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   883篇
  免费   45篇
  国内免费   2篇
林业   87篇
农学   15篇
基础科学   4篇
  306篇
综合类   151篇
农作物   24篇
水产渔业   56篇
畜牧兽医   199篇
园艺   25篇
植物保护   63篇
  2023年   6篇
  2022年   8篇
  2021年   18篇
  2020年   19篇
  2019年   20篇
  2018年   24篇
  2017年   33篇
  2016年   24篇
  2015年   19篇
  2014年   26篇
  2013年   37篇
  2012年   74篇
  2011年   71篇
  2010年   44篇
  2009年   42篇
  2008年   72篇
  2007年   75篇
  2006年   71篇
  2005年   44篇
  2004年   41篇
  2003年   41篇
  2002年   31篇
  2001年   13篇
  2000年   15篇
  1999年   7篇
  1998年   7篇
  1997年   8篇
  1996年   3篇
  1995年   11篇
  1994年   2篇
  1993年   2篇
  1992年   2篇
  1990年   4篇
  1989年   6篇
  1988年   1篇
  1986年   2篇
  1985年   1篇
  1983年   1篇
  1980年   2篇
  1979年   2篇
  1977年   1篇
排序方式: 共有930条查询结果,搜索用时 343 毫秒
81.
The biochemical profile [levels of calcium, phosphorus, magnesium, chlorides and iron, the activities of alkaline phosphatase (ALP), aspartate aminotransferase (AST) and alanine aminotransferase (ALT), and the concentrations of total protein, albumin, cholesterol, urea, glucose, and vitamins A and E] was studied in the blood serum of 40 anoestrous and 40 control inseminated animals in a production herd with an increased occurrence of anoestrus in gilts. The anoestrous gilts showed significantly lower levels of albumin (P less than 0.01) and glucose (P less than 0.01) and ALP activity (P less than 0.05), and significantly higher concentrations of urea (P less than 0.01), vitamin A (P less than 0.01) and vitamin E (P less than 0.05) and ALT activity (P less than 0.05), as compared with the inseminated controls. An extended enzymatological examination consisting of the evaluation of the activities of ALP, AST, ALT and gamma-glutamyl transferase (GMT) was performed in another set of 22 anoestrous and 20 mated gilts. The anoestrous gilts showed a statistically significant increase in the activities of AST (P less than 0.01), GMT (P less than 0.01) and ALT (P less than 0.05) and an insignificant increase in the activity of ALP in comparison with the control animals. The comparison of the obtained values of the studied biochemical criteria with literary data indicated a lower concentration of magnesium and a higher ALP and ALT activities in the anoestrous and inseminated gilts in both groups under study. A high acidity of fat and a medium to high fungus infestation (Mucor sp., Aspergillus sp.) were found by chemical and mycological examination of the administered feed mixtures. The histological examination of the ovaries of anoestrous animals showed cystically degenerative changes, proliferations of fibrous elements, and partial atrophy of ovarial cortex. It has been inferred from the observations that mycotoxins may be involved in the increase in the occurrence of anoestrus, either by a direct effect on sexual organs or by impairing the function of liver which, secondarily contributes to the rise of ovarial dysfunctions.  相似文献   
82.
83.
84.
An octaploid (Fragaria x ananassa cv. Calypso) genotype of strawberry was transformed with an antisense chalcone synthase (CHS) gene construct using a ripening related CHS cDNA from Fragaria x ananassa cv. Elsanta under the control of the constitutive CaMV 35S promoter via Agrobacterium tumefaciens. Out of 25 transgenic lines, nine lines showed a reduction in CHS mRNA accumulation of more than 50% as compared to the untransformed cv. Calypso control. The antisense CHS construct was found to be integrated into the genome, with a copy number ranging from one to four. The pigmentation of the fruit was only affected when less than 5% of the control CHS expression level was detected. A stable antisense phenotype over a period of 4 years was obtained in the primary transgenic lines at a rate of 1:20. As a consequence of the reduced activity of CHS, the levels of anthocyanins, flavonols, and proanthocyanidins were downregulated and precursors of the flavonoid pathway were shunted to the phenylpropanoid pathway leading to highly increased levels of cinnamoyl glucose (520% of control), caffeoyl glucose (816% of control), and feruloyl glucose (1092% of control) as well as p-coumaryl alcohol (363% of control) and p-coumaryl-1-acetate (1079% of control), which occur only as trace components in untransformed control fruits. These results demonstrate that the introduction of an antisense CHS construct in strawberry results in an unpredictable biochemical phenotype, thereby confirming that CHS function is an important regulatory point of substrate flow between the flavonoid and the phenylpropanoid pathways.  相似文献   
85.
The molecular organization of presynaptic active zones during calcium influx-triggered neurotransmitter release is the focus of intense investigation. The Drosophila coiled-coil domain protein Bruchpilot (BRP) was observed in donut-shaped structures centered at active zones of neuromuscular synapses by using subdiffraction resolution STED (stimulated emission depletion) fluorescence microscopy. At brp mutant active zones, electron-dense projections (T-bars) were entirely lost, Ca2+ channels were reduced in density, evoked vesicle release was depressed, and short-term plasticity was altered. BRP-like proteins seem to establish proximity between Ca2+ channels and vesicles to allow efficient transmitter release and patterned synaptic plasticity.  相似文献   
86.
During Cassini's close flyby of Enceladus on 14 July 2005, the High Rate Detector of the Cosmic Dust Analyzer registered micron-sized dust particles enveloping this satellite. The dust impact rate peaked about 1 minute before the closest approach of the spacecraft to the moon. This asymmetric signature is consistent with a locally enhanced dust production in the south polar region of Enceladus. Other Cassini experiments revealed evidence for geophysical activities near Enceladus' south pole: a high surface temperature and a release of water gas. Production or release of dust particles related to these processes may provide the dominant source of Saturn's E ring.  相似文献   
87.
To gain insight into the distribution of arabinoxylans (AX), endoxylanases, and endoxylanase inhibitors in industrial wheat roller milling, all streams, that is, 54 flour fractions, 4 bran fractions, and the germ, were analyzed for ash, starch, and protein contents, alpha-amylase activity levels, total (TOT-AX) and water-extractable arabinoxylan (WE-AX) contents, endoxylanase activity levels, and endoxylanase inhibitor (TAXI and XIP) contents. In general, bran fractions were significantly richer in TOT-AX and WE-AX contents, endoxylanase activity levels, and endoxylanase inhibitor contents than germ and, even more so, than flour fractions. In the 54 different flour fractions, minimal and maximal values for TOT-AX and WE-AX contents differed by ca. 2-fold, whereas they differed by ca. 15-fold for endoxylanase activity levels. The latter were positively correlated with ash and negatively correlated with starch content, suggesting that the endoxylanase activity in flour is strongly influenced by the level of bran contamination. TAXI contents in the flour fractions varied ca. 4-fold and were strongly correlated with bran-related parameters such as ash content and enzyme activity levels, whereas XIP contents varied ca. 3-fold and were not correlated with any of the parameters measured in this study. The results can be valuable in blending and optimizing wheat flour fractions to obtain flours with specific technological and nutritional benefits.  相似文献   
88.
Early reactivation of European rivers during the last deglaciation   总被引:1,自引:0,他引:1  
During the Last Glacial Maximum, the sea-level lowstand combined with the large extent of the Fennoscandian and British ice sheets led to the funneling of European continental runoff, resulting in the largest river system that ever drained the European continent. Here, we show an abrupt and early reactivation of the European hydrological cycle at the onset of the last deglaciation, leading to intense discharge of the Channel River into the Bay of Biscay. This freshwater influx, probably combined with inputs from proglacial or ice-dammed lakes, dramatically affected the hydrology of the region, both on land and in the ocean.  相似文献   
89.
90.
We observe second-harmonic generation from metamaterials composed of split-ring resonators excited at 1.5-micrometer wavelength. Much larger signals are detected when magnetic-dipole resonances are excited, as compared with purely electric-dipole resonances. The experiments are consistent with calculations based on the magnetic component of the Lorentz force exerted on metal electrons-an intrinsic second-harmonic generation mechanism that plays no role in natural materials. This unusual mechanism becomes relevant in our work as a result of the enhancement and the orientation of the local magnetic fields associated with the magnetic-dipole resonances of the split-ring resonators.  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