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   540篇
  免费   39篇
林业   36篇
农学   17篇
基础科学   5篇
  176篇
综合类   18篇
农作物   36篇
水产渔业   43篇
畜牧兽医   164篇
园艺   21篇
植物保护   63篇
  2023年   5篇
  2022年   6篇
  2021年   21篇
  2020年   19篇
  2019年   20篇
  2018年   15篇
  2017年   21篇
  2016年   16篇
  2015年   3篇
  2014年   12篇
  2013年   38篇
  2012年   33篇
  2011年   48篇
  2010年   17篇
  2009年   23篇
  2008年   40篇
  2007年   39篇
  2006年   28篇
  2005年   31篇
  2004年   30篇
  2003年   13篇
  2002年   23篇
  2001年   10篇
  2000年   6篇
  1999年   6篇
  1997年   3篇
  1996年   3篇
  1995年   2篇
  1994年   4篇
  1993年   2篇
  1992年   4篇
  1991年   7篇
  1989年   1篇
  1988年   3篇
  1986年   3篇
  1985年   2篇
  1983年   2篇
  1982年   1篇
  1981年   2篇
  1980年   3篇
  1978年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1973年   2篇
  1972年   2篇
  1969年   2篇
  1963年   2篇
  1961年   1篇
排序方式: 共有579条查询结果,搜索用时 15 毫秒
31.
32.
Marine sponges belonging to the phylum Porifera (Metazoa), evolutionarily the oldest animals are the single best source of marine natural products. The present review presents a comprehensive overview of the source, taxonomy, country of origin or geographical position, chemical class, and biological activity of sponge-derived new natural products discovered between 2001 and 2010. The data has been analyzed with a view to gaining an outlook on the future trends and opportunities in the search for new compounds and their sources from marine sponges.  相似文献   
33.
34.

Objective

To evaluate the effects of ketamine continuous rate infusions (CRI) at two dose rates on cardiovascular function and serum creatine kinase MB isoenzyme (CK-MB) and troponin I in healthy conscious dogs.

Study design

Experimental, prospective, crossover, randomized, blinded study.

Animals

Eight adult mixed-breed dogs, aged 6 ± 1 years and weighing 19 ± 8.6 kg (mean ± standard deviation).

Methods

Dogs were administered an intravenous bolus of ketamine (0.5 mg kg?1) followed by a ketamine CRI for 12 hours (20 μg kg?1 minute?1; treatment TC20 or 40 μg kg?1 minute?1; treatment TC40). Sedation, heart rate (HR), mean arterial pressure (MAP), electrocardiographic and echocardiographic parameters were evaluated at baseline (T0) and 1 (T1), 2 (T2), 4 (T4), 8 (T8), 12 (T12) and 24 (T24) hours after ketamine infusion started. Serum concentrations of CK-MB and troponin I were measured at baseline and 12, 24 and 48 hours after infusion started.

Results

HR increased over the first 4 hours, significantly at T1 in TC20 and at T4 in TC40 when compared with T0 (p < 0.05). MAP was significantly increased at T2 in TC40 when compared with TC20. Behavioral changes, such as stereotypical head movements and twitches, occurred within 4 hours in TC40. There were no significant changes in echocardiographic examinations in any dog when compared with baseline. There were no temporal changes in serum CK-MB activity either within or between treatments (p > 0.05). No troponin I was detected in any sample.

Conclusions and clinical relevance

No indication of myocardial injury resulting from ketamine infusion was detected in this study in healthy dogs. Further studies are needed to assess the ketamine infusion effects on antinociception and other organ function not evaluated in the present study.  相似文献   
35.
The availability of genetic linkage maps enables the detection and analysis of QTLs contributing to quality traits of the genotype. Proton Transfer Reaction Mass Spectrometry (PTR-MS), a relatively novel spectrometric technique, has been applied to measure the headspace composition of the Volatile Organic Compounds (VOCs) emitted by apple fruit genotypes of the progeny ‘Fiesta’ × ‘Discovery’. Fruit samples were characterised by their PTR-MS spectra normalised to total area. QTL analysis for all PTR-MS peaks was carried out and 10 genomic regions associated with the peaks at m/z = 28, 43, 57, 61, 103, 115 and 145 were identified (LOD > 2.5). We show that it is possible to find quantitative trait loci (QTLs) related to PTR-MS characterisation of the headspace composition of single whole apple fruits indicating the presence of a link between molecular characterisation and PTR-MS data. We provide tentative information on the metabolites related to the detected QTLs based on available chemical information. A relation between apple skin colour and peaks related to carbonyl compounds was established. The two authors contributed equally to this work.  相似文献   
36.
This paper proposes an evolutionary reading of rural development referred to cases of rapid industrial growth, where a strong concentration process has involved the main urban centers and the successful industrial districts. This territorial development pattern has gradually extinguished rural society and its institutional basis, creating a clear separation between new central and peripheral areas. The consequent effects on local economy and social dynamics reveal the long‐term risks raised in terms of development sustainability. An empirical study of two Italian provinces is also carried out to show how this framework can be helpful in interpreting real historical patterns.  相似文献   
37.
Summary In this study, the application of molecular markers to optimise genetic diversity in a polycross breeding program of perennial ryegrass (Lolium perenne L.) was evaluated. The genetic diversity among 98 potential parental plants from three maturity groups (early, intermediate and late flowering) was investigated using amplified fragment length polymorphism (AFLP) markers. For each maturity group, two polycrosses of six parental plants with contrasting levels of genetic diversity were composed. Average genetic diversity among parents selected for narrow polycrosses was 36% lower than among parents selected for wide polycrosses. Diversity within first generation synthetic progenies (Syn1) was proportional to the diversity within the respective parental polycrosses. However, differences were less pronounced with Syn1 progenies from narrow polycrosses showing 16% reduced diversity when compared to Syn1 progenies from wide polycrosses. Multivariate analyses allowed for a clear separation of the six Syn1 progenies based on AFLP markers and demonstrated their genetic distinctness. Evaluation of dry matter yield, date of ear emergence and stem length of Syn1 and Syn2 progenies showed progenies from wide polycrosses to be constantly higher yielding when compared to progenies from narrow polycrosses. On the other hand, there was no significant difference in variability for the two morphological traits between progenies of narrow- and wide polycrosses. The results presented here provide evidence for an efficient application of molecular markers to select genetically diverse polycross parents which resulted in an average yield increase of 3.8%.  相似文献   
38.
The efficiency of one- and two-dimensional NMR experiments in characterizing the content of the constituents of both herbal drugs and herbal drug preparations is demonstrated for kava-kava. These experiments directly detect active constituents represented by kavalactones in both a finely powdered herbal drug and a commercial extract. In addition, NMR spectroscopy can detect all other compounds present in the extract. As previously evidenced, NMR experiments can represent a generally applicable technique for rapid screening and are a complement to the classical analytical techniques such as high-performance thin-layer chromatography, high-performance liquid chromatography, capillary gas chromatography, and electrophoresis. These experiments can be considered a very simple and fast analytical method to obtain a fingerprint of the herbal drugs and their preparations, and to quantify the content of the active principles of the extract.  相似文献   
39.
Exceptionally high concentrations of Cd (100 to 200 ppm dry weight) were found in samples of Halobates collected off Baja California. In contrast, no detectable Cd was found in Rheumatobates collected from mangrove lagoons in the Gulf of California. Concentrations of five other heavy metals in the two marine insects are also presented.  相似文献   
40.
The objective of this work was to quantify the flavonoids present in foods most commonly consumed by the Brazilian population. The predominant flavonoids found in largest abundance in all of the analyzed vegetables were glycosides of quercetin. In lettuce, a small amount of luteolin was also detected. In sweet pepper, quercetin and luteolin were both present. White onion [48-56 mg/100 g of fresh weight (FW), expressed as aglycon], red onion (40-100 mg/100 g of FW), red lettuce (67-67.2 mg/100 g of FW), arugula (41-118 mg/100 g of FW), and chicory (18-38 mg/100 g of FW) were highest in total flavonoids. In fruits, the highest concentrations of flavonoids were found in the peel (125-170 mg/100 g of FW) and pulp (35-44 mg/100 g of FW) of oranges and in some apple varieties (14-36 mg/100 g of FW). Variability in flavonoid content due to time of harvesting was high for leafy vegetables and red onions. The estimated ingestion by Brazilian population ranged from 60 to 106 mg/day.  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