首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   773篇
  免费   31篇
  国内免费   45篇
  2024年   1篇
  2023年   16篇
  2022年   22篇
  2021年   18篇
  2020年   28篇
  2019年   25篇
  2018年   28篇
  2017年   26篇
  2016年   22篇
  2015年   22篇
  2014年   29篇
  2013年   34篇
  2012年   29篇
  2011年   33篇
  2010年   26篇
  2009年   29篇
  2008年   45篇
  2007年   49篇
  2006年   29篇
  2005年   41篇
  2004年   39篇
  2003年   26篇
  2002年   28篇
  2001年   6篇
  2000年   24篇
  1999年   14篇
  1998年   17篇
  1997年   11篇
  1996年   20篇
  1995年   8篇
  1994年   14篇
  1993年   7篇
  1992年   11篇
  1991年   11篇
  1990年   9篇
  1989年   10篇
  1988年   6篇
  1987年   5篇
  1986年   8篇
  1985年   5篇
  1984年   5篇
  1982年   3篇
  1981年   3篇
  1979年   1篇
  1978年   1篇
  1977年   1篇
  1972年   1篇
  1969年   1篇
  1968年   1篇
  1963年   1篇
排序方式: 共有849条查询结果,搜索用时 15 毫秒
1.
The phenotypic consequences in the house mouse (Mus musculus domesticus) of the transition from an ordinary field-dwelling rodent to a species that is dependent on human populations was studied by investigating the morphometric variation of non-commensal populations of M. m. domesticus from Syria, Jordan, SW Iran, and Libya and comparing them with that of conspecific commensal populations from Eastern Turkey, Greece, and Bulgaria. Commensal populations of M. musculus musculus from the Czech Republic were used as an outgroup. About 849 adult specimens of M. musculus were analysed by multivariate procedures based on standard molar, skull and body measurements. As expected, there was considerable variation among the studied populations and a good correspondence between morphometric and geographic distances. The resulting morphometric tree was consistent with the hypothesis that the original radiation of M. m. domesticus took place somewhere in the Near East. Commensal populations of M. m. domesticus form a single derived branch. Specimens originating from four different sites in eastern Syria showed the greatest similarity to one another and possessed relatively bigger molars than the other studied populations. Commensal populations were characterised by longer tails when compared to non-commensal populations, which suggests an adaptation for living in a more three-dimensionally heterogeneous environment for commensal populations.  相似文献   
2.
3.
Oral mucositis is a known complication of methotrexate (MTX) therapy, but a single efficacious intervention or agent for prophylaxis or management of this side effect has not yet been identified. We report a case of MTX-induced oral mucositis in a patient with rheumatoid arthritis, who was successfully treated with Wild chamomile mouthwashes.  相似文献   
4.
Nationalization and Globalization Trends in the Wild Mushroom Commerce of Italy with Emphasis on Porcini ( Boletus edulis and Allied Species). This paper presents an historical overview of wild mushroom commerce in Italy, with a focus on recent trends in the production of porcini (Boletus edulis and closely allied species). Over the past century, two major trends—nationalization and globalization—have been apparent in the wild mushroom commerce of Italy. First, a simplified national mushroom menu has emerged through processes of governmental regulation and culinary fashion, but it has come at the expense of differing, localized mushroom traditions which may suffer under the European Union’s free trade principles. Second, Italy has emerged as a focal point of a global market for a small number of mushroom species—particular porcini. While the name porcini has become synonymous with Italian cuisine, and in spite of a vibrant tradition of recreational mushroom collecting in Italy, most of the porcini commercially available in Italy or exported by Italy are no longer of Italian origin. Porcini and other mushrooms now flow into Italy from all over the world—especially from China and eastern Europe—and are then often exported as “Italian porcini.” This globalization of the wild mushroom trade, while offering significant income to rural producers and processors around the globe, has other effects as well, for example, a kind of national branding as “Italian” of globally-produced products, of which porcini is one, that is in direct opposition to some of the European Union’s rules for regional denominations.
Processi di Nazionalizzazione e Globalizzazione nel Commercio Italiano dei Funghi spontanei, con Particolare Riguardo ai Porcini ( Boletus edulis e Specie Affini). Questo articolo presenta una panoramica storica sul commercio dei funghi spontanei in Italia, con particolare riguardo alle recenti tendenze nella produzione dei porcini (Boletus edulis e specie affini). Nello scorso secolo si sono osservate due tendenze principali—di nazionalizzazione e di globalizzazione—nel commercio dei funghi spontanei in Italia. In primo luogo si è affermata nel territorio nazionale una tradizione limitata al consumo di un numero contenuto di specie, sia per effetto di alcune normative che di mode culinarie, ma ciò è avvenuto a discapito di tradizioni locali più ricche, che potrebbero ulteriormente risentire dei principi per il libero scambio all’interno dell’Unione Europea. In secondo luogo, l’Italia si è posta in evidenza come un punto nodale per il mercato globale di alcune specie fungine, in particolare dei porcini. Nonostante quest’ultimo termine sia tradizionalmente associato alla cucina italiana, e nonostante esista in Italia una vivace e radicata tradizione nella raccolta amatoriale dei funghi, la maggior parte dei porcini ivi commercializzati (allo stato fresco, essiccati o variamente conservati) o esportati verso altri paesi non sono più di origine locale. I porcini e altre specie fungine giungono attualmente in Italia da ogni parte del mondo—in modo particolare dalla Cina e dall’Europa orientale—e sono successivamente spesso esportati come “prodotti Italiani.” Questo processo di globalizzazione del mercato dei funghi spontanei, pur offrendo un significativo introito ai raccoglitori e commercianti rurali su tutto il globo, è responsabile di altri effetti, come per l’appunto una sorta di marchio nazionale “Italiano” su alcuni prodotti di provenienza globale, come per l’appunto i porcini, cosa che contrasta con alcune delle regole dell’Unione Europea in materia di denominazioni regionali.
  相似文献   
5.
Summary The competitive ability of the pollen of wild tomato species (Solanum pennellii, Lycopersicum hirsutum and L. minutum) and a cultivar of tomato to affect fertilization at low temperature was investigated. Pollen grains of wild species were mixed with those of the cultivar and pollinated onto the stigma of the cultivar. The number of seedlings resembling the wild species, excluding L. minutum, were significantly higher when the plants were maintained at a low temperature (8°–12° C) than when they were maintained at a normal temperature (22°–28° C). This indicates that the pollen of S. pennellii and L. hirsutum wild species can compete better with pollen of the cultivar at a low temperature, while there was no change in the competitive ability of L. minutum pollen under these conditions.  相似文献   
6.

Background

Gastric cancer exhibits familial clustering, and gastric cancer familial relatives (GCF) tend to present with corpus-predominant gastritis and precancerous lesions as SPEM or IM after H. pylori infection. The study determined whether the children of gastric cancer patients (GCA) had genomic single nucleotide polymorphisms (SNPs) predisposed to the gastric precancerous lesions as spasmolytic polypeptide-expressing metaplasia (SPEM) or intestinal metaplasia (IM).

Results

There were 389 family relatives of 193 non-cardiac GCA and 173 duodenal ulcer patients (DU), received blood sampling for DNA collection. The differences of the risk alleles of SNPs in the ITGA5, ITGB1, IL-10, COX-2, RUNX3, and TFF2 genes were compared between 195 children of GCA and 143 DU. The children of GCA had higher allele frequencies of ITGA5-1160 T-carrier (P = 0.006, OR[95% CI] = 2.2[1.2-4]), ITGB1-1949 A-carrier (P = 0.047; OR[95% CI] = 2.8[1.4-5.3]), ITGB1 + 31804 C-carrier (P = 0.013; OR[95% CI] = 4.7[1.7-13.0]), IL-10-592 AA (P = 0.014; OR[95% CI] = 2.3[1.4-4.0]) and COX-2-1195 G-carrier (P = 0.019; OR[95% CI] = 1.7[0.9-3.2]) than DU. The combined genotype with ITGA5-1160/ITGB1-1949/ITGB1 + 31804 as T/A/C carriers and COX-2-1195/IL-10-592 as G-carrier/AA was more prevalent in the children of GCA than in DU (P < 1×10−4), and predisposed with a 5.3-fold risk of getting SPEM in the H. pylori-infected children of GCA (P = 0.016). Such risk of getting SPEM increased to 112 folds, if combined with RUNX3 + 492/TFF2-308 as A-carrier/CC in this limited study scale (P = 1×10−4).

Conclusions

The SNPs of ITGA5-1160/ITGB1-1949/ ITGB1 + 31804 as T/A/C carriers and COX-2-1195/IL-10-592 as G-carrier/AA, or more specific to combine RUNX3 + 492/TFF2-308 as A-carrier/CC shall be host factor predisposing to gastric cancer during H. pylori infection, and serve as marker to identify high-risk subjects for H. pylori eradication.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-015-0121-7) contains supplementary material, which is available to authorized users.  相似文献   
7.

Background

Barley, globally the fourth most important cereal, provides food and beverages for humans and feed for animal husbandry. Maximizing grain yield under varying climate conditions largely depends on the optimal timing of flowering. Therefore, regulation of flowering time is of extraordinary importance to meet future food and feed demands. We developed the first barley nested association mapping (NAM) population, HEB-25, by crossing 25 wild barleys with one elite barley cultivar, and used it to dissect the genetic architecture of flowering time.

Results

Upon cultivation of 1,420 lines in multi-field trials and applying a genome-wide association study, eight major quantitative trait loci (QTL) were identified as main determinants to control flowering time in barley. These QTL accounted for 64% of the cross-validated proportion of explained genotypic variance (pG). The strongest single QTL effect corresponded to the known photoperiod response gene Ppd-H1. After sequencing the causative part of Ppd-H1, we differentiated twelve haplotypes in HEB-25, whereof the strongest exotic haplotype accelerated flowering time by 11 days compared to the elite barley haplotype. Applying a whole genome prediction model including main effects and epistatic interactions allowed predicting flowering time with an unmatched accuracy of 77% of cross-validated pG.

Conclusions

The elaborated causal models represent a fundamental step to explain flowering time in barley. In addition, our study confirms that the exotic biodiversity present in HEB-25 is a valuable toolbox to dissect the genetic architecture of important agronomic traits and to replenish the elite barley breeding pool with favorable, trait-improving exotic alleles.
  相似文献   
8.

Background

Root system architecture is important for water acquisition and nutrient acquisition for all crops. In soybean breeding programs, wild soybean alleles have been used successfully to enhance yield and seed composition traits, but have never been investigated to improve root system architecture. Therefore, in this study, high-density single-feature polymorphic markers and simple sequence repeats were used to map quantitative trait loci (QTLs) governing root system architecture in an inter-specific soybean mapping population developed from a cross between Glycine max and Glycine soja.

Results

Wild and cultivated soybean both contributed alleles towards significant additive large effect QTLs on chromosome 6 and 7 for a longer total root length and root distribution, respectively. Epistatic effect QTLs were also identified for taproot length, average diameter, and root distribution. These root traits will influence the water and nutrient uptake in soybean. Two cell division-related genes (D type cyclin and auxin efflux carrier protein) with insertion/deletion variations might contribute to the shorter root phenotypes observed in G. soja compared with cultivated soybean. Based on the location of the QTLs and sequence information from a second G. soja accession, three genes (slow anion channel associated 1 like, Auxin responsive NEDD8-activating complex and peroxidase), each with a non-synonymous single nucleotide polymorphism mutation were identified, which may also contribute to changes in root architecture in the cultivated soybean. In addition, Apoptosis inhibitor 5-like on chromosome 7 and slow anion channel associated 1-like on chromosome 15 had epistatic interactions for taproot length QTLs in soybean.

Conclusion

Rare alleles from a G. soja accession are expected to enhance our understanding of the genetic components involved in root architecture traits, and could be combined to improve root system and drought adaptation in soybean.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1334-6) contains supplementary material, which is available to authorized users.  相似文献   
9.
为了阐明我国中亚热带森林区土壤微生物量N的时空变异特征及其主要影响因子,在福建省武夷山国家自然保护区选择了常绿阔叶林(EBF,500 m)、针叶林(CF,1200m)、亚高山矮林(SDF,1800 m)和高山草甸(AM,2100 m)4种不同海拔植被类型土壤(0~10、10~25、25~40 cm)进行研究.结果表明,土壤微生物量N随着海拔高度的增加显著增加,在0~10 cm土层中EBF、CF、SDF、AM土壤微生物量N分别为106.7、140.8、254.9和355.8 mg·kg-1,不同海拔之间土壤微生物量N差异显著(P<0.05).土壤微生物量N在0~10 cm土壤表层最高,随着土壤深度的增加而逐渐减小.4种不同植被类型的3个土壤层次中土壤微生物量N均具有明显的季节动态变化,且变化规律一致,均表现为冬季最高,秋季次之,夏季最低.相关分析表明,在0~10 am土层影响土壤微生物量N沿海拔梯度空间变异的主要因子是土壤湿度、土壤有机质及全N含量,而影响土壤微生物量N季节性变异的主要因子是土壤水分与土壤温度.  相似文献   
10.
本研究选取黄麻属(Corchorus)2个栽培品种及其4个野生类型和3个野生近缘种为材料,采用常规根尖压片法对黄麻属供试材料的染色体数目和核型进行研究。结果表明:染色体数目均为2n=14。核型公式分别为:宽叶长果(长果黄麻栽培种)2n=2x=14=14m(4SAT);南阳野生长果(长果黄麻野生类型)2n=2x=14=14m(2SAT);坦桑尼亚野生长果(长果黄麻野生类型)2n=2x=14=2M+12m;闽麻5号(圆果黄麻栽培种)2n=2x=14=12m+2sm;爱店野生圆果(圆果黄麻野生类型)2n=2x=14=14m ;廉江野生圆果(圆果黄麻野生类型) 2n=2x=14=4M+10m;假黄麻(黄麻属野生近缘种)2n=2x=14=2M+12m;假长果(黄麻属野生近缘种)2n=2x=14=2M+12m;甜麻(黄麻属野生近缘种)2n=2x=14=14m。其中除了宽叶长果核型分类为1B外,其他的都为1A型。本文还讨论了黄麻野生近缘种甜麻的分类学地位。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号