| Yellow top weed(Flaveria bidentis (L.) Kuntze)is an invasive weed native to South America, firstly reported in Hengshui lake in Hebei province in 2003 and now rapidly spreading to 84 counties of Hebei province, five districts of Tianjin and Anyang city in Henan province, which has posed a potential threat to plant diversity and agricultural production. Field investigations, laboratory and greenhouse experiments were conducted to study ecological characteristics of the species systemically. The main results were as follows:1. Seeds of F. bidentis were positively photoblastic and light stimulating was necessary for seed germination, but light intensity for germination was not harsh. Seed germination at 30℃for 5 d were 6.7% and 98.3% in continuous darkness and alternating light / dark, while germination within 5 d were 67.0%, 88.0% and 95.8%, respectively, when incubated at 1000 lux and 30℃for 6 h, 12 h and 24 h before dark incubation. Seedling emergence percentages were 96%, 8% and 0, respectively, when incubated at a soil depth of 0 cm, 0.5 cm and 1 cm. Plant biomass and seed production fell with light intensity decreasing.2. Seed germination and seedling emergence had a wide temperature range, and were both stimulated by alternating temperatures. Under constant temperatures, seed germination and seedling emergence both o -ccurred from 15℃to 40℃with the optimum temperatures from 22.5℃to 35℃and 22.5℃to 40℃res -pectively. The germination threshold temperature was 14.73℃and the effective accumulative temperat -ure required to reach 90% germination was 40.36℃. Germination and seedling emergence was enhanc -ed by 12% to 100% and 10.8% to 100% individually from 12.5℃to 20℃fluctuating temperatures. Under perennial field natural temperature in North China, F. bidentis emerged from the first ten days of April to the middle of October, with its growth period being 140~87 d, namely the later seedlings emerg -ed, the shorter the growth period was. The length of growth period was regulated by vegetative growth stage because of larger plasticity compared with reproduction growth stage. The plant height, leaf numb–er, branch number, biomass and seed reproduction decreased as F. bidentis sowing date was postponed, but plants emerging before August 25 could produce mature seeds.3. F. bidentis seed germination inhibition was independent of soil moisture, salinity and pH solutions. Seed germination occurred from 10% to 40% soil relative water content, with maximum germination at 25% soil moisture. Seeds germinated over a wide range of NaCl solutions (0 to 320 mmol·L-1), and optimum germination occurred between 0 and 160 mmol·L-1 NaCl. Seed germination was not significantly affected by the tested range of pH (4.0~10.0) solutions.4. Aqueous extracts of F. bidentis had inhibitory effect on seed germination of many plants. Germination of 29 species was inhibited and radical length of 28 species was reduced to varying degree by the aqueous extract 0.1g·mL-1 concentration of F. bidentis at mature stage. There was a strong correlation between extract concentration and increased inhibition to test species. A gradually decrease of seed germination and radical length of E. crusgally and B. napus were observed with increasing the concentration of the extracts. Seed germination and radical length of E. crusgally and B. napus were inhibited more by extracts from leaves than roots and by extracts from mature plants than from young plants of F. bidentis.5. F. bidentis had strong regeneration capacity. The cutting of aboveground parts during seedling stage and squaring period achieved highest survival rate, and flowered and produced seeds subsequently, while the cutting rooting rate and survival rate decreased at later growth stage. The remained parts containing leaves had the ability to regenerate into a new plant.These results could help predict the growth area and potential spread area of F. bidentis and facilitate the development of its effective control programs.
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