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   370篇
  免费   13篇
  国内免费   18篇
林业   12篇
农学   15篇
基础科学   5篇
  109篇
综合类   79篇
农作物   60篇
水产渔业   35篇
畜牧兽医   56篇
园艺   6篇
植物保护   24篇
  2024年   3篇
  2023年   4篇
  2022年   3篇
  2021年   7篇
  2020年   8篇
  2019年   13篇
  2018年   6篇
  2017年   13篇
  2016年   9篇
  2015年   13篇
  2014年   21篇
  2013年   18篇
  2012年   27篇
  2011年   47篇
  2010年   16篇
  2009年   14篇
  2008年   26篇
  2007年   25篇
  2006年   18篇
  2005年   19篇
  2004年   9篇
  2003年   14篇
  2002年   5篇
  2001年   5篇
  2000年   7篇
  1999年   4篇
  1998年   1篇
  1997年   1篇
  1996年   5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401条查询结果,搜索用时 15 毫秒
21.
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   
22.
Antiproliferative activity and apoptosis induction of ethyl acetate of Eucalyptus citriodora resin (EAEER), and its major bioactive compound in melanoma B16F10 cells were investigated. 6-[1-(p-Hydroxy-phenyl)ethyl]-7-O-methyl aromadendrin (HEMA), a flavanol derivative, was isolated from EAEER and identified on the basis of its mass and NMR spectra. The results from MTT assay showed high antiproliferative effects of EAEER and HEMA on B16F10 cells. Moreover, EAEER- and HEMA-induced cell apoptosis was association with the decrease in the mitochondrial transmembrane potentials (Δψ(m)), increase in Bax/Bcl-2 ratio, and activation of caspase-3. Cells treated with EAEER and HEMA generated intracellular reactive oxygen species (ROS) and nitric oxide (NO), indicating that ROS and RNS play important roles in the induction of apoptosis in B16F10 cells. Taken together, EAEER and its major bioactive compound, HEMA, inhibited the proliferation of B16F10 cells via apoptosis and may be a potential antimelanoma agent.  相似文献   
23.
The purpose of this study was threefold: to investigate the types and amounts of medical waste from various hospitals in Taiwan; to analyze residues from incinerated medical waste for heavy metal content; and to measure the amount of infectious and general medical waste per bed per day and determine the ratio of infectious to general medical waste. Information was first obtained on the amounts of medical waste produced per bed per day and on waste handling and treatment. Next, residue samples were taken from incinerators and analyzed for heavy metal content. Third, daily records concerning waste content and weight from one hospital were kept over the period of one year. Chromium (Cr) and zinc (Zn) concentrations were significantly higher in medical waste than in general waste. Medical waste fluctuated significantly over the one-year period. Records showed that wastes containing dialysis material and wood/cotton materials were found to have the highest weight and wastes containing human organs had the lowest weight. Results of the present study underscore the need for all hospitals to effectively separate waste for proper treatment and disposal, particularly waste that produces residues which may be hazardous to the environment. In addition, hospitals also need to recycle or reuse certain materials in order to reduce waste production.  相似文献   
24.
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   
25.
4-Shogaol is one of the phytoconstituents isolated from dried red ginger, which is commercially available to consumers. Some active constituents from ginger have been found to have anti-inflammatory and antioxidant effects, but studies on 4-shogaol have been relatively rare. This is the first report describing the antimetastasis activities of 4-shogaol and the possible mechanisms. This study determined that 4-shogaol inhibits the migration and invasion of MDA-MB-231 and causes mesenchymal-epithelial transition (MET). In addition, 4-shogaol suppresses the activation of NF-κB and cell migration and invasion induced by TNF-α. Furthermore, 4-shogaol has been shown to inhibit the phosphorylation of IκB and the translocation of NF-κB/Snail in MDA-MB-231. This study shows that RKIP, an inhibitory molecule of IKK, is up-regulated after 4-shogaol treatment and prolongs the inhibitory effects of 4-shogaol. Inhibition of RKIP by shRNA transfection significantly decreases the inhibitory effect of 4-shogaol on the NF-κB/Snail pathway, together with cell migration and invasion, whereas overexpression of Snail suppresses 4-shogaol-mediated metastasis inhibition and E-cadherin upregulation. Finally, the animal model revealed that 4-shogaol effectively inhibits metastasis of MDA-MB-231 in mice. This study demonstrates that 4-shogaol may be a novel anticancer agent for the the treatment of metastasis in breast cancer.  相似文献   
26.
Ma  Chih-Ming  Ku  Young  Kuo  Yu-Lin  Chou  Yiang-Chen  Jeng  Fu-Tien 《Water, air, and soil pollution》2009,197(1-4):313-321
Water, Air, & Soil Pollution - The decomposition of gaseous isopropanol (IPA) by UV/TiO2 process in an annular photoreactor was studied under various conditions such as UV light intensity and...  相似文献   
27.
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   
28.
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   
29.
This study was performed to investigate the free radical scavenging active components from in vitro propagated medicinal herbs of the genus Dendrobium, namely, Dendrobium tosaense Makino and Dendrobium moniliforme SW, using a 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical antioxidative assay. Seeds of the capsules derived after 12 weeks of hand-pollination germinated asymbiotically (50-74%) on half-strength Murashige and Skoog's (MS) basal medium with 3% sucrose and solidified with 0.9% Difco agar. Active growth in the germinated seedlings was achieved by reculturing on full-strength MS basal medium supplemented with 8% banana homogenate, 8% potato homogenate, 8% coconut water, 1.5% sucrose, and 0.9% Difco agar. Healthy plantlets transferred to plastic trays containing moss or moss and tree fern successfully acclimatized (84-100%) in the greenhouse. Extracts were prepared from plants grown in the greenhouse for a period of 6 months. Methanolic extracts of D. tosaense and D. moniliforme scavenged DPPH at 95.9 and 83.4%, respectively, at a concentration of 0.4 mg/mL. Therefore, methanolic solubles of D. tosaense and D. moniliforme were subjected to bioguided fractionation and separation by column chromatographic methods individually. After chromatographic separation of these crude extracts, the obtained fractions (Dm 1, Dm 2, Dm 3, Dt 1, Dt 2, and Dt 3) were tested for their activity. Among them, fractions Dm 2 and Dt 1 showed significant antioxidant activity by DPPH radical antioxidative assay. Active fractions were purified further by column chromatography and resulted in identification of the antioxidant components alkyl ferulates from D. moniliforme and quercetin from D. tosaense.  相似文献   
30.
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   
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