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   4571篇
  免费   233篇
  国内免费   2篇
林业   199篇
农学   233篇
基础科学   13篇
  816篇
综合类   360篇
农作物   694篇
水产渔业   401篇
畜牧兽医   1780篇
园艺   69篇
植物保护   241篇
  2023年   29篇
  2022年   89篇
  2021年   163篇
  2020年   97篇
  2019年   121篇
  2018年   144篇
  2017年   134篇
  2016年   146篇
  2015年   150篇
  2014年   205篇
  2013年   269篇
  2012年   328篇
  2011年   383篇
  2010年   220篇
  2009年   199篇
  2008年   292篇
  2007年   296篇
  2006年   263篇
  2005年   219篇
  2004年   215篇
  2003年   187篇
  2002年   141篇
  2001年   131篇
  2000年   92篇
  1999年   57篇
  1998年   28篇
  1997年   17篇
  1996年   9篇
  1995年   9篇
  1994年   3篇
  1993年   8篇
  1992年   16篇
  1991年   14篇
  1990年   22篇
  1989年   17篇
  1988年   7篇
  1987年   9篇
  1986年   7篇
  1985年   5篇
  1984年   4篇
  1983年   5篇
  1980年   5篇
  1978年   11篇
  1975年   3篇
  1974年   5篇
  1972年   5篇
  1971年   3篇
  1970年   3篇
  1969年   3篇
  1968年   4篇
排序方式: 共有4806条查询结果,搜索用时 0 毫秒
51.
The effect of heat treatment on the antioxidant activity of extracts from Citrus unshiu peels was evaluated. Citrus peels (CP) (5 g) were placed in Pyrex Petri dishes (8.0 cm diameter) and heat-treated at 50, 100, or 150 degrees C for 10, 20, 30, 40, 50, and 60 min in an electric muffle furnace. After heat treatment, 70% ethanol extract (EE) and water extract (WE) (0.1 g/10 mL) of CP were prepared, and total phenol contents (TPC), radical scavenging activity (RSA), and reducing power of the extracts were determined. The antioxidant activities of CP extracts increased as heating temperature increased. For example, heat treatment of CP at 150 degrees C for 60 min increased the TPC, RSA, and reducing power of EE from 71.8 to 171.0 microM, from 29.64 to 64.25%, and from 0.45 to 0.82, respectively, compared to non-heat-treated control. In the case of WE from CP heat-treated at the same conditions (150 degrees C for 60 min), the TPC, RSA, and reducing power also increased from 84.4 to 204.9 microM, from 15.81 to 58.26%, and from 0.27 to 0.96, respectively. Several low molecular weight phenolic compounds such as 2,3-diacetyl-1-phenylnaphthalene, ferulic acid, p-hydroxybenzaldoxime, 5-hydroxyvaleric acid, 2,3-diacetyl-1-phenylnaphthalene, and vanillic acid were newly formed in the CP heated at 150 degrees C for 30 min. These results indicated that the antioxidant activity of CP extracts was significantly affected by heating temperature and duration of treatment on CP and that the heating process can be used as a tool for increasing the antioxidant activity of CP.  相似文献   
52.
To investigate the hepatic metabolism of the new insecticide flupyrazofos [O,O-diethyl O-(1-phenyl-3-trifluoromethylpyrazol-5-yl) phosphorothioate], isolated rat liver was perfused with flupyrazofos under single-pass conditions. In outflow perfusate and bile, 1-phenyl-3-trifluoromethyl-5-hydroxyprazole (PTMHP), PTMHP-sulfate and PTMHP-glucuronide conjugates were identified as the metabolites of flupyrazofos. However, O,O-diethyl O-(1-phenyl-3-trifluoromethylpyrazol-5-yl) phosphate (flupyrazofos oxon) was not detected. A HPLC method with UV detection was used to investigate the hepatic disposition of flupyrazofos and its metabolite PTMHP. The concentrations of flupyrazofos, PTMHP and PTMHP conjugates in outflow perfusate reached steady-state levels within 20 min after commencing perfusion of 7.3 microM flupyrazofos. At steady state, the mean extraction ratio of flupyrazofos was 0.93 (+/- 0.01) and clearance was 26.1 (+/- 0.2) ml min-1 which nearly approached perfusate flow rate (28 ml min-1). PTMHP accounted for 55.7 (+/- 5.8)% of eliminated flupyrazofos and was recovered as unchanged PTMHP, PTMHP-sulfate and PTMHP-glucuronide in the bile as well as the outflow perfusate.  相似文献   
53.
Kim DS  Cook RJ  Weller DM 《Phytopathology》1997,87(5):551-558
ABSTRACT Strain L324-92 is a novel Bacillus sp. with biological activity against three root diseases of wheat, namely take-all caused by Gaeumannomyces graminis var. tritici, Rhizoctonia root rot caused by Rhizoctonia solani AG8, and Pythium root rot caused mainly by Pythium irregulare and P. ultimum, that exhibits broad-spectrum inhibitory activity and grows at temperatures from 4 to 40 degrees C. These three root diseases are major yieldlimiting factors for wheat in the U.S. Inland Pacific Northwest, especially wheat direct-drilled into the residue of a previous cereal crop. Strain L324-92 was selected from among approximately 2,000 rhizosphere/rhizoplane isolates of Bacillus species isolated from roots of wheat collected from two eastern Washington wheat fields that had long histories of wheat. Roots were washed, heat-treated (80 degrees C for 30 min), macerated, and dilution-plated on (1)/(10)-strength tryptic soy agar. Strain L324-92 inhibited all isolates of G. graminis var. tritici, Rhizoctonia species and anastomosis groups, and Pythium species tested on agar at 15 degrees C; provided significant suppression of all three root diseases at 15 degrees C in growth chamber assays; controlled either Rhizoctonia root rot, takeall, or both; and increased yields in field tests in which one or more of the three root diseases of wheats were yield-limiting factors. The ability of L324-92 to grow at 4 degrees C probably contributes to its biocontrol activity on direct-drilled winter and spring wheat because, under Inland Northwest conditions, leaving harvest residues of the previous crop on the soil surface keeps soils cooler compared with tilled soils. These results suggest that Bacillus species with desired traits for biological control of wheat root diseases are present within the community of wheat rhizosphere microorganisms and can be recovered by protocols developed earlier for isolation of fluorescent Pseudomonas species effective against take-all.  相似文献   
54.
Canine distemper virus (CDV) has long been recognized as a cause of myocarditis; however, cases of myocarditis caused by naturally acquired CDV infection have been reported only rarely in dogs. We describe here our retrospective study of naturally acquired systemic CDV infection in 4 dogs, 4–7 wk old, that had myocarditis, with myocardial necrosis and fibrosis. One of the 4 dogs had intracytoplasmic eosinophilic inclusion bodies in cardiomyocytes. Other lesions included bronchointerstitial pneumonia (4 of 4), necrotizing hepatitis (2 of 4), splenic lymphoid necrosis (2 of 4), encephalitis (1 of 3; brain was not submitted in 1 case), and necrotizing gastroenteritis (1 of 4). The presence of CDV in the heart was confirmed by immunohistochemistry in all 4 dogs.  相似文献   
55.
We aimed to investigate the effects of inoculating Panax ginseng C.A. Meyer seedlings with arbuscular mycorrhizal fungi (AMF) by examining the root colonization, plant nutrition uptake, growth characteristics, and soil aggregation of P. ginseng seedlings inoculated at the time of transplantation. At 16 weeks, the AMF spore density per 30 g of fresh mycorrhizosphere in seedlings inoculated with AMF (AMF+ seedlings) was 256.8 and that in seedlings not inoculated with AMF (AMF− seedlings) was 186.3, respectively. The colonization rate of AMF in the lateral roots of AMF+ seedlings was approximately 19% higher than that in the lateral roots of AMF− seedlings. The patterns of AMF colonization in ginseng roots were similar to those of the Paris-type mycorrhizal association. Plant growth characteristics, such as plant height, root length, leaf area, number of lateral roots, fresh weight of shoots and roots, and chlorophyll content, were significantly enhanced in AMF+ seedlings compared to AMF− seedlings. The macronutrient content (P, K, and Ca) and micronutrient content (Cu, Fe, and Zn) of both shoots and roots were also significantly higher in AMF+ seedlings compared to AMF− seedlings. Furthermore, glomalin content and soil aggregation were significantly enhanced in AMF inoculated areas. Our results indicate that AMF inoculation may enhance the growth of ginseng seedlings by improving the uptake of mineral nutrients and the soil structure in mycorrhizosphere.  相似文献   
56.
The Bia owie a Primeval Forest, Poland, is the only forest area in temperate Europe where large tracts of natural forest communities have been conserved. The concept of silvatic mosaic [Oldeman, R.A.A., 1990. Forests: Elements of Silvology. Springer, Berlin] was applied in order to characterise the structure of stands representing oak–lime–hornbeam forest in both, the restricted area of Bia owie a National Park (BNP) and the commercial forests (CF), as well as to indicate a holistic approach towards protection and restoration of natural areas.Six developmental phases of stands were distinguished and defined: regeneration, young, pole, late pole, optimal, and terminal. Compositional and structural differences between the first five phases were also evaluated and visualised. Moreover, the considerable differences between analogous phases in BNP and CF were found. The uniform stand patches were mapped into two plots in BNP (25 ha each) and two in CF (25 and 20 ha), with a resolution of 10 m× 10 m pixels.The mosaic composition of natural communities indicates close-to-the equilibrium stage: the biostatic optimal phase occupies 33–40% of the area, while aggradational younger phases share nearly the same portion of the area as the senescent phase does. Although small, 0.01–0.04 ha eco-units dominate in BNP, there is a considerable portion of intermediate and large patches, indicating a complex character of factors responsible for the mosaic structure of natural stands in BNP. Both, composition and texture analyses indicate that the area of 25 ha is close to the minimum structure area specific for the communities studied.The comparison of natural and managed stands revealed large deficits of the optimal phase (only 19%) and a total absence of the terminal one in the managed area (20–35% in natural forest). Also the mosaic texture of CF plots was simplified because of standardisation of the management unit size. The modification of developmental phases, mosaic composition, and structure must lead to changes in the dynamics of the entire community. The present study demonstrates that the natural forest should be considered as a dynamic supersystem, covering a sufficient area to allow the developmental phases to perform in correct proportions. Because of differences in the durability of various phases, the natural cycle of forest development has an asynchronous and highly unpredictable character. Indeed, the applicability of multipurpose forestry rules as a tool of nature conservation has been questioned. An absolute ban on cutting in the remaining old-growth stands should be a basic rule for the restoration management.  相似文献   
57.
Thyroid disease is common in dogs and conventional ultrasonography is a standard diagnostic test for diagnosis and treatment planning. Strain elastography can provide additional information about tissue stiffness noninvasively after applying external or internal compression. However, natural carotid artery pulsations in the canine thyroid gland are too weak to maintain sufficient internal compression force. The objective of the present study was to describe the feasibility of strain elastography for evaluating the canine thyroid gland and the repeatability of dobutamine‐induced carotid artery pulsation as an internal compression method. In seven healthy Beagle dogs, strain on each thyroid lobe was induced by external compression using the ultrasound probe and internal compression using carotid artery pulsation after dobutamine infusion. The thyroid appeared homogeneously green and the subcutaneous fat superficial to the thyroid lobe appeared blue. Strain values and strain ratios did not differ among dogs or between the left and right lobes. Interobserver repeatability was excellent for both compression methods. Intraobserver repeatability of the strain ratio measured using the carotid artery pulsation method (intraclass coefficient correlation = 0.933) was higher than that measured using the external compression method (0.760). Mean strain values of thyroid lobes for the external compression method (142.93 ± 6.67) differed from the internal method (147.31 ± 8.24; P < 0.05). Strain ratios between the two methods did not differ. Strain elastography was feasible for estimating thyroid stiffness in dogs. Carotid artery pulsation induced by dobutamine infusion can be used for canine thyroid strain elastography with excellent repeatability.  相似文献   
58.
Evidence for ozone formation in human atherosclerotic arteries   总被引:1,自引:0,他引:1  
Here, we report evidence for the production of ozone in human disease. Signature products unique to cholesterol ozonolysis are present within atherosclerotic tissue at the time of carotid endarterectomy, suggesting that ozone production occurred during lesion development. Furthermore, advanced atherosclerotic plaques generate ozone when the leukocytes within the diseased arteries are activated in vitro. The steroids produced by cholesterol ozonolysis cause effects that are thought to be critical to the pathogenesis of atherosclerosis, including cytotoxicity, lipid-loading in macrophages, and deformation of the apolipoprotein B-100 secondary structure. We propose the trivial designation "atheronals" for this previously unrecognized class of steroids.  相似文献   
59.
Benign Theileria species of cattle are found in most parts of the world. The major piroplasm surface protein (MPSP), a conserved protein in all Theileria species, has been used as a maker for epidemiological and phylogenetical studies of benign Theileria species. Parasites with Ikeda- or Chitose-type MPSP genes are dominant in Japan, but we report here mixed infection cases of Theileria parasites with an additional MPSP type parasite infecting cattle in Abashiri District, Hokkaido. The MPSP gene sequence found in the additional type was closely related to MPSP genes of Theileria parasites found in Southeast Asian countries, including Thailand (Narathiwat) and Indonesia (Java). Theileria parasites from the blood sample were also distinguishable from the Ikeda or Chitose type parasites by the small subunit (SSU) rRNA gene sequence analysis, and they are grouped into the SSU rRNA types C/D found in Korea, North America, and Spain. The present finding of mixed infections of cattle with three different types of Theileria makes epidemiological feature of bovine theileriosis in Japan more complex. We have designed a set of primers specific to this MPSP type in order to conduct further epidemiological study.  相似文献   
60.
韩国位于中温带,年降雨量1300mm,平均温度10~20℃,这种特异气候条件适于野生蘑菇的大量生长。商业化生产中,侧耳菌(蠔蘑)占总产量的64.5%,为9000农民创造产值75680M/T。韩国目前有16种侧耳菌菌株适宜于农民种植。生产者同消费者一样倾向于喜爱灰色或深色的子实体,而不是白色的。用稻草培养的伞菌是种植的第二大类食用菌,占总产量的13.7%。乳色钮扣状蘑菇还有小规模的种植,用松树原木培养的香菇占总产量的3.2%。这些蘑菇在国内或世界市场鲜食、干食均可。瓶装锯末培养的金针菇、北风菌、叶苓及柱盖菌占总产量的17.2%。瓶装生产的优势是自动化、产量计划的可能性。除了这些食用蘑菇,医用真菌如灵芝、天麻、茯苓等其他经济真菌在韩国也有较大的生产规模。  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