全文获取类型
收费全文 | 42009篇 |
免费 | 1515篇 |
国内免费 | 1784篇 |
专业分类
45308篇 |
出版年
2023年 | 385篇 |
2022年 | 635篇 |
2021年 | 651篇 |
2020年 | 790篇 |
2019年 | 951篇 |
2018年 | 983篇 |
2017年 | 842篇 |
2016年 | 858篇 |
2015年 | 827篇 |
2014年 | 1849篇 |
2013年 | 3224篇 |
2012年 | 1311篇 |
2011年 | 1952篇 |
2010年 | 1412篇 |
2009年 | 1902篇 |
2008年 | 2076篇 |
2007年 | 2067篇 |
2006年 | 1757篇 |
2005年 | 1677篇 |
2004年 | 1348篇 |
2003年 | 1312篇 |
2002年 | 1082篇 |
2001年 | 865篇 |
2000年 | 773篇 |
1999年 | 722篇 |
1998年 | 754篇 |
1997年 | 701篇 |
1996年 | 688篇 |
1995年 | 650篇 |
1994年 | 661篇 |
1993年 | 604篇 |
1992年 | 565篇 |
1991年 | 501篇 |
1990年 | 465篇 |
1989年 | 456篇 |
1988年 | 407篇 |
1987年 | 423篇 |
1986年 | 292篇 |
1985年 | 650篇 |
1984年 | 932篇 |
1983年 | 646篇 |
1982年 | 719篇 |
1981年 | 569篇 |
1980年 | 500篇 |
1979年 | 444篇 |
1978年 | 278篇 |
1977年 | 273篇 |
1976年 | 224篇 |
1974年 | 185篇 |
1973年 | 179篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Phytoplasmas are noncultivable bacteria usually maintained in Catharanthus roseus shoots grown in vitro on MS medium with benzylaminopurine. The aim of our research was to examine the influence of indole-3-butyric acid (IBA) on C. roseus shoots infected with three different phytoplasma strains. Supplement of IBA in the medium supported plant growth, photosynthesis and remission of symptoms in all phytoplasma-infected shoots, but had no effect on the presence of EY-C and SA-I phytoplasma strains in tested tissue. However, HYDB phytoplasma was undetectable in approximately half of the tested shoots grown on the medium with IBA. After 1 year of IBA treatment, HYDB-infected periwinkle shoots were retransferred to the medium supplemented with benzylaminopurine. Some of the shoots showing remission of symptoms during the IBA treatment permanently escaped the infection and remained negative when tested for phytoplasma presence. This is the first report on the differential influence of plant growth regulators on phytoplasma-infected C. roseus shoots. 相似文献
992.
A number of bacteria in the family Enterobacteriaceae harbor the genes comprising well-developed pectinolytic pathways (e.g. Erwinia sp.) or abridged versions of this pathway (e.g. Yersinia sp.). One of the most enigmatic components present in some of these pathways is a small gene that encodes a predicted secreted protein of approximately 160 amino acid residues with unknown function. This protein shows distant amino acid sequence similarity over its entire length to galactose-specific family 32 carbohydrate-binding modules (CBMs). Here we demonstrate the ability of the Yersinia enterocolitica example, here called YeCBM32, to bind polygalacturonic acid containing components of pectin. This binding is selective for highly polymerized galacturonic acid and shows a complex mode of polysaccharide recognition. The high resolution X-ray crystal structure (1.35 A) shows YeCBM32s overall structural similarity to galactose specific CBMs and conserved binding site location but reveals a substantially different binding site topology, which likely reflects its unique polymeric and acidic ligand. The results suggest the possibility of a unique role for YeCBM32 in polygalacturonic acid transport. 相似文献
993.
Diverse subtypes and developmental origins of trophoblast giant cells in the mouse placenta 总被引:3,自引:0,他引:3
Trophoblast giant cells (TGCs) are the first terminally differentiated subtype to form in the trophoblast cell lineage in rodents. In addition to mediating implantation, they are the main endocrine cells of the placenta, producing several hormones which regulate the maternal endocrine and immune systems and promote maternal blood flow to the implantation site. Generally considered a homogeneous population, TGCs have been identified by their expression of genes encoding placental lactogen 1 or proliferin. In the present study, we have identified a number of TGC subtypes, based on morphology and molecular criteria and demonstrated a previously underappreciated diversity of TGCs. In addition to TGCs that surround the implantation site and form the interface with the maternal deciduas, we demonstrate at least three other unique TGC subtypes: spiral artery-associated TGCs, maternal blood canal-associated TGCs and a TGC within the sinusoidal spaces of the labyrinth layer of the placenta. All four TGC subtypes could be identified based on the expression patterns of four genes: Pl1, Pl2, Plf (encoded by genes of the prolactin/prolactin-like protein/placental lactogen gene locus), and Ctsq (from a placental-specific cathepsin gene locus). Each of these subtypes was detected in differentiated trophoblast stem cell cultures and can be differentially regulated; treatment with retinoic acid induces Pl1/Plf+ TGCs preferentially. Furthermore, cell lineage tracing studies indicated unique origins for different TGC subtypes, in contrast with previous suggestions that secondary TGCs all arise from Tpbpa+ ectoplacental cone precursors. 相似文献
994.
Trade-offs between seedling growth and survival in deciduous broadleaved trees in a temperate forest 总被引:1,自引:0,他引:1
Seiwa K 《Annals of botany》2007,99(3):537-544
BACKGROUND AND AIMS: In spatially heterogeneous environments, a trade-off between seedling survival and relative growth rate may promote the coexistence of plant species. In temperate forests, however, little support for this hypothesis has been found under field conditions, as compared with shade-house experiments. Performance trade-offs were examined over a large resource gradient in a temperate hardwood forest. METHODS: The relationship between seedling survival and seedling relative growth rate in mass (RGR(M)) or height (RGR(H)) was examined at three levels of canopy cover (forest understorey, FU; small gap, SG; and large gap, LG) and at two microsites within each level of canopy cover (presence or absence of leaf litter) for five deciduous broad-leaved tree species with different seed sizes. KEY RESULTS: Within each species, both RGR(M) and RGR(H) usually increased with increasing light levels (in the order FU < SG < LG), whereas little difference was observed based on the presence or absence of litter. Seedling survival in FU was negatively correlated with both RGR(M) and RGR(H) in both LG and SG. The trade-off between high-light growth and low-light survival was more evident in the relationship with LG as compared with SG. An intraspecific trade-off between survival and RGR was observed along environmental gradients in Acer mono, whereas seedlings of Betula platyphylla var. japonica survived and grew better in LG. CONCLUSIONS: The results presented here strongly support the idea of light gradient partitioning (i.e. species coexistence) in spatially heterogeneous light environments in temperate forests, and that further species diversity would be promoted by increased spatial heterogeneity. The intraspecific trade-off between survival and RGR in Acer suggests that it has broad habitat requirements, whereas Betula has narrow habitat requirements and specializes in high-light environments. 相似文献
995.
Thayyullathil F Chathoth S Shahin A Kizhakkayil J Hago A Patel M Galadari S 《Journal of cellular biochemistry》2011,112(4):1138-1153
Sphingosine (SPH) is an important bioactive lipid involved in mediating a variety of cell functions including apoptosis. However, the signaling mechanism of SPH-induced apoptosis remains unclear. We have investigated whether SPH inhibits survival signaling in cells by inhibiting Akt kinase activity. This study demonstrates that treatment of Jurkat cells with SPH leads to Akt dephosphorylation as early as 15 min, and the cells undergo apoptosis after 6 h. This Akt dephosphorylation is not mediated through deactivation of upstream kinases, since SPH does not inhibit the upstream phosphoinositide-dependent kinase 1 (PDK1) phosphorylation. Rather, sensitivity to the Ser/Thr protein phosphatase inhibitors (calyculin A, phosphatidic acid, tautomycin, and okadaic acid) indicates an important role for protein phosphatase 1 (PP1) in this process. In vitro phosphatase assay, using Akt immunoprecipitate following treatment with SPH, reveals an increase in Akt-PP1 association as determined by immunoprecipitation analysis. Moreover, SPH-induced dephosphorylation of Akt at Ser(473) subsequently leads to the activation of GSK-3β, caspase 3, PARP cleavage, and ultimately apoptosis. Pre-treatment with caspase 3 inhibitor z-VAD-fmk and Ser/Thr phosphatase inhibitor abrogates the effect of SPH on facilitating apoptosis. Altogether, these results demonstrate that PP1-mediated inhibition of the key anti-apoptotic protein, Akt, plays an important role in SPH-mediated apoptosis in Jurkat cells. 相似文献
996.
997.
Programme of senescence in petals and carpels of Pisum sativum L. flowers and its control by ethylene 总被引:3,自引:0,他引:3
The role of ethylene in the control of senescence of both petals and unpollinated carpels of pea was investigated. An increase
in ethylene production accompanied senescence, and the inhibitors of ethylene action were effective in delaying senescence
symptoms in different flower verticils. Pollination did not seem to trigger the senescence syndrome in the corolla as deduced
from the observation that petals from pollinated and unpollinated flowers and from flowers whose carpels had been removed
senesced at the same time. A cDNA clone encoding a putative ethylene-response sensor (psERS) was isolated from pea flowers, and the pattern of expression of its mRNA was studied during development and senescence of
different flower tissues. The levels of psERS mRNA paralleled ethylene production (and also levels of 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) mRNA) in both
petals and styles. Silver thiosulfate treatments were efficient at preventing ACO and psERS mRNA induction in petals. However, the same inhibitor showed no ability to modify expression patterns in pea carpels around
the anthesis stage, suggesting different controls for ethylene synthesis and sensitivity in different flower organs.
Received: 18 June 1998 / Accepted: 22 December 1998 相似文献
998.
The addition of 0.5 mM salicylic acid (SA) to the hydroponic growth solution of young maize (Zea mays L.) plants under normal growth conditions provided protection against subsequent low-temperature stress. This observation
was confirmed by chlorophyll fluorescence parameters and electrolyte leakage measurements. In addition, 1 d of 0.5 mM SA pre-treatment
decreased net photosynthesis, stomatal conductivity and transpiration at the growth temperature (22/20 °C). Since there was
only a slight decrease in the ratio of variable to maximal fluorescence (Fv/Fm) the decrease in photosynthetic activity is not due to a depression in photosystem II. The analysis of antioxidant enzymes
showed that whereas SA treatment did not cause any change in ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase
(EC 1.15.1.1) activities, there was a decrease in catalase (EC 1.11.1.6) activity, and an increase in guaiacol peroxidase
(EC 1.11.1.7) and glutathione reductase (EC 1.6.4.2) activities after the 1-d SA treatment at 22/20 °C. In native polyacrylamide
gels there was, among the peroxidase isoenzymes, a band which could be seen only in SA-treated plants. It is suggested that
the pre-treatment of maize plants with SA at normal growth temperature may induce antioxidant enzymes which lead to increased
chilling tolerance.
Received: 4 June 1998 / Accepted: 23 November 1998 相似文献
999.
We investigated the in vitro stimulatory effect of ganglioside (GM3, GD1a, GD1b, GT1b, or GQ1b)-containing liposomes on human immune cells. The effect of ganglioside-containing liposomes on the concentration of cytoplasmic free calcium ions ([Ca2+]1) in human immunocytes was examined using the confocal laser fluorescence microscopic method. The GD1a- and GT1b-containing liposomes significantly increased [Ca2+]1 of human T lymphocytes compared with the GM3-, GD1b- and GQ1b-containing ones. The response of CD8+ and CD4+ cells was significantly higher than that of CD20+ cells. Our results show that the increase in [Ca2+]i may be caused by not the number of sialic acids contained in the gangliosides but the conformation of the sialic acid moiety to protrude exteriorly from the liposomal membrane surface, and that a sort of receptor recognizing the sialic acid moiety exists on human T lymphocytes (both CD8+ and CD4+ cells), which may be involved in the activation of the cells. The present results are almost the same as those obtained for the rat T lymphocyte system previously reported. This clearly confirms that a sort of ganglioside surely stimulates T lymphocytes directly, which is not species-specific but conserved in humans and rats among animal species. 相似文献
1000.
Kawano M Ariyoshi W Iwanaga K Okinaga T Habu M Yoshioka I Tominaga K Nishihara T 《Biochemical and biophysical research communications》2011,(4):1222-580