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   1891篇
  免费   97篇
林业   160篇
农学   82篇
基础科学   9篇
  597篇
综合类   86篇
农作物   100篇
水产渔业   152篇
畜牧兽医   554篇
园艺   33篇
植物保护   215篇
  2024年   1篇
  2023年   24篇
  2022年   34篇
  2021年   59篇
  2020年   75篇
  2019年   77篇
  2018年   68篇
  2017年   78篇
  2016年   72篇
  2015年   60篇
  2014年   84篇
  2013年   148篇
  2012年   133篇
  2011年   176篇
  2010年   90篇
  2009年   87篇
  2008年   129篇
  2007年   114篇
  2006年   98篇
  2005年   84篇
  2004年   69篇
  2003年   59篇
  2002年   60篇
  2001年   12篇
  2000年   18篇
  1999年   15篇
  1998年   6篇
  1997年   11篇
  1996年   7篇
  1995年   1篇
  1994年   6篇
  1993年   3篇
  1992年   4篇
  1991年   3篇
  1990年   4篇
  1989年   1篇
  1988年   5篇
  1987年   1篇
  1986年   2篇
  1984年   2篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1976年   1篇
  1975年   1篇
  1971年   1篇
  1939年   1篇
排序方式: 共有1988条查询结果,搜索用时 15 毫秒
141.
Two new oleanene saponins were isolated from the MeOH extract of the shell of Argania spinosa.They possess protobassic acid and 16alpha-protobassic acid as aglycons. The disaccharide moiety linked to C-3 of the aglycon is made up of two glucose units; the pentasaccharide moiety linked to C-28 is made up of arabinose, xylose, and three rhamnose units. Their structures were elucidated by 1D and 2D NMR experiments including (1)H-(1)H (DQF-COSY, 1D TOCSY, and 2D HOHAHA) and (1)H-(13)C (HSQC and HMBC) spectroscopy along with mass spectrometry.  相似文献   
142.
A biomonitoring study using the ozone-sensitive bioindicator plant Nicotiana tabacum cv. Bel-W3 was conducted in the city of Valencia (eastern Spain) and surrounding areas in 2002. Plants were exposed to ambient air at seven sites, including four traffic-exposed urban sites, a large urban garden and a suburban and a rural station, for six consecutive 2-week periods using highly standardised methods. Foliar injury was registered at all stations in at least one of the exposure periods. The urban stations submitted to intense traffic showed lower ozone injury than the less traffic-exposed stations. Strong changes in the intensity of ozone injury were observed for the different exposure periods. Leaf injury was significantly related to both mean ozone values (24 and 12 h means) and cumulative exposure indices (AOT20, AOT40). However, correlation strength was moderate (r s?=?0.39 to 0.58), suggesting that the plant response to ozone was modified by environmental factors. The use of sensitive bioindicators like tobacco Bel-W3 in cities provides complementary information to that of continuously operating air quality monitors, as the impact of ambient ozone levels is directly measured.  相似文献   
143.
The effects of soil management on some microbiological properties and soil bacterial community structure were evaluated. Two field sites with the same soil type, located on the same geographic area adjacent to one other, have received different soil management practices and cultivation. One site has been subjected for 20 years to intensive horticulture under conventional tillage and irrigation with low quality salt-rich water; the second field site has been uncultivated for a long period and was turned to organic farming practices over the last 5 years and is currently cultivated with fruit orchard. Total bacterial counts, microbial ATP, microbial community metabolic (BIOLOG®) profiles, and DNA fingerprinting by PCR-DGGE were determined. Two-way ANOVA revealed that total bacterial counts were not significantly (P>0.3) affected by the two different management practices; ATP content was consistently and significantly (P<0.001) lower in salt-water irrigated soil than in organic soil at the three sampling times. The cluster analysis of community level physiological profiles indicated that microbial communities were much more uniform in organic soil than in irrigated one, suggesting that salt-water irrigation could have affected the size of the microbial population, its metabolic activities, as well as its composition. Molecular patterns fitted the BIOLOG® profile diversity. In particular, at any sampling time, PCR-DGGE patterns of bacterial DNA, extracted by an indirect method, significantly discriminated irrigated from organic soil samples. The PCR-DGGE patterns of total soil DNA, extracted by a direct method, showed a moderate to significant variation among irrigated and organic soil samples. Biochemical, microbiological and molecular data contributed to evidence a significantly different response of indigenous microflora to soil management by using saline water or organic farming.  相似文献   
144.
A new chemotype of the aromatic Verbenaceae species Lippia alba Mill. N. E. Br. from southeastern Brazil has recently been shown to have a high content of linalool in the leaf essential oil. Vegetative propagation of this chemotype was conducted at six different locations in Brazil, and the variation of the content and the optical purity of linalool in the oils were verified. Yields (0.6-0.9%, hydrodistillation), chemical composition, linalool content, and optical purity of the oils from all the plants were compared, using GC-FID, GC-MS, chiral chromatography, and retention index calculation. No plant exceeded the matrix in linalool content (46.5 to 90.7%), and the chemical profile of the oils was the same for all the samples. Purification of linalool to a content close to 100% was effected by vacuum distillation of the crude oil. Chiral analysis showed exclusively the presence of S-linalool in all the crude oils and in the distilled samples.  相似文献   
145.
146.
Artichoke (Cynara scolymus L.), an edible vegetable from the Mediterranean area, is a good source of natural antioxidants such as vitamin C, hydroxycinnamic acids, and flavones. The antioxidant activity of aqueous-organic extracts of artichoke were determined using three methods: (a) free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH(*)) scavenging, (b) ferric-reducing antioxidant power (FRAP), and (c) inhibition of copper(II)-catalyzed in vitro human low-density lipoprotein (LDL) oxidation. In addition, the present study was performed to investigate the ability of the edible portion of artichoke to alter in vivo antioxidative defense in male rats using selected biomarkers of antioxidant status. One gram (dry matter) had a DPPH(*) activity and a FRAP value in vitro equivalent to those of 29.2 and 62.6 mg of vitamin C and to those of 77.9 and 159 mg of vitamin E, respectively. Artichoke extracts showed good efficiency in the inhibition in vitro of LDL oxidation. Neither ferric-reducing ability nor 2,2'-azinobis(3-ethylbenzothiazolin-6-sulfonate) radical scavenging activity was modified in the plasma of the artichoke group with respect to the control group. Among different antioxidant enzymes measured (superoxide dismutase, gluthatione peroxidase, gluthatione reductase, and catalase) in erythrocytes, only gluthatione peroxidase activity was elevated in the artichoke group compared to the control group. 2-Aminoadipic semialdehyde, a protein oxidation biomarker, was decreased in plasma proteins and hemoglobin in the artichoke-fed group versus the control group. In conclusion, the in vitro protective activity of artichoke was confirmed in a rat model.  相似文献   
147.
The dark polymeric organic fraction rich in potassium recovered from olive oil mill waste waters (OMWW) and named polymerin and the potassium salified deglycosylated polymerin derivative (K-SDpolymerin) were easily transformed into their metal derivatives by saturation with various metals, including Na, Cu, Zn, Mn, Fe, and Al. Saturated metal polymerins were characterized by diffuse reflectance infrared Fourier transform spectroscopy and atomic absorption spectrometry. Tests on tomato plants of the various polymerins showed that only the soluble polymerin, K-SDpolymerin, and the insoluble Mn-SDpolymerin were significantly toxic. The toxic effects of OMWW on tomato at the original concentration and diluted 1:10 were much stronger than those of any polymerin. The possible exploitation of polymerins as bioamendments and/or metal biointegrators as a functon of their phytotoxic effects, their humic acid-like nature, and their richness in macro- and micronutrient metals is also discussed.  相似文献   
148.

Purpose

This work analyzes polychlorinated biphenyl (PCB) and heavy metal contamination in fluvial sediments and soils in an urban catchment, according to the geo-accumulation index and to soil and sediment quality guidelines. The catchment is located in Coimbra, Portugal, being affected by frequent flooding, and its main stream is a tributary to one of the major rivers in Portugal (Mondego). Given the presence of industrial activities over time, some inputs of pollutants are expected, but so far, the legacy of historic pollution in this catchment has not yet been investigated.

Materials and methods

Twenty-five samples were collected from nine sampling sites at the depths of 0–20, 20–40 and 40–60 cm (to provide a historic perspective) along longitudinal profiles (streamlines) and in soils downstream of pollution sources. These samples were analyzed for six heavy metals (Cu, Cr, Pb, Cd, Zn, Ni), organic carbon, pH and ten PCBs (IUPAC numbers 28, 30, 52, 101, 138, 153, 166, 180, 204, 209).

Results and discussion

Total PCB concentrations ranged 0.47–5.3 ng g?1 dry weight (dw), and levels increase from the bottom to the top layers, suggesting an increased input over the last 100 years. PCB congener distribution shows the dominance of hexachlorobiphenyls, especially PCB138, suggesting the existence of local sources. PCB levels did not exceed sediment quality levels, placing sediments/soil under class 1 (not contaminated) or class 2 (trace contamination) with respect to PCB. All six metals exceeded the lowest effect level for sediment quality criteria, and three (Cd, Pb and Zn) largely exceeded the clean levels for dredged materials, placing sediments in class 5 (heavily contaminated). Sampling site S1 presented the highest concentrations of Zn, Pb and Cd, and historic vehicle traffic was identified as the most likely source, given the vertical and horizontal profiles.

Conclusions

High levels of Pb, Cd and Zn were found in fluvial sediments at some locations of the Loreto catchment, likely from historic traffic sources. This urban area is frequently affected by flooding events and is currently being subject to urban redevelopment. During these events/actions, historic pollutants in the sediments might surface and be redistributed, impacting the downstream ecosystem of the major Mondego River or increasing the risk of exposure of the urban population.
  相似文献   
149.
The effect of 0, 1, 2, 3 and 4 intercross cycles using genetic male sterility on the mean yield and genetic variability of the CNA 5 irrigated rice synthetic population was assessed studying 300 S0:2 (1998/99) and S0:3 (1999/00) families grown in two locations (Lambari, MG and Goianira, GO). Each intercross was investigated using a 8 × 8 triple lattice design experiment containing 60 families and four control cultivars. Five experiments were carried out per location and generation, totaling 20 experiments. Individual, combined over locations, combined over generations and combined over locations and generations analyses of variance were performed. Linear regression equations were fitted to data of each location and generation as well as, genetic and phenotypic parameters were estimated. An average increase of 3.91% in grain yield was obtained with the increase in the number of intercrosses. However, when genetic gains obtained from selection of the five and ten highest yielding families were considered, negative b estimates were always obtained, suggesting a tendency of gain increase reduction with the increase in the number of intercrosses. This was confirmed by the analysis of the means of the best families, since increases in the number of intercrosses did not result in selection of families with greater mean grain yield. The estimates of the genetic variances and heritabilities did not suggest an increase in genetic variability with the increase in intercross cycles. Therefore, carrying out intercrosses on the base population did not improve the results of the irrigated rice recurrent selection program. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
150.
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