Interpreting Botanical Progress
July--September 2000

Age Of Maturity And Life Span In Herbaceous, Polycarpic Perennials Martin H. Bender, Jerry M. Baskin, and Carol C. Baskin.........................311 Bioactive Chemicals and Biological-Biochemical Activities and Their Functions in Rhizospheres of Wetland Plants Amir Neori, K. Ramesh Reddy, Hana Í Ková-Kon Alová and Moshe Agami.............351 Mercury Toxicity In Plants Manomita Patra and Archana Sharma..............................................379 A Foliar Morphometric Approach To The Study of Salicaceae Mélanie Thiébaut...............................................................423 AGE OF MATURITY AND LIFE SPAN IN HERBACEOUS, POLYCARPIC PERENNIALS MARTIN H. BENDER 1,2, JERRY M. BASKIN 1 and CAROL C. BASKIN 1,3 1 School of Biological Sciences University of Kentucky, Lexington, KY 40506-0225 USA 2 Present address: The Land Institute 2440 E. Water Well Road, Salina, KS 67401 USA 3 Department of Agronomy University of Kentucky, Lexington, KY 40506-0091 USA I. ABSTRACT A review of age of maturity in herbaceous, polycarpic perennials found that the most common year of earliest maturity for wild and cultivated conditions was the second year of life, followed by the first- and then the third year. A comparison of age of maturity in wild and cultivated conditions for individual taxa confirmed the assumption that perennials generally do not mature sooner in the wild than they do in cultivation. This validated the use of the pattern for maturity in cultivation (second-year or later) against which to judge that for maturity in the wild. For plants of the same age of maturity, those with clonal growth had longer life spans than those with little or no clonal growth. This difference in life span was more pronounced for plants of first- and second-year maturity than it was for those of later maturity. Her-baceous, polycarpic perennials in the wild generally were either short-lived with first-year matur-ity or long-lived with later maturity. These results were also true for non-clonal taxa only. For application to the real world, theoretical plant population models must take these results into account. Click Here to Go to Back to Top BIOACTIVE CHEMICALS AND BIOLOGICAL-BIOCHEMICAL ACTIVITIES AND THEIR FUNCTIONS IN RHIZOSPHERES OF WETLAND PLANTS AMIR NEORI K. RAMESH REDDY National Center for Mariculture Soil and Water Science Department Israel Oceanographic & Limnological Research University of Florida P.O. Box 1212 Gainesville, FL 32611 Eilat 88112 USA Israel HANA Í KOVÁ-KON ALOVÁ MOSHE AGAMI Section of Plant Ecology Plant Sciences Department Institute of Botany Faculty of Life Sciences Academy of Sciences of the Czech Republic Tel Aviv University Trebon, Czech Republic Tel Aviv 69978, Israel I. ABSTRACT Wetland soils provide anoxia-tolerant plants with access to ample light, water and nutrients. Intense competition, involving chemical strategies, ensues between the plants. The roots of wetland plants are prime targets for root-eating pests, while the wetland rhizosphere is an ideal environment for many other organisms and communities because it provides water, oxygen, organic food and physical protection. Consequently, the rhizosphere of wetland plants is densely populated by many specialized organisms, which considerably influence its biogeochemical functioning. The roots protect themselves against pests and control their rhizosphere organisms by bioactive chemicals, which often also have medicinal properties. Anaerobic metabolites, alkaloids, phenolics, terpenoids and steroids are bioactive chemicals abundant in roots and rhizospheres in wetlands. Bioactivities include allelopathy, growth regulation, extraorganismal enzymatic activities, metal manipulation by phytosiderophores and phytochelatines, various pest-control effects and poisoning. Complex biological- biochemical interactions between roots, rhizosphere organisms and the rhizosphere solution determine the overall biogeochemical processes in the wetland rhizosphere and in the vegetated wetlands. To comprehend how wetlands really function, it is necessary to understand these interactions. Such understanding requires further research. Click Here to Go to Back to Top MERCURY TOXICITY IN PLANTS - SOME ASPECTS MANOMITA PATRA AND ARCHANA SHARMA Centre of Advanced Study Department of Botany University of Calcutta 35 Ballygunge Circular Road Calcutta 700 019 India I. ABSTRACT Mercury poisoning has become a problem of current interest as a result of environmental pollution on a global scale. Natural emissions of mercury form two-thirds of the input while man-made release form about one-thirds. Considerable amounts of mercury may be added to agricultural land with sludge, fertilisers, lime, and manures. The most important sources of contaminating agricultural soil have been the use of organic mercurials as a seed coat dressing to prevent fungal diseases of seeds. In general, effect of treatment on the germination capacity is favourable when recommended dosages are used. Injury to the seed is more pronounced at higher rates, increasing in direct proportion to increasing rates of application. The availability of soil Hg to plants is low, and there is a tendency for Hg accumulation in the roots, indicating that the roots serve as a barrier to Hg uptake. Hg concentration in above ground parts of plants appears to largely depend on foliar uptake of Hg0 volatilised from the soil. Uptake of Hg has been found to be plant-specific in bryophytes, lichens, wetland plants, woody plants, and crop plants. Factors affecting plant uptake include soil or sediment organic content, carbon exchange capacity, oxide and carbonate content, redox potential, formulation used and total metal content. In general, mercury uptake in plants could be related to the pollution level. With lower levels of mercury pollution, the amounts in crops are below the permissible levels. Aquatic plants have shown to be bioaccumulators of Hg. Hg concentrations in the plants (stems + leaves) were always greater when the metal was introduced in organic form. In freshwater aquatic vascular plants, differences in uptake rate depended on the species of plant, the seasonal growth rate changes, and the metal ion being absorbed. A part of Hg emitted from the source into the atmosphere is absorbed by plant leaves, and migrates to humus through fallen leaves. Hg vapour uptake by leaves of the C3 species oats, barley, and wheat was five times greater than that by leaves of the C4 species corn, sorghum, and crabgrass. Such differential uptake by C3 and C4 species was largely attributable to internal resistance to Hg vapour binding. Airborne Hg thus seems to contribute significantly to the Hg content in crops and thereby to its human food intake. Accumulation, toxicity response, and Hg distribution differ between plants exposed through shoot or through root, even when internal Hg concentrations in the treated plants were similar. Throughfall and litterfall play a significant role in the cycling and deposition of Hg. The possible causal mechanisms of Hg toxicity are (i) Changes in the permeability of the cell membrane; (ii) reactions of sulphydryl (-SH) groups with cations; (iii) affinity for reacting with phosphate groups and active groups of ADP or ATP; and (iv) replacement of essential ions (mainly major cations). In general, inorganic forms are thought to be more available to plants than organic ones. The exposure of plants to mercurials may be by - ( 1) Direct administration as antifungal agents i.e. mainly to crop plants, through seed treatment or foliar spray. The end points screened are seed germination, seedling growth, relative growth of root and shoot and in some case studies of leaf area index, internode development and other anatomical characters. 2) Accidental exposures through soil, water, and air pollution. The level of toxicity is tested usually under laboratory conditions using a wide range of concentrations and different periods of exposure. Additional parameters include biochemical assays and genetical studies. The absorption of organic and inorganic Hg from soil by plants is low and there is a barrier to Hg translocation from plant roots to tops. Thus, large increases in soil Hg levels produce only modest increases in plant Hg levels by direct uptake from soil. The seed injury caused by organic mercurials to cereals has been characterized by abnormal germination and characteristic hypertrophy of the roots and coleoptile. Mercury affects both light and dark reactions of photosynthesis. Substitution of the central atom of chlorophyll, magnesium, by mercury in vivo prevents photosynthetic light- harvesting in the affected chlorophyll molecules, resulting in a breakdown of photosynthesis. The reaction varies with light intensity. A concentration and time dependent protective effect of GSH seems to be mediated by the restricted uptake of the metal involving cytoplasmic protein synthesis. Plant cells contain aquaporins, proteins that facilitate the transport of water, in the vacuolar membrane (tonoplast) and the plasma membrane. Many aquaporins are mercury sensitive, and in AQP1, a mercury-sensitive cysteine residue (Cys-189) is present adjacent to a conserved Asn-Pro-Ala motif. Mercury has a toxic effect at low concentrations on the degrading capabilities of microorganisms. Sensitivity to the metal was enhanced by a reduction in pH and tolerance to Hg by microorganisms was found to be in the order : total population > nitrogen fixers > nitrifiers. Numerous experiments have been carried out to study the genetic effects of mercury compounds in experimental test systems using a variety of genetic endpoints. The most noticeable and consistent effect was the induction of c-mitosis through disturbance of the spindle activity, resulting in the formation of polyploid and aneuploid cells, and c-tumours. Organomercurials had been reported to be 200 times more potent than inorganic mercury. Exposure to inorganic mercury reduced mitotic index in the root tip cells and increase the frequency of chromosomal aberrations in degrees directly proportional to the concentrations used and to the duration of exposure. The period of recovery after removal of Hg was inversely related to the concentration and duration of exposure. Bacterial plasmids encode resistance systems for toxic metal ions including Hg2+, functioning by energy-dependent efflux of toxic ions, through ATPases and chemiosmotic cation/proton antiporters. The inducible mercury resistance (mer) operon encodes both a mercuric ion uptake and a detoxification enzymes. In Gram-negative bacteria, a periplasmic protein, MerP, an inner-membrane transport protein, MerT, and a cytoplasmic enzyme, mercuric reductase (the MerA protein), are responsible for the transport of mercuric ions into cell and their reduction to elemental mercury, Hg(II). In Thiobacillus ferrooxidans, an acidophilic chemoautotrophic bacterium sensitive to Hg ions, a group of mercury resistant strains, which volatilized mercury, was isolated. The entire coding sequence of the mercury ion resistance gene was located within a 2.3kb fragment of chromosomal DNA (encoding 56,000 and 16,000 molecular weight proteins) from strain E-15 of Escherichia coli. Higher plants and Schizosaccharomyces pombe respond to heavy metal stress of Hg by synthesizing phytochelatins (PCs) that act as chelators. The strength of Hg(II) binding to glutathione and phytochelatins followed the order: gGlu-Cys-Gly<(gGlu-Cys)2Gly<(gGlu-Cys)3Gly<(gGlu-Cys)4Gly. Suspension cultures of haploid tobacco (Nicotiana tabacum) cells were subjected to ethyl methane sulfonate to raise Hg-tolerant plantlets. HgCl2-tolerant variants were selected from nitrosoguanidine (NTG)-treated suspension cell cultures of cow pea (Vigna unguiculata) initiated from hypocotyl callus, and incubated with 18mg/ml HgCl2. Experiments have been carried out to develop Hg-tolerant plants of Hordeum vulgare through previous exposure to low doses of Hg and subsequent planting in next generation in Hg-contaminated soil. Phytoremediation involves the use of plants to extract, detoxify and/or sequester environmental pollutants from soil and water. Transgenic plants cleave mercury ions from methyl-mercury complexes; reduce mercury ions to the metallic form; take up metallic mercury through their roots; and evolve less toxic elemental mercury. Genetically engineered plants contain modified forms of bacterial genes that break down methyl mercury and reduce mercury ions. The first gene successfully inserted into plants was merA, which codes for a mercuric ion reductase enzyme, reducing ionic mercury to the less toxic elemental form. MerB codes for an organomercurial lyase protein that cleaves mercury ions from highly toxic methyl mercury compounds. Plants with the merB gene have been shown to detoxify methyl mercury in soil and water. Both genes have been successfully expressed in Arabidopsis thaliana, Brassica (mustard), Nicotiana tabacum (tobacco) and Liriodendron tulipifera (tulip poplar). Plants currently being transformed include cattails, wild rice, and Spartina, another wetlands plant. The problem of mercury contamination can be reduced appreciably by combining the standard methods of phytoremediation of removal of Hg from polluted areas through scavenger plants with raising such plants both by routine mutagenesis and by genetic engineering. The different transgenics raised utilising the two genes merA and merB are very hopeful prospects. Click Here to Go to Back to Top A FOLIAR MORPHOMETRIC APPROACH TO THE STUDY OF SALICACEAE MÉLANIE THIÉBAUT Laboratoire de Biodiversité Evolution des Végétaux actuels et fossiles Bâtiment 401 A Université Claude Bernard Lyon 1, 43 boulevard du 11 Novembre 1918 69622 Villeurbanne cedex FRANCE I. Resume/Summary Un assemblage de feuilles de quatre espèces de saules (Salix viminalis, Salix alba, Salix fragilis, Salix caprea), deux de leurs hybrides (Salix alba X fragilis et Salix caprea X viminalis), ainsi que deux espèces de peupliers (Populus alba, Populus tremula), et deux espèces " externes " (Eleagnus angustifolia, Pyrus salicifolia) a été étudié. Le but était de savoir si des feuilles de forme très proche mais d'espèces différentes (en particulier les feuilles allongées de S. viminalis, S. alba, S. fragilis, E. angustifolia et P. salicifolia) pouvaient être discriminées par des caractères foliaires (donc végétatifs) continus, et ce malgré le très grand polymorphisme foliaire existant chez les Salicacées (surtout chez le genre Populus), et les problèmes d'hybridation. Il en a résulté que les analyses multivariées (ACP et Analyse agrégative) associées à un jeu de caractères approprié, permettent à plus de 98% le classement des feuilles dans leur propre espèce, même dans ces conditions difficiles. Il ressort de ce travail que l'ACP n'est un bon outil que lorsqu'elle conserve un maximum de la variabilité totale, c'est-à-dire ici, quand les taxa sont peu nombreux ; en revanche, l'Analyse Agrégative exprime le maximum de ses capacités en présence d'un grand nombre de taxa. On peut alors envisager l'application de cette approche morphométrique à la détermination d'empreintes fossiles, dans le but d'inclure même les restes fragmentaires dans les études paléobotaniques, ce qui donnerait accès à la réelle biodiversité des flores tertiaires et à la variabilité existant au sein des espèces. Summary An assemblage of leaves of four species of willows (Salix viminalis, Salix alba, Salix fragilis, Salix caprea), two of their hybrids (Salix alba X fragilis and Salix caprea X viminalis), two species of poplars (Populus alba, Populus tremula), and two "external" species (Eleagnus angustifolia, Pyrus salicifolia) has been studied. The study aimed to determine whether leaves that are very close in shape, but that belong to different species, (particularly the elongated leaves of S. viminalis, S. alba, S. fragilis, E. angustifolia and P. salicifolia) could be discriminated by continuous foliar characters (i.e. vegetative characters), despite both the great foliar polymorphism met in Salicaceae (especially in the genus Populus), and hybridisation problems. Our results show that multivariate analyses (PCA and Cluster Analysis) associated to an appropriate characters set, enable leaves to be classified in their own species at more than 98%, even in these difficult conditions. It can be seen from this work that PCA is a good tool when it keeps a maximum of total variability, i.e. here when there are few taxa ; on the other hand, Cluster Analysis is more appropriate with many taxa. One can then envisage the application of this morphometric approach to fossil imprints determination, where even the fragmentary rests in paleobotanical studies could be included. This would give access to real biodiversity of tertiary flora and to intraspecific variability. Click Here to Go to Back to Top