| Owning to exerting strong impacts on ecological environment, economy and human welfare, the exotic species invaded arouse extensive attention. Understanding the factors that contribute to the success of invasive species may facilitate the prediction of future invasions, control and management. Bidens alba and B. pilosa var. radiata (Asteraceae) are native weeds from tropical America, that have invaded roadside, cropland, waste ground in China, causing losses to local agriculture and environment. Mechanisms facilitating their invasion are not well understood. In this study, I conducted comparative study of seed germination, growth and reproduction, as well as phenotypic plasticity in the two alien weeds and their native congener under different environmental conditions, aimed to better understand the invasive mechanisms of this two alien Bidens weeds.The main results are as follows:1. The seed germination of the two alien Bidens weeds were different from their native congener under different condictions.The highest germination percentage of the three species were observed between25～30℃. At15℃with both the periodic light and complete dark, the seeds of the two alien Bidens weeds germinated over80%, but the native congener germinated less than20%. There was no significant difference in seed germination of the native congener between the periodic light and continuous dark treatments at each temperature (except35℃). No germination occurred in the native seeds at40℃. The germination percentage of B. alba at40℃was67%in periodic light treatment, which higher than that in complete dark treatment. The seed of B. pilosa var. radiata germinated30%at40℃with periodic light treatment, but did not germinate under complete dark treatment.There was no significant difference in germinate speed among the three species at25～30℃. The seed germination was fast and tidy, and reach the peak in the third or fourth day at25～30℃with both the periodic light and complete dark. Under the periodic light treatment, the seed germination of the two alien species reach the peak in the sixth day at15℃, while the native seed germination reach the peak in the ninth day. Under the complete dark conditions, the germination speed of three species was similar to that under the periodic light treatment at each temperature, except that the germination of the native at35℃delayed6days compared with that under the periodic light treatments.Seedling emergence of the three species was obviously influenced by seed burial depth. Maximum emergence occurred at the burial depth of0cm, with the highest germination percentage in B. alba and the lowest germination percentage in B. pilosa var. radiata. At the burial depth of1-4.5cm, the germination percentage of the native was significantly higher than that of the two alien Bidens weeds. No seedlings of B. alba emerged from seeds placed at a depth of3.5cm or deeper. Seedling emergence of B. biternata and B. pilosa var. radiata decreased dramatically at the burial depth of5.5cm.In summary, these results suggest that the two alien Bidens weeds have the ability to germinate under a broad range of temperature conditions when compared with their native congener, and the seed germination was influenced by seed burial depth.2. The growth characters of the two alien Bidens weeds were different from their native congener under different light and water condictions.The morphology and biomass under favorable condition was higher than those under limited condition. The sequence of plant height was B. pilosa var. radiata> B. biternata> B. alba. The total leaf area, total biomass, leaf area ratio and specific leaf area of the two alien Bidens weeds were higher than the native congener. Plant height, leaf area and biomass of B. pilosa var. radiata were the smallest in the limited conditions, while the leaf area ratio and the specific leaf area were the highest. B. pilosa var. radiata increased specific leaf area and reduced the leaf construction cost to survive in low light conditions, which showed the invasiveness of this species. The leaf area, biomass, specific leaf area and leaf area ratio of B. biternata were higher than those of B. alba. B. biternata might be a native weed with strong fitness.Under favorable light and water conditions, B. alba had the highest root biomass ratio, root mass/crown mass and leaf biomass ratio, but the parameters of B. biternata were the smallest. The stem biomass ratio was the highest in B. biternata and was the lowest in B. alba. Under limited light and water conditions, the root biomass ratio and root crown ratio of B. pilosa var. radiata were the highest; the two parameters were similar for B. alba and B. biternata. The stem biomass ratio was the highest in B. alba and was the lowest in B. biternata. The leaf biomass ratio of B. biternata was higher than that of B. pilosa var. radiata.The three Bidens species had high phenotypic plasticity under different light and water conditions, The phenotypic plasticity indices of plant height, leaf area, leaf area ratio, specific leaf area and biomass in B. pilosa var. radiata were higher than those in B. alba and in B. biternata. The phenotypic plasticity indices of leaf area, biomass, root biomass ratio and root crown ratio in B. alba were higher than those in B. biternata.In summary, the growth characters of the two alien Bidens weeds were better than their native congener under different light and water conditions. They had high phenotypic plasticity. This may be related with their strong invasiveness.3. The growth and reproductive characters of the two alien Bidens weeds were different from their native congener under different soil nutrient levels.The plant height, branch number, total leaf area, specific leaf area, total biomass of the three Bidens species under the high nutrient conditions were significantly higher than those under the low nutrient condictions. These parameters had no significant difference among the three species in low nutrients condition. In the high fertile conditions, the branches number of B. biternata were significantly higher than that in the two alien species; the total leaf area and specific leaf area was the highest in B. alba and was the lowest in B. pilosa var. radiata, and there was significant difference in the three species; the total biomass of B. biternata and B. alba were significantly higher than that of B. pilosa var. radiata.The biomass allocation had no obvious difference among the three species under the same nutrient levels. Both root biomass ratio and stem biomass ratio in different nutrients levels had significant difference in the alien Bidens weeds, but had no obvious difference in B. biternata. The leaf biomass ratio of the three species had no significant difference between low and high nutrient levels.The relative growth rate and mean leaf area ratio of the three species were higher in high nutrient treatment than those in lower nutrient treatments. While the net assimilation rate had no significant difference in the two nutrient conditions. There were no significant difference in the relative growth rate, net assimilation rate and mean leaf area ratio among the three species under the low nutrient level. Under the high nutrient level, the relative growth rate of B. biternata and B. alba were significant higher than B. pilosa var. radiata, the net assimilation rate was the highest in B. biternata and was the lowest in B. alba; the mean leaf area ratio of B. biternata were significant higher than that of B. pilosa var. radiata.The seed yield of the three species were significantly higher in high nutrient level than that in low nutrient level. Under both high and low nutrients level, the duration of reproductive phase and seed production of the two alien species were significantly higher than those of the native.In summary, long reproductive phase and high seed production in B. alba and B. pilosa var. radiata which were different from those in native weed B. biternata, were some of the important characters for invasiveness.The above results provide some advice for the management and control of the two alien Bidens weeds.(1) Eradicate alien Bidens weeds before they flower and set seeds, this can prevent the massive seeds occur.(2) Avoid the seeds spread in anywhere.(3) Plough deeply the farmland, this can prevent seeds germination.(4) Don’t throw the garbage with rich nutrients randomly. |
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