| Artemisia ordosica has gradually become the dominant species of the sand community in the process of plant succession. Some of early pioneer plants in the stabilization process of sand dunes have gradually withdrawed from the community and has a lower biodiversity with the continuous expansion of A. ordosica population. Repair desertification of land as soon as possible is the formidable task of ecological construction in our country. Therefore, the further research of ecology laws during the formation and succession of A. ordosica community plays a very important meaning in prevention and control of desertification. In this thesis,4 plots (very low coverage, low coverage, middle coverage and high plant coverage) were selected in standard plot of A。 ordosica of desert ecosystem research station in Yanchi County, Ningxia Hui Autonomous Region and the community characteristics, species composition and soil water contents were studied. On the basis of these, species of allelochemicals, release approach, allelopathy in the process of community succession and the allelopathy mechanism of A. ordosica had been a systematic researched. The main conclusions are as follows:(1) In the process of sand fixation, The average height and average coverage of plants gradually increased, Shannon-Weiner index and Simpson index showed an increase and then decrease trend. The population of A. ordosica was expanding constantly, and ultimately a single dominant species population was formed. Agriophyllum squarrosum、 Corispermum hyssopifolium、 Astragalus melitodes gradually exited plant community and Setaria viridis became a companion species of A. ordosica community. There were no significant difference except the soil moisture contents of 40-60 centimeters depth of the low coverage sample were significantly higher than those of the other plant samples with the same depth.(2) On the basis of single factor experiment, Method of aqueous extraction of chemical substances in roots, stems, leaves and seeds from A. ordosica were studied using the three factors (extraction time, extraction temperature and solid-liquid ratio) 3 levels orthogonal experiment and the chemical substances of extracting solution were detected by gas chromatography mass spectrometry (GC-MS). The results showed that the best extraction conditions in roots, stems, leaves and seeds were: 48 h、 40℃、 1:5,48 h、 40℃、 1:40,72 h、 40 ℃、1:50,72 h、 40℃、 1:5. The chemical substances detected by single factor test and orthogonal test were the same, but the contents of the orthogonal test were significantly higher than those of the single factor test. The extraction efficiencies of chemical substances from A. ordosica using the orthogonal experiment were higher than those of the single factor test and the reliability of chemicals detection could greatly be improved.(3) The non-volatile chemical substances were extracted from the roots, stems, leaves, seeds of A. ordosica and soil under optimal extraction conditions. The extracts were analyzed by GC-MS and detected 35 acidic organic compounds,6 alkaline organic compounds and 20 neutral organic compounds. They were phenols, low-molecular-weight organic acids and fatty acids in acidic substances, morpholine, amine, urea and pyrimidine in alkaline substances and alkene, alkyne, alcohol, ketone, ester, aldehyde, furan and bisabolol oxide in neutral substances. Then inhibitory activities of acidic, alkaline and neutral components of root extraction from A. ordosica on the germination of A. squarrosum were evaluated using the petri dishes method. The results showed that inhibitory effect of acidic components on the germination of A. squarrostm was the strongest. So the allelochemicals of aqueous extracts from A. ordosica were mainly acidic organic compounds.(4) Seed germination, seedling growth and dry mass tests from the five test species (A. ordosica, A. squarrosum, C. hyssopifolium, A. melitodes, S. viridis) were conducted with aqueous extracts of 3 concentration (0.05,0.10 and 0.20 g FW-mL-1) from the roots, stems, leaves and seeds of A. ordosica. The results showed that A. ordosica extracts had inhibitory effects on seed germination, seedling growth and dry mass of A. ordosica, A. squarrosum, C. hyssopifolium and A. melitodes. On the contrary, A. ordosica extracts had promotion effects on seed germination, seedling growth and dry mass of S. viridis. Autointoxication of aqueous extracts from the roots, stems, leaves and seeds of A. ordosica on root growth was the highest. In general, the inhibitory effects of extracts from leaves and stems were more pronounced than those of roots and seeds extracts. These illustrated that the autointoxication of A. ordosica mainly influenced the growth of root. Autotoxins chemicals mainly came from stems and leaves by rainfull or decomposition of litters. The chemical substances releasing from A. ordosica had noteworthy effects on A. squarrosum, C. hyssopifolium and A. melitodes in the whole course of an individual’s life, from seed germination to growth and development, and the effects increased with higher concentration of extracts. Among them, the growth of roots was the strongest inhibitory effects. These illustrated that the chemical substances into the soil first influenced the growth of roots of associated plant roots. The aqueous extracts of A. ordosica significantly promoted seed germination, seedling growth and dry mass of S. viridis, and the effects increased with higher concentration of extracts. After reaching the maximum, the promoting effects were decreased when concentrations were increased. But extracts of all parts of A. ordosica in test concentration range had positive effects.(5) Volatile organic compounds (VOCs) from A. ordosica under different growth seasons were collected using dynamic headspace collection methods and identified by TCT-GC-MS. The results indicated that terpenes were the dominant VOCs releasing from A. ordosica, in which had 23 VOCs. The main volatiles releasing from roots in the growing season were caryophyllene,2-ethyl hexanol, Limonene et al, and those of stems and leaves were Limonene, ocimene,3-carene et al. The main volatile releasing from roots in the mature season was caryophyllene, and those of stems and leaves were Limonene, ocimene, β-pinene, β-phellandrene et al. Seed germination and seedling growth tests from A. squarrosum and C. hyssopifolium were conducted with Limonene of 5 volume (0.10 mL,0.25 mL,0.50 mL,1.00 mL,2.00 mL) in dryer (diameter of 30 cm, height of 50 cm). The results indicated that the volatile of Limonene had significant negative effects on seed germination, seedling growth and dry mass of A. squarrosum, and the inhibitory effects increased with higher concentrations of volatile of Limonene. The volatile of Limonene displayed allelopathic effects on C. hyssopifolium and the effects were concentration-dependent. The effects of Limonene were positive on seed germination, seedling growth and dry mass when the concentrations of Limonene were low and negative when the concentrations were high.(6) The K+and NH4+ flux were measured in seedling roots of the four associated herbaceous plants(A. squarrosum, C. hyssopifolium, A. melitodes, S. viridis) using non-invasive micro-test technique (NMT) after being treated with the extracts of roots and stems from A. ordosica. The results indicated that exposure of roots (A. squarrosum, C. hyssopifolium, A. melitodes) to extracts from A. ordosica induced outflow of K+and NH4+, but inflow in S. viridis. The extracts of A. ordosica mainly influenced the absorption of NH4+.According to these results, this paper draws the following conclusions:A. ordosica can release the allelochemicals to influence the symbiotic plant seed germination, seedling growth and development, thus affecting the plant composition and distribution pattern. Therefore allelopathy plays a very important role. |
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