首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1140篇
  免费   64篇
  2024年   2篇
  2023年   12篇
  2022年   30篇
  2021年   50篇
  2020年   31篇
  2019年   35篇
  2018年   45篇
  2017年   42篇
  2016年   55篇
  2015年   80篇
  2014年   81篇
  2013年   96篇
  2012年   86篇
  2011年   76篇
  2010年   38篇
  2009年   48篇
  2008年   45篇
  2007年   55篇
  2006年   53篇
  2005年   44篇
  2004年   27篇
  2003年   33篇
  2002年   24篇
  2001年   23篇
  2000年   7篇
  1999年   14篇
  1998年   4篇
  1997年   4篇
  1996年   2篇
  1994年   4篇
  1993年   4篇
  1992年   5篇
  1990年   2篇
  1989年   3篇
  1988年   7篇
  1987年   2篇
  1986年   2篇
  1982年   2篇
  1981年   2篇
  1980年   2篇
  1979年   4篇
  1978年   2篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1971年   2篇
  1965年   1篇
  1941年   1篇
  1940年   1篇
  1903年   1篇
排序方式: 共有1204条查询结果,搜索用时 31 毫秒
891.
Nitrate uptake and nitrogen inclusion into amino acids were studied in the intact thallus and isolated bionts of the lichen Parmelia sulcata with the aid of mass spectroscopic tracing of heavy isotope 15N. The isolated photobiont, the green algae Trebouxia sp. did not take up nitrate, whereas the mycobiont and intact thalli were enriched in 15N when incubated with Na15NO3. Pulse feeding experiments with intact thalli followed by separation of photobiont showed that the labelled nitrate was originally assimilated by the mycobiont and only after that was detected in the photobiont. The isolated mycobiont after pulse labeling excreted labeled compounds into the incubation medium. Amino acids were detected in the exudate. The quantities of two amino acids considerably exceeded those of the others. One was identified as alanine, the other could not yet be identified with certainty. Both of these high-quantity compounds were also much more enriched in 15N than the others. These two compounds are proposed to be the transport forms of nitrogen within the Parmelia sulcata thallus.  相似文献   
892.
893.
Odonata have been widely used as indicators for the biomonitoring of terrestrial and aquatic habitats due to their sensitivity to environmental impacts. We aimed to determine whether the larval or adult phases of these insects were the best predictors of variation in habitat parameters and the loss of environmental integrity. Specimens were collected during three seasons (dry, rainy, and ebb) from 12 points in the Suiá-missu River basin, at the headwaters of the Xingu River in Mato Grosso, Brazil. The Protest analysis indicated a high degree of congruence between the assemblages of larvae and adults in streams with varying degrees of habitat integrity (R = 0.832, p < 0.001, m 2 = 0.307). When the congruence with environmental factors was analyzed, a significant association was found only for the larval phase (R = 0.318, p = 0.03, m 2 = 0.888). When the suborders were analyzed separately, congruence was confirmed for anisopteran adults (R = 0.338, p = 0.031, m 2 = 0.885) and larvae (R = 0.417, p = 0.003, m 2 = 0.826) and for the zygopteran adults (R = 0.345, p = 0.027, m 2 = 0.881) and larvae (R = 0.405, p = 0.011, m 2 = 0.836). These results indicate that both larvae and adults respond systematically to environmental impacts. We suggest that either life phase can be used for biomonitoring, given their effectiveness for the interpretation of disturbance in terrestrial and aquatic habitats. These findings further reinforce the effectiveness of this insect order for the detection of modifications to the environment, showing that they are good indicators of environmental conditions.  相似文献   
894.
The aim of this study was to develop and validate a discriminating in vitro release test to evaluate rivastigmine transdermal patches. The Exelon® Patch was chosen as a model transdermal product. The studies of in vitro release were designed to determine the impact of the official apparatus chosen (USP apparatus 5 and USP apparatus 6), the rotation speed, and the dissolution medium characteristics on the rivastigmine release profile from transdermal patches. Patches with different drug release profiles were tested in order to evaluate the discriminating power of the in vitro release test developed and validated. Variables such as the apparatus type, the dissolution medium, and the rotation speed have a significant influence on the drug release characteristics from a transdermal patch. The in vitro release methodologies using the USP apparatus 5 at 50 rpm and USP apparatus 6 at 25 rpm using the medium phosphate-buffered saline pH 7.4 were considered discriminative and adequate to characterize the rivastigmine (RV) release from a commercial transdermal patch, Exelon® Patch.  相似文献   
895.
The eucalypts comprise a group of woody plants used in commercial forest plantations owing to their high growth rates, adaptability to various ecological conditions and multiple applications. Despite the enormous amount of molecular data available for eucalypts, a basic understanding of the nature of its genome still requires information regarding the DNA amount in the genus. In this work, we estimated the genome size and base composition of 25 eucalypt species. With a comparative karyotype approach, we aimed to identify possible chromosomal alterations correlated with the genome size variation. Classical cytogenetic and genomic in situ hybridization experiments were conducted for this purpose. The studied species showed genome size ranging from 2C = 0.91 (Corymbia intermedia) to 2C = 1.37 pg (Eucalyptus paniculata) and AT/CG ratios varying from AT = 61.3 (Eucalyptus urophylla) to AT = 62.85% (C. intermedia). Comparative karyotype analysis revealed no remarkable differences in chromosome number (2n = 22) or morphology among eucalypt species despite considerable differences in nuclear DNA content. The genome in situ hybridization method did not distinguish non-homologous chromosomal regions of Eucalyptus baileyana and Corymbia citriodora, despite the difference of 0.45 pg between their genome sizes. The results found in the present work corroborate the consideration of small and dispersed DNA changes as the main cause of genome size variation in eucalypts.  相似文献   
896.
This study evaluated the effect of the protease inhibitor ritonavir (RIT) on Trichosporon asahii and Trichosporon inkin. Susceptibility to RIT was assessed by the broth microdilution assay and the effect of RIT on protease activity was evaluated using azoalbumin as substrate. RIT was tested for its anti-biofilm properties and RIT-treated biofilms were assessed regarding protease activity, ultrastructure and matrix composition. In addition, antifungal susceptibility, surface hydrophobicity and biofilm formation were evaluated after pre-incubation of planktonic cells with RIT for 15 days. RIT (200 μg ml?1) inhibited Trichosporon growth. RIT (100 μg ml?1) also reduced protease activity of planktonic and biofilm cells, decreased cell adhesion and biofilm formation, and altered the structure of the biofilm and the protein composition of the biofilm matrix. Pre-incubation with RIT (100 μg ml?1) increased the susceptibility to amphotericin B, and reduced surface hydrophobicity and cell adhesion. These results highlight the importance of proteases as promising therapeutic targets and reinforce the antifungal potential of protease inhibitors.  相似文献   
897.
The leaf anthracnose disease in sorghum, caused by Colletotrichum sublineolum (Henn. ex Sacc. & Trotter), is widely distributed throughout its Brazilian cultivation areas. The disease can cause significant losses in grain yield and quality. This study aimed to quantify the effects of leaf anthracnose on grain yield of different sorghum genotypes. Two elite inbred lines of sorghum, BR009 (susceptible) and BR008 (moderately resistant), and the hybrids, BR304 and MR43 (susceptible), BRS310 and DKB599 (moderately resistant) and BRS308 and AG1060 (resistant), were planted in a complete randomized block design with three replicates. The disease severity (DS) was evaluated weekly, starting from the onset of the first foliar symptoms, and yield losses were estimated using linear regression analysis. Leaf anthracnose significantly reduced sorghum yields in the susceptible genotypes. The highest yield loss of 86% was observed in the inbred line BR009, when the disease severity reached 100%. For the hybrids, the grain yield loss varied from 35% (BRS310) to 72% (BRS308). According to the adjusted model, a grain yield reduction of 23.48 kg/ha for BR304, 14.57 kg/ha for BRS310 and 15.91 kg/ha for DKB599 was observed for every 1% increase in disease severity. We demonstrate for the first time the effect of leaf anthracnose disease on grain sorghum yields under Brazilian conditions. The results from this study provide a starting point for developing new strategies for the integrated disease management of sorghum anthracnose.  相似文献   
898.
Geographic range size and genetic diversity are key correlates of extinction risk and evolutionary potential of a species, with species occupying smaller geographic ranges and showing limited genetic diversity assumed to be more threatened by environmental changes. The Italian Aesculapian snake Zamenis lineatus is a narrow‐range endemic of southern Italy and Sicily, once considered as part of the widespread species Z. longissimus. To date, we still lack comprehensive data on geographic range and intraspecific diversity of Z. lineatus. In this study, we analysed 106 Aesculapian snakes across the Italian Peninsula and Sicily in order to define the genetic diversity and distribution range of Z. lineatus, its possible range overlap with Z. longissimus and to assess whether hybridization occurs at the species’ range boundaries. We combined genetic data from mitochondrial and nuclear DNA markers with phenotypic data suitable for taxonomic identification. The observed phylogeographic pattern of Z. lineatus suggests: (i) a reduced peninsular range size, about a half of what is currently considered; (ii) limited genetic diversity and weak population structure; (iii) the occurrence of pervasive introgressive hybridization with Z. longissimus in the eastern contact zone. Together, results from this study indicate a higher extinction risk for Z. lineatus than previously appreciated and provide directions for future studies on the hybridization at the contact zone(s) between Z. lineatus and Z. longissimus.  相似文献   
899.

Purpose

The high consumption of electrical and electronic equipment motivated by the rapid technological advances seen over the years has lead to an increase in the generation of waste electrical and electronic equipment (WEEE). Such residues contain various dangerous substances and therefore deserve special attention. To that end, the Brazilian Policy on Solid Waste has provided guidelines on integrated and solid waste management, such as consumer electronics, aiming at their appropriate disposal and treatment through reverse logistics. In this context, the present work focuses on studying the recycling of some WEEE plastics.

Methods

This study was conducted using the methodological framework presented in the International Standard ISO 14040:2006 and aimed to determine the life cycle inventory (LCI) of a WEEE plastic recycling process in a company in Brazil. Having collected the data, it was possible to identify and quantify the environmental aspects caused by the recycling process of major plastics (acrylonitrile-butadiene-styrene (ABS) and high impact polystyrene (HIPS). The study was conducted in the only company in Brazil that operates WEEE plastic recycling in large scale.

Results and discussion

Some of the environmental aspects caused during the recycling process of the plastics under study were identified and quantified. As a result, besides presenting the inventory, it was also possible to determine a reduction in the consumption of energy and in CO2 emissions. When compared to the production of virgin ABS and HIPS, the recycling processes for such plastics showed a reduction in energy consumption by approximately 90% for both plastics and a reduction in CO2 emissions by approximately 84% for HIPS and 87% for ABS. The plastics recycled by the company retain over 90% of their virgin mechanical properties.

Conclusions

The study shows that recycling is highly relevant and that components present in WEEE received appropriate destination and treatment. Recycling avoids environmental impacts as it prevents WEEE from being disposed of in landfills and as the pellets of recycled plastics can re-enter the supply chain as raw materials. Considering the legislation in Brazil, the stage of collection/transport/treatment of WEEE conducted by the company under study presents strong indications of contributions to the environment, society, and economy of the country.
  相似文献   
900.
A comprehensive understanding of the presence and role of yeasts in bottled wines helps to know and control the organoleptic quality of the final product. The South Region of Brazil is an important wine producer, and the state of “Rio Grande do Sul” (RS) accounts for 90% of Brazilian wines. The state of “Santa Catarina” (SC) started the production in 1975, and is currently the fifth Brazilian producer. As there is little information about yeasts present in Brazilian wines, our main objective was to assess the composition of culturable yeasts associated to bottled wines produced in RS and SC, South of Brazil. We sampled 20 RS and 29 SC bottled wines produced between 2003 and 2011, and we isolated culturable yeasts in non-selective agar plates. We identified all isolates by sequencing of the D1/D2 domain of LSU rDNA or ITS1-5.8 S-ITS2 region, and comparison with type strain sequences deposited in GenBank database. Six yeast species were shared in the final product in both regions. We obtained two spoilage yeast profiles: RS with Zygosaccharomyces bailii and Pichia membranifaciens (Dekkera bruxellensis was found only in specific table wines); and SC with Dekkera bruxellensis and Pichia manshurica. Knowledge concerning the different spoilage profiles is important for winemaking practices in both regions.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号