全文获取类型
收费全文 | 6197篇 |
免费 | 554篇 |
国内免费 | 1篇 |
专业分类
6752篇 |
出版年
2024年 | 7篇 |
2023年 | 56篇 |
2022年 | 147篇 |
2021年 | 244篇 |
2020年 | 129篇 |
2019年 | 126篇 |
2018年 | 147篇 |
2017年 | 138篇 |
2016年 | 225篇 |
2015年 | 338篇 |
2014年 | 386篇 |
2013年 | 460篇 |
2012年 | 573篇 |
2011年 | 605篇 |
2010年 | 321篇 |
2009年 | 287篇 |
2008年 | 332篇 |
2007年 | 348篇 |
2006年 | 331篇 |
2005年 | 279篇 |
2004年 | 244篇 |
2003年 | 244篇 |
2002年 | 245篇 |
2001年 | 42篇 |
2000年 | 34篇 |
1999年 | 56篇 |
1998年 | 49篇 |
1997年 | 42篇 |
1996年 | 34篇 |
1995年 | 32篇 |
1994年 | 25篇 |
1993年 | 18篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 13篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1961年 | 5篇 |
1960年 | 5篇 |
排序方式: 共有6752条查询结果,搜索用时 15 毫秒
21.
22.
Rachel Y. Reams H. Leon Thacker Daniel D. Harrington Meliton N. Novilla Billie Wilson 《Mycopathologia》1996,135(2):115-118
Aspergillus flavus Link ex Fries and A. parasiticus Speare can invade peanut kernels and under certain environmental conditions produce unacceptable levels of the mycotoxin aflatoxin. A concerted effort is underway to reduce aflatoxin contamination in peanut and peanut products. A potentially effective method of control in peanut is the discovery and use of genes for resistance to either fungal invasion or aflatoxin formation. The objective of the present experimental study was to develop an effective and efficient procedure for screening individual plants or pods of single plants for resistance to invasion by the aflatoxigenic fungi and subsequent aflatoxin production. Methods of obtaining adequate drought-stress and fungal infection were developed through this series of experiments. By completely isolating the pods from the root zone and imposing drought-stress only on pegs and pods, high levels of fungal infection were observed. High amounts of preharvest aflatoxin accumulation were also produced by completely isolating the pods from the root zone. Mid-bloom inoculation with A. parasiticus-contaminated cracked corn and drought-stress periods of 40 to 60 days were the most effective procedures. This technique was used to assess peanut genotypes previously identified as being partially resistant to A. parasiticus infection or aflatoxin contamination, and segregating populations from four crosses. Variability in aflatoxin contamination was found among the 11 genotypes evaluated, however, none were significantly lower than the standard cultivars. Broad-sense heritability of four crosses was estimated through evaluation of seed from individual plants in the F2 generation. The heritability estimates of crosses GFA-2 × NC-V11 and Tifton-8 × NC-V11 were 0.46 and 0.29, respectively, but mean aflatoxin contamination levels were high (73,295 and 27,305 ppb). This greenhouse screening method could be an effective tool when genes for superior aflatoxin resistance are identified.Cooperative investigation of the USDA-ARS and the University of Georgia, College of Agriculture. 相似文献
23.
Measures of geographic range size: the effects of sample size 总被引:2,自引:0,他引:2
A number of methods have been used for quantifying the sizes of the geographic ranges of species. The consequences of different levels of sampling (the proportion of actual spatial occurrences) are explored for eight of these, using data on the occurrences of butterfly species on a 10 × 10 km grid across Britain. For all methods, the percentage error of estimation (PEE) decreases with the number of 10 × 10 km squares which a species occupies, most rapidly for extent measures, and more rapidly for area measures than for measures of numbers of units occupied. The rate of decline in PEE itself falls as sampling effort increases. At a given sampling level, rank correlations between range sizes measured by different methods are generally high, but there is no consistent change in the magnitude of these correlations as the level of sampling increases. The composition of the set of species with the smallest range sizes changes with the level of sampling. 相似文献
24.
The involvement of a vanadate-sensitive ATPase in plasma membranes of a salt tolerant cyanobacterium
Rachel Gabbay-Azaria Uri Pick Gozal Ben-Hayyim Elisha Tel-Or 《Physiologia plantarum》1994,90(4):692-698
Plasma membranes of the marine cyanobacterium Spirulina subsalsa were tested for ATPase activity, and for involvement in salt stress. Transition of cells from saline to hypersaline medium enhances the respiratory activity associated with extrusion of Na+ and Cl− , and persisting salt stress induces synthesis of respiratory enzymes in the plasma membranes. The membranes possess an ATPase, specific for ATP and Mg2+ and sensitive to orthovanadate and dicyclohexylcarbodiimide. Immunoblot analysis of plasma membrane polypeptides from Spirulina subsalsa with anti- Arabidopsis H+ -ATPase serum identified a single polypeptide of 100 kDa, which cross-reacted with the antibodies. An unusual feature of this ATPase is a specific stimulation by Na+ ions. Prolonged adaptation of S. subsals cells to hypersaline conditions induced an increase in ATPase activity in subsequent plasma membrane preparations, as well as a higher content of the 100 kDa polypeptide. It is suggested that the ATPase investigated is an H+ -pump, which is involved in extrusion of Na+ and in conferring resistance to salt stress. 相似文献
25.
The genetic code, once thought to be rigid, hag been found to be quite fiexible, permitting several different reading alternatives. One of these is translatlonal frameshifting, a process programmed in the mRNA sequence and which enables a +1 or -1 shift from the reading frame of the initiation codon. So far, the Involvement of translatlonal frameahifting in gene expression has been described mainly in viruses (particularly retroviruses), retrotransposons, and bacterial insertion elements, in this MicroReview., we present a survey of the cellular genes, mostly in Escherichia coil, which have been found to be expressed through a transiational frameshifting process, as well as a discussion of the regulatory implications of this process. 相似文献
26.
Yuval Cohen Vaishali P. Chitnis Rachel Nechushtai Parag R. Chitnis 《Plant molecular biology》1993,23(4):895-900
We studied assembly of the PsaE subunit of photosystem I into photosynthetic membranes of cyanobacterial mutant strains that lack specific photosystem I subunits. Radiolabeled PsaE was incubated with photosynthetic membranes, and their binding and assembly were assayed by resistance to removal by chaotropic agents and proteolytic digestion. PsaE incorporated into the wild-type membranes was resistant to these treatments. In the absence of PsaD, it was resistant to proteolytic digestion, but was removed by NaBr. When the membranes were isolated from a mutant strain in which the psaF and psaJ genes have been inactivated, PsaE assembled in vitro could not be removed. PsaE could associate with the membranes of the strain DF in which the psaD, psaJ and psaF genes have been mutated. However, the radiolabeled PsaE associated with these membranes was removed both by the proteolytic as well as by the chaotropic agents. Characterization of PsaE present in vivo revealed similar results. These observations suggest that PsaD and PsaF/J may interact with PsaE and stabilize it in the photosystem I complex. 相似文献
27.
Avraham Geier Rachel Beery Michal Haimsohn Rina Hemi Zvi Malik Avraham Karasik 《In vitro cellular & developmental biology. Animal》1994,30(12):867-874
Summary The ability of epidermal growth factor (EGF), insulinlike growth factor-1 (IGF-1), insulin, 12-O-tetradecanoylphorbol-13-acetate
(TPA), and aurintricarboxylic acid (ATA) to protect the human breast cancer cell line MDA-231 from death induced by the anticancer
drug adriamycin was investigated. Cell death was induced in the MDA-231 cells either by a short-time exposure to a high dose
of adriamycin (2 μg · ml−1 · 1 h−1) and further culturing in the absence of the drug, or by continuous exposure to a low dose of adriamycin (0.3μg/ml). Cell death was evaluated after 48 h of incubation by several techniques (trypan blue dye exclusion, lactic dehydrogenase
activity, cellular ATP content, transmission electron microscopy, and DNA fragmentation). EGF, TPA, and ATA, each at an optimal
concentration of 20 ng/ml, 5 ng/ml, and 100μg/ml respectively, substantially enhanced survival of cells exposed either to a high or low dose of adriamycin. Neither IGF-1
nor insulin, each at concentrations of 20 ng/ml, had an effect on cell survival. The three survival factors enhanced protein
synthesis in the untreated cells and attenuated the continuous decrease in protein synthesis in the adriamycin-treated cells.
Moreover, the three survival factors protected the MDA-231 cells from death in the absence of protein synthesis (cycloheximide
30μg/ml). These results suggest that EGF, TPA, and ATA promote survival of adriamycin pretreated cells by at least two mechanisms:
enhancement of protein synthesis and by a protein synthesis independent process, probably a posttranslational modification
effect. 相似文献
28.
Jessica N. Hightower Dolly L. Crawford Wayne E. Thogmartin Kyle R. Aldinger Sara Barker Swarthout David A. Buehler John Confer Christian Friis Jeffery L. Larkin James D. Lowe Martin Piorkowski Ronald W. Rohrbaugh Kenneth V. Rosenberg Curtis Smalling Petra B. Wood Rachel Vallender Amber M. Roth 《Diversity & distributions》2023,29(2):254-271
Aim
Climate change is affecting the distribution of species and subsequent biotic interactions, including hybridization potential. The imperiled Golden-winged Warbler (GWWA) competes and hybridizes with the Blue-winged Warbler (BWWA), which may threaten the persistence of GWWA due to introgression. We examined how climate change is likely to alter the breeding distributions and potential for hybridization between GWWA and BWWA.Location
North America.Methods
We used GWWA and BWWA occurrence data to model climatically suitable conditions under historical and future climate scenarios. Models were parameterized with 13 bioclimatic variables and 3 topographic variables. Using ensemble modeling, we estimated historical and modern distributions, as well as a projected distribution under six future climate scenarios. We quantified breeding distribution area, the position of and amount of overlap between GWWA and BWWA distributions under each climate scenario. We summarized the top explanatory variables in our model to predict environmental parameters of the distributions under future climate scenarios relative to historical climate.Results
GWWA and BWWA distributions are projected to substantially change under future climate scenarios. GWWA are projected to undergo the greatest change; the area of climatically suitable breeding season conditions is expected to shift north to northwest; and range contraction is predicted in five out of six future climate scenarios. Climatically suitable conditions for BWWA decreased in four of the six future climate scenarios, while the distribution is projected to shift east. A reduction in overlapping distributions for GWWA and BWWA is projected under all six future climate scenarios.Main Conclusions
Climate change is expected to substantially alter the area of climatically suitable conditions for GWWA and BWWA, with the southern portion of the current breeding ranges likely to become climatically unsuitable. However, interactions between BWWA and GWWA are expected to decline with the decrease in overlapping habitat, which may reduce the risk of genetic introgression. 相似文献29.
30.