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   17059篇
  免费   1877篇
  国内免费   2211篇
  2024年   44篇
  2023年   217篇
  2022年   540篇
  2021年   892篇
  2020年   732篇
  2019年   846篇
  2018年   856篇
  2017年   659篇
  2016年   780篇
  2015年   1102篇
  2014年   1288篇
  2013年   1338篇
  2012年   1549篇
  2011年   1367篇
  2010年   963篇
  2009年   794篇
  2008年   954篇
  2007年   831篇
  2006年   751篇
  2005年   637篇
  2004年   623篇
  2003年   605篇
  2002年   595篇
  2001年   392篇
  2000年   293篇
  1999年   285篇
  1998年   175篇
  1997年   171篇
  1996年   143篇
  1995年   132篇
  1994年   110篇
  1993年   82篇
  1992年   70篇
  1991年   57篇
  1990年   49篇
  1989年   47篇
  1988年   31篇
  1987年   27篇
  1986年   18篇
  1985年   12篇
  1984年   14篇
  1983年   15篇
  1982年   18篇
  1981年   12篇
  1980年   6篇
  1979年   8篇
  1978年   3篇
  1977年   4篇
  1974年   2篇
  1972年   2篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
91.
Prohibited pesticide residues have become one of the main factors affecting the quality and safety of Lycii Fructus, However, rarely studies focus on the rapid determination of these residues. Here, a total of 30 kinds of prohibited pesticide residues were determined by ultra-performance liquid chromatography tandem triple quadrupole mass spectrometry (UPLC-MS/MS) in five different process ways. Pretreatment methods, chromatographic separation and detection conditions in mass spectrometry were all optimized accordingly. Among the five different pretreatment methods, the first and third solid phase extraction failed to provide high recoveries of sulfosulfuron compounds (both lower than 60%). Recovery of chlorphenamidine by the Quick Easy Cheap Effective Rugged and Safe multiresidue method (QuEChERS) was lower than 60%, which did not meet the requirements of trace determination. The concentrations of 30 prohibited pesticides residues treated by straightforward and solid phase extraction showed good linearity in their corresponding ranges, with correlation coefficients over 0.99. The average recoveries of straightforward ranged from 78.13% to 110.9%, while RSD ranged from 1.3% to 16.9%, albeit poor purification was observed. The recovery yield from solid phase extraction was between 67.75% and 103.08% with RSD value from 0.8% to 14.0%, which met the requirements of trace determination, this method has good precision and stability. These results could be employed to other Traditional Chinese Medicines (TCMs) in detecting prohibited pesticide residues.  相似文献   
92.
Grape pistil has an important influence on fruit size and quality. However, there were few studies on grape ovary, and the development process of the ovary is still unclear. Therefore, in this paper, four different grape varieties with different lengths of small inflorescences, namely ‘Musct Hambourg’ grape (Vitis vinifera), ‘Concord’ grape (Vitis labrusca), ‘ShanPuTao’ grape (Vitis amurensis) and ‘GongNiang2Hao’ grape (Vitis amurensis × Vitis vinifera) were used as test materials. Four varieties ovary were significant differences by means of stereomicroscope, paraffin section. The expression of ovary determining gene VvAGAMOUS (VvAG) and its development related genes VvCRABS CLAW (VvCRC) andVvAGAMOUS-LIKE 11 (VvAGL11) with similar functions during the development of different grape varieties were preliminarily explored using fluorescence quantitative test. The relationship between VvAG and VvCRC, VvAG and VvAGL11 were analyzed using Y1H assay. Our results showed that there were obvious abdominal sutures on the surface of expect for ‘Musct Hambourg’ grape, and existing poly carpels. The ovary development of ‘ShanPuTao’ and ‘GongNiang2Hao’ grape was completed when the inflorescence length was less than 1 cm, while the ‘Concord’ and ‘Musct Hambourg’ grape were fully developed when the length of inflorescence was 3–4 and 4–5 cm, respectively. VvAG and VvCRC began to express in large quantities after the formation of stamen primordia, while VvAGL11 during the forming of ovule primordia. Therefore, VvAG and VvCRC mainly regulated the development of stamens and carpels and also promote the development of ovules, while VvAGL11 major regulated the development of ovules. The promoters of VvCRC and VvAGL11 were bound by VvAG. This study provides an important theoretical basis for further research on the molecular mechanism of grape ovary development.  相似文献   
93.
Yu Zai  Shaohua Liu 《Phyton》2023,92(1):225-236
Kosteletzkya virginica (K. virginica) is used for revegetation of salt-affected coastal tidal flats and as a raw material of biodiesel. K. virginica root tuber, a biowaste with low economic value, is rich in bioactive compounds. This study aimed to extract and identify flavonoids from K. virginica root tubers. The optimal extraction conditions were 1/25 (w/v) solid/liquid ratio, 40% ethanol concentration at 40°C for 60 min. Under these conditions, 65.2 ± 3.7 mg/g total flavonoid content was extracted from the roots, which were collected from salinized soil in late autumn of the third year. Antioxidant activity was evaluated through 1,1-diphenyl-2-picrylhydrazyl, hydroxyl radical, and superoxide anion scavenging assays. The extracted flavonoids exhibited antioxidant activity in a dose-dependent manner. Five flavonoids, glucoliquiritin apioside, licoisoflavone B, 5-methoxy-7,8-diprenyl- flavone, 7,2′-dihydroxy-6,8-dimethyl-4′,5′-methylenedioxyflavan, and 5,7,4′-trihydroxy-3′-methoxy-6,8-di-Cmethylflavanone, were identified by ultra-performance liquid chromatography–tandem mass spectrometry. Our results suggest that the flavonoids of K. virginica root tubers might be potent antioxidants and can be effectively applied as an ingredient in food and natural medicine.  相似文献   
94.
Wang  Zunxin  Wang  Xianyu  Liu  Siqin  Yang  Ying  Li  Yang  Chen  Siyuan  Wang  Guangpeng  Zhang  Xincheng  Ye  Yuxiu  Hu  Laibao  Zhou  Qing  Wang  Feibing  Chen  Xinhong 《Journal of Plant Growth Regulation》2023,42(1):294-303
Journal of Plant Growth Regulation - Zinc is an important micronutrient for the growth and development of human body and plants. Proper use of nitrogen fertilizer and foliar application of Zn have...  相似文献   
95.
The localization of mRNA encoding preproatrial natriuretic peptide (ANP) was investigated in cultured human umbilical vein endothelial cells (HUVEC) and tissue preparations of umbilical vein and artery. The techniques used were in situ hybridization and in situ hybridization combined with immunocytochemistry, using 32P-radiolabelled and non-radioactive digoxigenin labelled complementary RNA probes. Human ANP mRNAs are mainly localized in the endothelial cells of the umbilical vein and, to a lesser extent, in the endothelial cells of the umbilical artery. The autoradiographic labelling and the intensity of digoxigenin staining were significantly reduced by treatment with RNase before in situ hybridization. This study provides unequivocal evidence for the expression of the ANP gene in the endothelial cells of human umbilical vessels, confirming that these endothelial cells have the ability to synthesize this peptide. The functional significance of the presence of the ANP mRNA in the endothelial cells of human umbilical vessels is discussed.  相似文献   
96.
Gui  Runfei  Mo  Zhaowen  Zeng  Shan  Wen  Zhiqiang  Long  Weisi 《Journal of Plant Growth Regulation》2023,42(3):1604-1613

Compared with the standard method of manual fertilizer broadcasting (MFB), mechanized hill-drilling direct-seeding with deep application of slow-release nitrogen fertilizer (MHDDF) is an efficient method to integrate both fertilization and seeding. However, there are few studies that combine the use of slow-release fertilizer with MHDDF. We sought to explore the combined effect of MHDDF with slow-release fertilizer on rice yield and nitrogen, phosphorus, and potassium utilization, compared to MFB. We compared three different MHDDF methods (D30: 450 kg ha?1, D40: 600 kg ha?1, D50: 750 kg ha?1), with one MFB method (B50: 750 kg ha?1), and one control (CK: 0 kg ha?1). We found that the yield of all MHDDF method was higher than that of both the MFB method. Yield was the highest in the D50 treatment and was 14.14–46.03% higher than that in B50 treatment. Biomass accumulation, nutrient accumulation, and nutrient use efficiency were similarly higher in MHDDF method than both MFB and CK. Compared to B50, the D50 treatment increased nitrogen recovery efficiency by 170.53–231.50%, phosphorus recovery efficiency by 480.00–724.25%, and potassium recovery efficiency by 201.55–169.59%. Overall, we found that combining MHDDF with slow-release fertilizer was an effective method to increase rice yield and nutrient use efficiency compared with MFB.

  相似文献   
97.
儿童最大有氧活动能力的发展特征   总被引:4,自引:1,他引:3  
本文报告了我国463名10-19岁儿童青少年的最大有氧活动能力的发展特征。在青春早期,男女童的最大吸氧量绝对值均随年龄增长而增加,男童由1.75升/分增至3.10升/分,女童由1.44升/分增至2.07升/分,女童增长较少;以后女童即稳定于这一水平,男童仍略有增长。按身高及按最大心率计标的相对值与其有相似的特征。按体重和瘦体重计算的相对值,在男女童都未见随年龄增长的规律。男童VO2max绝对值及各  相似文献   
98.
建立了一个探讨灵长类视皮层从V1区到MT区的运动信息加工原理的计算模型,这个过程的突出特征是视觉运动信息经过了从局部检测进步到整体感知。模型的第一层由用于抽提运动模式的局部速度以及结构性质的Reichardt运动检测器组成,进一步的加工是通过Boltzmann Machine神经网络来实现的。这种网络的学习算法具有局部更新的显著性质,在学习阶段,网络不断地修改联结权重以形成对于记录在网络的显单元上  相似文献   
99.
单祥年  刘季和 《病毒学报》1993,9(4):345-351
从手术切除的24例女性和12例男性尖锐湿疣新鲜标本中,以及42例女性尖锐湿疣、16例男性外耳道乳头状瘤和4例女性假性湿疣的石蜡包埋标本中,提取组织的基因组DNA,用人工合成的人乳头瘤病毒6.11和16型E6区特异性寡聚核苷酸引物,通过PCR进行HPV DNA的分型检测。结果66例女性尖锐湿疣中,感染HPV6型者4例,感染11型者12例,6+11型混合感染者49例;阴性1例,总检出率达98.4%。4例女性假性湿疣中1例为HPV6型感染,阳性率25%。16例男性外耳道乳头状瘤中HPV6+11型感染者5例,6+16型感染者3例,6+11+16型多重感染者8例,阳性率100%。12例男性尖锐湿疣中,HPV11型感染者7例,6+11型4例,阴性1例,总阳性率91.6%。还对细胞学上空泡化和非典型空泡化尖锐湿疣标本的HPV感染做了比较,未发现差异。  相似文献   
100.
Beta-葡聚糖是由β-(1,3)和β-(1,4)糖苷键连接的非纤维素多糖,主要分布在谷类作物籽粒胚乳及糊粉层中,在高尔基体合成,经由囊泡运输到质膜,最终在细胞壁上沉积。通过增加胆汁酸排泄,延迟葡萄糖吸收,β-葡聚糖可有效降低胆固醇及血糖水平。Beta-葡聚糖合成酶基因家族成员最早在水稻(Oryza sativa)中得到鉴定,后在其他作物中陆续被发现。该基因家族包括3个主要成员:CslF、CslH和CslJ亚基因家族,起源于不同分支,经过趋同演化,执行合成β-葡聚糖的功能。Beta-葡聚糖基因家族成员均受到负选择压力,演化过程中序列高度保守。CslF亚家族基因成员相对较多,常在染色体上形成基因簇,CslF6是介导β-葡聚糖合成的主效基因。CslF亚家族在叶基部等幼嫩组织中表达水平相对较高,且明显受到光照强度的影响;CslH和CslJ亚家族成员较少,其中CslH亚家族在叶尖等成熟组织中的表达水平高,而CslJ亚家族在籽粒中有较高的表达水平。该文综述了β-葡聚糖合成酶基因家族成员的系统发育关系、表达模式,β-葡聚糖合成酶的亚细胞定位,以及作物中的定向育种研究进展,提出β-葡聚糖合成酶基因家...  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