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   5644篇
  免费   346篇
  国内免费   508篇
林业   472篇
农学   647篇
基础科学   313篇
  1061篇
综合类   1597篇
农作物   420篇
水产渔业   319篇
畜牧兽医   1140篇
园艺   190篇
植物保护   339篇
  2024年   19篇
  2023年   96篇
  2022年   219篇
  2021年   250篇
  2020年   228篇
  2019年   228篇
  2018年   163篇
  2017年   247篇
  2016年   184篇
  2015年   237篇
  2014年   224篇
  2013年   317篇
  2012年   352篇
  2011年   391篇
  2010年   285篇
  2009年   234篇
  2008年   263篇
  2007年   247篇
  2006年   251篇
  2005年   228篇
  2004年   158篇
  2003年   208篇
  2002年   243篇
  2001年   186篇
  2000年   182篇
  1999年   104篇
  1998年   85篇
  1997年   67篇
  1996年   61篇
  1995年   54篇
  1994年   36篇
  1993年   48篇
  1992年   37篇
  1991年   29篇
  1990年   36篇
  1989年   33篇
  1988年   21篇
  1987年   25篇
  1986年   21篇
  1985年   21篇
  1984年   13篇
  1983年   13篇
  1982年   12篇
  1981年   12篇
  1979年   9篇
  1971年   9篇
  1970年   8篇
  1965年   9篇
  1964年   14篇
  1963年   12篇
排序方式: 共有6498条查询结果,搜索用时 15 毫秒
31.
为探究施氮处理对不同筋型小麦的植株性状、籽粒产量和品质的影响,采用盆栽试验,选用强筋小麦品种中麦578(A1)和中麦5051(A2)、弱筋小麦品种扬麦15(A3)和扬麦24(A4)为供试品种,在施氮量相同的条件下进行底施(B1)和追施(B2)处理。结果表明:在其他栽培措施相同条件下,强筋小麦穗长、总小穗数、千粒重和籽粒产量均优于弱筋小麦,其中A1籽粒产量分别比A2、A3和A4高0.44%、49.81%和15.27%;施氮处理中B2的株高、穗长、穗粒数、总小穗数和籽粒产量均高于B1;不同处理组合中,强筋小麦品种A1B2的植株和产量性状优于其他处理;强筋小麦品种的籽粒蛋白质含量和蛋白质产量均高于弱筋小麦品种,且具有极显著差异(P<0.01)。本试验中强筋小麦品种中麦5051在氮肥追施处理中可以兼顾籽粒产量、蛋白质含量和蛋白质产量。  相似文献   
32.
Common wheat(Triticum aestiuum L.)is one of the most important crops because it provides about 20%of the total calories for humans.T.aestiuum is an excellent modern species for studying concerted evolution of sub-genomes in polyploid species,because of its large chromosome size and three well-known genome donors.Establishment of common wheat genome reference sequence and development of high-density SNP chips provide an excellent foundation to answer questions of wheat evolution and breeding at the genomic level.By genotyping more than 600 accessions of common wheat and their diploid and tetraploid ancestors using a Wheat660 K SNP array,we found dramatic genome changes due to tetraploidization and hexaploidization,in contrast to weaker influences of domestication and breeding on them.Further,since common wheat was introduced in China in 1500 BCE,Chinese landraces formed two subgroups(T.aestiuum-L1 and T.aestiuum-L2)with considerably diverse geographic distributions and agronomic traits.T.aestiuum-L2,mainly distributed in central and east China is found to have more but smaller oval grains with early maturity characteristics.We found that variation and selection in intergenic regions of the A and B sub-genomes dominated this differentiation,in which chromosomes 7 A and 3 B took the leading roles due to the existence of putative genes related to defense responses and environmental adaption in the highly differentiated regions.Large haplotype blocks were detected on 3 B(232.6-398.3 Mb)and 7 A(211.7-272.9 Mb)in the landraces,forming two distinct haplotypes,respectively.We discovered that artificial crosses in breeding promoted recombination in the whole genome,however,this recombination and differentiation was highly asymmetric among the three sub-genomes in homoeologous regions.In addition,we found that the wide use of European and northern American cultivars in breeding at early era,led dramatic changes in Chinese wheat genome,whereas,the recent breeding functioned to optimize it.This study will provide the insight for reconsideration of wheat evolution and breeding,and a new strategy for parent selection in breeding.  相似文献   
33.
WNT7A and β-catenin localisations and roles in regulating periimplantation ovine conceptus development under natural estrous conditions have been elaborated.However,their locations and expression patterns have not been reported under induction of oestrus.The localisation,expression and function of WNT7A and β-catenin in the uterine tissues of the early pregnant and non-pregnant sheep on days 10,12,14,16 and 18 following artificial induction of oestrus were investigated by means of in situ hybridisation,real-time RT-PCR,immuno-histochemistry and western blotting methods.WNT7A and β-catenin mRNA and protein were both restricted to the apical surfaces of the uterine luminal epithelium (LE) and glandular epithelium (GE).In pregnant sheep,protein localisation of WNT7A and β-catenin was observed both in the endometrial LE and GE.Their staining presented on day 10,increased between day 12 and day 16,and decreased on day 18.WNT7A and β-catenin mRNA and protein expression increased initially and then decreased from day 10 to day 18,peaking on day 16,and β-catenin reaching a peak on day 18 in the uterine tissues of pregnant sheep (p0.05).By contrast,no significant changes in WNT7A and β-catenin mRNA and protein expression levels were observed from day 10 to day 18 of the oestrus cycle in the uterine tissues of non-pregnant sheep (p0.05).Additionally,WNT7A and β-catenin mRNA and protein expression levels in the uterine tissues of the early pregnant sheep were significantly higher than those of non-pregnant sheep (p0.05).Treatment of endometrial epithelial cells with WNT7A increased the mRNA expressions of β-catenin,c-myc and Cyclin D1.These results provided an underlying mechanism of periimplantation ovine conceptus development under induction of oestrus.  相似文献   
34.
Landings in the blue crab, Portunus trituberculatus, fishery in Korean waters of the Yellow Sea have declined substantially from 11,000 t in the 1980s to 2,300 t in 2004. Blue crab habitat quality in the Yellow Sea has been degraded by anthropogenic activities including sand mining, land reclamation, and coastal pollution. Various traditional management measures have been implemented, including closed seasons during spawning and size limits, but these measures alone have been unsuccessful to conserve blue crab stocks. Consequently, a total allowable catch and a stock-rebuilding program using an ecosystem-based management approach were implemented in 2003 and 2006, respectively to rebuild blue crab stocks and restore habitats. This program involved assessment of both blue crab stock status and trammel-net fishery impacts at an ecosystem-level using an ecosystem-based fisheries assessment method ( [Zhang et al., 2009] and [Zhang et al., 2010]), which considered fishery data from catch and effort time-series, crab population biology, and ecosystem characteristics, including habitats and environmental conditions. Recent (2008) management status indices have shown significant positive change compared to conditions in 2000 with respect to sustainability of the stock and fishery and with regards to biodiversity and ecosystem habitat quality.  相似文献   
35.
36.
柴油机功率强化前后气缸盖的温度场模拟与试   总被引:6,自引:0,他引:6  
对功率强化前后的柴油机分别进行了性能仿真计算,进而确定了缸盖温度场计算的边界条件;利用有限元软件对气缸盖进行三维稳态热分析,并考虑了冷却水沸腾换热的影响;在该柴油机的单缸试验机上进行了稳态温度场测量试验,分析了试验和计算结果;得到了火力面边界条件的设置规律、稳态温度场随工况的变化关系和单缸机与多缸机的稳态温度场差别.  相似文献   
37.
矸石是在煤炭生产过程中产生的固体废弃物。我国是煤炭生产和消费的大国,全国每年矸石排放量约1.7亿t,已形成矸石山1500余座,压占大量土地,污染环境。文章在分析辽西北矿区矸石废弃地立地生态特征及研究该地区造林适用原则基础上,对造林关键技术进行研究分析,提出相应栽植技术要点,对该地区矸石废弃地植被恢复、改善生态环境具有实际指导意义。  相似文献   
38.
以黑龙江省30份主栽水稻品种或品系为试材,在孕穗期设置17℃冷水灌溉处理(20d),以常规栽培管理为对照,分析冷水胁迫对水稻干物质生产的影响。结果表明,冷水胁迫导致所有试材每穴实粒数、结实率、千粒重和产量下降,并根据相对结实率将试材耐冷性分为1、3、5、7和9级,以7和9级最多,1级最少,分别占总材料的30%和10%。在冷水处理下,全部材料抽穗期至成熟期干物质积累量及比例、群体生长率、净同化率、粒叶比、收获指数、叶面积指数、剑叶叶基角、剑叶和倒2叶与倒3叶披垂度均下降,但高效叶面积率增加。相关分析表明,冷水处理的相对结实率与收获指数(r=0.96**)、产量(r=0.91**)、粒叶比(r=0.84**)和干物质积累量(r=0.48**)的冷水反应指数(CRI)呈极显著正相关,与群体生长率(r=0.44*)、净同化率(r=0.44*)和干物质积累比例(r=0.43*)的CRI显著正相关。综上,孕穗期冷水胁迫对水稻干物质生产的影响在品种间存在很大差异,在冷水胁迫下耐冷性极强品种(系)干物质积累量及比例、群体生长率、净同化率、粒叶比和收获指数对冷水反应迟钝,这是耐冷性极强品种(系)保持较高产量的重要形态特征和生理原因。  相似文献   
39.
中加燕麦种质的遗传多样性和群体结构分析   总被引:2,自引:0,他引:2  
为揭示燕麦种质资源的遗传基础,采用内含子切接点引物和长随机引物的PCR分子标记技术,对加拿大和中国的64份燕麦种质的遗传多样性和群体结构进行了分析.结果表明,所用15条引物共扩增出171条带,平均每条引物扩增出11.4条带,其中多态性带共134务,多态性百分比为78.36%.燕麦各种质阃的Dice遗传相似系数分析表明,所有燕麦材料间的遗传相似系数变幅为0.699~0.934,平均值为0.81,表明参试燕麦种质间存在着一定的遗传差异,但遗传差异相对较小.群体结构分析结果表明,所研究的燕麦种质中81.25%的种质血缘相对比较单一,仅18.75%的种质拥有混合来源.  相似文献   
40.
文蛤染色体核型及三倍体诱导的初步研究   总被引:12,自引:1,他引:12  
常建波 《水产学报》1996,20(3):269-274
文蛤染色体核型及三倍体诱导的初步研究APRELIMINARYSTUDYONTHEKARYOTYPEANDTRIPLOIDINDUCTIONINMERETRIXMERETRIX常建波魏利平杨建敏宫向红孙逢贤(山东省海洋水产研究所,烟台264000)Ch...  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