Xanthium Suppression Under Maize||Sunflower Intercropping System

Weed control is one of the major constraint factors in crop production around the world. Field experiments were conducted during 2008-2009 under intercropping systems involving alfalfa （Medicago sativa L.）, sweet potato （lpomoea batatas [L.] Lain.）, peanut （Arachis hypogaea L.）, and sunflower （Helianthus annuus L.） in maize （Zea mays L.） without any weeds control methods taken. The results demonstrate that maizellsunflower is most effective on weed suppression and that it also has a more competitively inhibitory effect on Xanthium compared with the other patterns by evaluating the Xanthium density and dry weight under different intercropping systems with maize. Maizellpeanut, maizellalfalfa and maizel]sweet potato intercrops have no apparent inhibitory effect on weeds. To further investigate the effect of maize]lsunflower on weed control, indoor pot experiments were conducted by determining the effect of extractions on germination rate （GR）, root vigor, MDA （malondialdehyde）, SOD （superoxide dismutase） and POD （peroxidase） content of Xanthium. The results better prove that maizellsunflower extractions have more significant inhibitory effect on GR and young root vigor of Xanthium than maize monocrop extractions.

Al elopathic Effects of Melilotus officinalis Desr. against Weeds

In this study, the allelopathic effects of 100 g/L water extract of Melilotus offtcinalis Desr. on seed germination and seedling growth of Melilotus offwinalis Desr., Trifolium pretense L. , Medicago sativa L. cv. Victoria, Echinochloa crusgalli （ L. ） Beauv., Lxer/s chinensis （ Thunb. ） Nakai and Plantago asiatica L. were investigated by biological assay. Dried powder of Melilotus officinalis Desr. was applied to the field in spring and winter to verify the inhibitory effects of Me- lilotus officinalis Desr. on weed biomass in the experimental field and compare the effects between Melilotus offwinalis Desr. and a variety of commonly used chemi- cal herbicides on weed suppression. The results showed that water extract of Melilotm officinalis Desr. posed significant effects on seed germination and seedling growth of Trifolium pretense L. , Echinochloa crusgalli （ L. ） Beauv. , Lxeris chinensis （ Thunb. ） Nakm, Plantago asiatica L. , Melilotus o^tcinalis Desr. and Medi- cago sativa L. cv. Victoria, exhibiting strong allelopathic effects, but the inhibition rate varied greatly among different plant species. Therefore, the allelopathic effects of Melilotus o^winalis Desr. should be utilized appropriately in practical applieatians. Dried powder of Melilotus officinalis Desr. significantly reduced weed quantity in the field for a short duration, which should be administered multiple times in a sufficient amount for weed control in the production.

Phenotyping of Weedy Rice to Assess Root Characteristics Associated with Allelopathy

Weedy rice is a species of Oryza, and is a wild relative of cultivated rice. The weed possesses unique hardiness that allows them to thrive in dynamic and stressful environments. These characteristics suggest that weedy rice is a stored source of novel genes for competitive traits. One such trait is allelopathy, where a species releases secondary metabolites that suppress the growth and development of neighboring species. Weed competition is a limiting factor in rice production systems;therefore, it is critical to identify specific allelopathic weedy rice accessions to determine the genetic pathways and mechanisms associated with allelopathy to be used in breeding programs. Due to the complex nature of allelochemical production and the lack of knowledge of allelopathy mechanisms in weedy rice, phenotypic traits, particularly root traits, can be used to overcome this limitation and serve as target characteristics for breeding weed suppressive rice varieties. Five weedy rice accessions were chosen from preliminary screenings of larger sample sizes with the ability to suppress barnyardgrass weed seedling growth. Another five weedy rice with low barnyardgrass suppression was selected for the current root phenotypic study. Five cultivated rice lines were used as a comparison. All plants were propagated in a transparent germination pouch for four weeks. Roots were scanned and analyzed for root length and area covered. No differences were found in the seedling root area among weedy rice and rice accessions;however, allelopathic weedy rice plants exhibited a 14% increase in root length than non-allelopathic weedy rice plants. The allelopathic weedy rice accession B2 possessed the most extended root system (22.4 cm root length). The highly allelopathic weedy rice accessions (including B2) screened and phenotyped in this study are ideal candidates for identifying the genetic controls of early root length, a possible trait contributing to underground allelopathic production and competitive advantage.