全文获取类型
收费全文 | 4143篇 |
免费 | 246篇 |
国内免费 | 200篇 |
出版年
2024年 | 16篇 |
2023年 | 40篇 |
2022年 | 37篇 |
2021年 | 58篇 |
2020年 | 131篇 |
2019年 | 102篇 |
2018年 | 116篇 |
2017年 | 112篇 |
2016年 | 83篇 |
2015年 | 80篇 |
2014年 | 127篇 |
2013年 | 231篇 |
2012年 | 84篇 |
2011年 | 125篇 |
2010年 | 113篇 |
2009年 | 189篇 |
2008年 | 224篇 |
2007年 | 230篇 |
2006年 | 197篇 |
2005年 | 150篇 |
2004年 | 137篇 |
2003年 | 113篇 |
2002年 | 98篇 |
2001年 | 70篇 |
2000年 | 91篇 |
1999年 | 124篇 |
1998年 | 123篇 |
1997年 | 103篇 |
1996年 | 87篇 |
1995年 | 92篇 |
1994年 | 108篇 |
1993年 | 76篇 |
1992年 | 105篇 |
1991年 | 74篇 |
1990年 | 88篇 |
1989年 | 70篇 |
1988年 | 80篇 |
1987年 | 69篇 |
1986年 | 50篇 |
1985年 | 62篇 |
1984年 | 64篇 |
1983年 | 27篇 |
1982年 | 42篇 |
1981年 | 51篇 |
1980年 | 46篇 |
1979年 | 32篇 |
1978年 | 17篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1974年 | 8篇 |
排序方式: 共有4589条查询结果,搜索用时 15 毫秒
21.
Alfalfa (Medicago sativa L.) growth and nodulation in acid soil is reduced because the plant and its bacterial symbiontRhizobium meliloti cannot tolerate acid, aluminum-rich soil. A study was conducted to determine if a relatively acid-tolerant alfalfa germplasm combined with a relatively acid-tolerantR. meliloti strain could overcome these limitations. In a light room study, an acid-tolerant alfalfa germplasm inoculated with a more acid-tolerantR. meliloti strain produced greater top growth, nodule number and weight, and acetylene reduction values in an unlimed soil (pH 4.6) than the same germplasm inoculated with a relatively acid-sensitiveR. meliloti strain or an acid-sensitive germplasm inoculated with either a relatively acid-tolerant or acid-sensitiveR. meliloti strain. 相似文献
22.
Nitrate reduction in roots and shoots and exchange of reduced N between organs were quantitatively estimated in intact 13-d-old seedlings of two-row barley (Hordeum vulgare L. cv. Daisengold) using the 15N-incorporation model (A. Gojon et al. (1986) Plant Physiol. 82, 254–260), except that NH
+
4
was replaced by NO
-
2
. N-depleted seedlings were exposed to media containing both nitrate (1.8 mM) and nitrite (0.2 mM) under a light-dark cycle of 12:12 h at 20°C; the media contained different amounts of 15N labeling. Experiments were started either immediately after the beginning (expt. 1) or immediately prior to the end (expt. 2) of the light period, and plants were sampled subsequently at each light-dark transition throughout 36 h. The plants effectively utilized 15NO
-
3
and accumulated it as reduced 15N, predominantly in the shoots. Accumulation of reduced 15N in both experiments was nearly the same at the end of the experiment but the accumulation pattern in roots and shoots during each 12-h period differed greatly depending on time and the light conditions. In expt. 1, the roots accounted for 31% (light), 58% (dark), and 9% (light) of nitrate reduction by the whole plants, while in expt. 2 the contributions of the root were 82% (dark), 20% (light), and 29% (dark), during each of the three 12-h periods. Xylem transport of nitrate drastically decreased in the dark, but that of reduced N rather increased. The downward translocation of reduced 15N increased while nitrate reduction in the root decreased, whereas upward translocation decreased while nitrate reduction in the shoot increased. We conclude that the cycling of reduced N through the plant is important for N feeding of each organ, and that the transport system of reduced N by way of xylem and phloem, as well as nitrate reduction by root and shoot, can be modulated in response to the relative magnitude of reduced-N demands by the root and shoot, with the one or the other predominating under different circumstances.Symbols Anl
accumulation of reduced 15N from 15NO
-
3
in 14NO
-
3
-fed roots of divided root system
- Ar
accumulation in root of reduced 15N from 15NO
-
3
- As
accumulation in shoot of reduced 15N from 15NO
-
3
- Rr
15NO
-
3
reduction in root
- Rs
15NO
-
3
reduction in shoot
- Tp
translocation to root of shoot-reduced 15N from 15NO
-
3
in phloem
- Tx
translocation to shoot of root-reduced 15N from 15NO
-
3
in xylem 相似文献
23.
J. Masson D. Lancelin C. Bellini M. Lecerf P. Guerche G. Pelletier 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1989,78(2):153-159
Summary Five diploid potato clones have been transformed by electroporation of protoplasts with different selectable markers. The resulting diploid regenerated plants have been used in somatic hybridization. It has been shown that hybrid cell selection on the basis of antibiotic or herbicide resistances brought by the two parents of fusion is an efficient method for the recovery of tetraploid somatic hybrids. 相似文献
24.
Analysis of chloroplast and mitochondrial DNAs in asymmetric somatic hybrids between tobacco and carrot 总被引:1,自引:0,他引:1
M. A. Smith A. Pay D. Dudits 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1989,77(5):641-644
Summary Chloroplast and mitochondrial DNAs have been examined by comparison of restriction enzyme patterns in asymmetric hybrid plants, resulting from the fusion between leaf mesophyll protoplasts of Nicotiana tabacum (Solanaceae), and irradiated cell culture protoplasts of Daucus carota (Umbellifereae). These somatic hybrids with normal tobacco morphology were selected as a consequence of the transfer of methotrexate and 5-methyltryptophan resistance from carrot to tobacco. The restriction patterns of chloroplast DNAs in somatic hybrids were indistinguishable from the tobacco parent. However, we found somatic hybrids with mitochondrial DNA significantly different from either parent, as judged by analysis of fragment distribution after restriction enzyme digestion. The possible formation of altered mitochondrial DNA molecules as the result of parasexual hybrid production between two phylogenetically highly divergent plant species will be discussed. 相似文献
25.
Nitrate and nitrite was reduced by Escherichia coli E4 in a l-lactate (5 mM) limited culture in a chemostat operated at dissolved oxygen concentrations corresponding to 90–100% air saturation. Nitrate reductase and nitrite reductase activity was regulated by the growth rate, and oxygen and nitrate concentrations. At a low growth rate (0.11 h–1) nitrate and nitrite reductase activities of 200 nmol · mg–1 protein · min–1 and 250 nmol · mg–1 protein · min–1 were measured, respectively. At a high growth rate (0.55 h–1) both enzyme activities were considerably lower (25 and 12 nmol mg–1 · protein · min–1). The steady state nitrite concentration in the chemostat was controlled by the combined action of the nitrate and nitrite reductase. Both nitrate and nitrite reductase activity were inversely proportional to the growth rate. The nitrite reductase activity decreased faster with growth rate than the nitrate reductase. The chemostat biomass concentration of E. coli E4, with ammonium either solely or combined with nitrate as a source of nitrogen, remained constant throughout all growth rates and was not affected by nitrite concentrations. Contrary to batch, E. coli E4 was able to grow in continuous cultures on nitrate as the sole source of nitrogen. When cultivated with nitrate as the sole source of nitrogen the chemostat biomass concentration is related to the activity of nitrate and nitrite reductase and hence, inversely proportional to growth rate. 相似文献
26.
The ethanol-oxidizing, proton-reducing Pelobacter acetylenicus was grown in chemostat cocultures with either Acetobacterium woodii, Methanobacterium bryantii, or Desulfovibrio desulfuricans. Stable steady state conditions with tightly coupled growth were reached at various dilution rates between 0.02 and 0.14 h-1. Both ethanol and H2 steady state concentrations increased with growth rate and were lower in cocultures with the sulfate reducer < methanogen < homoacetogen. Due to the higher affinity for H2, D. desulfuricans outcompeted M. bryantii, and this one A. woodii when inoculated in cocultures with P. acetylenicus. Cocultures with A. woodii had lower H2 steady state concentrations when bicarbonate reduction was replaced by the energetically more favourable caffeate reduction. Similarly, cocultures with D. desulfuricans had lower H2 concentrations with nitrate than with sulfate as electron acceptor. The Gibbs free energy (G) available to the H2-producing P. acetylenicus was independent of growth rate and the H2-utilizing partner, whereas the G available to the latter increased with growth rate and the energy yielding potential of the H2 oxidation reaction. The critical Gibbs free energy (Gc), i.e. the minimum energy required for H2 production and H2 oxidation, was-5.5 to-8.0 kJ mol-1 H2 for P. acetylenicus,-5.1 to-6.3 kJ mol-1 H2 for A. woodii,-7.5 to-9.1 kJ mol-1 H2 for M. bryantii, and-10.3 to-12.3 kJ mol-1 H2 for D. desulfuricans. Obviously, the potentially available energy was used more efficiently by homoacetogens > methanogens > sulfate reducers. 相似文献
27.
A method to obtain plants from embryogenic callus of Brassica nigra and protoplasts of hypocotyl expiants is described. Callus was initiated on Murashige and Skoog medium containing kinetin (kn) and 2,4-dichlorophenoxy acetic acid (2,4-D). Lowering of auxin induced embryo formation. Supplementation with gibberellic acid (GA3) enhanced embryogenic response tenfold. Passage through liquid medium devoid of growth regulators was essential for the growth of embryos. Secondary embryos were produced on transfer to solid basal medium. Embryogenic callus retained its morphogenic ability even after 12 subcultures. Both primary and secondary embryos produced fertile plants. Hypocotyl-derived protoplasts were also regenerated to plants following the same protocol. The survival of plants on transfer to soil was about 80%. The seeds from plants derived from callus and protoplasts were viable.Abbreviations 2,4-D
2,4-dichlorophenoxy acetic acid
- NAA
naphthalene acetic acid
- IAA
indole acetic acid
- kn
kinetin
- GA3
gibberellic acid 相似文献
28.
Akira Hiraishi 《Archives of microbiology》1988,150(1):56-60
Nineteen established and one undesignated species of the Rhodospirillaceae were examined for fumarate reduction in connection with their quinone systems. The fumarate reductase activity with reduced methyl viologen (MVH) or FMNH2 as electron donor was found in membrane (chromatophore) preparations from phototrophically grown cells of all species containing menaquinone (MK) and/or rhodoquinone. The species having ubiquinone as the sole quinone contained no fumarate reductase activity, except some Rhodobacter species showing the FMNH2-dependent activity. The MVH-fumarate reductase activity of the MK-type species was not inhibited by Triton X-100 or acetone treatment, suggesting the presence of a fumarate reductase reacting directly with MVH, while such an enzyme was absent in the MK-lacking strains, with few exceptions. The FMNH2-fumarate reduction system was abolished by a detergent or acetone extraction in all bacteria but differed much among species with different quinone types as to the response to respiratory inhibitors. These differences in fumarate-reducing properties and quinone systems among the phototrophic bacteria are discussed from evolutionary and taxonomic viewpoints.Non-standard abbreviations RQ
rhodoquinone
- MK
menaquinone
- MVH
reduced methyl viologen
- HOQNO
2-n-heptyl-4-hydroxyquinoline-N-oxide
- TTFA
2-thenoyltrifluoroacetone 相似文献
29.
Crude extracts or supernatants of broken cells of Clostridium formicoaceticum reduce unbranched, branched, saturated and unsaturated carboxylates at the expense of carbon monoxide to the corresponding alcohols. The presence of viologens with redox potentials varying from E
0=-295 to-650 mV decreased the rate of propionate reduction. The more the propionate reduction was diminished the more formate was formed from carbon monoxide. The lowest propionate reduction and highest formate formation was observed with methylviologen. The carbon-carbon double bond of E-2-methyl-butenoate was only hydrogenated when a viologen was present. Formate as electron donor led only in the presence of viologens to the formation of propanol from propionate. The reduction of propionate at the expense of a reduced viologen can be followed in cuvettes. With respect to propionate Michaelis Menten behavior was observed. Experiments are described which lead to the assumption that the carboxylates are reduced in a non-activated form. That would be new type of biological reduction.Non-standard abbreviations glc
Gas liquid chromatography
- HPLC
high performance liquid chromatography
- RP
reverse phase; Mediators (the figures in parenthesis of the mediators are redox potentials E
0 in mV)
- CAV2+
carbamoylmethylviologen, 1,1-carbamoyl-4,4-dipyridinium dication (E
0=-296 mV)
- BV2+
benzylviologen, 1,1-dibenzyl-4,4-dipyridinium dication (E
0=-360 mV)
- MV
methylviologen, 1,1-dimethyl-4,4-dipyridinium-dication (E
0=-444 mV)
- DMDQ2+
dimethyldiquat, 4,4-dimethyl-2,2-dipyridino-1,1-ethylendication (E
0=-514 mV)
- TMV2+
tetramethylviologen, 1,1,4,4-tetramethyl-4,4-dipyridinium dication (E
0=-550 mV)
- PDQ2+
propyldiquat, 2,2-dipyridino-1,1-propenyl dication (E
0=-550 mV)
- DMPDQ2+
dimethylpropyldiquat, 4,4-dimethyl-2,2-dipyridino-1,1-propenyl dication (E
0=-656 mV)
- PN
productivity number=mmol product (obtained by the uptake of one pair of electrons) x (biocatalyst (dry weight) kg)-1×h-1 相似文献
30.
Ex planta, bacteroids of the sulla-symbiont Rhizobium hedysari strain HCNT 1 terminated reduction of nitrite at nitrous oxide irrespective of the presence or absence of acetylene. Nitrate was not reduced during the experimental period, but slight nitrate reductase activity occurred if incubation with nitrate was prolonged (up to 15 h). As was observed in free-living cells, exposure of the bacteroids to the metal chelator, diethyldithiocarbamate, prevented reduction of nitrite, indicating the presence of a copper-containing nitrite reductase. Pulses of 10–75 M nitrite transiently impeded O2 uptake in bacteroids, which resumed consumption of O2 when the nitrite had been reduced. Exposure to >1.0 mM nitrite for 24h greatly inhibited nitrogenase activity (assayed as acetylene reduction activity) of bacteroids in planta. Exposure to the same concentrations of nitrite after 1h of incubation in the presence of acetylene almost completely stopped ongoing ethylene production in bacteroids of strain HCNT 1 extracted from nodules. Free cells of the non-nitrite-reducing R. hedysari strain CC 1335 were lacking in nitrogenase (acetylene-reduction) activity, whereas identically cultured (low-oxygen) strain HCNT 1 cells reduced both nitrite and acetylene.Abbreviations PMS
phenazine methosulfate
- DDC
diethyldithiocarbamate 相似文献