The Effect of Light on the Structure and Organization of Lemna Peroxisomes

The effect of light on a number of Lemna minor enzyme activitieswas investigated. The levels of activity of glycolate oxidase,catalase and RuBPCase increased with increasing irradiance,paralleling the increase in Lemna growth rate. In contrast withresults obtained for other species, no glycolate oxidase activitycould be detected in etiolated Lemna fronds or when these weretreated with light or glycolate, in vivo or in vitro, for upto 24 h. The number of peroxisome profiles per cell section was determinedin Lemna grown under different light conditions. When frondswere grown under dim light, the number of peroxisome profilesper cell section appeared to increase with increasing irradiance,although no further increase in the peroxisome number was apparentwhen the fronds were grown under higher irradiances. The levelof glycolate oxidase activity per peroxisome was shown to increasewith increasing irradiance, whereas that of catalase remainedrelatively constant, indicating that differential addition ofenzymes to pre-existing peroxisomes is possible. Peroxisomes from Lemna grown under high irradiance were subjectedto serial sectioning and examined under the electron microscope.Some peroxisomes were found to have a three dimensional structuresuggesting either fission and/or fusion or branching of theseorganelles, supporting the hypothesis of a peroxisomal reticulum.The dynamic relationship between the various shapes is discussed. Key words: Peroxisomes, glycolate oxidase, catalase     

Integration of hydraulic and chemical signalling in the control of stomatal conductance and water status of droughted plants

We describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABA] are likely to involve the water flux through the plant. (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψ_r), it will not depend solely on the soil water potential (Ψ_s) but also on the flux of water through the soil-plant-atmosphere continuum, to which are linked the difference between Ψ_r and Ψ_s. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water-flux-dependent message, with a water-flux-dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short-term plant response to this message would depend on the evaporative demand.     

Bird densities are associated with household densities

Increasing housing density is an important component of global land transformation, with major impacts on patterns of biodiversity. However, while there have been many studies of the changes in biodiversity across rural–urban gradients, which are influenced in large part by housing densities, how biodiversity changes across the full range of regional variation in housing density remains poorly understood. Here, we explore these relationships for the richness and abundance of breeding birds across Britain. Total richness, and that of 27 urban indicator species, increased from low to moderate household densities and then declined at greater household densities. The richness of all species increased initially faster with household density than did that of the urban indicator species, but nonurban indicator species richness declined consistently after peaking at a very low housing density. Avian abundance showed a rather different pattern. Total abundance and that summed across all urban indicator species increased over a wide range of household densities, and declined only at the highest household densities. The abundance of individual urban indicator species generally exhibited a hump-shaped relationship with housing density. While there was marked intraspecific variation in the form of such relationships, almost invariably avian abundance declined at housing densities below that at which the UK government requires new developments to be built. Our data highlight the difficulties of maintaining biodiversity while minimising land take for new development. High-density housing developments are associated with declines in many of those species otherwise best able to exploit urban environments, and those components of native biodiversity with which human populations are often most familiar.     

Mechanism of Kasugamycin Resistance in <Emphasis Type="Italic">Escherichia coli</Emphasis>

Strains of Escherichia coli are resistant to the antibiotic kasugamycin due to the partial non-methylation of 16S ribosomal RNA. An RNA methylase activity, absent from resistant strains, is shown here to methylate in vitro the 16S RNA of resistant as well as sensitive strains.     

Ultrastructure of the labellar epidermis in selected Maxillaria species (Orchidaceae)

The ultrastructure of the labellar epidermis of 13 species of Maxillaria and one hybrid was examined using low-vacuum scanning electron microscopy (SEM). The labellum may be homogeneous and glabrous or papillose, comprising one type of cell only, or heterogeneous with papillae, uniseriate trichomes and/or glands in various combinations. The trichomes are unbranched and multicellular with pointed or truncated tips. Moreover, in some taxa, moniliform trichomes occur, and these are thought to fragment with the formation of pseudopollen. Homogeneous and heterogeneous labellar organization may represent separate evolutionary lines. Preliminary results suggest that labellar features may provide additional taxonomic characters allowing determination of intrageneric affinities.     

Defining hybrid poplar (Populus deltoides × Populus trichocarpa) tolerance to ozone: identifying key parameters

This study examined whether two genotypes of hybrid poplar ( Populus deltoides  ×  Populus trichocarpa ), previously classified as ozone tolerant and ozone sensitive, had differing physiological and biochemical responses when fumigated with 120 nL L⁻¹ ozone for 6 h per day for eight consecutive days. Isoprene emission rate, ozone uptake and a number of physiological and biochemical parameters were investigated before, during and after fumigation with ozone. Previous studies have shown that isoprene protects plants against oxidative stress. Therefore, it was hypothesized that these two genotypes would differ in either their basal isoprene emission rates or in the response of isoprene to fumigation by ozone.
Our results showed that the basal emission rates of isoprene, physiological responses and ozone uptake rates were all similar. However, significant differences were found in visible damage, carotenoids, hydrogen peroxide (H₂O₂), thiobarbituric acid reactions (TBARS) and post-fumigation isoprene emission rates. It is shown that, although the classification of ozone tolerance or sensitivity had been previously clearly and carefully defined using one particular set of parameters, assessment of other key variables does not necessarily lead to the same conclusions. Thus, it may be necessary to reconsider the way in which plants are classified as ozone tolerant or sensitive.     

Evidence for Black Duck Winter Distribution Change

ABSTRACT The Mississippi Flyway midwinter population survey (MWS) indicates that American black ducks (Anas rubripes) have been rapidly declining for the last 10 years. We found a negative relationship between MWS and Ontario (Canada) midwinter counts for black ducks. Thus, as number of black ducks in the MWS decreased, Ontario midwinter counts increased. A shift in midwinter distribution of black ducks may be partly responsible for the decreasing trend in MWS counts. We recommend that midwinter black duck surveys be expanded to more sites in southern Canada and northeastern United States that currently are not sampled to better assess winter habitat use and improve the midwinter black-duck population index.