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   4590篇
  免费   246篇
  国内免费   2篇
林业   200篇
农学   234篇
基础科学   15篇
  816篇
综合类   367篇
农作物   665篇
水产渔业   401篇
畜牧兽医   1827篇
园艺   69篇
植物保护   244篇
  2023年   30篇
  2022年   43篇
  2021年   163篇
  2020年   98篇
  2019年   122篇
  2018年   145篇
  2017年   136篇
  2016年   150篇
  2015年   153篇
  2014年   209篇
  2013年   277篇
  2012年   328篇
  2011年   384篇
  2010年   223篇
  2009年   202篇
  2008年   295篇
  2007年   299篇
  2006年   264篇
  2005年   218篇
  2004年   214篇
  2003年   186篇
  2002年   140篇
  2001年   132篇
  2000年   95篇
  1999年   59篇
  1998年   33篇
  1997年   22篇
  1996年   14篇
  1995年   12篇
  1994年   3篇
  1993年   13篇
  1992年   19篇
  1991年   15篇
  1990年   27篇
  1989年   19篇
  1988年   10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4838条查询结果,搜索用时 15 毫秒
991.
992.
This study examined the distribution of water-extractable Al in soils from forest sites near Nagoya in Aichi Prefecture. The mean concentrations of water-extractable Al in surface soils varied markedly among sites, with the values at site FGB (126 mg/kg) higher than those at sites FG (11.8 mg/kg) and FCB (33.8 mg/kg). Comparisons of the acid buffer capacity of vegetation at the sites revealed that forest cover has a higher capacity for neutralization than the denuded state. We found that the effects of acidic accumulation in the surface soils of suburban forests might depend on vegetation history, and that forest soils that once had no vegetation cover accumulated labile Al due to acidification. The phenomenon was particularly distinct on granite bedrock.  相似文献   
993.
Despite its recent expansion in eastern US forests, red maple (Acer rubrum L.) generally exhibits a low leaf photosynthetic rate, leaf mass per unit area (LMA) and leaf nitrogen concentration ([N]) relative to co-occurring oaks (Quercus spp.). To evaluate these differences from the perspective of leaf energy investment, we compared leaf construction cost (CC) and leaf maintenance cost (MC) with leaf photosynthetic rate at saturating photon flux density and ambient CO2 partial pressure (Amax) in red maple and co-occurring red oak (Quercus rubra L.) and chestnut oak (Quercus prinus L.). We also examined relationships among leaf physiological, biochemical and structural characteristics of upper-canopy leaves of these three species at lower (wetter) and upper (drier) elevation sites of a watershed in the Black Rock Forest, Cornwall, NY, USA. Although A(max), leaf [N], leaf carbon concentration ([C]) and LMA were significantly less in red maple than in either oak species at both sites, CC per unit leaf area of red maple was 28.2 and 35.4% less than that of red oak at the lower and upper site, respectively, and 38.8 and 32% less than that of chestnut oak at the lower and upper site, respectively. Leaf MC per unit leaf area, which was positively associated with leaf CC (r2 = 0.95), was also significantly lower in red maple than in either oak species at both sites. When expressed per unit leaf area, A(max) was positively correlated with both CC (r2 = 0.65) and MC (r2 = 0.59). The cost/benefit ratio of CC/Amax of red maple was significantly less than that of chestnut oak at the lower site, however, CC/A(max) did not exhibit any significant interspecific differences at the upper site. Expressed per unit leaf area, CC was correlated positively with LMA (r2 = 0.90), leaf [N] (r2 = 0.97), and leaf [C] (r2 = 0.89), and negatively correlated with leaf molar carbon to nitrogen ratio (r2 = 0.92). Combined with red maple's general success in many oak-dominated forests, our findings suggest that reduced leaf-level photosynthetic capacity and related leaf characteristics in red maple are partially balanced by lower energy and resource requirements for leaf biomass construction and maintenance, which could enhance the competitive success of this species.  相似文献   
994.
Conifer needles exposed to ambient air pollutants were observed by a scanning electron microscope (SEM). Predominant changes on needles were the breakdown and the aggregation of wax structures on stomatal chambers. However, the mode of destruction in stomatal wax structures was dependent on the pollutants. Ginkgo biloba seemed to be resistant to air pollutants despite higher amounts of sulfur in the foliage.  相似文献   
995.
Fish muscle, which accounts for 15%–25% of the total protein in fish, is a desirable protein source. Their hydrolysate is in high demand nutritionally as a functional food and thus has high potential added value. The hydrolysate contains physiologically active amino acids and various essential nutrients, the contents of which depend on the source of protein, protease, hydrolysis method, hydrolysis conditions, and degree of hydrolysis. Therefore, it can be utilized for various industrial applications including use in nutraceuticals and pharmaceuticals to help improve the health of humans. This review discusses muscle protein hydrolysates generated from the muscles of various fish species, as well as their amino acid composition, and highlights their functional properties and bioactivity. In addition, the role of the amino acid profile in regulating the biological and physiological activities, nutrition, and bitter taste of hydrolysates is discussed.  相似文献   
996.
Near-infrared reflectance spectroscopy (NIRS) was used as a rapid and nondestructive method to determine the oil content and fatty acid composition in intact seeds of perilla [Perilla frutescens var. japonica (Hassk.) Hara] germplasms in Korea. A total of 397 samples (about 2 g of intact seeds) were scanned in the reflectance mode of a scanning monochromator, and the reference values for the oil content and fatty acid composition were measured by gravimetric method and gas-liquid chromatography, respectively. Calibration equations for oil and individual fatty acids were developed using modified partial least-squares regression with internal cross validation (n = 297). The equations for oil and oleic and linolenic acid had lower standard errors of cross-validation (SECV), higher R2 (coefficient of determination in calibration), and higher ratio of unexplained variance divided by variance (1-VR) values than those for palmitic, stearic, and linoleic acid. Prediction of an external validation set (n = 100) showed significant correlation between reference values and NIRS estimated values based on the standard error of prediction (SEP), r2 (coefficient of determination in prediction), and the ratio of standard deviation (SD) of reference data to SEP. The models for oil content and major fatty acids, oleic and linolenic acid, had relatively higher values of SD/SEP(C) and r2 (more than 3.0 and 0.9, respectively), thereby characterizing those equations as having good quantitative information, whereas those of palmitic, stearic, and linoleic acid had lower values (below 2.0 and 0.7, respectively), unsuitable for screening purposes. The results indicated that NIRS could be used to rapidly determine oil content and fatty acid composition (oleic and linolenic acid) in perilla seeds in the breeding programs for development of high-quality perilla oil.  相似文献   
997.
998.
999.
Plants derived through agricultural biotechnology, or genetically modified organisms (GMOs), may affect human health and ecological environment. A living GMO is also called a living modified organism (LMO). Biotech cotton is a GMO in food or feed and also an LMO in the environment. Recently, two varieties of biotech cotton, MON 15985 and MON 88913, were developed by Monsanto Co. The detection method is an essential element for the GMO labeling system or LMO management of biotech plants. In this paper, two primer pairs and probes were designed for specific amplification of 116 and 120 bp PCR products from MON 15985 and MON 88913, respectively, with no amplification from any other biotech cotton. Limits of detection of the qualitative method were all 0.05% for MON 15985 and MON 88913. The quantitative method was developed using a TaqMan real-time PCR. A synthetic plasmid, as a reference molecule, was constructed from a taxon-specific DNA sequence of cotton and two construct-specific DNA sequences of MON 15985 and MON 88913. The quantitative method was validated using six samples that contained levels of biotech cotton mixed with conventional cotton ranging from 0.1 to 10.0%. As a result, the biases from the true value and the relative deviations were all within the range of +/-20%. Limits of quantitation of the quantitative method were all 0.1%. Consequently, it is reported that the proposed detection methods were applicable for qualitative and quantitative analyses for biotech cotton MON 15985 and MON 88913.  相似文献   
1000.
Hapten syntheses and antibody generation for a new herbicide, metamifop   总被引:1,自引:0,他引:1  
To develop a competitive indirect enzyme-linked immunosorbent assay for metamifop, a new aryloxyphenoxypropionic acid herbicide, three structurally related haptens were synthesized. Hapten conjugates to keyhole limpet hemocyanin and bovine serum albumin were used as immunogens and plate-coating antigens, respectively. Various sets of polyclonal antibodies from rabbits and the coating antigens were screened for the assay in simple homologous and heterologous ELISA formats. A selected heterologous ELISA was optimized to show an average IC50 value as low as 20.1 ng/mL, detection ranges of 1.0-350 ng/mL, and a lowest detection limit of 0.1 ng/mL. The cross-reactivities of other aryloxyphenoxypropionic acid herbicides to the antibodies were less than 0.5% in the assays except fenoxaprop-P and fenoxaprop-P ethyl, having a diaryl ether group identical to that of metamifop. Molecular modeling studies revealed that the physicochemical properties of the diaryl ether group are the most important determinants of sensitivity and selectivity. The results strongly indicate that the selected set of ELISA is a highly sensitive and convenient tool for detecting metamifop.  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