全文获取类型
收费全文 | 321篇 |
免费 | 73篇 |
出版年
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 15篇 |
2015年 | 15篇 |
2014年 | 17篇 |
2013年 | 23篇 |
2012年 | 23篇 |
2011年 | 20篇 |
2010年 | 16篇 |
2009年 | 7篇 |
2008年 | 13篇 |
2007年 | 13篇 |
2006年 | 8篇 |
2005年 | 9篇 |
2004年 | 7篇 |
2003年 | 5篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 8篇 |
1991年 | 7篇 |
1990年 | 13篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 4篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 10篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1927年 | 1篇 |
排序方式: 共有394条查询结果,搜索用时 15 毫秒
1.
J.J.M. Meulenberg E. Sellink W.A.M. Loenen N.H. Riegman M. Van Kleef P.W. Postma 《FEMS microbiology letters》1990,71(3):337-343
We have cloned genes from Klebsiella pneumoniae which are required for pyrroloquinoline quinone (PQQ) biosynthesis. The cloned 6.7 kb fragment can complement several chromosomal pqq mutants. Escherichia coli strains are unable to synthesize PQQ but E. coli strains containing the cloned 6.7 kb K. pneumoniae fragment can synthesize PQQ in large amounts and E. coli pts mutants can be complemented on minimal glucose medium by this clone. 相似文献
2.
Genetically stable cell lines of cucumber for the large-scale production of diploid somatic embryos 总被引:2,自引:0,他引:2
Marc Kreuger Wiert van der Meer Erik Postma Rob Abbestee Natasja Raaijmakers Gerrit-Jan van Holst 《Physiologia plantarum》1996,97(2):303-310
For the initiation of embryogenic cucumber ( Cucumis sativus L.) cell lines, from excised radicles, directly in liquid medium, the culture regime, explant density and type and concentration of hormones were adjusted so that pro-embryogenic masses (PEMs) were formed within about 8 weeks. The established cucumber cell lines were maintained tor several years without loss of embryogenic and genetic stability. The ploidy level of somatic embryos from different cucumber eell lines was either diploid or tetraploid and depended on the ploidy level of Ihe cell line. Cucumber cell lines that produced only diploid embryos were obtained by selecting completely diploid explant material and growing it in the dark during the initiation phase. Mixoploid explains could lead to tetraploid or mixoploid ceil lines. Isolation and additional selection and subculturing of single PEMs resulted in either completely diploid or tetraploid cell lines, indicating that all cells of individual PEMs are either diploid or tetraploid. The ernbryogenic cucumber cell Imes, differing only in ploidy level, were indistinguishable in growth rate and embryogetiic potential and were genetically stable over several years. 相似文献
3.
Complementarity in root architecture for nutrient uptake in ancient maize/bean and maize/bean/squash polycultures 总被引:1,自引:0,他引:1
Background and Aims
During their domestication, maize, bean and squash evolved in polycultures grown by small-scale farmers in the Americas. Polycultures often overyield on low-fertility soils, which are a primary production constraint in low-input agriculture. We hypothesized that root architectural differences among these crops causes niche complementarity and thereby greater nutrient acquisition than corresponding monocultures.Methods
A functional–structural plant model, SimRoot, was used to simulate the first 40 d of growth of these crops in monoculture and polyculture and to determine the effects of root competition on nutrient uptake and biomass production of each plant on low-nitrogen, -phosphorus and -potassium soils.Key Results
Squash, the earliest domesticated crop, was most sensitive to low soil fertility, while bean, the most recently domesticated crop, was least sensitive to low soil fertility. Nitrate uptake and biomass production were up to 7 % greater in the polycultures than in the monocultures, but only when root architecture was taken into account. Enhanced nitrogen capture in polycultures was independent of nitrogen fixation by bean. Root competition had negligible effects on phosphorus or potassium uptake or biomass production.Conclusions
We conclude that spatial niche differentiation caused by differences in root architecture allows polycultures to overyield when plants are competing for mobile soil resources. However, direct competition for immobile resources might be negligible in agricultural systems. Interspecies root spacing may also be too large to allow maize to benefit from root exudates of bean or squash. Above-ground competition for light, however, may have strong feedbacks on root foraging for immobile nutrients, which may increase cereal growth more than it will decrease the growth of the other crops. We note that the order of domestication of crops correlates with increasing nutrient efficiency, rather than production potential. 相似文献4.
5.
6.
Quantification of the regulation of glycerol and maltose metabolism by IIAGlc of the phosphoenolpyruvate-dependent glucose phosphotransferase system in Salmonella typhimurium. 总被引:1,自引:0,他引:1 下载免费PDF全文
The amount of IIAGlc, one of the proteins of the phosphoenolpyruvate:glucose phosphotransferase system (PTS), was modulated over a broad range with the help of inducible expression plasmids in Salmonella typhimurium. The in vivo effects of different levels of IIAGlc on glycerol and maltose metabolism were studied. The inhibition of glycerol uptake, by the addition of a PTS sugar, was sigmoidally related to the amount of IIAGlc. For complete inhibition of glycerol uptake, a minimal ratio of about 3.6 mol of IIAGlc to 1 mol of glycerol kinase (tetramer) was required. Varying the level of IIAGlc (from 0 to 1,000% of the wild-type level) did not affect the growth rate on glycerol, the rate of glycerol uptake, or the synthesis of glycerol kinase. In contrast, the growth rate on maltose, the rate of maltose uptake, and the synthesis of the maltose-binding protein increased two- to fivefold with increasing levels of IIAGlc. In the presence of cyclic AMP, the maximal levels were obtained at all IIAGlc concentrations. The synthesis of the MalK protein, the target of IIAGlc, was not affected by varying the levels of IIAGlc. The inhibition of maltose uptake was sigmoidally related to the amount of IIAGlc. For complete inhibition of maltose uptake by a PTS sugar, a ratio of about 18 mol of IIAGlc to 1 mol of MalK protein (taken as a dimer) was required. 相似文献
7.
Regulation of glycerol kinase by enzyme IIIGlc of the phosphoenolpyruvate:carbohydrate phosphotransferase system. 总被引:6,自引:6,他引:0 下载免费PDF全文
Wild-type glycerol kinase of Escherichia coli is inhibited by both nonphosphorylated enzyme IIIGlc of the phosphoenolpyruvate:carbohydrate phosphotransferase system and fructose 1,6-diphosphate. Mutant glycerol kinase, resistant to inhibition by fructose 1,6-diphosphate, was much less sensitive to inhibition by enzyme IIIGlc. The difference between the wild-type and mutant enzymes was even greater when inhibition was measured in the presence of both enzyme IIIGlc and fructose 1,6-diphosphate. The binding of enzyme IIIGlc to glycerol kinase required the presence of the substrate glycerol. 相似文献
8.
9.
Among‐year variation in selection during early life stages and the genetic basis of fitness in Arabidopsis thaliana 下载免费PDF全文
Incomplete information regarding both selection regimes and the genetic basis of fitness limits our understanding of adaptive evolution. Among‐year variation in the genetic basis of fitness is rarely quantified, and estimates of selection are typically based on single components of fitness, thus potentially missing conflicting selection acting during other life‐history stages. Here, we examined among‐year variation in selection on a key life‐history trait and the genetic basis of fitness covering the whole life cycle in the annual plant Arabidopsis thaliana. We planted freshly matured seeds of >200 recombinant inbred lines (RILs) derived from a cross between two locally adapted populations (Italy and Sweden), and both parental genotypes at the native site of the Swedish population in three consecutive years. We quantified selection against the nonlocal Italian genotype, mapped quantitative trait loci (QTL) for fitness and its components, and quantified selection on timing of germination during different life stages. In all 3 years, the local Swedish genotype outperformed the nonlocal Italian genotype. However, both the contribution of early life stages to relative fitness, and the effects of fitness QTL varied among years. Timing of germination was under conflicting selection through seedling establishment vs. adult survival and fecundity, and both the direction and magnitude of net selection varied among years. Our results demonstrate that selection during early life stages and the genetic basis of fitness can vary markedly among years, emphasizing the need for multiyear studies considering the whole life cycle for a full understanding of natural selection and mechanisms maintaining local adaptation. 相似文献
10.
In recent years the study of root phenotypic plasticity in response to sub-optimal environmental factors and the genetic control of these responses have received renewed attention. As a path to increased productivity, in particular for low fertility soils, several applied research projects worldwide target the improvement of crop root traits both in plant breeding and biotechnology contexts. To assist these tasks and address the challenge of optimizing root growth and architecture for enhanced mineral resource use, the development of realistic simulation models is of great importance. We review this research field from a modeling perspective focusing particularly on nutrient acquisition strategies for crop production on low nitrogen and low phosphorous soils. Soil heterogeneity and the dynamics of nutrient availability in the soil pose a challenging environment in which plants have to forage efficiently for nutrients in order to maintain their internal nutrient homeostasis throughout their life cycle. Mathematical models assist in understanding plant growth strategies and associated root phenes that have potential to be tested and introduced in physiological breeding programs. At the same time, we stress that it is necessary to carefully consider model assumptions and development from a whole plant-resource allocation perspective and to introduce or refine modules simulating explicitly root growth and architecture dynamics through ontogeny with reference to key factors that constrain root growth. In this view it is important to understand negative feedbacks such as plant–plant competition. We conclude by briefly touching on available and developing technologies for quantitative root phenotyping from lab to field, from quantification of partial root profiles in the field to 3D reconstruction of whole root systems. Finally, we discuss how these approaches can and should be tightly linked to modeling to explore the root phenome. 相似文献