全文获取类型
收费全文 | 7210篇 |
免费 | 492篇 |
国内免费 | 3篇 |
专业分类
7705篇 |
出版年
2023年 | 40篇 |
2022年 | 62篇 |
2021年 | 118篇 |
2020年 | 111篇 |
2019年 | 93篇 |
2018年 | 209篇 |
2017年 | 194篇 |
2016年 | 271篇 |
2015年 | 413篇 |
2014年 | 445篇 |
2013年 | 520篇 |
2012年 | 630篇 |
2011年 | 581篇 |
2010年 | 368篇 |
2009年 | 301篇 |
2008年 | 467篇 |
2007年 | 416篇 |
2006年 | 419篇 |
2005年 | 389篇 |
2004年 | 364篇 |
2003年 | 295篇 |
2002年 | 276篇 |
2001年 | 60篇 |
2000年 | 47篇 |
1999年 | 44篇 |
1998年 | 39篇 |
1997年 | 38篇 |
1996年 | 35篇 |
1995年 | 35篇 |
1994年 | 25篇 |
1993年 | 34篇 |
1992年 | 33篇 |
1991年 | 18篇 |
1990年 | 28篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 14篇 |
1986年 | 14篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 22篇 |
1982年 | 18篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 14篇 |
1978年 | 17篇 |
1976年 | 8篇 |
1975年 | 19篇 |
1974年 | 11篇 |
1973年 | 13篇 |
排序方式: 共有7705条查询结果,搜索用时 15 毫秒
81.
Amilcar Walter Saporetti-Junior Carlos Ernesto G. Reynaud Schaefer Agostinho Lopes de Souza Michellia Pereira Soares Dorothy Sue Dunn Araújo Jo?o Augusto Alves Meira-Neto 《Folia Geobotanica》2012,47(1):29-39
Distribution ranges of plant species are related to physical variables of ecosystems that limit plant growth. Therefore, each plant species response to physical factors builds up the functional diversity of an ecosystem. The higher the species richness of an ecosystem, the larger the probability of maintaining functions and the higher the potential number of plant functional groups (FGs). Thus, the richness potentially increases the number of functions of the highly diverse Atlantic Rainforest domain in Brazil. Severe plant growth limitations caused by stress, however, decrease species richness. In the Spodosols of the Mussununga, an associated ecosystem of Atlantic Rainforest, the percentage of fine sand is directly related to water retention. Moreover, the depth of the cementation layer in the Mussununga??s sandy soil is a physical factor that can affect the plants?? stress gradients. When a shallow cementation layer depth is combined with low water retention in soils and with low fine sand percentage, the double stresses of flooding in the rainy season and water scarcity in the dry season result. This study aimed to identify FGs among Mussununga plant species responding to water stress gradients of soil and to verify the effects of the gradients on plant species richness of the Mussununga. A canonical correspondence analysis (CCA) of species abundance and soil texture variables was performed on 18 plots in six physiognomies of the Mussununga. Species richness rarefactions were calculated for each vegetation form to compare diversity. The two main axes of the CCA showed two FGs responding to soil texture and cementation layer depth: stress tolerator species and mesic species. Physical variables affect plant diversity, with species richness rising as the fine sand proportion also rises in the Mussununga. The effect of the cementation layer is not significantly related to species richness variation. 相似文献
82.
QTL-seq identifies an early flowering QTL located near Flowering Locus T in cucumber 总被引:1,自引:0,他引:1
Hongfeng Lu Tao Lin Joël Klein Shenhao Wang Jianjian Qi Qian Zhou Jinjing Sun Zhonghua Zhang Yiqun Weng Sanwen Huang 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》2014,127(7):1491-1499
Key message
Next-generation sequencing enabled a fast discovery of a major QTL controlling early flowering in cucumber, corresponding to the FT gene conditioning flowering time in Arabidopsis.Abstract
Next-generation sequencing technologies are making it faster and more efficient to establish the association of agronomic traits with molecular markers or candidate genes, which is the requirement for marker-assisted selection in molecular breeding. Early flowering is an important agronomic trait in cucumber (Cucumis sativus L.), but the underlying genetic mechanism is unknown. In this study, we identified a candidate gene for early flowering QTL, Ef1.1 through QTL-seq. Segregation analysis in F2 and BC1 populations derived from a cross between two inbred lines “Muromskij” (early flowering) and “9930” (late flowering) suggested quantitative nature of flowering time in cucumber. Genome-wide comparison of SNP profiles between the early and late-flowering bulks constructed from F2 plants identified a major QTL, designated Ef1.1 on cucumber chromosome 1 for early flowering in Muromskij, which was confirmed by microsatellite marker-based classical QTL mapping in the F2 population. Joint QTL-seq and traditional QTL analysis delimited Ef1.1 to an 890 kb genomic region. A cucumber gene, Csa1G651710, was identified in this region, which is a homolog of the FLOWERING LOCUS T (FT), the main flowering switch gene in Arabidopsis. Quantitative RT-PCR study of the expression level of Csa1G651710 revealed significantly higher expression in early flowering genotypes. Data presented here provide support for Csa1G651710 as a possible candidate gene for early flowering in the cucumber line Muromskij. 相似文献83.
Transthyretin (TTR) protects against A-Beta toxicity by binding the peptide thus inhibiting its aggregation. Previous work showed different TTR mutations interact differently with A-Beta, with increasing affinities correlating with decreasing amyloidogenecity of the TTR mutant; this did not impact on the levels of inhibition of A-Beta aggregation, as assessed by transmission electron microscopy. Our work aimed at probing differences in binding to A-Beta by WT, T119M and L55P TTR using quantitative assays, and at identifying factors affecting this interaction. We addressed the impact of such factors in TTR ability to degrade A-Beta. Using a dot blot approach with the anti-oligomeric antibody A11, we showed that A-Beta formed oligomers transiently, indicating aggregation and fibril formation, whereas in the presence of WT and T119M TTR the oligomers persisted longer, indicative that these variants avoided further aggregation into fibrils. In contrast, L55PTTR was not able to inhibit oligomerization or to prevent evolution to aggregates and fibrils. Furthermore, apoptosis assessment showed WT and T119M TTR were able to protect against A-Beta toxicity. Because the amyloidogenic potential of TTR is inversely correlated with its stability, the use of drugs able to stabilize TTR tetrameric fold could result in increased TTR/A-Beta binding. Here we showed that iododiflunisal, 3-dinitrophenol, resveratrol, [2-(3,5-dichlorophenyl)amino] (DCPA) and [4-(3,5-difluorophenyl)] (DFPB) were able to increase TTR binding to A-Beta; however only DCPA and DFPB improved TTR proteolytic activity. Thyroxine, a TTR ligand, did not influence TTR/A-Beta interaction and A-Beta degradation by TTR, whereas RBP, another TTR ligand, not only obstructed the interaction but also inhibited TTR proteolytic activity. Our results showed differences between WT and T119M TTR, and L55PTTR mutant regarding their interaction with A-Beta and prompt the stability of TTR as a key factor in this interaction, which may be relevant in AD pathogenesis and for the design of therapeutic TTR-based therapies. 相似文献
84.
Ho-Shin Gwak Tae Hoon Kim Guk Heui Jo Youn-Jae Kim Hee-Jin Kwak Jong Heon Kim Jinlong Yin Heon Yoo Seung Hoon Lee Jong Bae Park 《PloS one》2012,7(10)
Radiation is a core part of therapy for malignant glioma and is often provided following debulking surgery. However, resistance to radiation occurs in most patients, and the underlying molecular mechanisms of radio-resistance are not fully understood. Here, we demonstrated that microRNA 21 (miR-21), a well-known onco-microRNA in malignant glioma, is one of the major players in radio-resistance. Radio-resistance in different malignant glioma cell lines measured by cytotoxic cell survival assay was closely associated with miR-21 expression level. Blocking miR-21 with anti-miR-21 resulted in radio-sensitization of U373 and U87 cells, whereas overexpression of miR-21 lead to a decrease in radio-sensitivity of LN18 and LN428 cells. Anti-miR-21 sustained γ-H2AX DNA foci formation, which is an indicator of double-strand DNA damage, up to 24 hours and suppressed phospho-Akt (ser473) expression after exposure to γ-irradiation. In a cell cycle analysis, a significant increase in the G2/M phase transition by anti-miR-21 was observed at 48 hours after irradiation. Interestingly, our results showed that anti-miR-21 increased factors associated with autophagosome formation and autophagy activity, which was measured by acid vesicular organelles, LC3 protein expression, and the percentage of GFP-LC3 positive cells. Furthermore, augmented autophagy by anti-miR-21 resulted in an increase in the apoptotic population after irradiation. Our results show that miR-21 is a pivotal molecule for circumventing radiation-induced cell death in malignant glioma cells through the regulation of autophagy and provide a novel phenomenon for the acquisition of radio-resistance. 相似文献
85.
86.
Host plant distributions and climate interact to affect the predicted geographic distribution of a Neotropical termite 下载免费PDF全文
Hélida F. da Cunha Érica D. Ferreira Geiziane Tessarolo João C. Nabout 《Biotropica》2018,50(4):625-632
Species distribution models (SDMs) have been widely used in the scientific literature. The majority of SDMs use climate data or other abiotic variables to forecast the potential distribution of a species in geographic space. Biotic interactions can affect the predicted spatial distribution of a species in many ways across multiple spatial scales, and incorporating these predictors in an SDM is a current topic in the scientific literature. Constrictotermes cyphergaster is a widely distributed termite in the Neotropics. This termite species nests in plants and more frequently nests in some arboreal species. Thus, this species is an excellent model to evaluate the influence of biotic interactions in SDMs. We evaluate the influences of climate and the geographic distribution of host plants on the potential distribution of C. cyphergaster. Three correlative models (MaxEnt) were built to predict the geographic distribution of the termite: (1) climate data, (2) biotic data (i.e., the geographic distribution of host plants), and (3) climate and biotic data. The models that were generated indicate that the potential geographic distribution of C. cyphergaster is concentrated in the Cerrado and Caatinga regions. In addition, path analysis and multiple regression revealed the importance of the direct effects of biological interactions in the geographic distribution of the termite, while climate affected the distribution of the termite mainly through indirect effects by influencing the geographic distributions of host plants. The current study endorses the importance of including biological interactions in SDMs. We recommend using biotic predictors in SDM studies of insect species, mainly because insects have important environmental services and biotic interaction data can improve the macroecological studies of this group. 相似文献
87.
Research Notes on Avian Biology 1994: Selected Contributions from the 21st International Ornithological CongressGeneral Biology: Migration
Subject: Migration 相似文献88.
Dora Batista Maria João Sousa Maria Salomé Pais 《In vitro cellular & developmental biology. Plant》1996,32(1):37-41
Summary Plant regeneration was achieved from both a spontaneous clone (Bragan?a) and Brewer's Gold variety ofHumulus lupulus. The results obtained for these two different genotypes were compared. The organogenic ability of petiole and stem segments
was tested on three different basal media supplemented with 0.025 mg (0.14 μM) indole-3-acetic acid/L and 2 mg (8.87 μM) 6-benzylaminopurine (N6-benzyladenine)/L. These conditions induced rather heterogeneous responses, which depended mainly on the explant source and
the genotype. Because of the high organogenic competence revealed by the spontaneous clone on modified Murashige and Skoog
medium, several hormones in different combinations were tested to optimize conditions for adventitious shoot regeneration
in this clone. The best relation between the average shoot number/callus and the regeneration rate was achieved with 0.025
mg (0.14 μM) indole-3-acetic acid/L and 2 mg (8.87 μM) 6-benzylaminopurine/L or with 0.02 mg (0.11 μM) indole-3-acetic acid/L and 1.5 mg (6.97 μM) kinetin/L, which enabled 72 and 59% of regeneration, respectively. The regenerated plantlets could be acclimatized with
90% success. 相似文献
89.
Longer lifespan in male mice treated with a weakly estrogenic agonist,an antioxidant,an α‐glucosidase inhibitor or a Nrf2‐inducer 下载免费PDF全文
Adam Antebi Clinton M. Astle Molly Bogue Martin S. Denzel Elizabeth Fernandez Kevin Flurkey Karyn L. Hamilton Dudley W. Lamming Martin A. Javors João Pedro de Magalhães Paul Anthony Martinez Joe M. McCord Benjamin F. Miller Michael Müller James F. Nelson Juliet Ndukum G. Ed. Rainger Arlan Richardson David M. Sabatini Adam B. Salmon James W. Simpkins Wilma T. Steegenga Nancy L. Nadon David E. Harrison 《Aging cell》2016,15(5):872-884
The National Institute on Aging Interventions Testing Program (ITP) evaluates agents hypothesized to increase healthy lifespan in genetically heterogeneous mice. Each compound is tested in parallel at three sites, and all results are published. We report the effects of lifelong treatment of mice with four agents not previously tested: Protandim, fish oil, ursodeoxycholic acid (UDCA) and metformin – the latter with and without rapamycin, and two drugs previously examined: 17‐α‐estradiol and nordihydroguaiaretic acid (NDGA), at doses greater and less than used previously. 17‐α‐estradiol at a threefold higher dose robustly extended both median and maximal lifespan, but still only in males. The male‐specific extension of median lifespan by NDGA was replicated at the original dose, and using doses threefold lower and higher. The effects of NDGA were dose dependent and male specific but without an effect on maximal lifespan. Protandim, a mixture of botanical extracts that activate Nrf2, extended median lifespan in males only. Metformin alone, at a dose of 0.1% in the diet, did not significantly extend lifespan. Metformin (0.1%) combined with rapamycin (14 ppm) robustly extended lifespan, suggestive of an added benefit, based on historical comparison with earlier studies of rapamycin given alone. The α‐glucosidase inhibitor, acarbose, at a concentration previously tested (1000 ppm), significantly increased median longevity in males and 90th percentile lifespan in both sexes, even when treatment was started at 16 months. Neither fish oil nor UDCA extended lifespan. These results underscore the reproducibility of ITP longevity studies and illustrate the importance of identifying optimal doses in lifespan studies. 相似文献
90.