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   908篇
  免费   48篇
  国内免费   1篇
林业   55篇
农学   25篇
基础科学   4篇
  328篇
综合类   42篇
农作物   73篇
水产渔业   82篇
畜牧兽医   279篇
园艺   26篇
植物保护   43篇
  2023年   5篇
  2022年   9篇
  2021年   24篇
  2020年   23篇
  2019年   25篇
  2018年   41篇
  2017年   33篇
  2016年   37篇
  2015年   31篇
  2014年   44篇
  2013年   54篇
  2012年   65篇
  2011年   81篇
  2010年   41篇
  2009年   48篇
  2008年   54篇
  2007年   48篇
  2006年   53篇
  2005年   47篇
  2004年   37篇
  2003年   27篇
  2002年   34篇
  2001年   14篇
  2000年   16篇
  1999年   4篇
  1998年   4篇
  1997年   2篇
  1996年   6篇
  1995年   4篇
  1992年   4篇
  1991年   3篇
  1990年   2篇
  1989年   2篇
  1983年   2篇
  1975年   2篇
  1974年   2篇
  1972年   2篇
  1967年   1篇
  1966年   1篇
  1965年   1篇
  1963年   1篇
  1928年   3篇
  1926年   1篇
  1925年   1篇
  1924年   4篇
  1913年   1篇
  1912年   1篇
  1910年   1篇
  1905年   1篇
  1903年   1篇
排序方式: 共有957条查询结果,搜索用时 15 毫秒
71.
72.
In wheat, the increase of yield and stability associated traits can be achieved by combining parents containing the stay-green trait and favorable alleles for grain yield. The aim of this work was to analyze the genetic dissimilarity between wheat lines from stay-green and synchronized maturation groups and elite cultivars. Moreover, to propose promising combinations seeking the selection of high-grain yield and high bread-making quality genotypes containing stay-green trait. The experiment was conducted in a randomized block design with three replications in 2003, 2004, and 2005, using sister-lines with the presence and absence of stay-green trait and elite cultivars. Genetic variability exists among wheat strains from the synchronized stay-green maturation group and elite cultivars. Genotypes of maturation group stay-green obtained an average performance superior to the synchronized group. Crosses between stay-green lines and the CEP 29 and BRS 177 cultivars are promising in the selection of genotypes carrying the stay-green trait with high yield and bread-making quality.  相似文献   
73.
74.
This study presents the morphology of the ovary, as well as the dynamics of the vitellogenesis process in oocytes of the cattle-tick Boophilus microplus. The ovary of these individuals is of the panoistic type; therefore, it lacks nurse cells. This organ consists of a single tubular structure, continuous, and composed of a lumen delimitated by a wall of small epithelial cells with rounded nuclei. In this tick species, the oocytes were classified into six stages varying from I to VI and according to: cytoplasm appearance and presence of the germ vesicle, yolk granules, and chorion. Oocytes of various sizes and at different developmental stages remain attached to the ovary through a cellular pedicel until completing stage V. Afterwards, they are liberated into the lumen and from there to the exterior. Some oocytes (classified as type VI) showed an atypical appearance indicating that some of the cellular components would be undergoing a degenerative process and/or reabsorption.  相似文献   
75.
The simultaneous detection of Staphylococcus aureus, Listeria monocytogenes, and Salmonella spp. has been approached by a new multiplex PCR-based procedure followed by capillary gel electrophoresis with laser-induced fluorescence detection (multiplex-PCR-CGE-LIF). As compared to slab gel electrophoresis, the use of CGE-LIF improved from 10- to 1000-fold the sensitivity of the multiplex PCR analysis, allowing the detection of 2.6 x 10(3) cfu mL(-1) of S. aureus, 570 cfu mL(-1) of L. monocytogenes, and 790 cfu mL(-1) of Salmonella in artificially inoculated food, without enrichment. Following 6 h of enrichment, as low as 260, 79, and 57 cfu mL(-1) of S. aureus, L. monocytogenes, and Salmonella, respectively, were detected. The CGE-LIF method is shown to be reproducible, providing relative standard deviation (RSD) values lower than 0.8% for analysis time and lower than 5.8% for peak areas. The multiplex-PCR-CGE-LIF proved a powerful analytical tool to detect various food pathogens simultaneously in a fast, reproducible, and sensitive way.  相似文献   
76.
A macro- and micromorphologic study was done on the soils from a stepped sequence of seven dated fluvial terraces in the lower Segre river valley (Lleida, northeast Spain) under a present-day semiarid Mediterranean climate. The soils have evolved from the Holocene through the early Pleistocene, providing an excellent morphostratigraphic framework for evaluating time-dependent factors influencing soil formation in a dry and calcareous environment. Throughout the chronosequence, some properties are regularly age-related specially carbonatation in subsurface horizons. The carbonates occur mainly as micrite, and although microsparite and sparite also appear in the oldest soils, they are replaced by fine-grained calcite by dissolution–reprecipitation processes (micritization process), which is active at present. Some pedological paleofeatures as the presence of sparite and recarbonated argillans in oldest terrace can be interpreted as the reflex of climatic changes during the Quaternary. In spite of this climate variability, the soils display progressive and systematic patterns of carbonate accumulation: on the lowest terraces, the soils do not yet have secondary carbonates but in the beginning Late Pleistocene calcic horizons, with carbonate pendents, are developed; these pendents increase its thickness with age although pendent growth rates decreases from Late Pleistocene to Middle Pleistocene. In the middle of the Late Pleistocene, calcic horizons evolved to petrocalcic horizons, which increase its thickness in the Middle and specially in Early Pleistocene. The presence of calcic and petrocalcic horizons is the primary basis for soil classification. This criterion is applicable not only to the soils of the lower Segre river basin, but also to many soils throughout the semiarid Mediterranean region.  相似文献   
77.
The effect of roasting on some nutritional characteristics of hazelnut lipidic fraction was investigated. Hazelnuts (Corylus avellana L.) were submitted to several different thermal treatments, comprising different temperatures (125-200 degrees C) and times of exposure (5, 15, and 30 min) and analyzed for their moisture and crude fat. Raw and roasted hazelnuts were also analyzed for their compositions in phytosterols and fatty acids (including trans isomers) by GC-FID, triacylglycerols by HPLC-ELSD, and tocopherols and tocotrienols by HPLC-DAD/fluorescence spectroscopy. Minor changes occurred in the fatty acid and triacylglycerol compositions. As temperatures and roasting periods increased, generally, a modest increase of oleic and saturated fatty acids and a decrease of linoleic acid, expressed as relative percentages, occurred. Similarly, an increase of triacylglycerols containing oleic acid moieties and a decrease of those containing linoleic acid moieties were found in the roasted samples. Roasting caused a modest decrease of the beneficial phytosterols (maximum 14.4%) and vitamin E homologues (maximum 10.0%) and a negligible increase of the trans fatty acids.  相似文献   
78.
The chemical conversion of phenylethylamine into phenylacetaldehyde in the presence of lipid oxidation products (LOPs) was studied to investigate the possibility that biogenic amines can be converted into Strecker aldehydes upon processing. Model systems of phenylethylamine and methyl 13-hydroperoxyoctadeca-9,11-dienoate (HP), 2,4-decadienal (DD), 4,5-epoxy-2-heptenal (EH), 4,5-epoxy-2-decenal (ED), 4-oxo-2-hexenal (OH), 4-oxo-2-nonenal (ON), or 4-hydroxy-2-nonenal (HN) were heated for 1 h at 180 °C and pH 3. Although HN and EH did not produce more phenylacetaldehyde than when phenylethylamine was heated alone, all other lipid oxidation products assayed increased the amount of phenylacetaldehyde produced by 300-900%, with ON being the most reactive compound for this reaction. The reaction was mainly produced at acidic pH values (<6) and was dependent upon the concentration of the LOPs involved, and the phenylacetaldehyde produced increased linearly as a function of the time and temperature. The E(a) values for the reactions between phenylethylamine and DD and ON were 54.8 and 53.8 kJ/mol, respectively. The reaction is proposed to take place by the formation of an imine between the phenylethylamine and the LOPs, which is later converted into another imine by an electronic rearrangement. This new imine is the origin of phenylacetaldehyde by hydrolysis. These results show a new pathway for Strecker aldehyde formation. This route provides a potential way to reduce biogenic amine content in foods when they can be thermally processed before consumption.  相似文献   
79.
Twenty-one free amino acids present in several samples of quince fruit (pulp and peel) and quince jam (homemade and industrially manufactured) were analyzed by GC/FID. The analyses showed some differences between quince pulps and peels. Generally, the highest content in total free amino acids and in glycine was found in peels. As a general rule, the three major free amino acids detected in pulps were aspartic acid, asparagine, and hydroxyproline. For quince peels, usually, the three most abundant amino acids were glycine, aspartic acid, and asparagine. Similarly, for quince jams the most important free amino acids were aspartic acid, asparagine, and glycine or hydroxyproline. This study suggests that the free amino acid analysis can be useful for the evaluation of quince jam authenticity. It seems that glycine percentage can be used for the detection of quince peel addition while high alanine content can be related to pear addition.  相似文献   
80.
OBJECTIVE: To evaluate dietary habits in Spanish children and adolescents based on a Mediterranean Diet Quality Index tool, which considers certain principles sustaining and challenging traditional healthy Mediterranean dietary patterns. DESIGN: Observational population-based cross-sectional study. A 16-item Mediterranean Diet Quality Index was included in data gathered for the EnKid study (in which two 24-hour recalls, a quantitative 169-item food-frequency questionnaire and a general questionnaire about socio-economic, demographic and lifestyle items were administered). SETTING: Spain. SUBJECTS: In total, 3850 children and youths aged 2-24 years residing in Spain. RESULTS: Of the sample, 4.2% showed very low KIDMED index results, 49.4% had intermediate values and 46.4% had high index results. Important geographical differences were seen, with subjects from the Northeast showing the most favourable outcomes (52% with elevated scores vs. 37.5% of those from the North). Lower percentages of high diet quality were observed in low socio-economic groups, compared with middle and upper income cohorts (42.8%, 47.6% and 54.9%, respectively). Large cities had more positive results and only slight variations were seen for gender and age. CONCLUSIONS: The KIDMED index, the first to evaluate the adequacy of Mediterranean dietary patterns in children and youth, confirms that this collective is undergoing important changes, which makes them a priority target for nutrition interventions. Results challenge certain commonly perceived notions tied to income level, population size and diet quality.  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