全文获取类型
收费全文 | 455篇 |
免费 | 29篇 |
国内免费 | 39篇 |
出版年
2024年 | 1篇 |
2023年 | 5篇 |
2022年 | 7篇 |
2021年 | 6篇 |
2020年 | 9篇 |
2019年 | 13篇 |
2018年 | 12篇 |
2017年 | 13篇 |
2016年 | 12篇 |
2015年 | 21篇 |
2014年 | 35篇 |
2013年 | 63篇 |
2012年 | 29篇 |
2011年 | 19篇 |
2010年 | 12篇 |
2009年 | 14篇 |
2008年 | 13篇 |
2007年 | 25篇 |
2006年 | 18篇 |
2005年 | 18篇 |
2004年 | 20篇 |
2003年 | 18篇 |
2002年 | 17篇 |
2001年 | 10篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 11篇 |
1997年 | 9篇 |
1996年 | 6篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有523条查询结果,搜索用时 15 毫秒
361.
对光周期敏感雄性核不育水稻的初步研究——修饰基因对光敏感雄性不育后代表现型值的影响 总被引:6,自引:0,他引:6
利用人工控制光周期程序(简称SPAC)处理或在北京自然长日下栽培,考察了15个群体的自交结实率和正常花粉率频率分布,得到了如下结果。 1.F_1的育性正常。F_1群体的表现型值呈双峰连续分布。分离出1个无法准确划分的“败育群”;2.在F_2的“败育群”或农垦58原种S群体内,雄性彻底败育的个体只是少数,其余多数个体败育不完全,表现型值各不相同,在低值端呈连续分布;3.由F_1选出的光敏感雄性不育单株繁衍来的F_4家系群体,其表现型值依然分离,在低值端呈连续分布。雄性完全败育个体的比例在各家系、株系、甚至同一株系内的植株间存在差别;4.仅F_4家系群体中的1个株系。农垦58S或鄂宜105S群体内表现型值基本一致,雄性彻底败育了。作者认为,连续分布是水稻光敏感雄性不育初级群体表现型值分布的普遍形式。它多半产生于修饰基因分离、重组造成的基因型值的连续变异。 相似文献
362.
Temperature‐dependent development of Aproceros leucopoda (Hymenoptera: Argidae), an invasive pest of elms in Europe
下载免费PDF全文
![点击此处可从《Journal of Applied Entomology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The temperature‐dependent development of Aproceros leucopoda Takeuchi, 1939 (Hymenoptera: Argidae), an invasive pest of elms in Europe, was studied in the laboratory on Siberian elm (Ulmus pumila) at six constant temperatures (10.9, 15.0, 19.5, 23.0, 24.3 and 27.0°C) and at a photoperiod of 16L:8D. The larvae of the species developed through 4–7 instars, and it was the 6th instar individuals of which continued their development at the highest rate. The developmental threshold (Tmin) and the thermal constant (K) were determined by the linear model and Lactin‐2 model for egg, larval, prepupal and pupal stages and one whole generation. The estimated values of Tmin and K by the two models for one generation were 7.3 and 7.1°C, and 426.5 and 432.7 degree‐days, respectively. Based on the survival and fecundity rates, the optimal temperature range for the species may be expected to be between 15.0 and 19.5°C. Allowing for the period of diapause, it was estimated that A. leucopoda might potentially develop through up to four or five generations per year in Hungary. These results may contribute to the better understanding of the biology of an invasive alien species in Europe. 相似文献
363.
Despite the cost of lost mating opportunities, biparental care is routinely observed across different groups of animals. Theory predicts that paternal care will increase reproductive success, yet many empirical studies have failed to find a direct link. Most studies have focused on the effect of paternal care on current breeding attempts, but male contributions may benefit future reproduction. Species producing clutches or litters more than once per season (i.e. multiple breeding attempts per season) are suitable for investigating whether paternal care contributes to current or future reproductive success, or both. We investigated the importance of the male's feeding contribution to multiple brooding in Japanese Tits Parus minor in northern Japan, where approximately 50% of pairs were known to reproduce multiple times within a breeding season. Male feeding contribution (proportion of provisioning delivered by the male) was positively correlated with the probability of the female multiple brooding and with nestling body mass. However, it was not correlated with fledging success in the current breeding attempt. Our results demonstrate the importance of male parental care for annual reproductive success in this multi‐brooded species and provide insights into the evolution of biparental care. 相似文献
364.
365.
毛球莸和小叶灰毛莸的化学成分 总被引:1,自引:0,他引:1
从毛球莸中分离出4个化合物,经MS、~1H-NMR和~(13)C-NMR等波谱分析方法分别鉴定为刺槐素(Ⅰ),香叶木素(Ⅱ),赪桐甾醇(Ⅲ)和一个环烯醚萜甙——8-acetyl ha-pagide。从小叶灰毛莸中除分离到上述环烯醚萜甙,还另外获得一个环烯醚萜甙ajugoside(Ⅴ)。 相似文献
366.
Freitas M. C. Afonso M. H. Almeida C. Alves L. C. Araújo M. F. Barreiros M. A. Seabra e Barros J. Costa M. Bordalo Gouveia M. A. Reis M. A. 《Biological trace element research》1994,(1):549-560
Under contract with the IAEA, the epiphytic lichenEvernia prunastri was collected to prepare a multielement lichen reference material for quality assurance of environmental studies. An intercomparison
run on trace and minor elements in this candidate research material (IAEA-336) was organized in which six analytical groups
of the National Institute of Engineering and Industrial Technology (INETI) took part. INAA, PIXE, XRF, AAS, and ICP-ES were
applied. The results obtained by different methods are compared, and their complementarity is discussed. As a quality control,
the IAEA cabbage research material (IAEA-359) was analyzed. The results agree quite well with the estimated values given by
the IAEA. 相似文献
367.
Cytokinins, donor plants and their time in vitro as well as basal media were investigated for their influence on shoot regenerative capacity of American elm (Ulmus americana L.) leaves. Leaves excised from six 2-year-old seedlings formed adventitious shoots when placed on Driver and Kuniyuki Walnut (DKW) medium supplemented with 7.5, 15 or 22.5 M of benzyladenine (BA) or thidiazuron (TDZ). Thidiazuron induced significantly higher regeneration percentages on elm leaves than BA, regardless of concentration used. Donor plant also affected the efficiency of shoot regeneration, with certain seedlings having 1.5 to 7 times more explants forming shoots as compared to other seedlings tested. By subculture 15, the average number of shoots per regenerating explant increased at least 3-fold for leaves on media with BA or TDZ for the one donor plant that survived continued subculturing. Leaf explants from donor plants with the highest regenerative capacity had a higher percentage of shoot formation on DKW than MS medium. Explants from productive donor plants should be placed on DKW medium supplemented with TDZ to improve shoot regeneration efficiency from American elm leaves. 相似文献
368.
The nucleotide excision repair (NER) machinery excises a variety of bulky DNA lesions, but with varying efficiencies. The structural features of the DNA lesions that govern these differences are not well understood. An intriguing model system for studying structure-function relationships in NER is the major adduct derived from the reaction of the highly tumorigenic metabolite of benzo[a]pyrene, (+)-anti-benzo[a]pyrene diol epoxide, with the exocyclic amino group of guanine ((+)-trans-anti-[BP]-N2-dG, or G*). The rates of incision of the stereochemically identical lesions catalyzed by the prokaryotic UvrABC system was shown to be greater by a factor of 2.3 ± 0.3 in the TG*T than in the CG*C sequence context [Biochemistry 46 (2007) 7006-7015]. Here we employ molecular dynamics simulations to elucidate the origin of the greater excision efficiency in the TG*T case and, more broadly, to delineate structural parameters that enhance NER. Our results show that the BP aromatic ring system is 5′-directed along the modified strand in the B-DNA minor groove in both sequence contexts. However, the TG*T modified duplex is much more dynamically flexible, featuring more perturbed and mobile Watson-Crick hydrogen bonding adjacent to the lesion, a greater impairment in stacking interactions, more dynamic local roll/bending, and more minor groove flexibility. These characteristics explain a number of experimental observations concerning the (+)-trans-anti-[BP]-N2-dG adduct in double-stranded DNA with the TG*T sequence context: its conformational heterogeneity in NMR solution studies, its highly flexible bend, and its lower thermal stability. By contrast, the CG*C modified duplex is characterized by a single BP conformation and a rigid bend. While current recognition models of bulky lesions by NER factors have stressed the importance of impaired Watson-Crick pairing/stacking and bending, our results highlight the likelihood of an important role for the local dynamics in the vicinity of the lesion. 相似文献
369.
Joseph D. Moore David E. Andersen Thomas R. Cooper Jeffrey P. Duguay Shaun L. Oldenburger C. Al Stewart David G. Krementz 《The Journal of wildlife management》2019,83(7):1617-1627
American woodcock (Scolopax minor; woodcock) migratory connectivity (i.e., association between breeding and wintering areas) is largely unknown, even though current woodcock management is predicated on such associations. Woodcock are currently managed in the Eastern and Central management regions in the United States with the boundary between management regions analogous to the boundary between the Atlantic and Mississippi flyways, based largely on analysis of band returns from hunters. Factors during migration influence survival and fitness, and existing data derived from banding and very high frequency telemetry provide only coarse-scale information to assess factors influencing woodcock migratory movement patterns and behavior. To assess whether current management-region boundaries correspond with woodcock migratory connectivity in the Central Management Region and to describe migration patterns with higher resolution than has been previously possible, we deployed satellite transmitters on 73 woodcock (25 adult and 28 juvenile females, and 8 adult and 12 juvenile males) and recorded 87 autumn or spring migration paths from 2014 to 2016. Marked woodcock used 2 primary migrations routes: a Western Route and a Central Route. The Western Route ran north-south, connecting the breeding and wintering grounds within the Central Management Region. The hourglass-shaped Central Route connected an area on the wintering grounds reaching from Texas to Florida, to sites throughout northeastern North America in both the Eastern Management Region and Central Management Region and woodcock following this route migrated through the area between the Appalachian Mountains and the Mississippi Alluvial Valley in western Tennessee during autumn and spring. Two of 17 woodcock captured associated with breeding areas in Michigan, Wisconsin, or Minnesota migrated to wintering sites in the Eastern Management Region and 12 marked woodcock captured on wintering areas in Texas and Louisiana migrated to breeding sites in the Eastern Management Region. Woodcock that used the Western Route exhibited high concentrations of stopovers during spring in the Arkansas Ozark Mountains and northern Missouri, and along the Mississippi River on the border between Wisconsin and Minnesota, and autumn concentrations of stopovers in southwestern Iowa, central Missouri, the Arkansas portion of the Ozark Mountains, and around the junction of Texas, Louisiana, Oklahoma, and Arkansas. Woodcock that used the Central Route exhibited high concentrations of stopovers during spring in northern Mississippi through western Tennessee, western Kentucky, and the Missouri Bootheel, and autumn concentrations of stopovers in northern Illinois, southwestern Ohio, and the portions of Kentucky and Tennessee west of the Appalachian Mountains. We suggest that current management of woodcock based on 2 management regions may not be consistent with the apparent lack of strong migratory connectivity we observed. Our results also suggest where management of migration habitat might be most beneficial to woodcock. © 2019 The Wildlife Society. 相似文献
370.
《Journal of molecular biology》2022,434(2):167350
Maturation of adenoviruses is distinguished by proteolytic processing of several interior minor capsid proteins and core proteins by the adenoviral protease and subsequent reorganization of adenovirus core. We report the results derived from the icosahedrally averaged cryo-EM structure of a cell entry defective form of adenovirus, designated ts1, at a resolution of 3.7 Å as well as of the localized reconstructions of unique hexons and penton base. The virion structure revealed the structures and organization of precursors of minor capsid proteins, pIIIa, pVI and pVIII, which are closely associated with the hexons on the capsid interior. In addition to a well-ordered helical domain (a.a. 310–397) of pIIIa, highlights of the structure include the precursors of VIII display significantly different structures near the cleavage sites. Moreover, we traced residues 4–96 of the membrane lytic protein (pVI) that includes an amphipathic helix occluded deep in the hexon cavity suggesting the possibility of co-assembly of hexons with the precursors of VI. In addition, we observe a second copy of pVI ordered up to residue L40 in the peripentonal hexons and a few fragments of density corresponding to 2nd and 3rd copies of pVI in other hexons. However, we see no evidence of precursors of VII binding in the hexon cavity. These findings suggest the possibility that differently bound pVI molecules undergo processing at the N-terminal cleavage sites at varying efficiencies, subsequently creating competition between the cleaved and uncleaved forms of VI, followed by reorganization, processing, and release of VI molecules from the hexon cavities. 相似文献