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   371篇
  免费   12篇
  国内免费   17篇
林业   13篇
农学   21篇
基础科学   5篇
  111篇
综合类   81篇
农作物   60篇
水产渔业   36篇
畜牧兽医   42篇
园艺   6篇
植物保护   25篇
  2024年   3篇
  2023年   4篇
  2022年   3篇
  2021年   8篇
  2020年   8篇
  2019年   12篇
  2018年   7篇
  2017年   12篇
  2016年   9篇
  2015年   13篇
  2014年   22篇
  2013年   17篇
  2012年   27篇
  2011年   47篇
  2010年   13篇
  2009年   13篇
  2008年   27篇
  2007年   28篇
  2006年   16篇
  2005年   16篇
  2004年   9篇
  2003年   16篇
  2002年   7篇
  2001年   5篇
  2000年   8篇
  1999年   4篇
  1998年   1篇
  1997年   1篇
  1996年   4篇
  1995年   1篇
  1991年   3篇
  1989年   4篇
  1988年   2篇
  1987年   3篇
  1985年   4篇
  1981年   1篇
  1979年   3篇
  1977年   1篇
  1976年   1篇
  1975年   2篇
  1974年   3篇
  1971年   1篇
  1970年   2篇
  1969年   3篇
  1968年   1篇
  1965年   1篇
  1963年   4篇
排序方式: 共有400条查询结果,搜索用时 15 毫秒
41.
Two new lactams, coixspirolactam D (1) and coixspirolactam E (2), and a new spiroenone, coixspiroenone (3), together with seven known compounds, coixspirolactam A (4), coixspirolactam B (5), coixspirolactam C (6), coixlactam (7), coixol (8), ethyl dioxindole-3-acetate (9), and isoindol-1-one (10), and two neolignans, zhepiresionol (11) and ficusal (12), were isolated from the bioactive subfraction of adlay bran ethanolic extract (ABE). Compounds 9 and 10 are the first isolates from natural resources. The structures of new compounds were identified by spectroscopic methods, including infrared (IR) spectrum, 1D and 2D nuclear magnetic resonance (NMR), and mass spectrum (MS). All of the isolated compounds were tested for antiproliferative effects on MCF-7, MDA-MB-231, and T-47D cells. Results showed that compounds 1, 3, 4, 6, and 7 at 50 μM significantly inhibited MCF-7 cell proliferation by 30.2, 19.2, 21.0, 13.5, and 32.4%, respectively; compounds 2, 4, and 7 significantly inhibited T-47D cells at 50 μM by 20.7, 24.8, and 28.9%; and compounds 1, 2, and 12 significantly inhibited MDA-MB-231 cells at 50 μM by 47.4, 25.3, and 69.3%, respectively. In conclusion, ABE has antiproliferative activities, and this effect is partially related to the presence of lactams and spiroenone.  相似文献   
42.
Twenty-four secondary metabolites, including 16 isoflavonoids, 7 astragalasides, and 1 benzoquinone, have been isolated from the roots of Astragalus membranaceus (Astragali radix). Among these isolated isoflavonoids, (-)-methylinissolin 3-O-β-d-(6'-acetyl)-glucoside (1), (-)-methylinissolin 3-O-β-d-{6'-[(E)-but-2-enoyl]}-glucoside (2), and calycosin 7-O-β-d-(6'-acetyl)-glucoside (3) have been identified as new compounds on the basis of spectroscopic analysis; (-)-methylinissolin 3-O-β-d-glucoside (4) was isolated from the natural products for the first time. The nitric oxide (NO) production inhibitory activity of the major compounds has been assessed in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. To identify A. membranaceus, a fingerprint method was developed by using a high-performance liquid chromatography-evaporative light scattering detector (HPLC-ELSD) method. Furthermore, characteristic peaks for the 11 major compounds in the chromatogram were unambiguously confirmed.  相似文献   
43.
Acacia confusa Merr. (Leguminosae) is traditionally used as a medicinal plant in Taiwan. In the present study, anti-inflammatory activity of extracts from the heartwood of A. confusa were investigated for the first time. Results demonstrated that ethanolic extracts of A. confusa heartwood strongly suppressed NO production in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages. Among all fractions derived from ethanolic extracts, the EtOAc fraction exhibited the best inhibitory activity. Following column chromatography and reverse-phase high-performance liquid chromatography, 13 specific phytocompounds including 5 new flavonoids (i.e., 7,8,3',4'-tetrahydroxy-4-methoxyflavan-3-ol, 7,8,3',4'-tetrahydroxyflavone, 7,8,3'-trihydroxy-3,4'-dimethoxyflavone, 7,3',4'-trihydroxyflavone, and 7,3',4'-trihydroxy-3-methoxyflavone) were isolated and identified from the EtOAc fraction. In addition, melanoxetin (3,7,8,3',4'-pentahydroxyflavone), a major compound in the EtOAc fraction, markedly suppressed LPS-induced NO and prostaglandin E 2 (PGE 2) production. Moreover, melanoxetin completely suppressed gene expression of inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) at 50 and 100 microM, respectively. This is the first report to identify the inhibitory bioactivities of melanoxetin on iNOS and COX-2.  相似文献   
44.
Hypercholesterolemia initiates the atherogenic process; however, chronic inflammation promotes atherogenesis. Monascus spp. fermented products are recognized for their anti-hypercholesterolemic effect, but their anti-inflammatory activity is not as significant as that of many plant-derived foods. To enhance the anti-inflammatory function of Monascus pilosus fermented products, ginger was added to the PDB medium at a ratio of 20% (v/v). The mycelia and broth were collected, freeze-dried, and extracted by ethanol for assays. Macrophage RAW264.7 was challenged with lipopolysaccharide (LPS) and coincubated with the extracts of fermented product cultured in ginger-supplemented medium (MPG) or extracts of fermented product cultured in regular PDB medium (MP) for 18 h. Human umbilical vein endothelial cell HUVEC was challenged with tumor necrosis factor (TNF)-α and coincubated with the extracts of either MPG or MP for 6 h. The results showed that MPG significantly (p<0.05) lowered the production of macrophage pro-inflammatory cytokines TNF-α, nitric oxide (NO), interleukin (IL)-1, IL-6, and prostaglandin E2 (PGE2) by 68.53%, 84.29%, 32.55%, 84.49%, and 69.49%, respectively; however, MP had no inhibitory effect. MPG significantly downregulated the expression of p-IκB, cyclooxygenase-2 (COX-2), and inducible nitric oxide synthase (iNOS) in macrophage by 42.16%, 50.87%, and 51.35%, respectively, while MP had no inhibition on COX-2 expression and only 16.64% and 19.22% downregulatory effect on iNOS and phosphorylated-IκB (p-IκB), respectively. Moreover, MPG significantly suppressed the expression of vessel cell adhesion molecule-1 (VCAM-1) and p-IκB in endothelial cell by 63.48% and 63.41%, respectively. LC/MS/MS analysis indicated that 6-gingerdiol was formed in the ginger-modified medium during fermentation. The results of this study will facilitate the development of Monascus spp. fermented products as antiatherosclerotic nutraceuticals.  相似文献   
45.
Phytochemical characteristics and antioxidant activities of the crude and fractionated plant extracts of Anoectochilus formosanus were evaluated using five different assay systems. An acid-treatment (2 N HCl in 95% ethanol) was employed to treat a butanol fraction (BuOH), creating an acid-hydrolyzed BuOH fraction. The IC(50) values for DPPH radicals in the BuOH and acid-hydrolyzed BuOH fractions were 0.521 and 0.021 mg/mL, respectively. The acid-hydrolyzed BuOH exhibited approximately 5-fold higher activity in scavenging superoxide anion than catechin. The acid-hydrolyzed BuOH fraction also effectively protected phi x174 supercoiled DNA against strand cleavage induced by H(2)O(2) and reduced oxidative stress in HL-60 cells. Metabolite profiling showed that the aglycones of flavonoid glycosides in BuOH were produced after acid hydrolytic treatment, and this resulted in a significant increase in antioxidant activities of acid-hydrolyzed BuOH. One new diarylpentanoid, kinsenone, and three known flavonoid glycosides and their derivatives were identified for the first time from A. formosanus, with strong antioxidant properties.  相似文献   
46.
Whereas non-leguminous cover crops such as cereal rye (Secale cereale) or annual ryegrass (Lolium multiflorium) are capable of reducing nitrogen (N) leaching during wet seasons, leguminous cover crops such as hairy vetch (Vicia villosa) improve soil N fertility for succeeding crops. With mixtures of grasses and legumes as cover crop, the goal of reducing N leaching while increasing soil N availability for crop production could be attainable. This study examined net N mineralization of soil treated with hairy vetch residues mixed with either cereal rye or annual ryegrass and the effect of these mixtures on growth and N uptake by cereal rye. Both cereal rye and annual ryegrass contained low total N, but high water-soluble carbon and carbohydrate, compared with hairy vetch. Decreasing the proportion of hairy vetch in the mixed residues decreased net N mineralization, rye plant growth and N uptake, but increased the crossover time (the time when the amount of net N mineralized in the residue-amended soil equalled that of the non-amended control) required for net N mineralization to occur. When the hairy vetch content was decreased to 40% or lower, net N immobilization in the first week of incubation increased markedly. Residue N was significantly correlated with rye biomass (r=0.81, P<0.01) and N uptake (r=0.83, P<0.001), although the correlation was much higher between residue N and the potential initial N mineralization rate for rye biomass (r=0.93, P<0.001) and N uptake (r=0.99, P<0.001). Judging from the effects of the mixed residues on rye N Concentration and N uptake, the proportion of rye or annual ryegrass when mixed with residues of hairy vetch should not exceed 60% if the residues are to increase N availability. Further study is needed to examine the influence of various mixtures of hairy vetch and rye or annual ryegrass on N leaching in soil. Received: 10 March 1997  相似文献   
47.
New dammarane-type saponins from the galls of Sapindus mukorossi   总被引:2,自引:0,他引:2  
Five new dammarane-type saponins, 3beta,7beta,20(S),22-tetrahydroxydammar-24-ene-3-O-alpha-l-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside, 3beta,7beta,20(S),22,23-pentahydroxydammar-24-ene-3-O-alpha-l-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside, 3beta,7beta,20(S),22,25-pentahydroxydammar-23-ene-3-O-alpha-l-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside, 25-methoxy-3beta,7beta,20(S),22-tetrahydroxydammar-23-ene-3-O-alpha-l-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside, and 25-methoxy-3beta,7beta,20(R)-trihydroxydammar-23-ene-3-O-alpha-l-rhamnopyranosyl-(1-->2)-beta-D-glucopyranoside, named sapinmusaponins A (1), B (2), C (3), D (4), and E (5), respectively, together with three known phenylpropanoid glycosides (6-8), were isolated from the galls of Sapindus mukorossi. The structures of these saponins were elucidated on the basis of spectroscopic analyses and chemical methods. Preliminary bioassay data revealed that saponins 1 and 3-5 showed moderate cytotoxic activity (ED50 approximately 9-18 microg/mL) against human tumor cell lines (Hepa59T/VGH, NCI, HeLa, and Med) and that 1-5 were inactive in vitro against HIV replication in H9 lymphocytes.  相似文献   
48.
Bacterial blight (BB) is the most economically damaging disease of rice in Asia and other parts of the world. In this study, a multiplex PCR genotyping method was developed to simultaneously identify genotypes of five BB resistance genes, Xa4, xa5, Xa7, xa13 and Xa21. The resistance R alleles were amplified using five functional markers (FMs) to generate amplicons of 217, 103, 179, 381 and 595 bp in IRBB66. Amplicons of 198, 107, 87, 391 and 467 bp corresponded to susceptible alleles in Taiwanese japonica rice cultivars. In backcross breeding programmes, the multiplex PCR assay was integrated into selection from a population using BB resistance donor IRBB66 crossed to rice cultivar ‘Tainung82’. Two plants with homozygosity for Xa4, xa5, Xa7, xa13 and Xa21 were selected from 1100 BC2F2 plants. In addition, the five BB resistance genes were also accurately identified in F2 populations. This multiplex PCR method provides a rapid and efficient method for detecting various BB resistance genes, which will assist in pyramiding genes to improve durability of BB resistance in Taiwanese elite rice cultivars.  相似文献   
49.
Heading Chinese cabbage (Brassica campestris ssp. pekinensis Rupr.) grown with low Ca or B developed dark tan lesions near the leaf margins. A synergistic effect of low Ca and low B accompanied the development of this symptom. Plants grown under humid conditions were free of tipburn. Plants grown under less humidity developed tipburn. Total chemical analysis revealed a steady decrease in total Ca from the outer to the inner leaves. Fractional chemical analysis showed 7 times more water-soluble Ca in the outer than in the inner leaves, and 2 times more 1 N NaCl-soluble Ca in the outer than in inner leaves. Threshold concentration of water-soluble forms of Ca for tipburn was around 3.0 mg/g dry weight. Data suggest that root pressure flow is required to move adequate amounts of water-soluble Ca to the wrapper leaves to prevent tipburn of Chinese cabbage.  相似文献   
50.
The method of recognizing color texture brought forth in the present study is to employ unsupervised learning network to automatically recognize the fabric type and the main texture types. Firstly, the color scanner is adopted to extract fabric image which is afterwards saved as the digital image. Secondly,CIE-Lab color model is taken to obtain the feature value and wavelet transform is utilized to display the texture of the fabric image. Thirdly, co-occurrence matrix is employed to figure out the feature values of the texture structure such as angular second moment, entropy, homogeneity, contrast. Finally, self-organizing map (SOM) network is used as the classifier. The experiment result shows that the study can automatically and accurately classify the fabric types (including shuttle-woven fabric, jersey fabric and non-woven fabric) and main texture type of the fabric (such as plain weave, twill weave, satin weave, single jersey, double jersey and non-woven fabric).  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