全文获取类型
收费全文 | 907篇 |
免费 | 65篇 |
出版年
2023年 | 7篇 |
2020年 | 14篇 |
2019年 | 5篇 |
2018年 | 15篇 |
2017年 | 25篇 |
2016年 | 23篇 |
2015年 | 20篇 |
2014年 | 26篇 |
2013年 | 65篇 |
2012年 | 78篇 |
2011年 | 93篇 |
2010年 | 49篇 |
2009年 | 33篇 |
2008年 | 24篇 |
2007年 | 20篇 |
2006年 | 15篇 |
2005年 | 17篇 |
2004年 | 11篇 |
2003年 | 12篇 |
2002年 | 18篇 |
2001年 | 17篇 |
2000年 | 9篇 |
1999年 | 23篇 |
1998年 | 16篇 |
1997年 | 7篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 11篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 5篇 |
1984年 | 16篇 |
1983年 | 20篇 |
1982年 | 16篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 16篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 7篇 |
1972年 | 8篇 |
排序方式: 共有972条查询结果,搜索用时 31 毫秒
51.
The results of the present study show that inoculation with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker and Couch potentially enhances the germination of Scots pine (Pinus sylvestris L.) somatic embryos in vitro. Stimulation by Pisolithus tinctorius was only observed in the absence of direct contact between the symbionts; mature embryos were not sufficiently robust for balanced interaction with the fungus on half-strength DCR medium. Subsequently, on MMN medium with a reduced sugar concentration, direct contact between somatic embryo-derived plants and the fungus resulted in in vitro formation of mycorrhiza. Ex vitro inoculation also improved adaptation of the somatic embryo-derived plants, even though mycorrhizal structures were not observed. The reactions to Pisolithus tinctorius varied between different Scots pine cell lines both in vitro and ex vitro. 相似文献
52.
A family of four closely related PDZ domain-containing membrane-associated guanylate kinase homologues (MAGUKs) is involved in the regulation of the amount and functional state of ionotropic glutamate receptors in excitatory synapses. To understand the mechanisms that determine the specificity of these interactions, we examined the structural basis of the highly selective association between the ionotropic GluR subunit GluR-A and synapse-associated protein 97 (SAP97). The C terminus of GluR-A bound to the PDZ domains of SAP97, but not to those of three related MAGUKs, PSD-93, PSD-95, and SAP102. Experiments with single PDZ domains indicated that the strongest contribution was by the second PDZ domain. Unexpectedly, mutation analysis of the GluR-A C terminus revealed that a tripeptide sequence SSG at position -9 to -11 plays an essential role in this binding, in addition to a C-terminal type I PDZ binding motif (leucine at C terminus and threonine at the -2 position). Analysis of the in vitro MAGUK-binding properties of a GluR-D mutant with a one-residue deletion at the C terminus provides further support for the view that an SSG sequence located N-terminally from a type I PDZ binding motif can mediate selective binding to SAP97 and suggest the existence of a novel variation of the PDZ domain-peptide interaction. 相似文献
53.
Albertsson PA 《Photosynthesis research》2003,76(1-3):217-225
The role of photosynthetic pigments in the development of separation methods in biochemistry during the period 1900-1980 is described beginning with M. Tswett who introduced separation of chlorophylls and carotenoids on columns and coined the term chromatography in 1906. In Uppsala, T. Svedberg developed the ultracentrifuge in the 1920s. A. Tiselius improved electrophoresis in the 1930s and developed chromatography of proteins in the 1940s and 1950s. Others of 'The Uppsala school in separation science' include J. Porath, P. Flodin and S. Hjertén who further developed various gel chromatographic methods. Hjertén introduced free zone electrophoresis in narrow tubes, a forerunner of capillary electrophoresis. Two proteins, phycoerythrin and phycocyanin, were used as test substances in all these methodological studies. Aqueous two-phase partitioning as a separation method was introduced in 1956 by the author. In this work, chloroplast particles were used, and the method was applied for the separation and purification of intact chloroplasts, inside-out thylakoid vesicles and plasma membranes. My research was carried out in cooperation with G. Blomquist, G. Johansson, C. Larsson, B. Andersson and H.-E. Akerlund during a 20-year period, 1960-1980. 相似文献
54.
Schiött A Johansson AC Widegren B Sjögren HO Lindvall M 《Cancer immunology, immunotherapy : CII》2000,48(10):579-587
The cytokine transforming growth factor β-1 (TGFβ1), was transfected into a TGFβ1-negative rat colon carcinoma. The growth
of isografts of TGFβ1-expressing tumors was compared to that of vector control transfectants. The TGFβ1 transfectant grew
significantly more slowly after intrahepatic isografting than did vector control and wild-type tumors. The TGFβ1-transfected
tumor tissue had significantly greater infiltration of both CD4+ and CD8+ T lymphocytes than did the vector control tumor. The tumor-infiltrating leukocytes (TIL) from TGFβ1-transfected tumor secreted
significantly more of the cytokines interleukin-10 (IL-10) and tumor necrosis factor α (TNFα) than did TIL from the vector
control tumor. The TGFβ1 transfectant also demonstrated a significantly slower outgrowth in immunodeficient SCID mice, supporting
a non-T-lymphocyte-dependent mechanism for the tumor retardation. In SCID mice, the TGFβ1-transfected tumor demonstrated significantly
greater infiltration of both granulocytes and macrophages than did the vector control transfectant. We also demonstrated a
direct inhibitory effect of rat TNFα on tumor proliferation in vitro. These results suggest that TGFβ1 induces a local secretion
of immunomodulating cytokines and that this may influence monocytes, lymphocytes and granulocytes to retard tumor outgrowth.
Received: 7 July 1999 / Accepted: 12 August 1999 相似文献
55.
Embryogenic cell masses of three Scots pine (Pinus sylvestris) cell lines K779, K884 and K1009 were cultivated with the ectomycorrhizal (ECM) fungi Laccaria bicolor, L. proxima,
Pisolithus tinctorius, Paxillus involutus and two strains of Suillus variegatus. The average growth ratio of the slowly proliferating cell line K1009 was improved by L. proxima and S. variegatus strain H, while of the rapidly proliferating lines K779 and K884 the non-mycorrhizal controls grew best. The fungi caused
two distinct reactions in embryogenic cultures. In the positive reaction, the shape and light yellow colour of the cultures
resembled the controls, while in the negative reaction the embryogenic cells became brown and necrotic and the fungi grew
aggressively over them. These reactions to the fungi did not correlate completely with effects on the growth ratio. All the
cell lines enhanced the radial growth of S. variegatus H and of P. tinctorius, while the Laccaria species and S. variegatus strain 1 thrived better alone. This study shows that early-stage embryogenic cells of Scots pine and ECM fungi are able to
interact. As some fungi produced a positive reaction or even increased proliferation, they could be used to enhance somatic
embryogenesis of Scots pine. Specific fungi might be used to induce the growth of slowly proliferating cell lines, and knowledge
of positive cell line-fungus interactions could be useful in work with later stages of somatic embryogenesis, such as rooting.
Accepted: 16 July 1998 相似文献
56.
Biomanipulation as an Application of Food-Chain Theory: Constraints, Synthesis, and Recommendations for Temperate Lakes 总被引:25,自引:2,他引:23
Lars-Anders Hansson Helene Annadotter Eva Bergman Stellan F. Hamrin Erik Jeppesen Timo Kairesalo Eira Luokkanen Per-Åke Nilsson Martin Søndergaard John Strand 《Ecosystems》1998,1(6):558-574
The aim of this review is to identify problems, find general patterns, and extract recommendations for successful biomanipulation.
An important conclusion is that the pelagic food chain from fish to algae may not be the only process affected by a biomanipulation.
Instead, this process should be viewed as the “trigger” for secondary processes, such as establishment of submerged macrophytes,
reduced internal loading of nutrients, and reduced resuspension of particles from the sediment. However, fish reduction also
leads to a high recruitment of young-of-the-year (YOY) fish, which feed extensively on zooplankton. This expansion of YOY
the first years after fish reduction is probably a major reason for less successful biomanipulations. Recent, large-scale
biomanipulations have made it possible to update earlier recommendations regarding when, where, and how biomanipulation should
be performed. More applicable recommendations include (1) the reduction in the biomass of planktivorous fish should be 75%
or more; (2) the fish reduction should be performed efficiently and rapidly (within 1–3 years); (3) efforts should be made
to reduce the number of benthic feeding fish; (4) the recruitment of YOY fish should be reduced; (5) the conditions for establishment
of submerged macrophytes should be improved; and (6) the external input of nutrients (phosphorus and nitrogen) should be reduced
as much as possible before the biomanipulation. Recent biomanipulations have shown that, correctly performed, the method also
achieves results in large, relatively deep and eutrophic lakes, at least in a 5-year perspective. Although repeated measures
may be necessary, the general conclusion is that biomanipulation is not only possible, but also a relatively inexpensive and
attractive method for management of eutrophic lakes, and in particular as a follow-up measure to reduced nutrient load.
Received 14 April 1998; accepted 31 August 1998 相似文献
57.
Jens Dauber Susannah Cass Doreen Gabriel Kate Harte Sandra Åström Erin O'Rourke Jane C. Stout 《Global Change Biology Bioenergy》2015,7(3):455-467
Increasing crop productivity to meet rising demands for food and energy, but doing so in an environmentally sustainable manner, is one of the greatest challenges for agriculture to date. In Ireland, Miscanthus × giganteus has the potential to become a major feedstock for bioenergy production, but the economic feasibility of its cultivation depends on high yields. Miscanthus fields can have a large number of gaps in crop cover, adversely impacting yield and hence economic viability. Predominantly positive effects of Miscanthus on biodiversity reported from previous research might be attributable to high crop patchiness, particularly during the establishment phase. The aim of this research was to assess crop patchiness on a field scale and to analyse the relationship between Miscanthus yield and species richness and abundance of selected taxa of farmland wildlife. For 14 Miscanthus fields at the end of their establishment phase (4–5 years after planting), which had been planted either on improved grassland (MG) or tilled arable land (MT), we determined patchiness of the crop cover, percentage light penetration (LP) to the lower canopy, Miscanthus shoot density and height, vascular plants and epigeic arthropods. Plant species richness and noncrop vegetation cover in Miscanthus fields increased with increasing patchiness, due to higher levels of LP to the lower canopy. The species richness of ground beetles and the activity density of spiders followed the increase in vegetation cover. Plant species richness and activity density of spiders on both MT and MG fields, as well as vegetation cover and activity density of ground beetles on MG fields, were negatively associated with Miscanthus yield. In conclusion, positive effects of Miscanthus on biodiversity can diminish with increasing productivity. This matter needs to be considered when assessing the relative ecological impacts of developing biomass crops in comparison with other land use. 相似文献
58.
59.
Åke Lindström Juliana Dänhardt Martin Green Raymond H. G. Klaassen Peter Olsson 《Journal of avian biology》2010,41(2):154-162
Today's intensive farming practices are known to have affected farmland biodiversity negatively in many different ways. As far as birds are concerned, they are known to have suffered during both summer and winter. Relatively little is known about the effects on birds during migration. We studied the stopover ecology of the Eurasian golden plover Pluvialis apricaria, a species listed in EU Birds Directive, in intensively farmed arable land in southernmost Sweden in the autumns of 2003–2007. We used key ecological variables (length of stay, fat deposition and moult) as fitness proxies to evaluate how the birds manage in this habitat. Eurasian golden plovers were present in large numbers mainly on arable fields from early August to November and radio‐tagged birds were found to stay in the area for up to three months. Adult birds carried out a substantial part of their flight feather moult during their stay. Body mass increased only somewhat during moult, but from the last stages of moult and onwards fuel loads corresponding to 24% above lean body mass (LBM) were accumulated at a rate of 0.5% of LBM per day, before the birds departed. Juveniles arrived later, from mid Sep., and had a similar pattern of fuel deposition. The fact that the birds choose to stay for long periods, moult in the area, and manage to store substantial fuel loads strongly suggests that Eurasian golden plovers do well in this intensively farmed arable land. 相似文献
60.
Benedicte R. Albrectsen Johanna Witzell Kathryn M. Robinson Sören Wulff Virginia M. C. Luquez Rickard Ågren Stefan Jansson 《Ecography》2010,33(3):483-493
According to the geographic mosaic theory of coevolution (GMTC), clines of traits reflecting local co‐adaptation (including resistance genes) should be common between a host and its parasite and should persist across time. To test the GMTC‐assumption of persistent clinal patterns we compared the natural prevalence of two parasites on aspen Populus tremula trees: mining moths of the genus Phyllocnistis and leaf rust Melampsora spp. Damage data were collated from the Swedish National Forest Damage Inventory (2004–2006). In addition, occurrence of the parasites was scored in field conditions in two common gardens in the north and south of Sweden over five growing seasons (2004–2008), then related to biomass (stem height and diameter) and to concentrations of eleven leaf phenolics. Phyllocnistis mainly occurred in the northern garden, a distribution range which was confirmed by the countrywide inventory, although Phyllocnistis was more abundant on southern clones, providing evidence for possible local maladaptation. Melampsora occurred all over the country and in both gardens, but built up more quickly on northern clones, which suggests a centre of local clone maladaptation in the north. Stem growth also followed a clinal pattern as did the concentration of three phenolic compounds: benzoic acid, catechin and cinnamic acid. However, only benzoic acid was related to parasite presence: negatively to Phyllocnistis and positively to Melampsora and it could thus be a potential trait under selection. In conclusion, clines of Phyllocnistis were stronger and more persistent compared to Melampsora, which showed contrasting clines of varying strength. Our data thus support the assumption of the GMTC model that clines exist in the border between hot and cold spots and that they may be less persistent for parasites with an elevated gene flow, and/or for parasites which cover relatively larger hot spots surrounded by fewer cold spots. 相似文献