全文获取类型
收费全文 | 1891篇 |
免费 | 118篇 |
出版年
2023年 | 8篇 |
2022年 | 22篇 |
2021年 | 40篇 |
2020年 | 28篇 |
2019年 | 41篇 |
2018年 | 60篇 |
2017年 | 49篇 |
2016年 | 116篇 |
2015年 | 258篇 |
2014年 | 212篇 |
2013年 | 298篇 |
2012年 | 216篇 |
2011年 | 114篇 |
2010年 | 107篇 |
2009年 | 111篇 |
2008年 | 58篇 |
2007年 | 52篇 |
2006年 | 48篇 |
2005年 | 44篇 |
2004年 | 20篇 |
2003年 | 31篇 |
2002年 | 19篇 |
2001年 | 12篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 8篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有2009条查询结果,搜索用时 453 毫秒
221.
Álvarez M Tamayo-Canul J Martínez-Rodríguez C López-Urueña E Gomes-Alves S Anel L Martínez-Pastor F de Paz P 《Animal reproduction science》2012,132(3-4):145-154
The objective of this study was to identify possible specificity in the extender formulation for the cryopreservation of ram spermatozoa recovered from three origins (ejaculate, electroejaculate or epididymis), by evaluating post-thawing sperm quality and fertility. Ejaculated, electroejaculated or epididymal spermatozoa samples obtained from identical rams (8) were cryopreserved in four different extenders (TES-Tris-fructose with one of two egg yolk concentrations: 10% Y10 and 20% Y20, and with one of two glycerol rates: 4% G4 and 8% G8). Samples were analyzed before and after cryopreservation by CASA (motility) and flow cytometry (viability with SYBR-14/PI and acrosomal status with PNA/PI). Spermatozoa obtained by electroejaculation were of poorer quality after freezing/thawing, demonstrating that protocols for these samples need to be optimized. Egg yolk at 20% was more appropriate for freezing sperm from any of the sources. In general, 4% glycerol improved the quality of post-thawing samples recovered from ejaculate and electroejaculate, while 8% glycerol was more appropriate for samples recovered from the epididymis. Based on these results, an analysis of fertility was conducted. Fertility rates were similar between ewe groups inseminated with post-thawed sperm obtained from two sources: ejaculate (cryopreserved in Y20+G4), and cauda epididymis (Y20+G8), and this rate was less in the electroejaculated sample (Y20+G4). 相似文献
222.
223.
Tania Silva María ángeles Abengózar María Fernández-Reyes David Andreu Kamran Nazmi Jan G. M. Bolscher Margarida Bastos Luis Rivas 《Amino acids》2012,43(6):2265-2277
Two antimicrobial cryptopeptides from the N1 domain of bovine lactoferrin, lactoferricin (LFcin17–30) and lactoferrampin (LFampin265–284), together with a hybrid version (LFchimera), were tested against the protozoan parasite Leishmania. All peptides were leishmanicidal against Leishmania donovani promastigotes, and LFchimera showed a significantly higher activity over its two composing moieties. Besides, it was the only peptide active on Leishmania pifanoi axenic amastigotes, already showing activity below 10?μM. To investigate their leishmanicidal mechanism, promastigote membrane permeabilization was assessed by decrease of free ATP levels in living parasites, entrance of the vital dye SYTOX Green (MW?=?600?Da) and confocal and transmission electron microscopy. The peptides induced plasma membrane permeabilization and bioenergetic collapse of the parasites. To further clarify the structural traits underlying the increased leishmanicidal activity of LFchimera, the activity of several analogues was assessed. Results revealed that the high activity of these hybrid peptides seems to be related to the order and sequence orientation of the two cryptopeptide moieties, rather than to their particular linkage through an additional lysine, as in the initial LFchimera. The incorporation of both antimicrobial cryptopeptide motifs into a single linear sequence facilitates chemical synthesis and should help in the potential clinical application of these optimized analogues. 相似文献
224.
Juan M. Rubiales César Morales-Molino Salvia García álvarez Mercedes García-Antón 《Vegetation History and Archaeobotany》2012,21(4-5):397-412
Palaeoecological evidence indicates that highland pines were dominant in extensive areas of the mountains of Central and Northern Iberia during the first half of the Holocene. However, following several millennia of anthropogenic pressure, their natural ranges are now severely reduced. Although pines have been frequently viewed as first-stage successional species responding positively to human disturbance, some recent palaeobotanical work has proposed fire disturbance and human deforestation as the main drivers of this vegetation turnover. To assess the strength of the evidence for this hypothesis and to identify other possible explanations for this scenario, we review the available information on past vegetation change in the mountains of northern inland Iberia. We have chosen data from several sites that offer good chronological control, including palynological records with microscopic charcoal data and sites with plant macro- and megafossil occurrence. We conclude that although the available long-term data are still fragmentary and that new methods are needed for a better understanding of the ecological history of Iberia, fire events and human activities (probably modulated by climate) have triggered the pine demise at different locations and different temporal scales. In addition, all palaeoxylological, palynological and charcoal results obtained so far are fully compatible with a rapid human-induced ecological change that could have caused a range contraction of highland pines in western Iberia. 相似文献
225.
The availability of most edible ectomycorrhizal mushrooms depends on their natural fructification. Sporocarp formation of
these fungi is linked to habitat characteristics and climate conditions, but these data alone do not explain all the trends
of fungal fruiting and dynamics. It could be hypothesized that the amount of soil mycelia could also be related to the production
of carpophores. Soil samples (five cylinders of 250 cm3 per plot) were taken monthly, from September to November, in five fenced permanent plots (5 × 5 m) in Pinar Grande (Soria,
Spain), a Pinus sylvestris stand situated in the north of the Sistema Ibérico mountain range. Plots were chosen to establish a gradient of Boletus edulis productivity from 0 to 38.5 kg/ha year, according to the mean fresh weight of sporocarps collected during the last 10 years.
B. edulis ectomycorrhizal root tips were identified in each soil sample according to its morphology and counted. DNA extractions were
performed with the PowerSoilTM DNA Isolation Kit and quantification of extraradical soil mycelium by real-time polymerase chain reaction using specific
primers and a TaqMan? probe. The concentration of soil mycelium of B. edulis (mg mycelium/g soil) did not differ significantly between plots (p = 0.1397), and sampling time (p = 0.7643) within the fructification period. The number of mycorrhizal short roots per soil volume showed significant differences
between the plots (p = 0.0050) and the three sampling times (p < 0.0001). No significant correlation between the number of mycorrhizas and the productivity of the plot (kg of B. edulis/ha year) was detected (p = 0.615). A statistically significant positive correlation (p = 0.0481) was detected between the concentration of mycelia of B. edulis in the soil samples and the presence of short roots mycorrhizal with B. edulis in these samples. The productivity of the plots, in terms of sporocarps produced during the last 10 years, was not correlated
either with the concentration of soil mycelium or with the presence or abundance of ectomycorrhizas. 相似文献
226.
García-Posadas L Contreras-Ruiz L López-García A Villarón Álvarez S Maldonado MJ Diebold Y 《Histochemistry and cell biology》2012,137(2):165-176
The purpose of this study was to demonstrate the presence of the receptor for hyaluronan-mediated motility (RHAMM) in human
conjunctival epithelium and in two widely used cell lines from human corneal (HCE) and conjunctival (IOBA-NHC) epithelia.
We compared the distribution of RHAMM proteins and mRNAs in human ocular surface tissues (corneal, limbal and conjunctival),
HCE and IOBA-NHC cell lines, and corneal and conjunctival epithelia primary samples from healthy donors with the previously
identified hyaluronan receptor CD44. We also aimed to determine if soluble CD44 (sCD44) was present in human tears, as it
could have a role in the interaction of the tear fluid with hyaluronan. Protein expression was evaluated by Western blots
and immunofluorescence microscopy. mRNA expression was evaluated by RT-PCR and Q-PCR. sCD44 was analyzed by ELISA in culture
supernatants and in human tears. We describe the expression of RHAMM in human healthy conjunctiva and in HCE and IOBA-NHC
cells at both protein and mRNA levels, and the presence of sCD44 in human tears. Furthermore, we detected CD44 and sCD44 expression
variations in in vitro inflammatory conditions. This study also focused on the necessary caution with which the conclusions
extracted from cell lines should be made, and in the great value of using primary samples as often as possible. 相似文献
227.
Carmen Rojo Miguel álvarez-Cobelas José Benavent-Corai María Mercedes Barón-Rodríguez María A. Rodrigo 《Biodiversity and Conservation》2012,21(10):2453-2476
Wetlands are considered vulnerable ecosystems of both high species richness and socio-economic value. In semi-arid regions, these ecosystems often experience long drought periods that are usually aggravated by local water overexploitation. Drought leads to: (i) reduced flooding area, (ii) isolation of water bodies, (iii) increased areas of dry sediments and shoreline length, and (iv) increased ionic concentration. These processes affect aquatic populations in a species-specific way and can have antagonistic effects on taxon richness. Here, we highlight long-term (1997–2008) trade-off effects on plankton species richness linked to drought in a semi-arid wetland (Las Tablas de Daimiel National Park, central Spain). Annual average phytoplankton species richness increased from wet- (1997–1998) to dry years (2001–2002) and taxon richness diminished again when drought was more severe (2007–2008). Zooplankton changes were more complex depending on taxonomic groups and the body size of the organisms, total species loss being related to hydrological conditions. Half of the algal species recorded in 2007–2008 and one-tenth of total zooplankton taxa were new comers in the wetland, because salinization, eutrophication and submerged macrophytes occurring in different sites enhanced species turnover and mitigated homogenization of beta diversity. Maintenance of one water body with a constant water level and macrophytes was the key to preventing the collapse of plankton richness. Our study has demonstrated that plankton can be very useful for tracking environmental changes of wetlands, thus giving the environmental manager another tool to enhance the conservation of wetlands and their biota. 相似文献
228.
Cysteine homeostasis plays an essential role in plant immunity 总被引:1,自引:0,他引:1
229.
Navascués J Pérez-Rontomé C Sánchez DH Staudinger C Wienkoop S Rellán-Álvarez R Becana M 《The New phytologist》2012,193(3):625-636
? Aluminum (Al) toxicity is a major limiting factor of crop production on acid soils, but the implication of oxidative stress in this process is controversial. A multidisciplinary approach was used here to address this question in the forage legume Lotus corniculatus. ? Plants were treated with low Al concentrations in hydroponic culture, and physiological and biochemical parameters, together with semiquantitative metabolic and proteomic profiles, were determined. ? The exposure of plants to 10 μM Al inhibited root and leaf growth, but had no effect on the production of reactive oxygen species or lipid peroxides. By contrast, exposure to 20 μM Al elicited the production of superoxide radicals, peroxide and malondialdehyde. In response to Al, there was a progressive replacement of the superoxide dismutase isoforms in the cytosol, a loss of ascorbate and consistent changes in amino acids, sugars and associated enzymes. ? We conclude that oxidative stress is not a causative factor of Al toxicity. The increased contents in roots of two powerful Al chelators, malic and 2-isopropylmalic acids, together with the induction of an Al-activated malate transporter gene, strongly suggest that both organic acids are implicated in Al detoxification. The effects of Al on key proteins involved in cytoskeleton dynamics, protein turnover, transport, methylation reactions, redox control and stress responses underscore a metabolic dysfunction, which affects multiple cellular compartments, particularly in plants exposed to 20 μM Al. 相似文献
230.
Rosana AR Ventakesh M Chamot D Patterson-Fortin LM Tarassova O Espie GS Owttrim GW 《Plant & cell physiology》2012,53(4):646-658
Inactivation of the DEAD box RNA helicase, crhR, has dramatic effects on the physiology and morphology of the photosynthetic cyanobacterium, Synechocystis sp. PCC 6803. These effects are observed at both normal growth temperature (30°C) and under cold stress (20°C), indicating that CrhR performs crucial function(s) at all temperatures. A major physiological effect is the rapid cessation of photosynthesis upon temperature downshift from 30 to 20°C. This defect does not originate from an inability to transport or accumulate inorganic carbon or a deficiency in photosynthetic capacity as the mutant has sufficient electron transport and enzymatic capacity to sustain photosynthesis at 30°C and inorganic carbon (Ci) accumulation at 20°C. Oxygen consumption in the presence of methyl viologen indicated that while electron transport capacity is sufficient to accumulate Ci, the mutant does not possess sufficient activity to sustain carbon fixation at maximal rates. These defects are correlated with severely impaired cell growth and decreased viability, cell size and DNA content at low temperature. The ΔcrhR mutant also progressively accumulates structural abnormalities at low temperature that cannot be attributed solely to reactive oxygen species (ROS)-induced photooxidative damage, suggesting that they are manifestations of pre-existing defects that are amplified over time. The data indicate that the observed physiological and morphological effects are intimately related to crhR mutation, implying that the lack of CrhR RNA unwinding/annealing activity results in the inability to execute one or more vital steps in photosynthesis that are required at all temperatures but are crucial at low temperature. 相似文献