| Biological invasion is one of the most important problems that the global natural ecosystem faces. It not only causes the biodiversity decrease, but also harms the global ecological environment and the economic development.Asteraceae (Compositae) is not only one of the biggest families in angiosperm, but also the most harmful family with greatest number of invasive species in China. Referred to the related documents and wide spot investigation, a list of58Asteraceae invasive species in China was recorded in this dissertation. Among them,12species were listed as the quarantine pests that forbidden import into China in2007.This dissertation focuses on the study of breeding system of12Asteraceae invasive species in China. On this basis, the detailed study is made on the correlation between reproductive traits of3species and invasiveness, and on the contribution of the characteristics of seeds in10Asteraceae invasive species to invasiveness. The main objective of this reseach is to elucidate the reproductive traits of Asteraceae and provide the useful information on the management of Asteraceae invasive species.According to Baker’s Law, species with uniparental reproduction have a higher chance of establishment after long-distance dispersal than species relying on other mates and pollinators. But there has always been the disagreement on the applicability of Baker’s law to invasive plants. We randomly select12Asteraceae invasive species to study their breeding system, which are Ageratina adenophora (Eupatorium adenophorum), Ageratum conyzoides, Aster subulatus, Bidens pilosa, Conyza bonariensis, C. canadensis, C. sumatrensis, Coreopsis lanceolata, Erigeron annuus, Solidago canadensis, Sonchus asper and S. oleraceus. In total, ten species of them (83.3%) set viable seeds in capitula enclosed by paper bags and show the self-compatibility and autonomous self-fertilization. Integrated experiments of the breeding system and pollen-pistil compatibility reveal that two species (Ageratina adenophora and Erigeron annuus) are apomixis (16.7%), three species (Ageratum conyzoides, Conyza canadensis, C. sumatrensis) are autogamy (25%), five species (Aster subulatus, Bidens pilosa, Conyza bonariensis, Sonchus asper and S. oleraceus) are partial autogamy and self-compatibility (41.7%) and other two species (Coreopsis lanceolata and Solidago canadensis) are self-incompatibility (16.7%). Although the latter two species are self-incompatible, they can vegetatively reproduce by robust rhizome.Our results demostrated that:(1) Baker’s law is propitious to explain the invasiveness of all12Asteraceae species studied in terms of vegetative reproduction and not requiring a mate or pollinator for seed production;(2)Ten of the12species of Asteraceae (83.3%) were capable of seed production when pollinators were excluded with paper bags, and their seed-set percentages were more than40%, suggesting that the autonomous seed production through autogamy, autonomous self-fertilization and apomixis play an important role in plant invasiveness;(3) In our study,12invasive species demonstrated the high correlation between breeding system and invasiveness and harmfulness. A few species whose breeding system are apomixis or autogamy are species of severe invasiveness, were listed as noxious or regional noxious weeds in the related literature, such as Ageratina adenophora, Erigeron annuus, Ageratum conyzoides, Conyza canadensis and C. sumatrensis. A few annual species whose breeding system were partial autogamy and self-compatibility are general invasive species or weeds, such as Aster subulatus, Bidens pilosa, Conyza bonariensis, Sonchus asper, S. oleraceus;(4) It was recently reported that autonomous seed production plays an important role in plant invasiveness. Our result combining with survey of published literature about the correlation of life form with autonomous seed production indicated that eleven of the12annual Asteraceae invasive species were hermaphroditic. They are capable of autonomous seed production as a consequence of self-compatibility or apomictic. Among the12perennial species of Asteraceae with successful invasion, nine of them showed self-incompatibility or partial self-incompatibility. Thus, autonomous seed production playing an important role in plant invasiveness may relate more to annual Asteraceae invasive species and less to perennial Asteraceae species.A total of49Asteraceae species were reported as invasive species in China, but few studies on reproductive biology have been reported. In order to understand the role of sexual reproductive traits in successful invasion in Asteraceae invasive species, we studied the reproductive traits in3Asteraceae invasive species (Conyza sumatrensis, Sonchus asper and Bidens pilosa), in terms of the biology and phenology of capitula and florets, pollen/ovule (P/O) ratio, pollen viability, stigma receptivity, mating system, flower visitors, association of these traits with invasiveness. The result demonstrated that these species are self-compatible and have a mixed mating regime of self-and cross-pollination, both are propitious to invasion. Apomixis was not discovered in decapitated inflorescence. The reproductive traits of autonomous seed production, versatile mating system of self-and cross-pollination and generalized pollination system may contribute to successful invasion. The invasiveness is increased by the high and rapid production, high percentage and rapid germination rate and high dispersal capabilities of achenes (seeds).Understanding of seed biology of invasive plants will be helpful to predicting the trend of invasiveness and evaluating the strategy on sustainable management. Seed dispersal by wind is the prevailing dispersal pattern in Asteraceae invasive plants. In this study, the dispersal and germination characteristics of the seeds with pappus in10Asteraceae invasive plants are determined. The physical character, settlement velocity in the still air and the dispersal potential of these seeds are compared. Also, the germination of seeds which are buried in soil at different depths, and the character of seedling establishment in these invasive plants were studied. The results showed that, the settlement velocity of seeds of ten Asteraceae invasive species ranks from Ageratum conyzoides, Galinsoga parviflora, Erigeron annuus, Ageratina adenophora, Aster subulatus, Sonchus asper, Sonchus oleraceus, Conyza bonariensis and Conyza canadensis to Conyza sumatrensis. The settlement velocity of the seed in the still air is interactively affected by1000-seed weight and the pappus character. The result of correlation analysis demonstrated that, the dispersal ability of seeds is positively related to1000-seed weight (R=0.4767, P=0.0038), whereas is negatively related to the length and number of pappus, and the number of barbs on pappus. The result of germination in incubator indicated that, except Aster subulatus and Erigeron annuus, the germination of the other8seed species was all above50%at25℃and under12hours’ illumination, and the germination was rapid at early stage after sowing. The equations of cumulative germination of the10seed species between the timing of incubation are fitted to cubic curve. The germination of the10seed species was the highest at the surface layer soil, and decreased with the increasing depths of burial. When the depth of burial was above3cm, all seeds could not germinate. The results of dispersal ability and germination character mentioned above may provide theoretical basis for the efficient control of these invasive plants. |
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