| Yellow-top (Flaveria bidentis (L.) Kuntze) weed was originated from South America and was transmitted into our country in the 1990s. By the end of 2007, F. bidentis was found in 89 counties (or districts, cities) and the distribution was estimated to cover an area of 40,000 hectare in central and southern Hebei Province. There is a high potential of F. bidentis spreading to other provinces around Hebei. The studies on this plant reported have been mainly focused on its morphological characteristics, some biological characteristics, damages and control strategies etc, but the adaptation natures has not been investigated or reported. In the present study, greenhouse pot-grown plants were used to analyze the drought resistance, alkali tolerance, salt tolerance and adaptivity to light intensity by yellow-top plants.1. Drought resistance by F. bidentisDifferent soil moisture contents (SWC) was obtained by stoping water supply for diffferent timeperiods. The water contents in the aerial parts of plants (PWC), proline (Pro) content, malondialdehyde (MDA), water-soluble sugar (WSS), superoxide dismutase (SOD) activity, peroxidase (POD) activity and catalase (CAT) activity in leaves of yellow-top plants were measured to evaluate drought resistance by the plants. The results showed that the water content in the soil was decreased by stopping water supplies to the soil. The water content in the aerial parts of F. bidentis and all the biochemical parameters measured were also significantly increased (P<0.05). The SWC, PWC, Pro, MDA, WSS, SOD, POD and CAT of the control plants at 10 days after treatment were 25.0%,77.4%,106.7μg/g FW,4.7μmol/g FW,0.17 mg/g FW,70.8 U/g FW, 2.1 U/g FW, and 0.07 U/g FW, respectively. When the soil was not watered for 10 days, the SWC and PWC were decresead to 5.6% and 17.3% while the other 6 biochemical contents were decreased to 199.2μg/g FW,6.3μmol/g FW,0.24 mg/g FW,122.9 U/g FW,6.2 U/g FW and 0.10 U/g FW, respectively. When the soil was not watered for 15 days, the SWC and PWC were futher lowered to 2.8% and 6.7% while the other biochemical paramters were futher increased to 941.4μg/g FW,19.6μmol/g FW,0.85 mg/g FW,185.3 U/g FW,41.1 U/g FW and 0.28 U/g FW, respectively. Comparing these data with those of other draought-resistant plants such as Ficus tikoua Bur., Euonymus fortunei (Turcz.) Hand.-Mazz., Trachelospermum jasminoides (Lindl.) Lem., Hedera nepalensis K. Koch., and Festuca arundinacea Schreb., yellow-top plant is more drought-resistant and is thus a high drought-resistant species.2. Alkali tolerance by F. bidentisThe test was done that F. bidentis height, conductivity, proline (Pro), malondialdehyde (MDA), soluble sugar (WSS) content, and peroxidase (POD) activity in leaf of F. bidentis after it was grown in soils of 7 different pH values (pH7.00,8.77,9.12,9.50,9.90,10.28,10.57) to evaluate alkali tolerance of F. bidentis. Results showed that the POD, REC, Pro, WSS and MDA in yellow-top plants increased as the rises of pH values and treatment timeperiods. When pH8.77 the grwoth of yellow-top was significantly (P<0.05) promoted while it was markedly suppressed when pH>9.90. Changes of these biochemical parameters were most distinct 20 days after treament; the POD, RC, Pro, WSS and MDA in the control plants were measured 2.1 U/g FW,2.2%,98.6μg/g FW,0.17 mg/g FW and 4.1μmol/g FW while those in the pH9.90-treated plants were increased to 13.0 U/g FW,11.8%,194.0μg/g FW,0.28 mg/g FW and 8.0μmol/g FW, respectively at this time. Morphologically, yellow-top plants grew and reproduced normally, no yellowing, wilt or senescence was observed under high pH value conditions. In comparison with other alkalescent plant species, yellow-top was even more alkali-tolerant. Therefore, it can be concluded that yellow-top is of a high alkali tolerant species.3. Salt tolerance by F. bidentis:Five salt treatments (NaCl concentrations:0,100,200,300 and 400 m mol/L) was designed and the plant hight (PL), relative electrocunductivity (REC), proline (Pro), malondialdehyde (MDA), soluble sugar (WSS) content, and peroxidase (POD) activity in leaf were measured to illustrate salt tolerance of yellow-top plants. The POD, REC, Pro, WSS and MDA in yellow-top plants increased as the rises of salt concentration and treatment time. At 100 mmol/L, the plant grwoth was significantly promoted (P<0.05), but it was obviously suppressed at 200 mmol/L and higher concentration (P<0.05). These biochemical parameters reached the highest values at 21 days after treatment; they were 25.4%,29.0 U/g FW,230.8μg/g FW,0.32 mg/g FW and 7.9μmol/g FW in plants at 200 mmol/L whilst those were 8.1 U/g FW,2.6%,76.3μg/g FW,0.12 mg/g FW and 5.3μmol/g FW respectively in the plants of the control treatment. It was observed that yellow-top seedlings wilted and senesced when the salt concentration reached 400 mmol/gFW. It is clear that yellow-top plants can tolerate 200-300 mmol/L of salt in the soils. This degree of salt-tolerance is higher than that of cotton (Gossypium hirsutum L.) and sugerbeet) Beta lgaris L.) and is equal to the halophyte Suaeda helroptera Kitag. Therefore, yellow-top can be thought of as a halophyte.4. Adaptivity to light intensity by F. bidentisSix light intensity gradients (0,2330,7700,6240,3770 and 10230 lux) were set in separate grwoth chambers to assess the adaptivity of F. bidentis to different light intensity conditions. The plant hight (PL), stem diameter at the soil surface (SD), photosynthesis rate (PR), water-soluable proteins (WSP) and photosynthetic pigments (total chlorophyll Ct and Carotenoids Car) were measured for each treatment. The results demonstrated that light treatments affected germination of yellow-top seeds significantly (P<0.05); the germination rates were 31.6%,90.0%,82.0%,83.2%, 92.4% and 62.0% respectivly, in the 5 treaments recorded 5 days after sowing. After seeds were sown for 20 days, all plants in the no light treatment were senesced, but seedlings grew healthily and the the plants were measured 9.6,10.3,11.1,10.9 and 10.6 cm and the stems were 1.87,1.97, 2.32,2.25 and 2.49 mm in diameter, respectively under the 5 light intensities. The WSP, Ct and Car were mostly much highered under different light intensities than those under darkness (control); the WSPs were 0.04,0.67,1.10,1.39 and 1.56 mg/g FW; Cts were 1.03,1.19,2.39,2.52 and 2.73 mg/g FW; Cars were 0.21,0.18,0.42,0.58 and 0.65 mg/g FW; and the PRs were 19.7,20.7,20.7,21.0 and 22.7μmol/m2 s, repsectively in the 6 treatments in 20 days after sowing. These data indicate clearly that light is necessary for thee germination of yellow-top seeds and for seedling growth. The photosynthesis potential of yellow-top plants is much higher than those of Anoectochilus formosanus Hay, Amaranthus retroflexus L., magnolia grandiflora L., Amorphophallus sinensis Belval, Malus sieversii (Led.) Roem and Camellia sinensis L. It can be concluded that F. bidentis is a strong light-demanding species and higher radiance is important to its seed germination and seedling growth.These results indicate that F. bidentis is strong light-demanding, halophilic, alkali-tolerant, drought-resistant plant and thus it is highly invasive and well adapted to verious ecological niches of its invaded areas. The findings, together with the recorded data, are valuable and useful in risk analysis and prediction of distribution of F. bidentis plants.
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