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   263篇
  免费   7篇
林业   25篇
农学   13篇
基础科学   1篇
  65篇
综合类   17篇
农作物   14篇
水产渔业   3篇
畜牧兽医   96篇
园艺   5篇
植物保护   31篇
  2023年   1篇
  2022年   1篇
  2021年   3篇
  2020年   1篇
  2019年   7篇
  2018年   6篇
  2017年   3篇
  2016年   5篇
  2015年   13篇
  2014年   11篇
  2013年   6篇
  2012年   12篇
  2011年   15篇
  2010年   9篇
  2009年   14篇
  2008年   12篇
  2007年   16篇
  2006年   17篇
  2005年   18篇
  2004年   21篇
  2003年   12篇
  2002年   11篇
  2001年   5篇
  2000年   2篇
  1999年   3篇
  1998年   4篇
  1997年   1篇
  1996年   2篇
  1995年   6篇
  1994年   2篇
  1993年   4篇
  1992年   1篇
  1990年   1篇
  1989年   1篇
  1988年   1篇
  1987年   1篇
  1985年   1篇
  1984年   1篇
  1981年   2篇
  1979年   2篇
  1978年   2篇
  1976年   3篇
  1975年   2篇
  1973年   1篇
  1972年   1篇
  1971年   1篇
  1970年   2篇
  1969年   2篇
  1955年   2篇
排序方式: 共有270条查询结果,搜索用时 15 毫秒
261.
An arable soil with organic matter formed from C3-vegetation was amended initially with maize cellulose (C4-cellulose) and sugarcane sucrose (C4-sucrose) in a 67-day laboratory incubation experiment with microcosms at 25 °C. The amount and isotopic composition (13C/12C) of soil organic C, CO2 evolved, microbial biomass C, and microbial residue C were determined to prove whether the formation of microbial residues depends on the quality of the added C source adjusted with NH4NO3 to the same C/N ratio of 15. In a subsequent step, C3-cellulose (3 mg C g−1 soil) was added without N to soil to determine whether the microbial residues formed initially from C4-substrate are preferentially decomposed to maintain the N-demand of the soil microbial community. At the end of the experiment, 23% of the two C4-substrates added was left in the soil, while 3% and 4% of the added C4-cellulose and C4-sucrose, respectively, were found in the microbial biomass. The addition of the two C4-substrates caused a significant 100% increase in C3-derived CO2 evolution during the 5-33 day incubation period. The addition of C3-cellulose caused a significant 50% increase in C4-derived CO2 evolution during the 38-67 day incubation period. The decrease in microbial biomass C4-C accounted for roughly 60% of this increase. Cellulose addition promoted microorganisms strongly able to recycle N immediately from their own tissue by “cryptic growth” instead of incorporating NO3 from the soil solution. The differences in quality of the microbial residues produced by C4-cellulose and C4-sucrose decomposing microorganisms are also reflected by the difference in the rates of CO2 evolution, but not in the rates of net N mineralization.  相似文献   
262.
The Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) imaging spectrometer observed the northern circumpolar regions of Mars at a resolution of a few kilometers. An extended region at 240 degrees E, 85 degrees N, with an area of 60 kilometers by 200 kilometers, exhibits absorptions at wavelengths of 1.45, 1.75, 1.94, 2.22, 2.26, and 2.48 micrometers. These signatures can be unambiguously attributed to calcium-rich sulfates, most likely gypsum. This region corresponds to the dark longitudinal dunes of Olympia Planitia. These observations reveal that water alteration played a major role in the formation of the constituting minerals of northern circumpolar terrains.  相似文献   
263.
The Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) investigation, on board the European Space Agency Mars Express mission, is mapping the surface composition of Mars at a 0.3- to 5-kilometer resolution by means of visible-near-infrared hyperspectral reflectance imagery. The data acquired during the first 9 months of the mission already reveal a diverse and complex surface mineralogy, offering key insights into the evolution of Mars. OMEGA has identified and mapped mafic iron-bearing silicates of both the northern and southern crust, localized concentrations of hydrated phyllosilicates and sulfates but no carbonates, and ices and frosts with a water-ice composition of the north polar perennial cap, as for the south cap, covered by a thin carbon dioxide-ice veneer.  相似文献   
264.
265.
266.
The effects of dietary soya bean trypsin inhibitor (SBTI) and of the daily injection of pure CCK or porcine intedstinal extract have been studied on the rat pancreas. The following parameters were measured: weight, DNA, RNA, trypsin, chymotrypsin, amylase, lipase, insulin and glucagon. The effects of dietary SBTI on the level of gastrin in the antrum have also been studied. Oral SBTI and i.p. CCK caused similar hypertrophy and hyperplasia but not the same activities of the enzymes. It is suggested that SBTI stimulates the secretion of at least two gastro-intestinal hormones: CCK and gastrin.  相似文献   
267.
Dietary intervention with anthocyanins may confer benefits in brain function, including vision. Research to date indicates that animals have only a limited capacity to absorb anthocyanins, compared to other types of flavonoids. Pigs, which are a suitable model for human digestive absorption, were used to examine the deposition of anthocyanins in tissues including the liver, eye, and brain tissue. Pigs were fed diets supplemented with 0, 1, 2, or 4% w/w blueberries ( Vaccinium corymbosum L. 'Jersey') for 4 weeks. Prior to euthanasia, pigs were fasted for 18-21 h. Although no anthocyanins were detected in the plasma or urine of the fasted animals, intact anthocyanins were detected in all tissues where they were sought. LC-MS/MS results are presented for the relative concentration of 11 intact anthocyanins in the liver, eye, cortex, and cerebellum. The results suggest that anthocyanins can accumulate in tissues, including tissues beyond the blood-brain barrier.  相似文献   
268.
OBJECTIVE: To determine whether injection of a mesenteric lymph node with iodinated aqueous contrast medium results in radiographic delineation of the thoracic duct and its branches, ascertain the ideal interval between injection and radiographic imaging, and evaluate mesenteric lymphadenography performed via laparoscopic and surgical approaches in dogs. ANIMALS: 10 adult dogs. PROCEDURE: In each dog, a right paracostal laparotomy or a right laparoscopic approach was performed to identify a mesenteric lymph node for injection of an iodinated aqueous contrast agent (0.22 mL/kg [81.4 mg of iodine/kg]). Lateral radiographic views were obtained at 60, 120, 180, 240, and 300 seconds after injection. RESULTS: A mesenteric lymph node was identified and injected with contrast medium in each dog. Via paracostal laparotomy, lymph node injection resulted in successful lymphangiographic evaluation in 4 of 5 dogs, whereas via the laparoscopic approach, lymph node injection resulted in successful lymphangio-graphic evaluation in 2 of 5 dogs. In successful radiographic evaluations, injected lymph nodes, mesenteric lymphatics, and the thoracic duct and its branches were delineated. Radiographs obtained at 60 and 120 seconds after injection of contrast medium provided the most detail. CONCLUSIONS AND CLINICAL RELEVANCE: Injection of a mesenteric lymph node directly with contrast medium appears to be a feasible technique for delineation of the thoracic duct and its branches in dogs and might be useful in small animals in which mesenteric lymphatic catheterization can be difficult and lymphangiography is more likely to fail. Refinement of the laparoscopic technique may provide a minimally invasive approach to lymphadenography.  相似文献   
269.
This study investigated the effect of inhaled fluticasone on lower airway inflammation and bronchial responsiveness (BR) to inhaled carbachol in cats with very mild, chronic bronchitis (n = 5) that were compared with healthy cats serving as controls (n = 6). Chest radiographs, BR tests performed non-invasively by barometric whole body plethysmography (BWBP) and bronchoalveolar lavage (BAL) were performed before and after treatment. BR was quantified by calculating the concentration of carbachol inducing bronchoconstriction (C-Penh300%), defined as a 300% increase of baseline Penh, an index of bronchoconstriction obtained by BWBP. BAL fluid was analyzed cytologically and the oxidant marker 8-iso-PGF2alpha was determined. At test 1, healthy cats and cats with bronchitis were untreated, whereas for test 2 inhalant fluticasone (250 microg once daily) was administrated for 2 consecutive weeks to cats with bronchitis. Control cats remained untreated. Inhaled fluticasone induced a significant increase in C-Penh300% and a significant decrease of BAL fluid total cells, macrophages, neutrophils and 8-iso-PGF2alpha in cats with bronchitis, whilst untreated control cats did not show significant changes over time. This study shows that a 2-week fluticasone treatment significantly reduced lower airway inflammation in very mild bronchitis. BR could be successfully monitored in cats using BWPB and decreased significantly in response to inhaled fluticasone. 8-Iso-PGF2alpha in BAL fluid was responsive to treatment and appeared as a sensitive biomarker of lower airway inflammation in cats.  相似文献   
270.
Studying molecular genetic relationships can substantially contribute to the understanding of the pathways of domestication of a species. Although an increasing number of molecular genetic studies have been performed on Lablab purpureus (hyacinth bean), many covered germplasm of restricted geographic origin or limited intra-specific systematic position. Integrating the molecular diversity found with phenotypic or morpho-agronomic diversity is also deficient. This investigation combines findings of eight molecular genetic studies that include about 400 accessions of both wild and cultivated germplasm, thus providing the largest assessment of diversity in Lablab purpureus to date. In particular, results from a recent molecular investigation (Robotham and Chapman 2015) are revisited and reinterpreted by integrating them with known phenotypic diversity. Wild accessions clearly fall into two types, with characteristic pods—2-seeded and 4-seeded. The large majority of cultivated types are more closely related to 4-seeded pod-types. Certain cultivated 2-seeded pod-type accessions from Ethiopia are genetically closer to wild 2-seeded pod-types. These two major phenotypes are reflected in two chloroplast DNA haplotypes A and B. Hence, two domestication events appear to exist in L. purpureus based on this combined data. No other geographic patterns of diversity, which might assist to trace the dispersal of L. purpureus, were found as cultivated accessions predominantly fell into 2-3 major groups. In all studies, the greatest genetic diversity was found in Africa, making Ethiopia one of the probable centers of domestication.  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