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   183篇
  免费   47篇
  国内免费   36篇
  266篇
  2025年   1篇
  2023年   2篇
  2022年   3篇
  2021年   3篇
  2020年   2篇
  2019年   5篇
  2018年   2篇
  2017年   3篇
  2015年   7篇
  2014年   16篇
  2013年   11篇
  2012年   5篇
  2011年   6篇
  2010年   2篇
  2009年   11篇
  2008年   10篇
  2007年   16篇
  2006年   19篇
  2005年   12篇
  2004年   4篇
  2003年   9篇
  2002年   9篇
  2001年   9篇
  2000年   13篇
  1999年   10篇
  1998年   7篇
  1997年   3篇
  1996年   2篇
  1995年   5篇
  1994年   8篇
  1993年   1篇
  1992年   8篇
  1991年   6篇
  1990年   3篇
  1989年   1篇
  1988年   4篇
  1986年   4篇
  1985年   5篇
  1984年   5篇
  1983年   1篇
  1982年   2篇
  1981年   2篇
  1980年   3篇
  1979年   1篇
  1978年   1篇
  1976年   1篇
  1974年   1篇
  1973年   1篇
  1953年   1篇
排序方式: 共有266条查询结果,搜索用时 15 毫秒
1.
Hai L  Wagner C  Friedt W 《Genetica》2007,130(3):213-225
Genetic diversity in spring bread wheat (T.aestivum L.) was studied in a total of 69 accessions. For this purpose, 52 microsatellite (SSR) markers were used and a total of 406 alleles were detected, of which 182 (44.8%) occurred at a frequency of <5% (rare alleles). The number of alleles per locus ranged from 2 to 14 with an average of 7.81. The largest number of alleles per locus occurred in the B genome (8.65) as␣compared to the A (8.43) and D (5.93) genomes, respectively. The polymorphism index content (PIC) value varied from 0.24 to 0.89 with an average of 0.68. The highest PIC for all accessions was found in the B␣genome (0.71) as compared to the A (0.68) and D␣genomes (0.63). Genetic distance-based method (standard UPGMA clustering) and a model-based method (structure analysis) were used for cluster analysis. The two methods led to analogical results. Analysis of molecular variance (AMOVA) showed that 80.6% of the total variation could be explained by the variance within the geographical groups. In comparison to the diversity detected for all accessions (H e = 0.68), genetic diversity among European spring bread wheats was H e = 0.65. A comparatively higher diversity was observed between wheat varieties from Southern European countries (Austria/Switzerland, Portugal/Spain) corresponding to those from other regions.  相似文献   
2.
    
The work reported in this paper aims at developing an accurate, specific, repeatable and robust HPTLC method for the determination of galanthamine in different Amaryllidaceae plant extracts.  相似文献   
3.
Spring wheat (Triticum aestivum) was grown in the field under ambient and supplemental levels of ultraviolet-B (UV-B, 280–315 nm) radiation to determine the potential for alteration in plant nutrients, decomposition, leaf quality and dry matter yield. Supplemental UV-B radiation simulating a 12, 20 and 25% stratospheric ozone depletion significantly decreased dry matter yield, but had no significant impact on harvest index. UV-B radiation resulted in an increase of the concentrations of N and K in all plant parts; changes of the concentrations of P, Mg, Fe and Zn varied in a tissue-dependent manner, as the decrease of P in leaves and stems, and its increase in spikes and grains. The mass of N, P, K, Mg, Fe and Zn in various plant parts and whole plant was generally decreased except leaf N mass was increased by enhanced UV-B radiation. Enhanced UV-B radiation decreased the concentrations of soluble carbohydrates in leaves and increased that of holocellulose and soluble proteins. After 60 and 100 days of decomposition of leaves and stems in the field, enhanced UV-B radiation stimulated the loss of organic C. As a consequence, the nutrient content of soils might be less diminished under enhanced UV-B radiation.  相似文献   
4.
This paper analyses the dependence of the first spring arrival dates of short/medium- and long-distance migrant bird species on climate warming in eastern Europe. The timing of arrival of the selected species at the observation site correlates with the North Atlantic Oscillation (NAO) index, air temperature, atmospheric pressure, precipitation and wind characteristics. A positive correlation of fluctuations in winter and spring air temperatures with variations in the NAO index has been established in eastern Europe. Positive winter NAO index values are related to earlier spring arrival of birds in the eastern Baltic region and vice versa—arrival is late when the NAO index is negative. The impact of climate warming on the bird’s life cycle depends on local or regional climate characteristics. We tested the hypothesis that differences in climate indices between North Africa and Europe can influence the timing of spring arrival. Our results support the hypothesis that differences in first spring arrival dates between European populations occur after individuals cross the Sahara. We assume that the endogenous programme of migration control in short/medium-distance migrants synchronises with the changing environment on their wintering grounds and along their migration routes, whereas in long-distance migrants it is rather with environmental changes in the second part of their migratory route in Europe. Our results strongly indicate that the mechanism of dynamic balance in the interaction between the endogenous regulatory programme and environmental factors determines the pattern of spring arrival, as well as migration timing.  相似文献   
5.
Due to being a virtually monomorphic wader species, migration dynamics and sex-related migration patterns in the Wood Sandpiper (Tringa glareola) have rarely been investigated. We captured spring migrants at an important stopover site in northeastern Austria. Birds were individually color-marked, and sex was determined by an analysis of DNA from tail feather material. Among temporary residents (birds seen again after day of capture), males migrated on average 3 days earlier than females. However, since sexes did not differ in fat score, the length of stay and the proportion of transients (birds not seen again after day of capture) and temporary residents, we suggest that males and females adopt similar migration strategies in the spring. The large number of transients captured as well as shorter stopover durations in later temporary residents indicate that Wood Sandpipers minimize time at this stage of their northbound migration. Temporary residents earlier in the season exhibited lower fat stores than later ones. Nevertheless, since the fat stores of transients and temporary residents were similar even after the progress of the season had been accounted for, we assume that Wood Sandpipers may afford to exhibit individual flexibility in migration strategy and the use of stopover sites, especially early in the season. This variability may be a necessary adaptation to cope with possible varying environmental conditions at dynamic and unpredictable inland stopover sites. After having reached North Mediterranean regions, mean body mass of spring migrants gradually increases during successive stopovers, indicating that Wood Sandpipers follow a ‘hopping’ migration strategy. This emphasizes the high conservation value of even small artificial mudflat pools as important stepping stones in order to maintain a continuous network of wetland habitats for this continental migrant.  相似文献   
6.
The aim of this study was to investigate patterns of abundance, distribution, temporal changes and species composition of the dominant ice-associated copepods in the spring annual pack ice, platelet ice and water column at Terra Nova Bay, Ross Sea, during late spring 1997. Ice cores were drilled for temporal and spatial scales. Stephos longipes and Harpacticus furcifer dominated the sea ice meiofauna in terms of numbers in the lower few centimeters of the bottom ice associated with high chlorophyll a and phaeopigment levels. Nauplii dominated the S. longipes population (91.6%) and occurred in extremely high concentrations. In contrast, copepodids were the dominant stages in H. furcifer. How H. furcifer carries out its entire life cycle and how it differs from ecologically similar species such as Drescheriella glacialis should be examined in more detail.  相似文献   
7.
In contrast to the attention attracted by temporal trends of phenology, the spatial patterns of arrivals, departures or stays of trans-Saharan birds are still nowadays largely unknown in most of their European breeding areas. In the case of the white stork (Ciconia ciconia), some studies have attempted to describe its migratory patterns throughout some European countries but, to our knowledge, no one has related these patterns to some kind of explanatory variable which offers an ecologically-based explanation for the heterogeneous phenology observable among populations. Here, arrivals, departures and stays of this species, recorded in hundreds of Spanish localities, were related to a set of environmental, geographical, biological and spatial predictors, and modeled by multiple regression. The best model for arrival dates accounted for up to 34% of variability of data and pointed towards an earlier arrival in those populations located in south-western Iberia and with higher population densities. This last relationship is probably due to the competition for nest-site fidelity maintenance. However, no variable was able to explain properly the blurred spatial pattern recorded for departure dates. Departure decisions are strongly influenced by social behavior in this species and depend on collective decisions influenced by peculiar local environmental conditions of each year rather than macrogeographical gradients. Environmental, geographical or spatial variables also did not capture much of the observed variability in the length of the stays among populations. However, this variable was strongly related to the arrival and departure dates of populations. White storks stay longer in localities with earlier arrivals and, especially, later departures.  相似文献   
8.
  总被引:3,自引:0,他引:3  
The lysosomal enzyme alpha-mannosidase-1 is one of the earliest developmentally controlled gene products in Dictyostelium discoideum. Although this enzyme is synthesized throughout the first 20 h of development, it is not required for complete morphogenesis, since structural gene (manA) mutants lacking activity develop normally. We isolated six strains deficient in alpha-mannosidase-1 activity which, unlike structural gene mutants, fail to aggregate. Fruiting revertants of these strains accumulate wild-type levels of alpha-mannosidase-1 activity, suggesting that both the enzymatic and morphological defects are caused by single mutations in nonstructural genes essential for early development. Direct genetic evidence for mutations outside of the structural gene was obtained by complementation analysis. We used alpha-mannosidase-1-specific monoclonal antibodies to analyze the biochemical defects in these mad (alpha-mannosidase-1-deficient) mutants. All mad mutants show a significantly reduced relative rate of enzyme precursor biosynthesis. The mad-404 mutation results in a complete lack of precursor biosynthesis, as well as a lack of functional alpha-mannosidase-1 mRNA. In some cases, however, the enzymatic defect results from improper post-translational modification which affects precursor processing. We conclude that a small number of aggregation-essential genes are involved in regulating the synthesis, modification, and processing of alpha-mannosidase-1 during development.  相似文献   
9.
The publication in 1962 of Rachel Carson's Silent Spring marks the mid-point in a century that saw, in its first half, the emergence of public health concerns related to human exposures to chemicals, and, in its second half, the emergence of public policies to deal with those concerns. Those policies made it imperative that the scientific community come to grips with the problem of identifying exposure levels not likely to cause harm. This problem was not significantly discussed within the scientific community until the 1950s, and well-described methods for practical solutions to it did not appear until the 1970s. An important report from the National Academy of Sciences, published in 1983 (Risk Assessment in the Federal Government), provided an analysis of these emerging methods, and recommended a useful framework for the assessment and management of risk. This framework remains central to public health and regulatory decision-making. A high-level perspective is offered on events leading to and following the 1983 report. The article describes early thinking about chemical toxicity and the scientific path that thinking followed through the 20th century, and to the present.  相似文献   
10.
Production of C3 as a marker of lymphokine-mediated macrophage activation   总被引:1,自引:0,他引:1  
C3 production was assayed using an enzyme-linked immunosorbent assay (ELISA) in cell-free supernatants harvested from thioglycollate-elicited macrophages exposed to a variety of macrophage stimulating and activating agents. Macrophage monolayers treated with the stimulating agents starch, glycogen, and zymosan secreted three- to four-fold less C3 (mean 12 ng/10(5) cells/12 hr) than macrophages exposed to lymphokines containing macrophage-activating factor (MAF) (mean C3 production 44 ng/10(5) cells/12 hr). The increased production of C3 in macrophages exposed to MAF parallels the ability of these macrophages to acquire tumoricidal capacity as monitored in an in vitro 72 hr tumor cell cytotoxicity assay using B16 melanoma cells. Macrophages previously rendered tumoricidal by exposure to MAF and which are refractory to further challenge by MAF following decay of their tumoricidal properties, do not produce C3 on rechallenge with MAF. Exposure of refractory macrophages to liposome-encapsulated MAF overcomes the refractory state and induces re-expression of the tumoricidal phenotype and C3 production. We conclude that quantitative detection of macrophage-generated C3 antigen provides a useful biochemical marker for monitoring the acquisition of tumoricidal properties in macrophages exposed to MAF and offers a sensitive assay for screening novel agents that activate macrophages via mechanisms similar to MAF.  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