水生维管植物研究:Ⅱ、水生维管植物对干涸的适应

根据实地考察和文献分析,研究了水生维管植物对干涸的不同的适应对策,归纳为3类：（1）直接适应型（或称生理适应型）;（2）间接适应型;（3）休眠适应型。这有助于认识水生植物的适应特征,揭示干涸对部分水生植被演替的作用。     

谈谈水生维管植物的观察

我国地域广阔,江河纵横,湖泊、水库、池塘等大小水体星罗棋布,为水生维管植物的繁衍提供了有利条件。我国水生维管植物是比较丰富的,它们在工农业生产、食品、医药、科教、环保等方面业已起着十分重要的作用。然而,目前我国专门从事水生维管植物研究的人并不多,工作尚少,各高等院校的植物分类学教学对水生维管植物或多或少有所忽略,实习多在山地进行,很少进行水生植物实习。我们现在对纠正了前人的错误观点。再如,菱属(Trapa)植物的沉水茎上的纤维状根状物(或称羽毛伏根状物),英国植物学家哈钦松曾认为是沉水叶,我国各权威性文献及中学课本中也都认为是沉水叶,而国内外有些学者认为是不定根,它到底是什么呢?笔者进行徒手切片,观察到其横切结构是典型的根的结构,从而澄清了这个问题。由此可见,水生维管植物的野外及实验室观察是何等重要! 在水生维管植物的观察过程中,要善于发现问题,解决问题。两栖寥(Polygonum amphi-bium)是我国长江以北地区较常见的水生草本植物。由于它能水陆两栖,形态变化很大,往往不易识别,有人曾将陆生的两栖寥误定为两栖寥的一个变种。笔者通过两年野外考察,看到两栖寥的植株类型一共有4种,除了文献中描述的陆生直立草本和浮水草本外,尚有挺水草本和异枝型草本。所谓两栖寥异枝型草本就是同一植株上具有不同类型分枝,如同时具有陆生直立草本型和浮水草本型两个分枝的植株。在自然界,由于水位的涨落,导致两栖寥各类型植株间的转变。笔者经栽培实验也得出了同样的结果。将水生植株栽在旱地上观察,其原有的浮水叶枯死,新生叶发育成典型的陆生直立草本型叶子,具有粗硬毛,有黑斑等。陆生直立草本淹水生长,则老叶烂死,新生叶发育成典型的浮水草本型,光滑无毛,无黑斑等。另外,笔者在观察中一直没有见到两栖寥陆生直立草本开花,这种类型的植株是否开花,有待进一步的观察和研究。石龙尾(Limnophila sessiliflora)是玄参科水生草本植物。有的分类学文献中说它“花无梗或稀可具1.5毫米之梗”。笔者在观察中发现,石龙尾在沉水生长期花有梗,长度一般超过1.5毫米,最长可达3毫米;气生茎生长出来后,其上的花无梗或稀可具长不过1.5毫米的梗。我们每年在7月份实习,此时的石龙尾仅有沉水茎,采到的标本与上述文献的描述不符,导致无法准确鉴定。笔者于1991年6月15日至9月23日进行了连续观察,终于解决了这个问题。可见,观察时间对观察结果具有一定影响。观察水生维管植物,要选没有污染、没有养鱼的池塘或河流,芦苇沼泽是较理想的地方。对水生植物的观察,要在不同季节进行,坚持经常。在春季可以观察水生维管植物的萌生及幼苗的生物学特性;在冬季,可观察水生维管植物的冬态及果实和营养繁殖器官的特性;至于夏秋季,可观察的东西就更丰富了,诸如各种水生植物群落、各种植物的花和营养器官的形态等等。例如,在早春当芦苇长到20厘米左右时,可以看到水芹、盒子草等植物的水播现象。再晚些时候,当芦苇高约80厘米时,可以看到野大豆(Glycine soja)幼苗出土的有趣现象。笔者此时曾在芦苇沼泽中见到,几十株甚至—二百株野大豆幼苗密集成撮萌发,而最后往往每撮仅有1—3株能存活下来,继续生长发育。这种现象说明了什么问题呢?据笔者推测:由于野大豆两片子叶较肥大,单株或少数幼苗一起,往往难以克服春季较干硬的土层阻力,而多数幼苗成撮萌发,破土而出就容易多了。但挤在一起的幼苗之间面临着激烈的生存竞争,只有少数胜利者才能继续生存,其它幼苗死后可以作为肥料供给幸存者。因此每一个野大豆成株,都可能是百里挑一的佼佼者。难怪野大豆有着极强的适应性和抗逆性。在冬季,笔者观察到一个奇怪的现象,生长在较深水(30—50厘米)中的慈姑,其匍匐茎及球茎生长在泥里(地下)的很少,大部分生长在水中,水中的较泥里的颜色深,鳞叶大而厚。而文献上一般认为慈姑球茎只生长在地下。一般认为水鳖(Hydrocharis dubia)的蒴果冬季沉入水底,笔者观察到大量蒴果漂浮在水面,将来被冻在冰层里。水鳖的蒴果与一般蒴果不同,不裂开,只能待烂掉后才能散出种子,冻在冰层里有助于这些蒴果在春季腐烂,散出种子。在冬季,还可观察狸藻(Utriculariauulgaris)的冬芽。它们直径约3毫米或较小,质地紧密,硬度大,比重较大。秋冬季节,植株烂掉后,冬芽沉向水底,待翌春发育成一个新植株。对水生植物可进行现场观察,也可将其采回进行实验室观察或栽培观察。水生维管植物标本的采集方法与陆生植物标本采集制作方法略有不同,应注意以下几点: 一、一定要做好野外记录,因为水生植物的腊叶标本,与原来植株相比,形态变化大,尤其花的形态及颜色,往往面目全非。没有野外记录的标本,其保留可供研究的价值很小。二、有些植物不同时期形态变化较大,要注意分期适时采集,如菱,其幼期沉水茎上有叶和托叶,花果期沉水茎上叶和托叶脱落而出现了大量的不定根。三、沉水植物和一些浮水植物,应在水中将其叶等细弱部分展开,洗净,摆好位置,用台纸托出,这样就直接上了台纸,待以后压干后稍加固定就行了。四、花最好单独采集一些,装在小瓶内用水泡好,带回实验室解剖观察。五、大部分挺水植物及部分浮水植物和部分沉水植物具有地下茎,采集时一定要注意采挖,否则会使鉴定困难。六、有些器官不宜制成腊叶标本,可做成浸制标本,如藕、慈姑等。七、每种植物一般采集3—5份标本,太少不利于鉴定、研究。另外,要避免一下子采光,留下的以后可以继续观察和采集。八、水生植物水分大,在压制过程中要勤翻、勤换纸,防止标本变色、发霉甚至腐烂。     

水生维管植物克隆繁殖方式的多样性

克隆繁殖是植物界的一种重要的繁殖方式 ,具有很大的多样性 ,特别是水生维管植物更是如此。通过对水生维管植物克隆繁殖方式的进行深入分析 ,不仅揭示了克隆繁殖在水生维管植物适应环境中的意义 ,而且也阐明了克隆繁殖方式作为水生维管植物的生存对策之一 ,在水生维管植物的生态和演化以及进化过程中的重要作用。     

长江三角洲地区水生维管植物的多样性

根据野外调查及相关资料,分析了长江三角洲地区水生植物区系的种类组成、地理成分以及濒危现状.该区现有水生维管植物39科90属185种,其生活型以挺水植物为主(占64.32％),主要分布于田边沟渠、农田以及池塘等不同水体.属、种等级的地理成分分析结果表明:其区系性质主要为温带性质.根据水生植物的濒危现状,参照世界自然保护联盟(IUCN)红色名录的等级和标准(3.1版本),将该区的植物分为9大类,其中地区性灭绝1种,极危种3种,濒危种8种,易危种10种,并简要讨论了该区水生植物受威胁的主要原因.     

水生昆虫

介绍了水生昆虫适应水生的生物学特征,环境转换与昆虫的适应,生态类型和意义。     

广西水生维管植物研究Ⅰ:一些新资料

初步研究了广西的水生维管植物 ,发现若干个新记录 ,包括 1个新记录科 ,1个存疑新记录属 ,3个新记录种 ,1个区内分布新资料。这对认识广西植物种类的多样性有一定意义     

四川邛海湖湿地水生维管植物的现状调查

本研究对邛海湖湿地水生维管植物种类及分布状况进行了调查.结果表明,邛海湖湿地现有水生维管束植物77种,隶属于25科50属.其中,蕨类植物5种,隶属2科3属;被子植物72种,隶属23科47属.从植物生态类型和生活型方面来划分,邛海水生维管束植物可分为湖区水生维管植物、河口滩涂植物和湿生植物三种类型.它们主要分布在湖泊的北面、西面和南面.区系分析结果表明,邛海水生维管束植物共有7种分布类型,其中以世界性分布型植物种最多.邛海湿地自然着生的水生植物群落破坏严重,挺水植物以上群落基本消失,浮水植物和沉水植物分布区锐减,分布深度退缩,分布密度减小.针对邛海湿地水生维管束植物存在问题,笔者提出了相应的解决方法和建议.该研究为邛海湿地的恢复和重建提供了理论依据.     

汉中地区汉水流域水生维管植物种类及地理分布

经野外调查,采集标本与鉴定分析,汉中地区汉水流域共有水生维管植物69种,包括蕨类植物3种,种子植物66种,分别隶属于32科48属。其中挺水植物41种,浮水及浮叶植物11种,沉水植物17种。66种种子植物分别隶属于46属,有7种地理分布类型,其中以世界分布类型成分居多,共21属,占本地区水生种子植物总属数的45．7％;热带分布类型成分13属,占28．3％,温带分布类型12属,占26．0％。调查分析发现8种6该区域的新分布种,即：竹叶眼子菜,南方眼子菜,小茨藻,大茨藻,水毛花,穗花狐尾藻,有梗石龙尾的狸藻;新记录属4个,即：茨藻属,狐尾藻属,石龙尾属和狸藻属;新记录科2个,即;茨藻科和小二仙草科。     

植物与草食动物之间的协同适应及进化

通常协同进化是指一个物种 (或种群 )的遗传结构由于回应于另一个物种 (或种群 )遗传结构的变化而发生的相应改变。广义的理解 ,协同进化是相互作用的物种之间的互惠进化。生物之间、特别是植物与草食动物之间的协同适应与进化 ,已经成为生物进化、生态、遗传等学科十分关注的问题 ,可能成为生物学中各学科研究的交汇点或结点。作者具体阐述了 :(1)生物之间协同进化的研究意义 ,包括对生物学与生态学的价值 ;(2 )生物之间协同进化研究的限制或困难 ,诸如时间、研究对象、进化等级尺度和研究方法的限制 ;(3)植物与草食动物之间协同进化的主要研究对象 (系统 ) ,即昆虫传粉系统、昆虫诱导植物反应系统、种子散布系统、以及大型草食动物采食与植物反应系统 ;(4 )植物与草食动物之间协同进化的主要研究内容 ,包括适应特征 (性状 )——物种的可塑性 ,以及适应机制——物种适应过程与策略两个方面 ;(5 )植物与草食动物之间协同进化研究的存在问题及研究方向     

水生维管束植物对滇池水体的净化效应

利用天然湖塘、湖湾放养水生维管束植物以净化污染水体的研究尚少见报道。本文在滇池北端的草海边利用天然濒湾、湖塘分别放养水葫芦、荇菜荇茭白、莲、满江红5种植物,了解其对富营养及重金属污染水体的净化作用;同时讨论在草海生态系统的水、植物、底泥三部分中,8种元素(N、P、Pb、Cd、Cr、Zn、Cu,Ni)的分布及迁移特征。结果:(1)得出5种植物在一定生长期间内对水和底泥中N、P及重金属的去除率以及对水体COD、浊度、pH值的改善情况;(2)计算了在5月份生长期时,几种植物对N、P及重金属的净化效率;(3)从水中N、P及重金属的分布、迁移特征,讨论了在水-底泥-植物之间元素含量的相互关系。为建立低投资、高效率、易管理的植物净化处理设施提供参考。     

Phylogenetic tree of vascular plants reveals the origins of aquatic angiosperms

Although aquatic plants are discussed as a unified biological group, they are phylogenetically well dispersed across the angiosperms. In this study, we annotated the aquatic taxa on the tree of vascular plants, and extracted the topology of these aquatic lineages to construct the tree of aquatic angiosperms. We also reconstructed the ancestral areas of aquatic families. We found that aquatic angiosperms could be divided into two different categories: the four aquatic orders and the aquatic taxa in terrestrial orders. Aquatic lineages evolved early in the radiation of angiosperms, both in the orders Nymphaeales and Ceratophyllales and among basal monocots (Acorales and Alismatales). These aquatic orders do not have any extant terrestrial relatives. They originated from aquatic habitats during the Early Cretaceous. Asia would have been one of the centers for early diversification of aquatic angiosperms. The aquatic families within terrestrial orders may originate from other areas besides Asia, such as America or Australia. The lineages leading to extant angiosperms diversified early in underexploited freshwater habitats. The four extant aquatic orders were relicts of an early radiation of angiosperm in aquatic environments. Their extinct ancestors might be aquatic early angiosperms.     

湖南莽山自然保护区的水生维管束植物——多样性及其生境特征

莽山自然保护区位于湖南省宜章县境内 (2 4°52′0 0″～ 2 5°0 3′12″N,112°4 3′19″～ 113°0 0′10″E) ,属南岭支脉。作者对该地区水生维管束植物及其生境进行了考察 ,采集到水生沼生维管束植物30科 4 7属 6 7种 ,其中 30种为莽山地区新记录。在比较沼泽和湖泊水生植物群落结构特点的基础上 ,讨论了沼泽作为水生植物重要生境的意义 ,建议对作为珍稀濒危水生植物重要产地的长江以南山地沼泽采取有效的保护措施     

激素对水生植物生理生态的影响及其应用

激素代谢是植物传导信号和调节生长发育的重要途径.陆地植物五大类激素在水生植物中也有分布,尽管近年来环境污染导致水生植物衰退的问题日益得到重视,但水生植物激素的研究和应用却远滞后于陆生植物.在总结了近年来激素类物质在水生植物中的研究成果,分别从激素的种类、激素的生理生态作用、激素生物合成的途径及作用的部位和机制、激素之间的相互作用.激素类物质在实验和实践上的应用等进行了全面阐述,指出了水生植物激素生理生态学研究的发展方向,从利用激素类物质诱导水生植物抗性的表达,提高抗逆性,恢复水生植被,以及研究和开发适于水生植物生产和管理的生长调节剂等方面,就水生植物激素的进一步研究和应用进行了探讨.     

Anatomical patterns of aerenchyma in aquatic and wetland plants

A well-developed aerenchyma is a major characteristic of aquatic plants. However, because such tissues are also found in wetland and terrestrial plants, it is not always possible to use their presence or absence to distinguish aquatic species. Whereas patterns of aerenchyma in roots have been studied in detail, those of the shoots have not. We collected and tested 110 species of various aquatic and wetland plants, including ferns (5), basal angiosperms (5), monocots (65), and eudicots (35). Three common and two rare types of aerenchyma were observed in their roots (three schizogeny and two lysigeny), plus five types of schizogeny in their shoots. We re-confirmed that, although a well-developed aerenchyma is more common in most organs of aquatic plants than in wetland plants, this presence cannot be used as strict evidence for the aquatic quality of vascular plants. Here, aerenchyma patterns were stable at the genus level, and the consistency of pattern was stronger in the roots than in the shoots. Furthermore, significant trends were verified in several higher taxa, and those consistencies of patterns partially coincided with their phylogeny.     

CAM photosynthesis in submerged aquatic plants

Crassulacean acid metabolism (CAM) is a CO₂-concentrating mechanism selected in response to aridity in terrestrial habitats, and, in aquatic environments, to ambient limitations of carbon. Evidence is reviewed for its presence in five genera of aquatic vascular plants, includingIsoëtes, Sagittaria, Vallisneria, Crassula, andLittorella. Initially, aquatic CAM was considered by some to be an oxymoron, but some aquatic species have been studied in sufficient detail to say definitively that they possess CAM photosynthesis. CO₂-concentrating mechanisms in photosynthetic organs require a barrier to leakage; e.g., terrestrial C₄ plants have suberized bundle sheath cells and terrestrial CAM plants high stomatal resistance. In aquatic CAM plants the primary barrier to CO₂ leakage is the extremely high difrusional resistance of water. This, coupled with the sink provided by extensive intercellular gas space, generates daytime CO₂(pi) comparable to terrestrial CAM plants. CAM contributes to the carbon budget by both net carbon gain and carbon recycling, and the magnitude of each is environmentally influenced. Aquatic CAM plants inhabit sites where photosynthesis is potentially limited by carbon. Many occupy moderately fertile shallow temporary pools that experience extreme diel fluctuations in carbon availability. CAM plants are able to take advantage of elevated nighttime CO2 levels in these habitats. This gives them a competitive advantage over non-CAM species that are carbon starved during the day and an advantage over species that expend energy in membrane transport of bicarbonate. Some aquatic CAM plants are distributed in highly infertile lakes, where extreme carbon limitation and light are important selective factors. Compilation of reports on diel changes in titratable acidity and malate show 69 out of 180 species have significant overnight accumulation, although evidence is presented discounting CAM in some. It is concluded that similar proportions of the aquatic and terrestrial floras have evolved CAM photosynthesis. AquaticIsoëtes (Lycophyta) represent the oldest lineage of CAM plants and cladistic analysis supports an origin for CAM in seasonal wetlands, from which it has radiated into oligotrophic lakes and into terrestrial habitats. Temperate Zone terrestrial species share many characteristics with amphibious ancestors, which in their temporary terrestrial stage, produce functional stomata and switch from CAM to C₃. Many lacustrineIsoëtes have retained the phenotypic plasticity of amphibious species and can adapt to an aerial environment by development of stomata and switching to C₃. However, in some neotropical alpine species, adaptations to the lacustrine environment are genetically fixed and these constitutive species fail to produce stomata or loose CAM when artificially maintained in an aerial environment. It is hypothesized that neotropical lacustrine species may be more ancient in origin and have given rise to terrestrial species, which have retained most of the characteristics of their aquatic ancestry, including astomatous leaves, CAM and sediment-based carbon nutrition.     

Allelopathic aquatic plants for aquatic weed management

This report presents, results of a feasibility study of use of allelopathic aquatic plants for aquatic weed management. In order to establish a list of potential allelopathic plants, we selected 16 aquatic plants native to the southeastern United States and subjected them to two bioassays — one involving lettuce seedlings and one involving the aquatic plantLemna minor as the target species. The lettuce seedling bioassay was selected because it is a widely used, experimentally simple assay to determine allelopathic activity. However, it uses lettuce, a terrestrial plant, as the target species, and thus may be less appropriate for use with aquatic plants. TheL. minor assay involves an aquatic plant as the target species and so is more appropriate for our goals, but it is experimentally much more complex and time-consuming. The plants selected for study wereBrasenia schreberi, Cabomba caroliniana, Ceratophyllum demersum, Eleocharis acicuiaris, Eleocharis obf usa, Hydrilla verticillata, Juncus repens, Limnobium spongia, Myriophyllum aquaticum, Myriophyllum spicatum, Najas guadalupensis, Nymphaea odorata, Nymphoides cordata, Potamogeton foliosus, Sparganium americanum, and Val/isneria americana.Nymphaea odorata (leaves and petioles) inhibited 78 % of lettuce seedling radicle growth and 98 % ofL. minor frond production. Brasenia schreberi inhibited 82 % of lettuce seedling radicle growth and 68 % of L. minor frond production. These results suggest thatN. odorata andB. schreberi are both highly inhibitory and are therefore candidates for use in aquatic weed management. Results also suggest that the simple lettuce seedling assay is a reasonable first “easy” one to use in an attempt to determine allelopathic potential of aquatic plants.     

Photosynthetic pathways in freshwater aquatic plants

Recent studies show that generalizations about photosynthetic pathways, derived from terrestrial plant studies, do not apply to aquatic plants. Crassulacean acid metabolism (CAM) photosynthesis is of selective value not only in arid environments, where it enhances water-use efficiency, but also in aquatic plants of oligotrophic waters, where it enhances competitive ability in carbon acquisition. C(4) photosynthesis is present in many aquatic species, but in these species it is not coupled with the specialized anatomy of terrestrial C(4) plants. The ratio of the stable carbon isotopes, (13)C/(12)C, in the biomass of terrestrial plants is a marker of their photosynthetic pathway. In aquatic environments, additional resistances to carbon-isotope fractionation make this technique of limited use in detecting photosynthetic pathways.     

Chromium accumulation and toxicity in aquatic vascular plants

Chromium poisoning among leather tanners has long been known. The workers have been found to suffer from ulcers, allergic dermatitis, lung cancer, and liver necrosis due to prolonged contact with chromium salts. One of the highly catastrophic incidences of lung cancer as a result of inhaling dust containing Cr (VI) was reported in 1960 from the Kiryama factory of the Nippon-Denko concern on the island of Hokkaido, Japan. Pollution of water resources, both surface and underground, by indiscriminate discharge of spent wastes of chromium-based industries has become a serious global concern, for it has created an acute scarcity of safe drinking water in many countries. In August 1975 it was observed that underground drinking water in Tokyo near the chromium (VI))-containing spoil heaps contained more than 2000 times the permissible limit of chromium. In Ludhiana and Chennai, India, chromium levels in underground water have been recorded at more than 12 mg/L and 550–1500 ppm/L, respectively. Chromium is widely distributed in nature, occupying 21st position in the index of most commonly occurring elements in the earth’s crust. Chromium occurs in nature in the form of a compound (chromium + oxygen + iron) known as “chromite.” The geographical distribution of chromite mines is uneven. Over 95% of economically viable chromite ores are situated in the southern part of Africa. Its annual global production is ca. 9 million tons, mainly mined in the former Soviet Union, Albania, and Africa. In India, over 90% of chromite deposits are located in Sukinda Valley of Orissa. Chromium occurs in several oxidation states, ranging from Cr²⁺ to Cr⁶⁺, with trivalent and hexavalent states being the most stable and common in the terrestrial environment. Chromium (III) is used for leather tanning because it forms stable complexes with amino groups in organic material. In the presence of excessive oxygen, chromium (III) oxidizes into Cr (VI), which is highly toxic and more soluble in water than are other forms. Chromium (VI) can easily cross the cell membrane, whereas the phosphate-sulphate carrier also transports the chromite anions. On the other hand, Cr (III) does not utilize any specific membrane carrier and hence enters into the cell through simple diffusion. The diffusion is possible only after the formation of appropriate lipophilic ligands. Use of chromium as industrial material was discovered only 100 years ago. It was used for the first time in the production of corrosion-resistant steel (stainless steel) and coatings. Subsequently, chromium was widely deployed in various industries; namely, electroplating, dyes and pigments, textiles, photography, and wood processing. The tanning industry is one of the major users of chromium (III) salts. During leather processing the conversion of putrefactive proteinaceous matter, skin, into non-putricible is carried out by the treatment of chromium sulphate solution. According to an estimate, ca. 32 tons of chromium sulphate salts are used annually in Indian tanneries. As a result of unplanned disposal of spent tannery wastes, ca. 2000–3200 tons of chromium as element escapes into the environment. This has raised severe ecological concern and reduced the forest cover considerably. Aquatic vascular plants play an important role in the uptake, storage, and recycling of metals. The uptake of metals depends on the chemical form present in the system and on the life form of the macrophytes (floating, free floating, well rooted, or rootless). The free-floating species (Eichhornia, Lemna, Pistia) absorb elements through the roots/leaves, whereas the rootless speciesCeratophyllum demersum absorbs mainly through the finally divided leaves. Submerged species showed higher chromium accumulation than do floating and emergent ones. The order is:Elodea canadensis > Lagarosiphon major > Potamogeton crispes > Trapa natans > Phragmitis communis. Roots of water hyacinth (Eichhornia crassipes) showed an accumulation of 18.92 μmol (g dry tissue wt^-1) Cr. AlthoughCeratophyllum demersum andHydrodictyon reticulatum showed lower levels of chromium accumulation, their bioconcentration factor values were very high. Floating-species duckweeds (Lemna, Spirodela) are potential accumulators of heavy metals. They have bioconcentrated Fe and Cu, as high as 78 times their concentration in wastewater. Duckweeds have also shown the ability to accumulate chromium substantially. Although duckweeds attain higher concentrations of chromium in their tissues than do other macrophytes, their bioconcentration factor (BCF) values were much lower than those reported in other aquatic species. A moderate accumulation of chromium has been found in emergent species. Plants ofScirpus validatus andCyperus esculentus accumulated 0.55 kg and 0.73 kg^-1 Cr, respectively. InBacopa monnieri andScirpus lacustris accumulations of 1600 and 739 μg g^-1 dw Cr, respectively, have been reported when exposed to 5 mg L^-1 Cr for 168 hours in solution culture. The accumulation of Cr was greater in the root than the shoot. Higher accumulations of chromium in roots and least in shoots of emergent species have also been recorded. Phytotoxicity of chromium in aquatic environment has not been studied in detail. The mechanism of injury in terms of ultrastructural organization, biochemical changes, and metabolic regulations has not been elucidated. It has been pointed out that while considering the toxicity of heavy metals, a distinction should be made between elements essential to plants and metals that have no proven beneficial biochemical effects. For example, an increased level of chromium may actually stimulate growth without being essential for any metabolic process. In aquatic species—namely,Myriophyllum spicatum— the maximum increase in shoot length was found at 50 μgl^-1 Cr. Higher concentrations up to 1000 μ gl^-1 caused an almost linear reduction both in shoot weight and length. Duckweeds showed relatively greater tolerance to chromium. However, an inhibition of growth inSpirodela andLemna was found at 0.02 mM and 0.00002 mM Cr concentrations, respectively. Mortality ofL. aequinoctialis was found at 0.005 mM Cr and higher concentrations. The effective chromium concentrations (EC-50) for some aquatic species have been reported as follows:Lemna minor, 5.0 mg L^-1, 14 days EC;L. Paucicostata, 1.0 mg L-¹, 20 days EC;Myriophyllum spicatum, 1.9 mg L^-1, 32 days EC; andSpirodela polyrrhiza, 50 mg L^-1, 14 days EC. Chromium toxicity on biochemical parameters showed a reduction in photosynthetic rate at 50 μgl^-1 Cr inMyriophyllum spicatum. Decrease in chlorophyll and protein contents were also recorded inNajas indica, Vallisneria spiralis, andAlternanthera sessilis with an increase in chromium concentration. InLimnanthemum cristatum, a slight reduction in chlorophyll and almost no change in control were found due to chromium toxicity. Submerged species (Ceratophyllum demersum, Vallisneria spiralis) and an emergent one (Alternanthera sessilis) showed decreases in chlorophyll as well as in protein contents when treated with chromium. Chromium-induced morphological and ultrastructural changes have been reported in several aquatic vascular plants: InLemna minor andCeratophyllum demersum, chromium-induced changes in chloroplast fine structure disorganized thylakoids with loss of grain and caused formation of many vesicles in the chloroplast. Chromium (VI) has caused stunting and browning of roots produced from the chromium-treated excised leaves ofLimnanthemum cristatum. At 226 μg/g Cr dry wt leaf tissue concentration, development of brown coloration in the hydathodes of juvenile leaves ofLimnanthemum cristatum is a characteristic chromiuminduced alteration. Aquatic vascular plants and algae may serve as effective bioindicators in respect to metals in aquatic environments. Chromium-induced morphological and ultrastructural changes inLimnanthemum cristatum have significant indicator values and could be used for assessing the level of chromium in ambient water.Wolffia globosa, a rootless duckweed, showed substantial chromium accumulation and high concentration factor (BCF) value at very low ambient chromium concentrations, suggesting its feasibility in detecting chromium pollution in water resources. Methylene blue-stained cells ofScenedesmus acutus become uniformly dark blue during chromium (VI) treatment. This may serve as an indicator of chromium pollution.     

异质生境中水生植物表型可塑性的研究进展

水生植物是一类以草本植物为主、与水紧密相关的生态类群, 大多数具有克隆性。面对水环境的变化, 水生植物在形态、行为和生理上表现出多样化的表型可塑性, 对异质生境具有很强的适应能力。表型可塑性研究已在陆生植物的多个类群展开, 然而目前对异质生境下水生植物的生态适应对策, 尤其是表型可塑性的研究尚重视不够。本文在阐明克隆植物表型可塑性主要实现方式及其关系、水生环境异质性及其特点的基础上, 重点从形态可塑性、觅食行为、克隆整合、克隆分工和风险分摊等5个方面讨论了水生植物如何通过表型可塑性适应异质性水生环境。在今后的水生植物表型可塑性研究中, 建议着重探讨以下问题: (1)表型可塑性的变化规律及机理; (2)克隆整合对群落和生态系统的影响; (3)克隆整合与克隆片段化的权衡; (4)不同克隆构型的表型可塑性及其内在机制; (5)表型可塑性的适应性进化; (6)水生植物与其他类群/营养级物种的关系; (7)水生生态系统对全球变化的响应。