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1.
Sequential medical trials involving paired data 总被引:2,自引:0,他引:2
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Long-term changes of diatoms and chemistry in headwater streams polluted by atmospheric deposition of sulphur and nitrogen compounds 总被引:4,自引:0,他引:4
1. Samples for the analysis of attached diatoms and surface water chemistry were taken at thirty-five stations in sixteen soft-water streams in the Netherlands during autumn 1990. Most were located in pine forest and heathland catchments, fed by deep groundwater (median residence time 87 years), with no direct influence of agricultural or urban drainage water. 2. The chemical data from sixteen stations were compared with data collected in 1974 and 1981. There were no significant increases in pH (6.3 in 1974, 6–4 in 1990) or in sulphate (200 mmol m?3 in 1974, 229 mmol m?3 in 1990). Over the same period nitrate increased significantly from 17 to 158 mmol m?3, while alkalinity decreased significantly from 355 to 251 meq m?3. 3. The most important correlates with the distribution of diatoms, as revealed by canonical correspondence analysis, were pH, nitrate, cross-sectional area of the stream and the Ca/(Ca + Cl) ratio. 4. Diatom assemblages of upstream stations indicated more acid conditions than those of downstream stations. Also there was considerable variation in the diatom assemblages of different habitats, 5. A diatom-pH transfer function was developed using the weighted-averaging method. The function was applied to comparable pairs of samples which were collected in 1974 and 1990 at ten stations. There was a significant decrease in the median diatom-inferred pH from 6.78 in 1974 to 6.55 in 1990. 6. The diatom assemblages are diverse and contain a high proportion of filamentous diatoms, particularly Aulacoseira crenulata and A. alpigena, which are rare in Western Europe. This is a response to the low current velocity (median 5 cm?1) and the relatively constant deep groundwater discharge. 相似文献
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Novel strategies for assessing and managing the risks posed by invasive alien species to global crop production and biodiversity 总被引:3,自引:0,他引:3
RICHARD BAKER RAY CANNON PAUL BARTLETT IAN BARKER 《The Annals of applied biology》2005,146(2):177-191
International actions to combat the threat posed by invasive alien species (IAS) to crops and biodiversity have intensified in recent years. The formulation of 15 guiding principles on IAS by the Convention on Biological Diversity (CBD) stimulated the International Plant Protection Convention (IPPC) to review its role in protecting biodiversity. IPPC standards now demonstrate clearly that the risks posed by any organism that is directly or indirectly injurious to cultivated or uncultivated plants can be assessed and managed under the IPPC. Since the IPPC, unlike the CBD, constitutes an international legal instrument recognised by the World Trade Organization, greater protection from the introduction of IAS is now available. However, phytosanitary measures can only be enacted if they can be justified by risk analysis and we outline some novel strategies to improve the assessment and management of the risks posed by IAS, highlighting some of the key challenges which remain. 相似文献
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We investigated the synthesis and translocation of amino compounds in Parasponia, a genus of the Ulmaceae that represents the only non-legumes known to form a root nodule symbiosis with rhizohia. In the xylem sap of P. andersonii we identified asparagine. aspartate. glutamine, glutamated significant quantities of a non-protein amino acid. 4-methylglutamte(2-amino-4-methylpentanedioic acid). This identification was confirmed by two methods, capillary gas chromatography (GC) electron ionization (El) mass spectrometry (MS) and reverse phase high pressure liquid chromatography (HPLC) analysis of derivatized compounds. In leaf, root and nodule samples from P. andersonii and P. parviflora we also identified the related compounds 4-methyleneglutamate and 4-methyleneglulamine. Using 15N2 labelling and GC-Ms analysis of root nodule extracts we followed N2 fixation and ammonia assimilation in P. andersonii root nodules and observed Label initially in glutamine and subsequently in glutamate, suggesting operation of the glutamine synthetase/glutamine:2-oxoglutarate aminotransferase (GS/GOGAT) pathway. Importantly, we observed the incorporation of significant quantities of 15N into 4-methylglutamate in nodules, demonstrating the de nova synthesis of this non protein amino acid and suggesting a role in the translation of N in symbioticParasponia. 相似文献
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MICHAEL D. BAKER CHARLES R. VOSSBRINCK JAMES J. BECNEL JOSEPH V. MADDOX 《The Journal of eukaryotic microbiology》1997,44(3):220-225
ABSTRACT. Sequences of the small subunit rRNA genes of Amblyospora californica and an Amblyospora sp. from Culex salinarius were determined. These sequences were compared phylogenetically with 16 other microsporidia. The results suggest Amblyospora forms a sister taxon to the rest of the microsporidia examined. The basal position of Amblyospora is discussed with respect to the evolution of microsporidian life cycles. These sequences represent the longest microsporidian small subunit rRNA genes sequenced to date, 1,359 and 1,358 bp, respectively. Structural features and GC content (49% for both) are comparable to those of other microsporidia which have been sequenced. 相似文献
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Measurement of mass flow of water in the stems of herbaceous plants 总被引:15,自引:2,他引:13
Abstract. Heat balance methods of stem flow measurement offer the opportunity to measure directly the mass flow rate of water in plants. We have tested one such approach; the constant power heat balance method of Sakuratani (1981). The results supported his statement of an approximate accuracy of 10% when measuring the transpiration rate of herbaceous plants. The response to sudden changes in stem flow rate is not instantaneous, but investigation of the time constant shows that it decreases as stem flow increases, to the extent that, at flow rates typical of daytime conditions the system is capable of accurately tracking changes in stem flow within 5 min or less.
We describe a new gauge design that is relatively rugged, simple to use with an appropriate digital datalogger and suitable for field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to continuous and direct recording of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should improve both the accuracy and the dynamic response of the system, is proposed. 相似文献
We describe a new gauge design that is relatively rugged, simple to use with an appropriate digital datalogger and suitable for field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to continuous and direct recording of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should improve both the accuracy and the dynamic response of the system, is proposed. 相似文献
10.
Modifications to the photosynthetic apparatus of higher plants in response to changes in the light environment 总被引:1,自引:0,他引:1
NEIL R. BAKER MARC McKIERNAN 《Biological journal of the Linnean Society. Linnean Society of London》1988,34(3):193-203
A brief review of the photosynthetic apparatus of higher plants is given, followed by a consideration of the modifications induced in this apparatus by changes in light intensity and light quality. Possible strategies by which plants may optimize photosynthetic activity by both long- and short-term modifications of their photosynthetic apparatus in response to changing light regimes are discussed. 相似文献