全文获取类型
收费全文 | 630篇 |
免费 | 54篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2019年 | 10篇 |
2018年 | 13篇 |
2017年 | 9篇 |
2016年 | 13篇 |
2015年 | 31篇 |
2014年 | 36篇 |
2013年 | 46篇 |
2012年 | 35篇 |
2011年 | 32篇 |
2010年 | 35篇 |
2009年 | 38篇 |
2008年 | 30篇 |
2007年 | 27篇 |
2006年 | 27篇 |
2005年 | 31篇 |
2004年 | 23篇 |
2003年 | 7篇 |
2002年 | 19篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 12篇 |
1998年 | 12篇 |
1997年 | 12篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 10篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 18篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1969年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有684条查询结果,搜索用时 482 毫秒
121.
122.
Javed Iqbal Magdalena Necpalova Sotirios V. Archontoulis Robert P. Anex Marie Bourguignon Daryl Herzmann David C. Mitchell John E. Sawyer Qing Zhu Michael J. Castellano 《Global Change Biology》2018,24(1):e303-e317
The frequency and intensity of extreme weather years, characterized by abnormal precipitation and temperature, are increasing. In isolation, these years have disproportionately large effects on environmental N losses. However, the sequence of extreme weather years (e.g., wet–dry vs. dry–wet) may affect cumulative N losses. We calibrated and validated the DAYCENT ecosystem process model with a comprehensive set of biogeophysical measurements from a corn–soybean rotation managed at three N fertilizer inputs with and without a winter cover crop in Iowa, USA. Our objectives were to determine: (i) how 2‐year sequences of extreme weather affect 2‐year cumulative N losses across the crop rotation, and (ii) if N fertilizer management and the inclusion of a winter cover crop between corn and soybean mitigate the effect of extreme weather on N losses. Using historical weather (1951–2013), we created nine 2‐year scenarios with all possible combinations of the driest (“dry”), wettest (“wet”), and average (“normal”) weather years. We analyzed the effects of these scenarios following several consecutive years of relatively normal weather. Compared with the normal–normal 2‐year weather scenario, 2‐year extreme weather scenarios affected 2‐year cumulative NO3? leaching (range: ?93 to +290%) more than N2O emissions (range: ?49 to +18%). The 2‐year weather scenarios had nonadditive effects on N losses: compared with the normal–normal scenario, the dry–wet sequence decreased 2‐year cumulative N2O emissions while the wet–dry sequence increased 2‐year cumulative N2O emissions. Although dry weather decreased NO3? leaching and N2O emissions in isolation, 2‐year cumulative N losses from the wet–dry scenario were greater than the dry–wet scenario. Cover crops reduced the effects of extreme weather on NO3? leaching but had a lesser effect on N2O emissions. As the frequency of extreme weather is expected to increase, these data suggest that the sequence of interannual weather patterns can be used to develop short‐term mitigation strategies that manipulate N fertilizer and crop rotation to maximize crop N uptake while reducing environmental N losses. 相似文献
123.
Lifestyle Intervention Decreases Urine Trimethylamine N‐Oxide Levels in Prepubertal Children with Obesity 下载免费PDF全文
María J. Leal‐Witt Marina Llobet Sara Samino Pol Castellano Daniel Cuadras Josep C. Jimenez‐Chillaron Oscar Yanes Marta Ramon‐Krauel Carles Lerin 《Obesity (Silver Spring, Md.)》2018,26(10):1603-1610
Objective
Early lifestyle interventions in children with obesity decrease risk of obesity and metabolic disorders during adulthood. This study aimed to identify metabolic signatures associated with lifestyle intervention in urine samples from prepubertal children with obesity.Methods
Thirty‐four prepubertal children with obesity were studied before and after a 6‐month lifestyle intervention program, and anthropometric, metabolic, and nutritional variables were collected. A nuclear magnetic resonance approach was applied to obtain the metabolomic profile from urine samples. Partial least squares‐discriminant analysis (PLS‐DA) was used to achieve group classification and variable importance on projection (VIP) for biomarker selection.Results
The intervention reduced caloric intake by 10% (P < 0.05) and BMI standard deviation score by 0.47 SD (P < 0.001). PLS‐DA identified trimethylamine N‐oxide (TMAO, VIP = 2.21) as the metabolite with the highest discrimination properties between groups. Urine TMAO levels were reduced after the intervention (P < 0.05). TMAO is a biomarker of cardiovascular disease risk and is a product of gut microbiota‐dependent metabolism of certain dietary compounds, including choline. Notably, changes in TMAO levels after the intervention did not correlate to differences in choline intake but were inversely associated with fiber intake (P < 0.05).Conclusions
These results indicate that lifestyle intervention decreases TMAO levels in children with obesity.124.
Sandra Santasusagna Isabel Moreno Alfons Navarro Joan J. Castellano Francisco Martinez Raquel Hernández Carmen Muñoz Mariano Monzo 《Translational oncology》2018,11(3):715-721
BACKGROUND: The analysis of exosomes in blood obtained from the tumor-draining mesenteric vein (MV) can identify tumor biomarkers before they reach target organs and form the premetastatic niche where circulating tumor cells can anchor. Our group has recently shown that microRNAs in plasma from the MV—but not the peripheral vein (PV)—have been related to liver metastases in colon cancer (CC) patients. Here we examine the exosomal protein cargo in plasma from the MV and paired PV in 31 CC patients. PATIENTS AND METHODS: The study included patients who were initially diagnosed with stage I-III CC and 10 healthy controls. Exosomes from the MV and PV of all patients and controls were isolated by ultracentrifugation and confirmed by cryogenic transmission electron microscopy. High-throughput proteomic analysis by mass spectrometry was used to identify expression levels of exosomal proteins. Findings were confirmed by Western blot. RESULTS: Exosomal ECM1 protein was more highly expressed in patients than in controls and was 13.55 times higher in MV from relapsed than relapse-free patients. High exosomal ECM1 expression was associated with liver metastases. Patients with high exosomal ECM1 expression in MV—but not PV—plasma had shorter time to relapse than those with low ECM1 expression (P = .04). CONCLUSION: High levels of exosomal ECM1 protein can identify CC patients with a higher risk of relapse. The analysis of exosomes isolated from the tumor-draining MV is a promising method for the identification of biomarkers before they reach the target organ. 相似文献
125.
126.
Caccone A; Moriyama EN; Gleason JM; Nigro L; Powell JR 《Molecular biology and evolution》1996,13(9):1224-1232
Drosophila melanogaster belongs to a closely related group of eight species
collectively known as the melanogaster subgroup; all are native to
sub-Saharan Africa and islands off the east coast of Africa. The
phylogenetic relationships of most species in this subgroup have been well
documented; however, the three most closely related species, D. simulans,
D. sechellia, and D. mauritiana, have remained problematic from a
phylogenetic standpoint as no data set has unambiguously resolved them. We
present new DNA sequence data on the nullo and Serendipity-alpha genes and
combine them with all available nuclear DNA sequence data; the total data
encompass 12 genes and the ITS of rDNA. A methodological problem arose
because nine of the genes had information on intraspecific polymorphisms in
at least one species. We explored the effect of inclusion/exclusion of
polymorphic sites and found that it had very little effect on phylogenetic
inferences, due largely to the fact that 82% of polymorphisms are
autapomorphies (unique to one species). We have also reanalyzed our
previous DNA-DNA hybridization data with a bootstrap procedure. The
combined sequence data set and the DNA-DNA hybridization data strongly
support the sister status of the two island species, D. sechellia and D.
mauritiana. This at least partially resolves what had been a paradox of
parallel evolution in these two species.
相似文献
127.
128.
INTRoDUCTIoNlYho1iumrePensL,whiteclover,isaneconomicallyimportantplantspeciesintemperatepastures.Asbrieflyreportedby[1],ithas16pairsofchromosomes(2n=32).Asyet,nodetailedcytologicalexaminationofthisspecies,suchasC-banding,hasbeenrep0rted.Inthelastdecade,thetechnique0fC-bandinghasbeenusedt0examinehighlyrepeatedsequencesinplantchrom0s0mesandhasprovidedausefultoolf0rtheanalysis0fcyt0geneticstructureincr0pplants[2-71.Inplants,thechr0m0s0mall0calizationofhighlyrepeatedDNAsequencesbyinsituhybr… 相似文献
129.
130.
The period (per) locus has received much attention in molecular evolution
studies because it is one of the best studied "behavioral genes" and
because it offers insight into the evolution of repetitive sequences. We
studied most of the coding region of per in Drosophila willistoni and
confirmed previously observed patterns of conservation and divergence among
distantly related species. Five regions are so highly diverged that they
cannot be aligned, whereas a region encompassing the PAS domain is very
conserved. Structural and nucleotide polymorphism patterns in the
willistoni group are not the same as those observed in previously studied
species. We sequenced the region homologous to the highly polymorphic
threonine-glycine repeat of D. melanogaster in multiple strains of D.
willistoni, as well as in other members of willistoni group, and found an
unusual amount of conservation in this region. However, the next
nonconserved region downstream in the sequence is quite variable and
polymorphic for the number of repeated glycines. The glycine codon usage is
significantly different in this glycine repeat as compared to other parts
of the gene. We were able to plot the directionality of change in the
glycine repeat region onto a phylogeny and find that the addition of
glycines is the general trend with the diversification of the willistoni
group.
相似文献