全文获取类型
收费全文 | 117891篇 |
免费 | 2739篇 |
国内免费 | 901篇 |
专业分类
121531篇 |
出版年
2021年 | 279篇 |
2019年 | 230篇 |
2018年 | 12054篇 |
2017年 | 10890篇 |
2016年 | 7829篇 |
2015年 | 1314篇 |
2014年 | 1036篇 |
2013年 | 1371篇 |
2012年 | 5432篇 |
2011年 | 13951篇 |
2010年 | 12820篇 |
2009年 | 8910篇 |
2008年 | 10663篇 |
2007年 | 12266篇 |
2006年 | 1191篇 |
2005年 | 1442篇 |
2004年 | 2000篇 |
2003年 | 1958篇 |
2002年 | 1693篇 |
2001年 | 971篇 |
2000年 | 860篇 |
1999年 | 690篇 |
1998年 | 373篇 |
1997年 | 395篇 |
1996年 | 372篇 |
1995年 | 325篇 |
1994年 | 276篇 |
1993年 | 332篇 |
1992年 | 639篇 |
1991年 | 544篇 |
1990年 | 553篇 |
1989年 | 545篇 |
1988年 | 458篇 |
1987年 | 450篇 |
1986年 | 392篇 |
1985年 | 395篇 |
1984年 | 422篇 |
1983年 | 337篇 |
1982年 | 294篇 |
1981年 | 281篇 |
1980年 | 248篇 |
1979年 | 332篇 |
1978年 | 273篇 |
1977年 | 286篇 |
1976年 | 219篇 |
1975年 | 241篇 |
1974年 | 241篇 |
1973年 | 211篇 |
1972年 | 407篇 |
1971年 | 456篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Osmotic stress and endogenous hormone levels may have a role in shoot organogenesis, but a systematic study has not yet to
investigate the links. We evaluated the changes of the endogenous indole-3-acetic acid (IAA) and abscisic acid (ABA) levels
in rice (Oryza sativa L. cv. Tainan 5) callus during shoot organogenesis induced by exogenous plant growth regulator treatments or under osmotic
stress. Non-regenerable callus showed low levels of endogenous ABA and IAA, with no fluctuation in level during the period
evaluated. The addition of 100 μM ABA or 2 mM anthranilic acid (IAA precursor) into Murashige and Skoog basal induction medium
containing 10 μM 2,4-D enhanced the regeneration frequency slightly, to 5 and 35%, respectively, and their total cellular
ABA or IAA levels were increased significantly, correspondingly to the treatments. However, the regeneration frequency was
greatly increased to 80% after treatment with 0.6 M sorbitol or 100 μM ABA and 2 mM anthranilic acid combined. Both treatments
produced high levels of total cellular ABA and IAA at the callus stage, which was quickly decreased on the first day after
transfer to regeneration medium. Thus, osmotic stress-induced simultaneous accumulation of endogenous ABA and IAA is involved
in shoot regeneration in rice callus. 相似文献
993.
994.
Simon J. Watson Pinky Langat Scott M. Reid Tommy Tsan-Yuk Lam Matthew Cotten Michael Kelly Kristien Van Reeth Yu Qiu Ga?lle Simon Emilie Bonin Emanuela Foni Chiara Chiapponi Lars Larsen Charlotte Hjulsager Iwona Markowska-Daniel Kinga Urbaniak Ralf Dürrwald Michael Schlegel Anita Huovilainen Irit Davidson ádám Dán Willie Loeffen Stephanie Edwards Michel Bublot Thais Vila Jaime Maldonado Laura Valls ESNIP Consortium Ian H. Brown Oliver G. Pybus Paul Kellam 《Journal of virology》2015,89(19):9920-9931
995.
Delfien Van Dyck Megan Teychenne Sarah A. McNaughton Ilse De Bourdeaudhuij Jo Salmon 《PloS one》2015,10(3)
Background
Mental health conditions are among the leading non-fatal diseases in middle-aged and older adults in Australia. Proximal and distal social environmental factors and physical environmental factors have been associated with mental health, but the underlying mechanisms explaining these associations remain unclear. The study objective was to examine the contribution of different types of physical activity in mediating the relationship of social and physical environmental factors with mental health-related quality of life in middle-aged and older adults.Methods
Baseline data from the Wellbeing, Eating and Exercise for a Long Life (WELL) study were used. WELL is a prospective cohort study, conducted in Victoria, Australia. Baseline data collection took place in 2010. In total, 3,965 middle-aged and older adults (55–65 years, 47.4% males) completed the SF-36 Health Survey, the International Physical Activity Questionnaire, and a questionnaire on socio-demographic, social and physical environmental attributes. Mediation analyses were conducted using the MacKinnon product-of-coefficients test.Results
Personal safety, the neighbourhood physical activity environment, social support for physical activity from family or friends, and neighbourhood social cohesion were positively associated with mental health-related quality of life. Active transportation and leisure-time physical activity mediated 32.9% of the association between social support for physical activity from family or friends and mental health-related quality of life. These physical activity behaviours also mediated 11.0%, 3.4% and 2.3% respectively, of the relationship between the neighbourhood physical activity environment, personal safety and neighbourhood social cohesion and mental health-related quality of life.Conclusions
If these results are replicated in future longitudinal studies, tailored interventions to improve mental health-related quality of life in middle-aged and older adults should use a combined strategy, focusing on increasing physical activity as well as social and physical environmental attributes. 相似文献996.
One of the major problems regarding consumer acceptance of genetically modified organisms (GMOs) is the possibility that their transgenes could have adverse effects on the environment and/or human health. Genome editing, represented by the CRISPR/Cas9 system, can efficiently achieve transgene-free gene modifications and is anticipated to generate a wide spectrum of plants. However, the public attitude against GMOs suggests that people will initially be unlikely to accept these plants. We herein explored the bottlenecks of consumer acceptance of transgene-free food crops developed by genome editing and made some recommendations. People should not pursue a zero-risk bias regarding such crops. Developers are encouraged to produce cultivars with a trait that would satisfy consumer needs. Moreover, they should carefully investigate off-target mutations in resultant plants and initially refrain from agricultural use of multiplex genome editing for better risk–benefit communication. The government must consider their regulatory status and establish appropriate regulations if necessary. The government also should foster communication between the public and developers. If people are informed of the benefits of genome editing-mediated plant breeding and trust in the relevant regulations, and if careful risk–benefit communication and sincere considerations for the right to know approach are guaranteed, then such transgene-free crops could gradually be integrated into society. 相似文献
997.
G. Tomlinson N. Buchmann R. Siegwolf P. Weber A. Thimonier E. Graf Pannatier M. Schmitt M. Schaub P. Waldner 《Trees - Structure and Function》2016,30(3):627-638
Key message
For long-term environmental investigations, tree-ring δ 15 N values are inappropriate proxies for foliar δ 15 N for both Fagus sylvatica and Picea abies under moderate N loads.Abstract
Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (δ15N) and carbon (δ13C) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring δ15N and δ13C values were less negative compared to foliar δ15N and δ13C values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar δ15N were not reflected in tree-ring δ15N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar δ13C, while there were no significant correlations between environmental variables and either tree-ring or foliar δ15N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring δ15N values are inappropriate proxies of foliar δ15N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring δ15N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.998.
Ronivaldo Rodrigues da Silva Tatiane Beltramini Souto Tássio Brito de Oliveira Lilian Caroline Gonçalves de Oliveira Daniel Karcher Maria Aparecida Juliano Luiz Juliano Arthur H. C. de Oliveira André Rodrigues Jose C. Rosa Hamilton Cabral 《Journal of industrial microbiology & biotechnology》2016,43(8):1059-1069
In this study, we detail the specificity of an aspartic peptidase from Rhizomucor miehei and evaluate the effects of this peptidase on clotting milk using the peptide sequence of k-casein (Abz-LSFMAIQ-EDDnp) and milk powder. Molecular mass of the peptidase was estimated at 37 kDa, and optimum activity was achieved at pH 5.5 and 55 °C. The peptidase was stable at pH values ranging from 3 to 5 and temperatures of up 45 °C for 60 min. Dramatic reductions in proteolytic activity were observed with exposure to sodium dodecyl sulfate, and aluminum and copper (II) chloride. Peptidase was inhibited by pepstatin A, and mass spectrometry analysis identified four peptide fragments (TWSISYGDGSSASGILAK, ASNGGGGEYIFGGYDSTK, GSLTTVPIDNSR, and GWWGITVDRA), similar to rhizopuspepsin. The analysis of catalytic specificity showed that the coagulant activity of the peptidase was higher than the proteolytic activity and that there was a preference for aromatic, basic, and nonpolar amino acids, particularly methionine, with specific cleavage of the peptide bond between phenylalanine and methionine. Thus, this peptidase may function as an important alternative enzyme in milk clotting during the preparation of cheese. 相似文献
999.
Vanesa Amarelle Federico Rosconi Juan Manuel Lázaro-Martínez Graciela Buldain Francisco Noya Mark R. O’Brian Elena Fabiano 《Biometals》2016,29(2):333-347
Ensifer meliloti is a nitrogen-fixing symbiont of the alfalfa legume able to use heme as an iron source. The transport mechanism involved in heme acquisition in E. meliloti has been identified and characterized, but the fate of heme once inside the cell is not known. In silico analysis of E. meliloti 1021 genome revealed no canonical heme oxygenases although two genes encoding putative heme degrading enzymes, smc01518 and hmuS, were identified. SMc01518 is similar to HmuQ of Bradyrhizobium japonicum, which is weakly homologous to the Staphylococcus aureus IsdG heme-degrading monooxygenase, whereas HmuS is homolog to Pseudomonas aeruginosa PhuS, a protein reported as a heme chaperone and as a heme degrading enzyme. Recombinant HmuQ and HmuS were able to bind hemin with a 1:1 stoichiometry and displayed a Kd value of 5 and 4 µM, respectively. HmuS degrades heme in vitro to the biliverdin isomers IX-β and IX-δ in an equimolar ratio. The HmuQ recombinant protein degrades heme to biliverdin IX-δ only. Additionally, in this work we demonstrate that humS and hmuQ gene expression is regulated by iron and heme in a RirA dependent manner and that both proteins are involved in heme metabolism in E. meliloti in vivo. 相似文献
1000.
Zhansheng Zhu Jing Yan Chunsong Geng Dagang Wang Chaoyang Li Shuai Feng Huiping Wang 《Cellular and molecular neurobiology》2016,36(6):981-988
Stroke was regarded as a severe disorder with high morbidity and high mortality worldwide, ischemic stroke (IS) accounts for 85 to 90 % of new increased stroke cases. Partial mechanisms were elucidated by genetic factors including genomic instability such as single nucleotide polymorphism (SNP). Previous reports demonstrated that inflammation was involved in IS, NLRP3 [nucleotide-binding domain (NOD)-like receptor protein 3], acting as a specific inflammatory gene, however, its function and influence on IS was not well clarified. In this study, a case–control study including 1102 IS patients and 1610 healthy controls was conducted to investigate the association between IS susceptibility with a SNP (rs10754558) in 3′UTR of NLRP3. Logistic regression analysis showed that the heterozygote and the homozygote GG confer a significantly increased risk of CRC after controlling for other covariates (adjusted OR = 1.52, 95 % C.I. 1.19–1.97, P = 0.002; adjusted OR = 2.22, 95 % C.I. 2.18–3.67, P < 0.001, respectively). Carriage of G allele was associated with a greatly increased risk of developing the disease (OR = 1.69, 95 % C.I. 1.31–1.83, P < 0.001). Stratification analysis found that hypertension had interaction with rs10754558 to modulate IS risk. Further in vitro assay revealed that rs10754558 can affect mRNA level of NLRP3, suggesting its possible functional significance. Our data suggested that genetic polymorphisms in NLRP3 may influence IS risk in Chinese population. Replication of our studies in other populations and further functional studies are required for complete comprehension of the roles of NLRP3 polymorphisms in IS risk. 相似文献