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   32408篇
  免费   2035篇
  国内免费   3354篇
林业   2729篇
农学   2685篇
基础科学   1976篇
  3881篇
综合类   14175篇
农作物   2341篇
水产渔业   1632篇
畜牧兽医   4635篇
园艺   2150篇
植物保护   1593篇
  2024年   186篇
  2023年   564篇
  2022年   1257篇
  2021年   1413篇
  2020年   1336篇
  2019年   1292篇
  2018年   901篇
  2017年   1508篇
  2016年   1124篇
  2015年   1670篇
  2014年   1689篇
  2013年   1988篇
  2012年   2657篇
  2011年   2780篇
  2010年   2587篇
  2009年   2337篇
  2008年   2323篇
  2007年   2139篇
  2006年   1817篇
  2005年   1421篇
  2004年   903篇
  2003年   664篇
  2002年   799篇
  2001年   710篇
  2000年   585篇
  1999年   287篇
  1998年   166篇
  1997年   136篇
  1996年   76篇
  1995年   89篇
  1994年   98篇
  1993年   61篇
  1992年   53篇
  1991年   54篇
  1990年   29篇
  1989年   20篇
  1988年   22篇
  1987年   14篇
  1986年   5篇
  1985年   5篇
  1984年   4篇
  1981年   7篇
  1980年   2篇
  1979年   4篇
  1965年   1篇
  1962年   5篇
  1957年   1篇
  1956年   5篇
  1955年   1篇
  1932年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
AIM: To investigate the role of nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC) and protein kinase C (PKC) signaling in tumor necrosis factor-α (TNF-α)-induced cardioprotection against hypoxia/reoxygenation (H/R) injury. METHODS: Neonatal rat ventricular myocytes were pretreated with TNF-α or sodium nitroprusside (SNP) or L-arginine (L-Arg), respectively, for 12 h and then subjected to continuous hypoxia for 12 h, followed by reoxygenation for 6 h. The manganese superoxide dismutase (Mn-SOD) activity of the cells was measured after H/R. Myocyte injury was determined by the release of lactic dehydrogenase (LDH). RESULTS: TNF-α (105 U/L) significantly increased the Mn-SOD activity and decreased release of LDH from ventricular myocytes. The cardioprotection against H/R injury was induced by the pretreatment with SNP (5 μmol/L) or L-Arg (5 mmol/L), which was blocked by ODQ (10 μmol/L), the specific sGC inhibitor, and Chel (5 μmol/L), the specific PKC inhibitor. Pretreatment with L-NAME (100 μmol/L), ODQ, Chel, antoxidant 2-MPG (400 μmol/L) or tyrosine kinase inhibitor genistein (50 μmol/L) attenuated the increased Mn-SOD activity and reduced LDH level induced by TNF-α. CONCLUSION: The results suggest that NO may play a role in TNF-α-induced cardioprotection, which is mediated by sGC and PKC.  相似文献   
92.
AIM: To elucidate the mechanism of arrhythmia in healed myocardial infarction (HMI), and to investigate the changes of action potential duration (APD),transient outward potassium current (Ito), delayed rectifier potassium current (IK) and inward rectifier potassium current (IK1) of left ventricular myocytes in noninfarcted zone of HMI. METHODS: 12 rabbits were randomly assigned in two groups: HMI group (thoracotomy and ligation of the circumflex coronary); sham-operated group (thoracotomy but no conorary ligation). 3 months after operation, whole cell patch clamp technique was used to record APD, Ito, IK and IK1 of ventricular myocytes in non-infarcted zone. RESULTS: Membrane capacitance was larger in HMI group than that in sham-operated group. Action potential duration was lengthened significantly in HMI group and early after depolarization (EAD) appeared in HMI group. The densities of Ito, IK,tail and IK1 were reduced significantly in HMI group (P<0.01), from (6.72±0.42) pA/pF, (1.54±0.13) pA/pF and (25.6±2.6) pA/pF in Sham-operated group to (4.03±0.33) pA/pF, (1.14±0.11) pA/pF and (17.6±2.3) pA/pF, respectively. CONCLUSION: The reduced densities of Ito, IK,tail and IK1 in ventricular myocytes of non-infarcted zone in HMI are responsible for the prolongation of APD and the presentation of EAD, which play important roles in the malignant arrhythmia of HMI.  相似文献   
93.
中国北方沙尘天气原因探讨   总被引:2,自引:1,他引:2  
我国是沙尘暴易发的国家,进入20世纪90年代以来,强沙尘暴有频率增加、强度加大、范围扩展、危害程度加剧的趋势。沙尘暴的发生危害范围亦逐渐从西北干旱经济落后区扩展到经济发达的北京、天津及华北平原区,近年来的沙尘暴造成了社会经济和人民生命财产的重大损失。受大气环流场和季风气候类型影响,中国冬、春盛吹西北风,强劲而干燥。发生沙尘暴的天数与大风日数和地面热力稳定程度相关,3-5月中国西北内陆地表增温大,是风力场和热力场极不稳定的季节,地表气候容易发生蠕动,沙尘暴便由此启动并易发。除自然要素外,不合理的人类活动也将诱发沙尘暴天气的发生,有些情况下还起到比自然要素更重要的作用。从生态地理区域的角度看,沙尘暴的源地在干旱和半干旱地区,由于大气环流的作用,影响到下风向的半湿润甚至湿润地区。无论何地干旱、半干旱地区都将存在沙尘暴发生的自然条件,而其下风向必将受到不同程度的影响,只是因为中国的季风气候,决定西北干旱、半干旱地区发生的沙尘暴必然影响东南部地区。在现有的科学技术条件下,人类所能做的就是调节自身的行为。因此,在沙尘暴源地实施生态建设是减缓沙尘暴发生的最重要措施,也是消除东部城市沙尘天气的主要途径。根据生态地理区域的特征,沙尘暴源地的干旱区、半干?  相似文献   
94.
日本刀角瓢虫形态特征及生物学特性研究   总被引:3,自引:0,他引:3  
日本刀角瓢虫是粉虱类害虫的重要捕食性天敌之一、本文详细地描述了日本刀角瓢虫的形态特征,并附有各虫态的特征图。且对日本刀角瓢虫的生活习性进行了观察、以烟粉虱卵为食料,日本刀角瓢虫世代发育历期平均为22.56天,成虫平均寿命为9l94天,平均每雌产卵量为564.8粒:以烟粉虱若虫为食料,日本刀角虫世代发育历期平均为21.70天,成虫平均寿命为81.62天,平均每雌产卵量为650.0粒:瓢虫可捕食烟粉虱的卵、若虫、“蛹”、成虫各虫态,是烟粉虱的有效天敌,具有很好的保护利用价值。  相似文献   
95.
AIM: To explore a new method of hepatocyte growth factor (HGF) inducing bone marrow mesenchymal stem cells (MSC) to differentiate into cardiomyocytes. METHODS: Bone marrow MSC was cultured with DMEM media (10% fetal calf serum) 4-6 passages, and induced by HGF (10 μg/L) for 30 d. Automatical beating of the differentiated cells was observed daily with transverse microscopy, or under condition of 0.1% isoproterenol or cal-cium-deprived incubation. Specific cardiac myosin in the cells was indentified by immunochemistry. RESULTS: At 14-20 d of differentiation, bone marrow mesenchymal stem cells formed clones, in 10%-50% of which spontaneous beating cell-mass had come to continuously exist. Isoproterenol increased the beating rate and calcium-deprived media inhibited the beating. The cells were identified to be cardiomyocytes by expression of cardiac myosin heavy chain. CONCLUSION: HGF may induce bone marrow mesenchymal stem cells into cardiomyocytes with high efficiency, but the differentiating pathway of stem cells remains to be further studied.  相似文献   
96.
秦巴山区养羊户适度经营规模的研究   总被引:1,自引:0,他引:1  
通过对秦巴山区陕西省西乡县15个典型陕南白山羊养殖专业户的资金投入、羊群结构、草地资源、经济效益等情况的调查,分析了养羊户的资源利用和生产经营状况,确定了与山羊生产密切相关的12个变量,即种公羊数、基础母羊数、后备母羊数、育肥羊数、羊舍面积、草地补种及施肥费用、医杂费、氨化或青贮饲料量、补饲精料量、工作日、投资额,并确定了各变量的参数值,以利润最大为目标函数,然后运用线性规划模型,计算出不同投资、不同人力及草地资源等条件下的适度经营规模。结果表明,养羊户的投资额是决定适度规模的主要因素。根据投资和羊群基础母羊数量设计的三种典型经营条件下,基础母羊全部留种的经营方式可获最大投资收益率(140.1%);基础母羊数量达到20只以上,自繁种公羊的经营方式的最大投资收益率为74.0%;资金有限,人工授精配种的经营方式的投资收益率最低,为73.1%。  相似文献   
97.
采用22种限制性内切酶分析了三江白猪线粒体DNA的多态性。结果表明:在60头个体中检出30种限制性态型.归结为7种基因单倍型;其间的差异来自少数限制性位点的突变。单倍型间平均遗传距离为0.35%,群体遗传多态程度为0.044%。三江白猪遗传多样性非常贫乏,说明三江白猪与地方种猪起源于一个共同祖先,在品种形成早期受到创立者效应的制约。  相似文献   
98.
Captive juvenile African black-footed penguins (Spheniscus demersus) housed in an outdoor enclosure at the Baltimore Zoo have an average 50% mortality from avian malarial (Plasmodium sp.) infection each year without intense monitoring for disease and chemotherapeutic intervention. During the 1996 malaria transmission season, the safety and efficacy of an anti-circumsporozoite (CSP) DNA vaccine encoding the Plasmodium gallinaceum CSP protein against P. relictum were studied. The goal was to reduce clinical disease and death without initiating sterile immunity after release into an area with stable, endemic avian malaria. The birds were monitored for adverse clinical signs associated with vaccination, the stimulation of an anti-CSP antibody response, and protection afforded by the vaccine. The presence of P. relictum in trapped culicine mosquitoes within the penguin enclosure was monitored to assess parasite pressure. Among the vaccinated penguins, the parasitemia rate dropped from approximately 50% to approximately 17% despite intense parasite pressure, as determined by mosquito infection rate. During the year of the vaccine trial, no mortalities due to malaria occurred and no undesirable vaccination side effects occurred. This is the first trial of an antimalarial vaccine in a captive penguin colony.  相似文献   
99.
对捕食线虫性真菌——少孢节丛孢菌A1分离株(Arthrobotrys oligospora A1)的18S rDNA基因序列进行了研究。结果表明:其18S rDNA全基因序列为1769bp,在进化树上与同种国外分离株A.oligospora var oligospora 1最为接近,同源性为94.7%。这与二者都具有捕食性结构—菌网的特点相一致。证实捕食线虫性真菌的捕食性结构与捕食线虫性真菌种系发生有关。从少孢节丛孢菌3个分离菌株(Arthrobotrys oligospora A1、A.oligospora 1、A.oligospora2)的进化树及同源性来看。不同国家地区的丛孢菌属(Arthrobotrys)分离株确实存在差异。  相似文献   
100.
蚕粪中提取叶绿素铜钠盐的稳定性研究   总被引:10,自引:3,他引:10  
以蚕粪为原料,95%的乙醇为溶剂提取叶绿素,并通过化学法制得叶绿素的铜纳盐。采用721分光光度计对叶绿素铜钠盐进行定性、定量测定,确定该色素测定系统中的最大波长为414nm。通过测量吸光值研究了几种常用食品添加剂对叶绿素铜钠盐稳定性的影响,并对其耐光、耐热、耐酸碱及耐氧化性、耐还原性进行探讨。结果表明:叶绿素铜钠盐在70℃以下稳定性较好;光照对叶绿素铜钠盐稳定性的影响较大;还原剂(Na2S2O3)对叶绿素铜钠盐稳定性的影响较大,氧化剂在短时间内对其稳定性的影响不大,但放置的时间不宜超过30min;叶绿素铜钠盐在pH值为6以上的溶液中较为稳定,且有机酸及单糖对其稳定性无明显影响。  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