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   6369篇
  免费   4890篇
  国内免费   639篇
林业   81篇
农学   685篇
基础科学   81篇
  1429篇
综合类   3588篇
农作物   386篇
水产渔业   10篇
畜牧兽医   2810篇
园艺   341篇
植物保护   2487篇
  2025年   430篇
  2024年   379篇
  2023年   385篇
  2022年   775篇
  2021年   764篇
  2020年   677篇
  2019年   685篇
  2018年   485篇
  2017年   393篇
  2016年   566篇
  2015年   599篇
  2014年   751篇
  2013年   854篇
  2012年   1179篇
  2011年   1054篇
  2010年   810篇
  2009年   682篇
  2008年   206篇
  2007年   40篇
  2006年   20篇
  2005年   19篇
  2004年   8篇
  2003年   5篇
  2002年   6篇
  2001年   13篇
  2000年   5篇
  1999年   11篇
  1998年   8篇
  1997年   6篇
  1996年   5篇
  1995年   5篇
  1994年   4篇
  1993年   1篇
  1992年   1篇
  1991年   1篇
  1988年   5篇
  1987年   10篇
  1986年   14篇
  1983年   1篇
  1982年   2篇
  1981年   2篇
  1978年   1篇
  1976年   1篇
  1974年   1篇
  1971年   1篇
  1963年   1篇
  1962年   6篇
  1956年   18篇
  1955年   3篇
排序方式: 共有10000条查询结果,搜索用时 20 毫秒
1.
2.
American sloughgrass (Beckmannia syzigachne (Steud.) Fernald) is one of the most competitive and malignant weeds in rice-wheat rotation fields in China. American sloughgrass populations in the Jiangsu Province of China became less sensitive to acetohydroxyacid synthase (AHAS) inhibitors after repeated application for many years in these areas. Two suspected resistant American sloughgrass populations (R1 and R2) collected in the field were detected the resistance to inhibitors of AHAS in whole-plant dose-response assays, compared to the susceptible (S) population. These assays indicated that R1 showed low resistance to mesosulfuron-methyl (3.32-fold), imazapic (2.84-fold) and pyroxsulam (1.55-fold), moderate resistance to flazasulfuron (4.67-fold) and pyribenzoxim (7.41-fold), and high resistance to flucarbazone (11.73-fold). However, using a combination of the cytochrome P450 inhibitor, malathion, with mesosulfuron-methyl resulted in a reduction in R1 resistance relative to mesosulfuron-methyl alone. Furthermore, R2 was highly resistant to flazasulfuron (34.90-fold), imazapic (11.30-fold), flucarbazone (49.20-fold), pyribenzoxim (12.94-fold), moderately resistant to mesosulfuron-methyl (9.77-fold) and pyroxsulam (6.26-fold), and malathion had no effect on R2 resistance to mesosulfuron-methyl. The full-length of AHAS genes was sequenced and the AHAS enzymes were assayed in vitro in order to clarify the mechanism of resistance to AHAS inhibitors in R1 and R2 populations. The results demonstrated that R2 had a Pro-197-Ser mutation in the AHAS gene, and the sensitivity of R2 to the five AHAS inhibitors was decreased, which may result in R2 resistance to AHAS inhibitors. There was no mutation in the AHAS gene of R1, and there were no significant differences in enzyme sensitivity between susceptible (S) and resistant (R1) populations. An enhanced metabolism may be the main mechanism of R1 resistance to AHAS inhibitors.  相似文献   
3.
为研究滇西北剑湖沉积物钙镁空间分布特征和释放风险,采用微波辅助消解法对剑湖沉积物样品进行前处理,用电感耦合等离子体发射光谱仪(ICP−OES)测定其钙(Ca)、镁(Mg)含量,并结合克里金空间插值和磷释放贡献计算等方法对其空间分布和释放风险进行深入研究。结果表明:剑湖表层沉积物Ca、Mg平均含量分别为18.91、8.20 g/kg,在湿地恢复区含量均较低,且水平空间变异系数均大于10%。剑湖柱状沉积物Ca、Mg含量分别为2.03~42.55 g/kg和1.30~73.60 g/kg,表层(0~5 cm)沉积物Ca、Mg含量均高于底层(70~75 cm);同时,Ca、Mg快释放贡献率平均值分别为15.84%、3.71%,慢释放贡献率平均值分别为20.93%、4.20%,因此剑湖沉积物中Ca、Mg在短时间(25 a)和长时间(150 a)尺度上均处于释放状态。  相似文献   
4.
To understand the genetic diversity and indica-japonica differentiation in Bangladesh rice varieties, a total of 151 accessions of rice varieties mostly Bangladesh traditional varieties including Aus, Boro, broadcast Aman, transplant Aman and Rayada varietal groups were genotyped using 47 rice nuclear SSRs. As a result, three distinct groups were detected by cluster analysis, corresponding to indica, Aus and japonica rice. Among deepwater rice varieties analyzed some having particular morphological features that mainly corresponded to the japonica varietal group. Some small seeded and aromatic varieties from Bangladesh also corresponded to the japonica varietal group. This research for the first time establishes that the japonica varietal group is a prominent component of traditional varieties in Bangladesh, particularly in deepwater areas.  相似文献   
5.
核线粒体假基因(NUMT)是线粒体DNA插入到核基因组中的DNA片段。目前,已经发现越来越多的真核生物基因组存在线粒体假基因现象。猪线粒体假基因在核基因组中的分布尚未见报道。研究通过猪线粒体基因组全序列与核基因组全序列进行比对,由BLAST程序鉴别出132个NUMT。结果表明:这些NUMT的大小在37~4 453bp,其中90%的NUMT长度处于40~1 000 bp的范围内,NUMT与相应的线粒体DNA片段之间的同源性数值范围为66%~100%。此外,鉴定出的NUMT序列几乎涵盖了包括线粒体DNA控制区在内的全部线粒体基因组,包括30个完整的线粒体基因,分布于猪的1、4、5、7、11、13、14、15、17号染色体上,近50%的NUMT定位在14号染色体上。  相似文献   
6.
阿魏酸酯酶可通过打断阿魏酸和二聚阿魏酸等酚酸类木质素成分通过酯键与植物细胞中半纤维素分子形成的致密网状交联结构,进而促进栖居在草食动物胃肠道中微生物对所进食饲料细胞壁的降解效率。作者在分析和归纳总结了产阿魏酸酯酶菌株来源、筛选方法及不同表达体系的基础上,利用生物信息学的方法,对酯酶蛋白的拓扑空间结构、系统进化及作用机理方面取得的最新研究进展进行了阐述。  相似文献   
7.
    
Brackish water (7 dS m−1) is frequently utilized to drip-irrigate crops in the Negev desert of Israel, the practice being to use deep sandy soils (96% sand) to avoid soil salinization. When muskmelon (Cucumis melo L.), a moderately salt-sensitive crop species, was grown using brackish irrigation under these conditions, yields declined due to a significant reduction in fruit size, but fruit quality parameters improved markedly. In the present study, we tested the hypothesis that the use of fresh irrigation water during the early vegetative phase would increase canopy size and leaf area index (LAI) and hence the potential productivity of the melon plant. The application of brackish water during the reproductive phase, on the other hand, would improve fruit quality. Using multiple irrigations within a 24-h period, applied with drip irrigation, we examined the timing, the duration, and the concentration of brackish irrigation water as tools to optimize fruit yield and quality in late-summer melons. Indeed, the combination of fresh (1.2 dS m−1) and brackish (7 dS m−1) irrigation water increased the yield level to that of fresh water plants whereas it brought about the improvement of fruit quality typical to brackish water plants, thus providing an attractive approach to optimize late-summer melon production. Our results demonstrate the trade-off between fruit size and fruit quality as related to the timing and the duration of brackish irrigation water. The use of a milder (<4.5 dS m−1) salinity level of irrigation water from plant emergence until harvest may be considered as well.  相似文献   
8.
    
The offspring of super-multiparous sows face problems such as decreased growth performance, poor meat quality and even diseases in animal husbandry. Gama-aminobutyric acid(GABA) has long been known to promote growth and suppress inflammation, but little is known about the mechanisms. A total of 72 growing-finishing pigs from the 8th generation were randomly allotted to 2 groups with 6 replicates per treatment to receive a corn-soybean basal diet or the basal diet supplemented 20 mg/kg GABA for 6...  相似文献   
9.
Dietary supplementation with high lev- els of zinc oxide (ZnO) is an effective method to prevent diarrhea and promote growth in weaned pig- lets. However, the mechanisms behind these beneficial effects are not well understood. Recent advances in this field indicated that the effects of ZnO on piglet growth are achieved through multiple regulatory path- ways rather than a single pathway. This review focu- ses on recent advances in the effects of high level ZnO on: 1 ) Zn status in weaned piglets; 2 ) intestinal microbe flora regulation; 3 ) intestinal structure and function;4) ion secretion in the intestine epithelium;5) intestinal immunology; and 6) secretion of brain-in- testine peptides. ZnO appears to regulate the intestinal microflora and decreases the release of histamine to prevent diarrhea. ZnO also increases the expression of intestinal insulin-like growth factor-1 ( IGF-1 ) and the IGF-1 receptor to ameliorate intestinal injury associat- ed with weaning, and mediates secretion of brain-gut peptides to stimulate food intake, to promote the growth of piglets. Better understanding of the underly-1 ing mechanisms should inspire us to supplement the diet for weaned piglets with ZnO and investigate new approaches to prevent diarrhea and enhance growth.  相似文献   
10.
    
Among different inbred chickens’ lines, we previously showed that lines P and N of Institute for Animal Health, Compton, UK are the most susceptible and the least affected lines, respectively, following infection with very virulent infectious bursal disease virus (vvIBDV). In this study, the differential expressions of 29 different immune-related genes were characterized. Although, birds from both lines succumbed to infection, line P showed greater bursal lesion scores and higher viral copy numbers compared to line N. Interestingly, line N showed greater down-regulation of B cell related genes (BLNK, TNFSF13B and CD72) compared to line P. While up-regulation of T-cell related genes (CD86 and CTLA4) and Th1 associated cytokines (IFNG, IL2, IL12A and IL15) were documented in both lines, the expression levels of these genes were different in the two lines. Meanwhile, the expression of IFN-related genes IFNB, STAT1, and IRF10, but not IRF5, were up-regulated in both lines. The expression of pro-inflammatory cytokines (IL1B, IL6, IL18, and IL17) and chemokines (CXCLi2, CCL4, CCL5 and CCR5) were up-regulated in both lines with greater increase documented in line P compared to line N. Strikingly, the expression of IL12B was detected only in line P whilst the expression of IL15RA was detected only in line N. In conclusion, the bursal immunopathology of IBDV correlates more with expression of proinflammatory response related genes and does not related to expression of B-cell related genes.  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