| Yellow-top (Flaveria bidentis) weed, originated from South America, is a new invasive plant species in northern China. Since F. bidentis was first recorded in 2001 in China, it has now been distributed widely in the northern China provinces including Hebei, Tianjin, Shandong and Henan. It can cause serious economic losses of some dryland crops and damages to ecological environment and reduction in biodiversity. The objective of the present work was to study how the allelopathic effects of yellow-top plant disrrupt seed germination and seedling growth of maize (Zea mays).1. Preparation of yellow-top plant extract. Fresh yellow-top plants were collected from the infested fields. The shoots were dried, powdered. The powder was soaked in distilled water (100 g in 1000 mL) for 72 hours at room temperature to extract the allelopathic compounds. The extract was filtered through the miro-porous film, and this filtered extract was diluted to obtain solutions of concentrations 0.10,0.05 and 0.01 g/mL for maize seed treatment.2. Effects of yellow-top plant extract on maize seed germination. Seeds of maize (Jiyuan 1) were treated with four concentrations (0.10,0.05,0.01 and 0.00 g/mL) of yellow-top extract. Results showed that seed germination rate, the moisture content, length of radicles and germs of maize were significantly inhibited by extract of the three concentrations in comparison with the control (0.00 g/mL). The inhibition effects became more serious as the concentration of the extract increased. The seed germination rate was 25-30%after being treated while that of the control was 95.4%recorded 48 h after treatment. The length of radicles and germs of maize were, respectively, less than 0.2 cm and 0.1 cm in the extract treatments were respectively,5.5cm and 1.2cm measured 72 h after treatment. The moisture content of radicle and germ were much lower than that of oontrol (90%). The yellow-top plant extract also affected the contents of malondialdehyde (MDA), soluble protein (SP) and soluble sugar (WSS), proline content (Pro), acticities of antioxidant enzymes in the germinating maze seeds and seedlings. The contents of soluble protein and the activities of POD decreased significantly whereas the content of Pro decreased first and increased then during 24-72h period. However, the contents of MDA and WSS, activities of CAT and SOD were significantly increased within 60 h following treatment.3. Effects of yellow-top plant extract on maize seedling growth. The maize seedlings grown in Hoagland nutrient solution were treated with the yellow-top extract of three concentrations (0.1,0.05 and 0.01g/mL) and distilled water as control. The MDA, WSS, Pro, POD, SOD, CAT, chlorophylls (CHL), photosynthetic rate of the seedling leaves were evaluated every four days. The yellow-top extract of the three concentrations had negative effects on the growth of maize. The negative effects were enhanced with the increase of extract concentration. The level of MDA contents, POD, SOD and CAT activities were significantly higher than those of the control plants, but the content of CHL and the photosynthetic rate were significantly less than those of the control plants. The CHL content at the highest concentration was 0.68mg/g, and this was 39%of CHL contents in the control leaves. The photosynthetic rate was 26%in control plants (108μmol·O2/dm2·h). The SP content (1μg/g) in yellow-top extract treatment was less than that of control plant (5.58μg/g). WSS and Pro contents had the same changable trend, the growing rate of WSS was more than 45% and the growing rate of Pro was about 30%. Contents of WSS and Pro in treatment were more than control plant (9%). As a result of these physiological changes, the seedling growth of maize was strongly inhibited by yellow-top extract treatment:the growth seedlings from seeds treated with extract of the three concentrations was almost completely stopped.4. Effects of yellow-top extract on ultrastructure of germinating seed and seedling root. With the scanning electron microscopy (SEM) and transmission electron microscopy (TEM), effects of yellow-top extract (0.1g/mL) on seed germination showed that the radicle and germ were shorter, and numbers of lateral roots were scarced. Structure of aleurone layer and scytellum were destoryed, the inclusion of the measured aleurone layer leaked badly and that of scutellum was consumed slower than control seed. The radicle transection cells were weakened with irregular arrangement. Those were the reasons that lead to lower seed vigor of the measured maize. Seedling leaf was in etiolation and stunted with some physiological speckles. Seedling root tissues were broken easily and swollen with several lateral roots. Besides, Root tips had many empty cells and skin excoriation cells which were dry up quickly. Ultrastructual changes in the treated seeds and seedling root showed that action of yellow-top extract was multifaceted from the changes of magnitude, position, and shape of organelles, such as the lost karyotheca, increasd vacuoles, and hollow dictyosomes. Those were the reasons that lead to lower maize seed vigor of the measured maize and the stunted maize seedling.5. Interruption of yellow-top extract on some growth regulators in germinating cron seed and seedling. Using the same methods of treatment described above 1 and 2, effects of yellow-top extract at three concentrations (0.1,0.05 and O.Olg/mL) on the contents of abscisic acid (ABA) and gibberellin A (GA) in seed and leaf of maize seedling were further investigated, distilled water as control. ABA and GA are functional antagonism. The results showed that, ABA content was increased in varying degrees, whereas GA content decreased significantly along with concentrations of yellow-top extract and responsing time. GA content in maize seed germination expanded by 116.2% and ABA content was higher to 234.9%. The result of treated maize seedling showed that GA content was up to 50% and ABA content was higher to 30% with treatment of three concentrations of yellow-top extract. The allelopathy effects were obviously with time and increasing concentrations.In conclusion, the results from this study showed that yellow-top plant had strong allelopathic inhibition on germination and growth of maize. After treated the extract from yellow-top plants, the morphology, anatomic structures and ultrastructures in the genminating maize seeds and in tissues of maize seedlings were changed and disfunctioned. Additionally, giberrelin A was significantly lowered and abscisic acid was increased significantly in the maize seeds and seedlings. These results revealed apparently the allelopathic effects and mechanism of yellow-top plant on maize. |
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