全文获取类型
收费全文 | 17127篇 |
免费 | 1215篇 |
国内免费 | 1篇 |
专业分类
18343篇 |
出版年
2023年 | 118篇 |
2022年 | 229篇 |
2021年 | 439篇 |
2020年 | 306篇 |
2019年 | 373篇 |
2018年 | 461篇 |
2017年 | 434篇 |
2016年 | 608篇 |
2015年 | 955篇 |
2014年 | 961篇 |
2013年 | 1238篇 |
2012年 | 1468篇 |
2011年 | 1359篇 |
2010年 | 919篇 |
2009年 | 817篇 |
2008年 | 981篇 |
2007年 | 951篇 |
2006年 | 869篇 |
2005年 | 815篇 |
2004年 | 783篇 |
2003年 | 660篇 |
2002年 | 662篇 |
2001年 | 159篇 |
2000年 | 111篇 |
1999年 | 136篇 |
1998年 | 186篇 |
1997年 | 119篇 |
1996年 | 94篇 |
1995年 | 89篇 |
1994年 | 92篇 |
1993年 | 86篇 |
1992年 | 68篇 |
1991年 | 46篇 |
1990年 | 64篇 |
1989年 | 43篇 |
1988年 | 42篇 |
1987年 | 45篇 |
1986年 | 37篇 |
1985年 | 58篇 |
1984年 | 57篇 |
1983年 | 38篇 |
1982年 | 43篇 |
1981年 | 42篇 |
1980年 | 35篇 |
1979年 | 28篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1975年 | 22篇 |
1973年 | 22篇 |
1972年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
101.
Armengol-Díaz Javier Esparcia Angeles Vicente Eduardo Miracle Maria Rosa 《Hydrobiologia》1993,255(1):381-388
Vertical distribution of planktonic rotifers is described in relation to temperature and oxygen in Lake La Cruz, a single-doline, closed karstic lake (121 m diameter and 25 m maximum depth) which shows iron meromixis. Samples were taken by peristaltic pumping at 10 cm depth intervals in the oxycline zone from June 1987 to September 1988. A model of rotifer vertical structure in stratified lakes is proposed. Rotifers concentrate their populations at the depths with intense gradients. As stratification develops some rotifer populations show a downward migration following the thermocline and some others show an upward migration following the oxycline. The production-respiration balance in the lake, and so the position of the oxycline with respect to the thermocline and the layer of maximum production, depends on meteorological conditions. A shift in the dominance of congeneric or related species can occur in consecutive years. In Lake La Cruz, mixing conditions and subterranean inflow in spring were much more intense in 1988 than 1987, and the distance between production and decomposition depths was smaller in 1988. Anuraeopsis miraclei, an oxycline-bound species with high abundance in 1987, was displaced by A. fissa in 1988. A. fissa, which was a metalimnetic species during early summer, reached peak densities (3 × 104 ind l–1) at the oxycline, equaling the abundance of A. miraclei the preceeding year. 相似文献
102.
Jean-Luc J. Pellegrin Eduardo Ortega-Barria Reginaldo P. Prioli Mary Buerger Richard G. Strout Joseph Alroy Miercio E. A. Pereira 《Glycoconjugate journal》1993,10(1):57-63
Sporozoites and merozoites of three species ofEimeria, E. tenella, E. maxima, andE. necatrix, that cause diarrhea in chickens worldwide, were examined for their expression of sialidase (SA) activity. The enzyme was found in three species, and the activity of merozoites was 10–20 times higher than that of sporozoites. The enzyme was resistant to degradation by proteases that are normally present in the intestine, a site inhabited by theEimeria parasites, and it was relatively resistant to heat, with optimum activity being at 40°C, which is within the range of temperature in the chicken intestine (40–43°C).E. tenella SA was immuniprecipitated by monoclonal and polyclonal antibodies raised against theTrypanosoma cruzi SA (TCSA), and enzyme activity was neutralized by these antibodies.E. tenella SA was identified by immunoblots as a doublet of molecular weight 190 000 and 180 000 using, as a probe, anti-TCSA antibodies and antibodies against a synthetic peptide (TR) derived from the long tandem repeat domain of TCSA. Binding of the monoclonal and polyclonal antibodies toE. tenella was completely blocked by TR, but not by an irrelevant peptide (BR). Therefore,E. tenella expresses a developmentally regulated SA that is structurally related to theT. cruzi counterpart. Because of the high SA activity in merozoites, and by analogy with other SA-producing microbes that inhabit mucin-rich epithelia, we suggest that theEimeria SA plays a role in desialylating intestinal mucins to reduce viscosity of the local environment and thereby facilitate parasite migration. The enzyme could also play a role in host cell-parasite interaction.Abbreviations SA
sialidase (neuraminidase)
- Neu5Ac
N-acetylneuraminic acid
- 4-MU-Neu5Ac
2-(4-methylumbelliferyl)--N-acetyl-d-neuraminic acid
- BSA
bovine serum albumin
- PBS
phosphate buffered saline
- PMSF
phenylmethylsulfonyl fluoride
- PNA
peanut agglutinin
- Ab
antibody
- TCN-2
monoclonal antibody toT. cruzi sialidase, anti-Ars, monoclonal antibody top-azophenylarsonate
- TCSA
Trypanosoma cruzi sialidase 相似文献
103.
Eduardo Moreno-Jiménez Fernando T. Maestre Maren Flagmeier Emilio Guirado Miguel Berdugo Felipe Bastida Marina Dacal Paloma Díaz-Martínez Raúl Ochoa-Hueso César Plaza Matthias C. Rillig Thomas W. Crowther Manuel Delgado-Baquerizo 《Global Change Biology》2023,29(2):522-532
Soil micronutrients are capital for the delivery of ecosystem functioning and food provision worldwide. Yet, despite their importance, the global biogeography and ecological drivers of soil micronutrients remain virtually unknown, limiting our capacity to anticipate abrupt unexpected changes in soil micronutrients in the face of climate change. Here, we analyzed >1300 topsoil samples to examine the global distribution of six metallic micronutrients (Cu, Fe, Mn, Zn, Co and Ni) across all continents, climates and vegetation types. We found that warmer arid and tropical ecosystems, present in the least developed countries, sustain the lowest contents of multiple soil micronutrients. We further provide evidence that temperature increases may potentially result in abrupt and simultaneous reductions in the content of multiple soil micronutrients when a temperature threshold of 12–14°C is crossed, which may be occurring on 3% of the planet over the next century. Altogether, our findings provide fundamental understanding of the global distribution of soil micronutrients, with direct implications for the maintenance of ecosystem functioning, rangeland management and food production in the warmest and poorest regions of the planet. 相似文献
104.
Mayrel Palestino-Domínguez Alejandro Escobedo-Calvario Soraya Salas-Silva Moises Vergara-Mendoza Veronica Souza-Arroyo Roberto Lazzarini Roxana Miranda-Labra Leticia Bucio-Ortiz María Concepción Gutiérrez-Ruiz Luis E. Gomez-Quiroz 《Journal of biochemical and molecular toxicology》2023,37(4):e23302
Alcohol-induced pancreas damage remains as one of the main risk factors for pancreatitis development. This disorder is poorly understood, particularly the effect of acetaldehyde, the primary alcohol metabolite, in the endocrine pancreas. Hepatocyte growth factor (HGF) is a protective protein in many tissues, displaying antioxidant, antiapoptotic, and proliferative responses. In the present work, we were focused on characterizing the response induced by HGF and its protective mechanism in the RINm5F pancreatic cell line treated with ethanol and acetaldehyde. RINm5F cells were treated with ethanol or acetaldehyde for 12 h in the presence or not of HGF (50 ng/ml). Cells under HGF treatment decreased the content of reactive oxygen species and lipid peroxidation induced by both toxics, improving cell viability. This effect was correlated to an improvement in insulin expression impaired by ethanol and acetaldehyde. Using a specific inhibitor of Erk1/2 abrogated the effects elicited by the growth factor. In conclusion, the work provides mechanistic evidence of the HGF-induced-protective response to the alcohol-induced damage in the main cellular component of the endocrine pancreas. 相似文献
105.
Sonja K. Glasser Martín Hesajim de Santiago-Hernández Oliverio Delgado-Carrillo Luis Alberto Villanueva Espino Adonaji Cortés Pérez Antonio González-Rodríguez Rafael Lira-Saade Mauricio Quesada 《American journal of botany》2023,110(5):e16170
Premise
Domestication of plant species results in phenotypic modifications and changes in biotic interactions. Most studies have compared antagonistic plant-herbivore interactions of domesticated plants and their wild relatives, but little attention has been given to how domestication influences plant-pollinator interactions. Floral attributes and interactions of floral visitors were compared between sister taxa of the genus Cucurbita (Cucurbitaceae), the domesticated C. moschata, C. argyrosperma ssp. argyrosperma and its wild progenitor C. argyrosperma ssp. sororia in the place of origin.Methods
We conducted univariate and multivariate analyses to compare floral morphological traits and analyzed floral reward (nectar and pollen) quantity and quality between flowers of wild and domesticated Cucurbita taxa. Staminate and pistillate flowers of all three taxa were video recorded, and visitation and behavior of floral visitors were registered and analyzed.Results
Most floral morphological characteristics of flowers of domesticated taxa were larger in both staminate and pistillate flowers. Staminate and pistillate flowers presented distinct correlations between floral traits and integration indices between domesticated and wild species. Additionally, pollen quantity and protein to lipid ratio were greater in domesticated species. Cucurbit pollen specialists, Eucera spp., had the highest probability of visit for all Cucurbita taxa.Conclusions
We provide evidence that floral traits of domesticated and wild Cucurbita species experienced different selection pressures. Domesticated Cucurbita species may have more resources invested towards floral traits, thereby increasing attractiveness to pollinators and potentially plant reproductive success. Wild ancestor plant populations should be conserved in their centers of origin to preserve plant-pollinator interactions. 相似文献106.
Karen Valverde-Méndez Eduardo J. Hernández Dennis Matamoros Natalia Barboza 《The Annals of applied biology》2023,183(3):231-243
Begomoviruses (Geminiviridae family) are characterized by their high recombination rate and a wide range of hosts, making their control difficult. In Costa Rica, various species of bipartite begomoviruses have been reported, which are Pepper golden mosaic virus (PepGMV), Tomato yellow mottle virus (ToYMoV), Tomato leaf curl Sinaloa virus (ToLCSiV) and the monopartite begomovirus Tomato yellow leaf curl virus (TYLCV). Since the TYLCV first report in Costa Rica, neither additional knowledge has been produced on how this begomovirus has spread in the country's territory nor on the distribution of the other bipartite species. A total of 429 tomato samples collected during the years 2015–2016 were used to study these aspects. Each sample was georeferenced and analysed with various techniques such as nucleic acid hybridization, polymerase chain reaction (PCR) and sequencing for the begomoviruses previously reported in Costa Rica. It was found that the presence/absence of the different species can vary, depending on the province. TYLCV is present in the six provinces analysed in this work, with a proportion from 3.7 to 86.6 per cent. Alajuela, Cartago, and Heredia are the provinces most affected by tomato-infecting begomoviruses. Fourteen different haplotypes of TYLCV were detected, but all were identified as TYLCV-IL. The distribution of TYLCV was related to the presence of the whitefly Bemisia tabaci MED, especially in the country's main tomato production areas. This information allows the phytosanitary surveillance services to develop strategies for the integrated management of the disease and to contribute data to the genetic improvement programmes of the crop. 相似文献
107.
108.
A Rhizobium tropici DNA region carrying the amino-terminal half of a nodD gene and a nod-box-like sequence confers host-range extension 总被引:1,自引:0,他引:1
Carolina Sousa Jorge Luis Folch Purificación Boloix Manuel Megías Noreide Nava Carmen Quinto 《Molecular microbiology》1993,9(6):1157-1168
Rhizobium tropici CIAT899 is a broad-host-range strain that, in addition to Phaseolus, nodulates other plant legumes such as Leucaena and Macroptilium. The narrow-host-range of Rhizobium leguminosarum biovars phaseoli (strain CE3) and trifolii (strain RS1051) can be extended to Leucaena esculents and Phaseolus vulgaris plants, respectively, by the introduction of a DNA fragment 521 bp long, which carries 128 amino acids of the amino-terminal region of a nodD gene from R. tropici, as well as a putative nod-box-like sequence, divergently oriented. The 521 bp fragment, in the presence of L. esculenta or P. vulgaris root exudates, induced a R. leguminosarum bv. viciae nodA-lacZ fusion in either a CE3 or RS1051 background, respectively. 相似文献
109.
Molecular analysis of a microaerobically induced operon required for hydrogenase synthesis in Rhizobium leguminosarum biovar viciae 总被引:5,自引:0,他引:5
Luis Rey Jesus Murillo Yolanda Hernando Elena Hidalgo Ezequiel Cabrera Juan Imperial Tomás Ruiz-Argüeso 《Molecular microbiology》1993,8(3):471-481
The nucleotide sequence (6138 bp) of a microaerobically inducible region (hupV/VI) from the Rhizobium leguminosarum bv. viciae hydrogenase gene cluster has been determined. Six genes, arranged as a single operon, were identified, and designated hypA, B, F, C, D and E based on the sequence similarities of all of them, except hypF, to genes from the hydrogenase pleiotropic operon (hyp) from Escherichia coli. The gene products from hypBFCDE were identified by in vivo expression analysis in E. coli, and their molecular sizes were consistent with those predicted from the nucleotide sequence. Transposon Tn5 insertions into hypB, hypF, hypD and hypE resulted in R. leguminosarum mutants that lacked any hydrogenase activity in symbiosis with peas, but still were able to synthesize the polypeptide for the hydrogenase large subunit. The gene products HypA, HypB, HypF and HypD contained CX2C motifs characteristic of metal-binding proteins. In addition, HypB bore a long histidine-rich stretch of amino acids near the N-terminus, suggesting a possible role in nickel binding for this protein. The gene product HypF, which was translationally coupled to HypB, presented two cysteine motifs (CX2CX81CX2C) with a capacity to form zinc finger-like structures in the N-terminal third of the protein. A role in nickel metabolism in relation to hydrogenase synthesis is postulated for proteins HypB and HypF. 相似文献
110.
Alison Ke Evaristo Cedeño Jordan Karubian Luis Carrasco Fernando Castillo Nelson Gonzales Daniel S. Karp 《Biotropica》2023,55(5):1045-1057
Most studies comparing biodiversity between natural and human-modified landscapes focus on patterns in species occurrence or abundance, but do not consider how different habitat types meet species' breeding requirements. Organisms that use or nest in tree cavities may be especially threatened by habitat conversion due to the loss of their nesting sites. Although cavity-nesting bird diversity is highest in the tropics, little is known about how tropical birds use cavities, how agriculture affects their reproductive biology, and how effective nest boxes could be as a conservation strategy in tropical agriculture. Here, we explored how habitat conversion from tropical forests to pasture affects the abundance, nesting habitat availability, and nest success of cavity-nesting birds in Northwest Ecuador. We conducted bird surveys and measured natural cavity availability and use in forest and agriculture. We also added artificial nest boxes to forest and agriculture to see whether cavity limitation in agriculture would elicit higher use of artificial nest boxes. We found evidence of cavity limitation in agriculture—there were many more natural cavities in forest than in agriculture, as well as more avian use of nest boxes placed in agriculture as compared to forest. Our results suggest that it is important to retain remnant trees in tropical agriculture to provide critical nesting habitat for birds. In addition, adding nest boxes to tropical agricultural systems could be a good conservation strategy for certain species, including insectivores that could provide pest-control services to farmers. Abstract in Spanish is available with online material. 相似文献