11-20%, 20-29% and 20-27%, respectively; however wheat emergence was unaffected. Moreover, seedling biomass of wheat, canola, wild oat and canary grass was reduced in the range of 41-48%, 53-61%, 31-45% and 30-45% by parthenium residues incorporation. In second study, soil collected from a rhizospheric depth of 15 cm and collar distance of 15 cm reduced the emergence and seedling growth by 15% and 40%, respectively averaged across different test species. Parthenium residues incorporation and infested rhizospheric soil increased the soil phenolics, electrical conductivity, organic carbon and nitrogen contents over control soils with the exception of pH that was declined. All test species manifested reduced chlorophyll and increased phenolic contents in response to parthenium residues incorporation and infested rhizospheric soil. The inhibition in emergence and seedling growth of all test species was associated with increase in phenolic contents. Parthenium residues incorporation at 8 g kg(-1) soil and upper parthenium-infested rhizospheric soil (15 cm soil depth and 15 cm collar distance) were more phytotoxic for all test species.