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   718篇
  免费   41篇
  国内免费   1篇
林业   42篇
农学   44篇
  114篇
综合类   48篇
农作物   92篇
水产渔业   57篇
畜牧兽医   328篇
园艺   9篇
植物保护   26篇
  2023年   5篇
  2022年   2篇
  2021年   26篇
  2020年   15篇
  2019年   13篇
  2018年   18篇
  2017年   25篇
  2016年   22篇
  2015年   30篇
  2014年   39篇
  2013年   46篇
  2012年   43篇
  2011年   51篇
  2010年   30篇
  2009年   28篇
  2008年   59篇
  2007年   47篇
  2006年   38篇
  2005年   43篇
  2004年   40篇
  2003年   36篇
  2002年   29篇
  2001年   20篇
  2000年   14篇
  1999年   7篇
  1998年   2篇
  1997年   3篇
  1996年   3篇
  1993年   2篇
  1992年   7篇
  1991年   2篇
  1990年   4篇
  1989年   1篇
  1987年   1篇
  1986年   1篇
  1985年   3篇
  1984年   2篇
  1983年   2篇
  1968年   1篇
排序方式: 共有760条查询结果,搜索用时 546 毫秒
41.
Various pigment colors were produced by Monascus fermentations with separate addition of 20 amino acids. The color characteristics and structures of the pigment derivatives were investigated. When each amino acid was added to the fermentation broth as a precursor, pigment extracts with different hue and chroma values were obtained depending on the content ratios of yellow, orange, and red colors in the fermentation broth. The yellow and orange pigments were identical regardless of amino acid addition. The red compounds varied on the basis of the type of amino acid added. LC-MS and (1)H and (13)C NMR structural analyses confirmed that the derivative pigments contained the moieties of the added amino acids. L, a, and b values of the CIELAB color system for the derivative pigments were measured. Values of hue and chroma were then calculated. The colors of the derivative pigments were in the range of orangish red to violet red. The hydrophilicities/hydrophobicities of the derivative pigments could be predicted from their log P values, which were estimated using computer programs.  相似文献   
42.
Pine wilt disease (PWD) represents a major threat to forest ecosystems worldwide. Although PWD is now better understood, effective control measures for this disease have still not been devised. Here, we report several years of field studies on preventative silvicultural control of PWD. Silvicultural control through preventative clear-cutting and the manual removal of logs was implemented between 2005 and 2009 in 16 Korean districts that had newly PWD-infected stands. Preventative clear-cutting of neighboring asymptomatic pine trees (within a 10-50-m radius of wilt trees) and the removal of felled logs or branches suppressed spread of PWD. Occurrences of PWD wilt pines in districts (city or county) subjected to this silvicultural control method were significantly reduced compared with those in districts using conventional controls (physical or chemical treatment of wilt pine trees). Through silvicultural control, PWD was successfully suppressed in 11 of 16 districts investigated. In contrast, successful control was achieved in only 1 among 18 districts subjected to conventional control. Our results will be of considerable interest to those engaged in the very difficult battle against the global spread of PWD.  相似文献   
43.
Breakdown and macroinvertebrate colonization of conifer needles (Cryptomeria japonica) and deciduous broadleaves (Euptelea polyandra) were investigated using litter bags in two study sites in streams flowing through a conifer plantation of C. japonica in Shikoku, southwestern Japan (one site with conifer canopy and another with mixed conifer and broadleaved canopy). Breakdown rates and macroinvertebrate densities were compared between litter species (conifer needle vs broadleaf) and between the two sites (conifer vs mixed canopy) to determine (1) whether breakdown rate of broadleaves is higher than conifer needles, (2) whether macroinvertebrates prefer broadleaves to conifer needles, and (3) whether the difference in riparian canopy is reflected in macroinvertebrate abundance. The results indicated that breakdown rates of broadleaves were higher than those of conifer needles, suggesting poorer quality of the latter as food for macroinvertebrates. Differences in macroinvertebrate density between needles and broadleaves were generally consistent with those in breakdown rates: broadleaves tended to have higher densities than needles, suggesting that conifer needles were not preferred by macroinvertebrates. However, total macroinvertebrate density in the conifer site was not significantly different from that in the mixed site, although the dominant shredder taxon differed (conifer site: gammarids; mixed site: lepidostomatids). Although conifer needles are low-quality food for macroinvertebrates, this may offer some advantages. Conifer needles remain on the streambed for longer periods owing to their lower breakdown rates, being a constantly available resource. In addition, accumulations of conifer litter may effectively trap and retain particulate organic matter.  相似文献   
44.
Single- and bi-solute sorption and desorption of 2,4-dichlorophenol (2,4-DCP) and 2,4,5-trichlorophenol (2,4,5-TCP) in montmorillonite modified with hexadecyltrimethylammonium (HDTMA) were investigated using multi-step sorption and desorption procedure. Effect of pH on the multi-step sorption and desorption was investigated. As expected by the magnitude of octanol-water partition coefficient, K ow , both sorption and desorption affinity of 2,4,5-TCP was higher than that of 2,4-DCP at pH 4.85 and 9.15. For both chlorophenols, the protonated speciation (at pH 4.85) exhibited a higher affinity in both sorption and desorption than the predominant deprotonated speciation (about 95% and 99% of 2,4-dichlorophenolate and 2,4,5-trichlophenolate anions at pH 9.15, respectively). Desorption of chlorinated phenols was strongly dependent on the current pH regardless of their speciation in the previous sorption stage. Freundlich model was used to analyze the single-solute sorption and desorption data. No appreciable desorption-resistant (or non-desorbing) fraction was observed in organoclays after several multi-step desorptions. This indicates that sorption of phenols in organoclay mainly occurs via partitioning into the core of the pseudo-organic medium, thereby causing desorption nearly reversible. In bisolute competitive systems, sorption (or desorption) affinity of both chlorophenols was reduced compared to that in its single-solute system due to the competition between the solutes. The ideal adsorbed solution theory (IAST) coupled to the single-solute Freundlich model successfully predicted bisolute multi-step competitive sorption and desorption equilibria.  相似文献   
45.
Used with one of two surfactants (SDS, an anionic surfactant, and Triton X-100, a nonionic surfactant), the ligand, I? was evaluated as a washing agent for the desorption of Cd from naturally and artificially contaminated soils. Increasing amounts of the ligand, I?, with a surfactant, specifically removes higher levels of Cd but not Cu, Zn and Pb. After seven washings, the ligand, I? with the nonionic surfactant, Triton X-100, removed 65 and 90% of the Cd from soils I and II, containing respectively, to 15 and 1275 mg of Cd/kg. The ligand, I?, and the anionic surfactant, SDS, removed 35 and 70% of the Cd from soils I and II, respectively. Before washing, the carbonate fraction of soil I contained the most Cd (48%) while the exchangeable and carbonate fractions of soil II contained 29 and 33% of the total Cd, respectively. For soil I, SDS with/ without the ligand desorbed Cd mainly from the carbonate and oxide fractions, while only Triton X-100 with ligand I? could remove Cd from the exchangeable fraction. For soil II, Cd was desorbed from the exchangeable fraction only when either surfactant was used in combination with the ligand. Thus, a surfactant with ligand can extract specific heavy metals from soils and selective sequential extraction is useful in identifying which fraction can be targeted by the surfactant – ligand agent.  相似文献   
46.
We used radio-frequency spectroscopy to study pairing in the normal and superfluid phases of a strongly interacting Fermi gas with imbalanced spin populations. At high spin imbalances, the system does not become superfluid even at zero temperature. In this normal phase, full pairing of the minority atoms was observed. Hence, mismatched Fermi surfaces do not prevent pairing but can quench the superfluid state, thus realizing a system of fermion pairs that do not condense even at the lowest temperature.  相似文献   
47.
用非注山毛豆叶丙酮提取物或鱼藤酮点滴处理菜粉蝶幼虫,出现畸形幼虫和畸形蛹,对畸形幼虫和畸形蛹的真皮细胞和内外表皮分别进行了超显微结构观察,发现鱼藤酮及其类似物影响昆虫旧表皮的分解和新表皮的合成是导致虫体畸形的主要原因,并对畸形蛹体壁的几丁质,粗脂肪,粗蛋白的含量进行测定,结果表明处理组与对照组存在明显的差异。  相似文献   
48.
Somatic cell nuclear transfer (SCNT) technology has recently been used to generate animals with a common genetic composition. In this study, we report the derivation of a pluripotent embryonic stem (ES) cell line (SCNT-hES-1) from a cloned human blastocyst. The SCNT-hES-1 cells displayed typical ES cell morphology and cell surface markers and were capable of differentiating into embryoid bodies in vitro and of forming teratomas in vivo containing cell derivatives from all three embryonic germ layers in severe combined immunodeficient mice. After continuous proliferation for more than 70 passages, SCNT-hES-1 cells maintained normal karyotypes and were genetically identical to the somatic nuclear donor cells. Although we cannot completely exclude the possibility that the cells had a parthenogenetic origin, imprinting analyses support a SCNT origin of the derived human ES cells.  相似文献   
49.
Variants of NOD2, an intracellular sensor of bacteria-derived muramyl dipeptide (MDP), increase susceptibility to Crohn's disease (CD). These variants are thought to be defective in activation of nuclear factor kappaB (NF-kappaB) and antibacterial defenses, but CD clinical specimens display elevated NF-kappaB activity. To illuminate the pathophysiological function of NOD2, we introduced such a variant to the mouse Nod2 locus. Mutant mice exhibited elevated NF-kappaB activation in response to MDP and more efficient processing and secretion of the cytokine interleukin-1beta (IL-1beta). These effects are linked to increased susceptibility to bacterial-induced intestinal inflammation and identify NOD2 as a positive regulator of NF-kappaB activation and IL-1beta secretion.  相似文献   
50.
苦楝果实中化学成分进一步研究及生物活性测定   总被引:19,自引:0,他引:19  
采用生物活性追踪法,从苦楝果实中分离得苦楝新醇(Ⅰ)、苦楝酮(Ⅱ)、苦楝醇(Ⅲ)、苦楝二醇(Ⅳ)、苦楝三醇(Ⅴ)、香草酸(Ⅵ)、香草醛(Ⅶ)和川楝素(Ⅷ),并根据波谱(1R,MS,′H—NMR,~(13)C—NMR)分析和理化常数测定确定其结构.其中Ⅱ、Ⅲ、Ⅳ和Ⅴ的21—H存在差向异构现象.生测结果表明,Ⅱ~Ⅴ和Ⅷ对菜青虫(Pieris rapae)有明显的拒食活性,Ⅷ还表现出明显的胃毒活性;Ⅰ、Ⅳ和Ⅶ仅表现出一定的拒食活性.  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