|其他摘要||Tithonia diversifolia (Asteraceae), a Mexico and central American native, has being widely spred into the tropical and subtropical regions in Asia, Africa and Pacific. The species is now a harmful perenial weed invading the grassland, riparian and agricultural fields in Southeast Asia, South Africa and Pacific. In China, T. diversifolia has been already widely naturalized in the tropical and subtropical areas in Yunnan, Guangxi, Guangdong, Fujian, Hainan, Hong Kong and Taiwan province, and it also has a great trend spreading into the higher altitude and further north latitude. In fact, Tithonia diversifolia has already seriously threatened the biodiversity and agroforestry production in China. In order to further understanding its invasiveness, the allelopathy and genetic diversity of Tithonia diversifolia distributed in China have been studied.
The results of effects of water extraction from different organs of T. diversifolia on seed germination, radicle and plumular axis elongation of B. chinensis and Bidens pilosa revealed, that the aqueous extract from different parts of the species could inhibit the seed germination, and the inhabitation was enforced along with the concentration increasement. At same concentration levels, the extractions from the leaves of T. diversifolia presented the highest inhabitation on seed germination rate and seed germination speed of the two species. However, the radicle and hypocotyl growth of the two plants were occasionally promoted at lower concentrations of the extractions or not significantly related with the extractions.
Observations on cytologies of 13 T. diversifolia populations from China (11 from Yunnan, and other 2 from Xiamen and Hainan) indicated, that the chromosome numbers of all the populations were 2n = 34 at metaphase, except some chimera cell type 2n = 32 or 68 (less than 5%) in Hekou population of Yunnan, and the basic chromosome number was x = 17. The karyotype variation at the diploid level presented a few distinguishable cytological characters.
Genetic diversity of 329 individuals from 16 T. diversifolia populations (14 from China and 2 from Laos) by using ISSR molecular markers was investigated. Ten ISSRprimers were employed and some 65 clear and reproducible DNA fragments were generated. The percentage of polymorphic loci PPB was 84.62%, Nei’s (1973) gene diversity H was 0.3599, and Shannon’s Information index I was 0.5197. The coefficient of genetic differentiation (GST) was 0.1173 and the gene flow (Nm) was 2.2071. Analysis of molecular variance(AMOVA) showed that a large proportion of genetic variation of 81.59% ( P<0.001) occurred within the populations. The higher gene flow imply that gene exchanging between the populations was rather frequent. Furthermore, there was no significant associations between genetic diversity and geographic distance among the investigated T. diversifolia populations(r =0.1592, P=0.1490＞0.05). The association test between the five ecological factors (longtitude, latitude, elevation, annual average temperature, annual average pecitation) and Nei’s gene diversity H，suggested that the genetic diversity of T. diversifolia was increased towarding the further north latitude(r=0.646，P=0.007). However, there were no significant correlations between the other four ecological factors and genetic diversity.
In summary, allelopathic activity of the water extraction infer that and rain-water could bring the allelopathic compounds of T. diversifolia into the soil, and thus the seed germination and seedling growth of other surrounding plants migh be inhabited. The chromosome numbers from all the investigated populations were same and the karyotype variation was limited. However, genetic diversity T. diversifola investigated by ISSR presented the extremely high level of genetic diversity, and therefore the species has a rather wide adaptability to the environment. The strong allelopathic activity and adaptability to the environment might be the factors associated with its invasiveness.|