紫茎泽兰生态学特征及入侵策略

紫茎泽兰（Eupatorium adenophorum）是臭名昭著的世界恶性杂草之一，目前全世界已有30多个国家和地区遭受到它的入侵危害，因此引起了全社会各方面广泛的关注。我国西南部是紫茎泽兰入侵并造成严重危害的地区之一，本文以四川省攀西地区紫茎泽兰入侵危害严重的生态系统为研究对象，分别对不同生境条件下各年龄紫茎泽兰的生长状况、幼苗生长动态、不同类型群落中紫茎泽兰的种群变化规律以及泽兰实蝇防治紫茎泽兰现状进行研究，分析紫茎泽兰生态学特征及入侵策略，以揭示紫茎泽兰的入侵机制。 　　不同生境条件下相同年龄紫茎泽兰其生长状况有很大的不同：当年生紫茎泽兰幼苗在偏阴和灌丛遮荫环境下生长状况优于偏阳生境下，一年生以上的成熟植株生长情况则相反;紫茎泽兰在其单优群落中的生长状况明显优于灌丛中伴生的紫茎泽兰，说明一定程度的遮荫、植被覆盖及竞争对紫茎泽兰生长有一定的抑制作用。不同种类的灌木对紫茎泽兰生长的影响亦有所不同，其中，马桑（Coriaria sinica）等冠幅较大、性喜阴湿的落叶类灌木对紫茎泽兰幼苗萌发、种群更新有庇护和促进作用，而其他类冠幅小、常绿或落叶、且生于干旱生境下的灌木，则不利于紫茎泽兰幼苗萌发及种群更新，有些甚至会产生化感物种抑制紫茎泽兰的生长。 　　紫茎泽兰种子萌发属投机式萌发，一年内只要有适合的温湿条件都可以萌发。种子萌发的高峰期主要集中在雨季，冬春干旱季节萌发率很低。紫茎泽兰为常绿半灌木，终年可持续生长，生长速率受光照、湿度和温度影响显著。秋季萌发的紫茎泽兰幼苗在越冬及干旱季节中，各项生长指标（包括主茎长度、总叶面积、基径等）增长缓慢，一年之内平均月增长量由高到低的顺序分别为：偏阳生境＞全荫湿润生境＞灌丛遮荫生境，生长旺期为雨季约6 ~10月份;秋季萌发越年生实生苗生长节律与当年生实生苗相似，但生长周期不同于雨季萌发苗，需经过两个冬季才能开花结实完成其生活史;紫茎泽兰生活史循环过程，通过有性生殖与无性生殖相互补的繁殖策略进行种群的更新与扩散，进而达到入侵的目的。 　　泽兰实蝇（Procecidochares utilis）作为天敌控制紫茎泽兰已经在国内外得到广泛的应用，但关于它对紫茎泽兰控制的有效性和防治现状的研究尚不深入。本文通过对攀西地区紫茎泽兰入侵危害严重的路域生态系统中泽兰实蝇寄生状况的抽样调查，初步研究了泽兰实蝇对紫茎泽兰生长，特别是生殖能力的影响。研究结果表明：1）植株寄生率与枝条寄生率有显著差异（p﹤0.05），分别为71.67% 和17.30%，前者显著高于后者;样方调查结果显示，成熟群落中枝条寄生率为17.48枝•m-2;1虫瘿•枝条-1的枝条占所有寄生枝条的92.30%;2）湿润生境下紫茎泽兰的枝条寄生率为20.27%，显著高于干旱生境下的枝条寄生率（9.33%）（p﹤0.05）;3）不同年龄植株枝条寄生率有差异，0 ~1年生植株枝条寄生率分别为36.36%和21.56%，显著高于2 ~4年生的植株枝条寄生率，后者分别为13.50%，8.82%及12.16%（p﹤0.05）;4）在目前的寄生强度下，泽兰实蝇对紫茎泽兰枝条的直径、花枝量、头状花序数及结实量均无显著影响（p﹥0.05）。因此可以推断，目前单一的引进泽兰实蝇进行天敌控制不能达到预期的防治目标，天敌的引入也应慎重考虑。 　　紫茎泽兰能够利用与当地大多数植物种类生长节律的时间差异，通过首先占领时间生态位而达到占据空间生态位，最终导致严重的入侵危害。在不同类型的群落中，紫茎泽兰种群利用不同的适应策略进行入侵。自然和人工播种样地中，紫茎泽兰种群与群落状况的变化规律均呈相反趋势。对于一个成熟的紫茎泽兰种群，一旦其定居后即能很好的利用时间和空间生态位的空缺的来促进其优势地位的增强和巩固，从而逐渐增加其入侵程度，造成越来越大的灾害;而对于一个郁蔽度较好且较完整的群落，紫茎泽兰通过幼苗进行扩张以达到入侵的途径通常比较困难，只有在发生人为活动干扰时，才有可能通过投机式繁殖方式进入群落内部定居，继而通过克隆生殖的方式进行入侵。

Effects of the Spanwise Characteristic Length on the Transition Feature in the Wake of a Cylinder

The transition features of the wake behind a uniform circular cylinder at Re = 200, which is just beyond the critical Reynolds number of 3-D transition, are investigated in detail by direct numerical simulations of 3-D incompressible Navier-Stokes equations. The spanwise characteris-tic length determines the transition features and global properties of the wake.

Transition from steady to oscillatory convection with chaotic feature in thermocapillary convection

The transition process from steady to turbulent convection via subharmonic bifurcation in thermocapillary convection of half floating zone was studied by numerical simulation and experimental test. Both approaches gave structure of period doubling bifurcations in the present paper, and the Feigenbaum universal law was checked for the system of thermocapillary convection.

Donut-Shaped Fingerprint In Homologous Polypeptide Relationships-A Topological Feature Related To Pathogenic Structural Changes In Conformational Disease

Features of homologous relationship of proteins can provide us a general picture of protein universe, assist protein design and analysis, and further our comprehension of the evolution of organisms. Here we carried Out a Study of the evolution Of protein molecules by investigating homologous relationships among residue segments. The motive was to identify detailed topological features of homologous relationships for short residue segments in the whole protein universe. Based on the data of a large number of non-redundant Proteins, the universe of non-membrane polypeptide was analyzed by considering both residue mutations and structural conservation. By connecting homologous segments with edges, we obtained a homologous relationship network of the whole universe of short residue segments, which we named the graph of polypeptide relationships (GPR). Since the network is extremely complicated for topological transitions, to obtain an in-depth understanding, only subgraphs composed of vital nodes of the GPR were analyzed. Such analysis of vital subgraphs of the GPR revealed a donut-shaped fingerprint. Utilization of this topological feature revealed the switch sites (where the beginning of exposure Of previously hidden "hot spots" of fibril-forming happens, in consequence a further opportunity for protein aggregation is Provided; 188-202) of the conformational conversion of the normal alpha-helix-rich prion protein PrPC to the beta-sheet-rich PrPSc that is thought to be responsible for a group of fatal neurodegenerative diseases, transmissible spongiform encephalopathies. Efforts in analyzing other proteins related to various conformational diseases are also introduced. (C) 2009 Elsevier Ltd. All rights reserved.