全文获取类型
收费全文 | 18320篇 |
免费 | 1543篇 |
国内免费 | 1640篇 |
专业分类
21503篇 |
出版年
2024年 | 65篇 |
2023年 | 317篇 |
2022年 | 676篇 |
2021年 | 1136篇 |
2020年 | 755篇 |
2019年 | 845篇 |
2018年 | 840篇 |
2017年 | 617篇 |
2016年 | 826篇 |
2015年 | 1159篇 |
2014年 | 1409篇 |
2013年 | 1406篇 |
2012年 | 1733篇 |
2011年 | 1481篇 |
2010年 | 981篇 |
2009年 | 794篇 |
2008年 | 874篇 |
2007年 | 853篇 |
2006年 | 715篇 |
2005年 | 600篇 |
2004年 | 511篇 |
2003年 | 410篇 |
2002年 | 347篇 |
2001年 | 281篇 |
2000年 | 266篇 |
1999年 | 257篇 |
1998年 | 155篇 |
1997年 | 152篇 |
1996年 | 126篇 |
1995年 | 107篇 |
1994年 | 133篇 |
1993年 | 85篇 |
1992年 | 112篇 |
1991年 | 91篇 |
1990年 | 64篇 |
1989年 | 82篇 |
1988年 | 45篇 |
1987年 | 48篇 |
1986年 | 34篇 |
1985年 | 45篇 |
1984年 | 14篇 |
1983年 | 19篇 |
1982年 | 10篇 |
1981年 | 2篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 3篇 |
1975年 | 1篇 |
1970年 | 1篇 |
1950年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
41.
In previous studies with tropical plant species, low root zone temperature (RZT) induced boron (B) deficiency, but it is not known if the same response to RZT will be expressed in temperate species, like oilseed rape, that are more tolerant of low temperature. The present experiments investigated the effect of RZT (10 and 20°C) on oilseed rape ( Brassica napus L. cv. Hyola 42) response to B in solution culture, in summer and winter. Regardless of canopy growth conditions, low RZT (10°C) promoted the partitioning of shoot B to the actively growing leaves, especially when B supply was low. However, low RZT did not significantly alter net B uptake rates or plant biomass. Low RZT decreased the shoot-to-root ratio, countering the effects of low B which increased it, leading to a decreased demand for B in the shoot at low RZT. At low B supply, B-deficiency symptoms appeared later at 10 than at 20°C, corresponding with higher B concentrations in the youngest fully opened leaves (YOLs) at 10°C RZT. Thus 10°C RZT increased the tolerance to low B supply. As a result, it is concluded that the effect of decreasing RZT on the responses of the temperate species, oilseed rape, to low B supply depends on whether the low RZT is above or below the optimal root temperature for growth. 相似文献
42.
Gejiao Wang Stefano Castiglione Ying Chen Ling Li Yifan Han Yingchuan Tian Dean W. Gabriel Yinong Han Kequiang Mang Francesco Sala 《Transgenic research》1996,5(5):289-301
Insect-resistant poplar (Populus nigra L.) plants have been produced by infecting leaves withAgrobacterium tumefaciens strains carrying a binary vector containing different truncated forms of aBacillus thuringiensis (B.t.) toxin gene under a duplicated CaMV 35S promoter. Putative transgenic plants were propagated by cuttings at two experimental farms (in Beijing and Xinjiang, China). At 2–3 years after transformation, 17 of them were selected on the bases of insect-tolerance and good silvicultural traits, and evaluated for insect resistance, for the presence of theB.t. toxin DNA fragment (Southern blots and PCR) and for the expression of the transgene (western and northern blots). Somaclonal variation, as suggested by the appearance of permanent changes in the shape of the leaves, was also investigated with molecular tools (RFLP (restriction fragment length polymorphism), RAPD (random amplified polymorphic DNA) and microsatellite DNA).Bioassays withApochemia cineraius andLymantria dispar on the leaves of the selected clones showed different and, in some cases, high levels of insecticidal activity. The molecular analysis demonstrated integration and expression of the foreign gene. Somatic changes were correlated to extensive genomic changes and were quantified in dendrograms, in terms of genomic similarity. The analysis of control plants suggested that genomic changes were correlated to thein vitro culture step necessary forA. tumefaciens-mediated gene transfer, rather than to the integration of the foreign genes.Three transgenic clones (12, 153 and 192), selected for insect resistance, reduced morphological changes and promising silvicultural traits, are now under large-scale field evaluation in six different provinces in China. 相似文献
43.
Abstract The present paper deals with a new species Habrophlebiodes zijinensis sp. nov. collected in Nanjing, Jiangsu Povince, China. 相似文献
44.
The alpha- and beta-tubulin folding pathways 总被引:4,自引:0,他引:4
The alpha-beta tubulin heterodimer is the subunit from which microtubules are assembled. The pathway leading to correctly folded alpha- and beta-tubulins is unusually complex: it involves cycles of ATP-dependent interaction of newly synthesized tubulin subunits with cytosolic chaperonin, resulting in the production of quasi-native folding intermediates, which must then be acted upon by additional protein cofactors. These cofactors form a supercomplex containing both alpha- and beta-tubulin polypeptides, from which native heterodimer is released in a GTP-dependent reaction. Here, we discuss the current state of our understanding of the function of cytosolic chaperonin and cofactors in tubulin folding. 相似文献
45.
46.
47.
Species of Cunninghamella, Gliocladium deliquescens, Trichoderma harzianum and T. koningii were isolated from rotten wood chips. When grown on medium containing cellulose, all except Cunninghamella produced the three primary enzymes (exoglucanase, endoglucanase and -glucosidase) of the cellulase complex. The patterns for enzyme production, changes in mycelial mass and pH of the induction medium for T. harzianum and T. koningii were closely similar, and were distinguishable from those of G. deliquescens. 相似文献
48.
本文研究TSH和forskolin对原代培养的猪甲状腺细胞[Ca~(2+)]_i和钙调蛋白的影响。结果表明,TSH可引起甲状腺细胞[Ca~(2+)]_1急性升高。此反应是剂量依赖关系,而与细胞外钙的存在与否无关。其反应性在细胞单层高于细胞是液,近汇合细胞单层高于汇合细胞单层。TSH作用3天,可使甲状腺细胞的钙调蛋白含量增高,此作用与TSH对甲状腺细胞数的影响无关。Forskolin对甲状腺细胞的[Ca~(2+)]_i和钙调蛋白均无明显的影响。 相似文献
49.
G H Tian G W Mainwood G P Biro K E Smith K W Butler D Lawrence R Deslauriers 《Canadian journal of physiology and pharmacology》1991,69(11):1760-1768
High buffer cardioplegia may provide protection against ischemic damage by reducing the extent of intracellular acidosis. Secondary cardioplegia may improve postischemic recovery by restoration of high energy phosphates, ionic gradients, and intracellular pH. To test these hypotheses, pig hearts were arrested with high buffer (150 mM MOPS) cardioplegia or modified St. Thomas' solution II and then kept ischemic at 12 degrees C for 8 h. High energy phosphates and intracellular pH were followed during the period of ischemia, using 31P nuclear magnetic resonance spectroscopy, and functional recovery was followed during reperfusion. The hearts arrested by high buffer cardioplegia showed significantly higher intracellular pH than hearts preserved with St. Thomas' solution, but there were no significant differences in high energy phosphates. There were no significant differences in functional recovery. We found, however, that secondary cardioplegia abolished ventricular fibrillation, and resulted in improved functional recovery after 8 h of ischemic preservation compared with the hearts reperfused with Krebs-Henseleit solution alone. Our results suggest that despite attenuating the decreases in intracellular pH, high buffer cardioplegia does not improve recovery following 8 h of preservation at 12 degrees C. Secondary cardioplegia reduces the incidence of ventricular fibrillation and improves postischemic functional recovery of the myocardium. 相似文献
50.
Summary The effect of protoplast-isolating enzymes on spontaneous fusion of maize protoplasts (Zea mays L. cv. Black Mexican Sweet) was investigated using a convenient ethidium bromide nuclear staining procedure. After 2–2.5 hour digestion in an enzyme solution containing 1% Cellulysin, 0.5% Rhozyme, and 0.02% Pectolyase Y-23, 50–75% of the protoplasts contained multiple nuclei. The cellulase Cellulysin was identified as the factor causing the spontaneous protoplast fusion; when Cellulysin was replaced by CELF cellulase, most protoplasts were uninucleate. Calcium and other components in the enzyme solution did not affect spontaneous fusion. Cellulysin also increased the percentage of multinucleate protoplasts from rice and asparagus suspensions. Presence of multiple nuclei might affect genetic manipulations involving protoplasts. 相似文献