全文获取类型
收费全文 | 7712篇 |
免费 | 516篇 |
国内免费 | 485篇 |
出版年
2024年 | 15篇 |
2023年 | 99篇 |
2022年 | 114篇 |
2021年 | 122篇 |
2020年 | 208篇 |
2019年 | 203篇 |
2018年 | 210篇 |
2017年 | 244篇 |
2016年 | 224篇 |
2015年 | 196篇 |
2014年 | 245篇 |
2013年 | 480篇 |
2012年 | 226篇 |
2011年 | 331篇 |
2010年 | 224篇 |
2009年 | 395篇 |
2008年 | 440篇 |
2007年 | 421篇 |
2006年 | 346篇 |
2005年 | 357篇 |
2004年 | 326篇 |
2003年 | 251篇 |
2002年 | 240篇 |
2001年 | 209篇 |
2000年 | 219篇 |
1999年 | 190篇 |
1998年 | 167篇 |
1997年 | 163篇 |
1996年 | 150篇 |
1995年 | 141篇 |
1994年 | 148篇 |
1993年 | 163篇 |
1992年 | 140篇 |
1991年 | 138篇 |
1990年 | 121篇 |
1989年 | 103篇 |
1988年 | 68篇 |
1987年 | 83篇 |
1986年 | 57篇 |
1985年 | 85篇 |
1984年 | 92篇 |
1983年 | 48篇 |
1982年 | 68篇 |
1981年 | 56篇 |
1980年 | 54篇 |
1979年 | 36篇 |
1978年 | 28篇 |
1977年 | 28篇 |
1976年 | 16篇 |
1973年 | 6篇 |
排序方式: 共有8713条查询结果,搜索用时 15 毫秒
931.
Jianwei Shi Zhengquan Wang Shuiqiang Yu Xiankui Quan Yue Sun Shuxia Jia Li Mei 《生物学前沿》2008,3(4):496-506
Fine root turnover is a major pathway for car-bon and nutrient cycling in forest ecosystems. However, to estimate fine root turnover, it is important to first understand the fine root dynamic processes associated with soil resource availability and climate factors. The objectives of this study were: (1) to examine patterns of fine root production and mortality in different seasons and soil depths in the Larix gmelinii and Fraxinus man-dshurica plantations, (2) to analyze the correlation of fine root production and mortality with environmental factors such as air temperature, precipitation, soil temperature and available nitrogen, and (3) to estimate fine root turn-over. We installed 36 Minirhizotron tubes in six mono-specific plots of each species in September 2003 in the Mao'ershan Experimental Forest Station. Minirhizotron sampling was conducted every two weeks from April 2004 to April 2005. We calculated the average fine root length, annual fine root length production and mortality using image data of Minirhizotrons, and estimated fine root turnover using three approaches. Results show that the average growth rate and mortality rate in L. melinii were markedly smaller than in F. mandshurica, and were high-est in the surface soil and lowest at the bottom among all the four soil layers. The annual fine root production and mortality in F. mandshurica were significantly higher than in L. gmelinii. The fine root production in spring and summer accounted for 41.7% and 39.7% of the total annual production in F. mandshurica and 24.0% and 51.2% in L. gmelinii. The majority of fine root mortality occurred in spring and summer for F. mandshurica and in summer and autumn for L. gmelinii. The turnover rate was 3.1 a-1 for L. gmelinii and 2.7 a-1 for F. mandshurica. Multiple regression analysis indicates that climate and soil resource factors together could explain 80% of the varia-tions of the fine root seasonal growth and 95% of the seasonal mortality. In conclusion, fine root production and mortality in L. gmelinii and F. mandshurica have dif-ferent patterns in different seasons and at different soil depths. Air temperature, precipitation, soil temperature and soil available nitrogen integratively control the dynamics of fine root production, mortality and turnover in both species. 相似文献
932.
《Animal : an international journal of animal bioscience》2008,2(6):933-941
The effect of the rate of increase in concentrate allowance after calving with two concentrate mixes (A and B) differing in composition was evaluated using 64 Finnish Ayrshire cows during the first 100 days of lactation. After calving, the concentrate allowance of multiparous cows was increased stepwise from 4 to 17 kg/day, and of primiparous cows from 3 to 13.5 kg/day over 12 days (F rate of increase; multiparous 1.08 kg/day, primiparous 0.88 kg/day) or 24 days (S rate of increase; mutiparous 0.54 kg/day, primiparous 0.44 kg/day). The concentrates were formulated to have similar crude protein and metabolizable energy concentrations but differing starch and NDF concentrations. For concentrate A the starch and NDF concentrations were 421 and 167 g/kg dry matter (DM) and for concentrate B 258 and 251 g/kg DM. All cows received grass silage ad libitum. The higher concentrate intake during weeks 1 to 4 of lactation with F compared with the S rate of increase caused higher DM, energy and protein intake. The higher concentrate intake for F than for S treatment in early lactation did not cause a large decrease in silage intake (8.8 v. 8.3 kg DM/day). The intake of concentrate A and B after calving did not differ for S treatment. However, for F treatment the intake of fibrous concentrate B increased faster than starch-rich concentrate A during weeks 1 to 4 of lactation. The concentrate composition had no effect on energy-corrected milk (ECM) yield during weeks 1 to 4 of lactation for S treatments, but with F treatments the cows fed B concentrate produced more milk. The F rate of increase in concentrate allowance compared with the S rate increased the calculated energy balance after calving. The rate of increase in concentrate feeding post partum or concentrate composition had no effect on DM, energy or protein intake during the whole 100-day experiment. The average ECM yield over days 1 to 100 of lactation was higher for S than for F treatments and tended to be higher with concentrate B than A. Results of this study showed that by the fast rate of increase in concentrate allowance after calving on a grass silage diet, it was possible to improve the energy status of the cows in early lactation. This had, however, no effect on production later in lactation. 相似文献
933.
934.
Quillaguamán J Doan-Van T Guzmán H Guzmán D Martín J Everest A Hatti-Kaul R 《Applied microbiology and biotechnology》2008,78(2):227-232
High poly(3-hydroxybutyrate) (PHB) content and volumetric productivity were achieved by fed-batch culture of Halomonas boliviensis using a defined medium. Initial shake flask cultivations in a minimal medium revealed that the growth of H. boliviensis was supported only when the medium was supplemented with aspartic acid, glycine, or glutamine. Addition of 0.1% (w/v) glutamine in the medium resulted in the highest cell dry weight (CDW; 3.9 g l−1). Glutamine was replaced by the less expensive monosodium glutamate (MSG) in the medium without any notable change in the
final cell density. Effect of initial concentrations of NH4Cl and K2HPO4 on cell growth and PHB accumulation by H. boliviensis was then analyzed using a fed-batch fermentation system. The best conditions for PHB production by H. boliviensis were attained using 0.4% (w/v) NH4Cl and 0.22% (w/v) K2HPO4 and adding MSG intermittently to the fermentor. Poly(3-hydroxybutyrate) content and CDW reached 90 wt.% and 23 g l−1, respectively, after 18 h of cultivation. In order to increase CDW and PHB content, MSG, NH4Cl, and K2HPO4 were initially fed to the fermentor to maintain their concentrations at 2%, 0.4%, and 0.22% (w/v), respectively, and subsequently their feed was suppressed. This resulted in a CDW of 44 g l−1, PHB content of 81 wt.%, and PHB volumetric productivity of 1.1 g l−1 h−1. 相似文献
935.
Nisin-controlled extracellular production of apidaecin in <Emphasis Type="Italic">Lactococcus lactis</Emphasis> 总被引:1,自引:0,他引:1
Apidaecins are heat-stable, nonhelical antibacterial peptides isolated from lymph fluid of the honeybee (Apis mellifera). These peptides are active against a wide range of gram-negative bacteria and they are the most prominent components of
the honeybee humoral defense against microbial invasion. In the present study, one isoform of apidaecin, apidaecin Ho, was
expressed extracellularly in the food-grade bacterium Lactococcus lactis. Results showed that expression driven by the lactococcal nisA promoter and Usp45 signal peptide resulted in efficient secretion of apidaecin in L. lactis subsp. cremoris NZ9000. Recombinant apidaecin was purified by gel filtration and semipreparative RP-HPLC, and about 10 mg active recombinant
apidaecin was obtained from 1,000 ml culture. This is the first report on the nisin-controlled extracellular production of
active apidaecin in L. lacits. The expression and delivery of apidaecin in the food-grade L. lactis may provide a clue to facilitate the widespread application of apidaecin in the control and prevention of gram-negative bacteria
infections of human and animals. 相似文献
936.
Xu Y Yasin A Tang R Scharer JM Moo-Young M Chou CP 《Applied microbiology and biotechnology》2008,81(1):79-87
Functional expression of lipase B from Pseudozyma antarctica (PalB) in the cytoplasm of Escherichia coli BL21(DE3) and its mutant derivative Origami B(DE3) was explored. Coexpression of DsbA was found to be effective in enhancing
PalB expression. The improvement was particularly pronounced with Origami B(DE3) as a host, suggesting that both folding and
disulfide bond formation may be major factors limiting PalB expression. Fusion tag technique was also explored by constructing
several PalB fusions for the evaluation of their expression performance. While the solubility was enhanced for most PalB fusions,
only the DsbA tag was effective in boosting PalB activity, possibly by both enhanced solubility and correct disulfide bond
formation. Our results suggest that PalB activity is closely associated with correct disulfide bond formation, and increased
solubilization by PalB fusions does not necessarily result in activity enhancement. 相似文献
937.
安徽滁州雌性丽斑麻蜥繁殖特征 总被引:1,自引:1,他引:0
在安徽滁州地区丽斑麻蜥(Eremias argus)年产两窝卵。窝卵数及窝卵重与雌体体长呈正相关,相对窝卵重与雌体体长无关,卵重与窝卵数无关。窝卵数、窝卵重及卵重在窝序间无明显的差异。卵长径与卵短径呈正相关,卵长径与窝卵数呈负相关,而卵短径与窝卵数无关。雌体主要通过增加窝卵数增加繁殖输出。 相似文献
938.
Attila Fábián Katalin Jäger Éva Darkó Beáta Barnabás 《Acta Physiologiae Plantarum》2008,30(5):737-744
A procedure has been developed for the cryopreservation of wheat female gametes. The procedure involves loading the cells with 25% concentrated vitrification solution consisting of 30% glycerol, 10% sucrose, 120 mM ascorbic acid (AA) and 5% propylene glycol (PG), dehydration in 80% concentrated vitrification solution, droplet vitrification and storage in liquid nitrogen, unloading and rehydration of the cells by gradual addition of isolation solution. Supplementation with AA significantly increased the proportion of viable egg cells after de- and rehydration. During the early phase of rehydration AA reduced the probability of membrane damage caused by rapid water uptake. Maintaining the temperature of the cells at 0°C during the de- and rehydration processes increased cell survival. Microscopic examination of the semi-thin sections of untreated and viable cryopreserved cells revealed that the vitrification process might cause changes in cell structure. 相似文献
939.
The isoeugenol monooxygenase gene of Pseudomonas putida IE27 was inserted into an expression vector, pET21a, under the control of the T7 promoter. The recombinant plasmid was introduced
into Escherichia coli BL21(DE3) cells, containing no vanillin-degrading activity. The transformed E. coli BL21(DE3) cells produced 28.3 g vanillin/l from 230 mM isoeugenol, with a molar conversion yield of 81% at 20°C after 6 h.
In the reaction system, no accumulation of undesired by-products, such as vanillic acid or acetaldehyde, was observed. 相似文献
940.
A novel two-step fermentation process for improved arachidonic acid production by Mortierella alpina
A novel two-step fermentation process was developed to enhance arachidonic acid (ARA) production by Mortierella alpina ME-1 in a 5 l fermentor. Agitation speed and aeration rate were adjusted from 180 to 40 rpm and from 0.6 to1 vvm, respectively,
after 5 days cultivation, to decrease physical damage to the mycelia and to extend the stationary phase. Moreover, 3% (w/v)
and 2% (w/v) ethanol were fed after 5 and 7 days cultivation, respectively, to enhance ARA content of total lipid. Eventually,
an ARA yield of 19.8 g/l was achieved, which was 1.7 times higher than that of a one-step fed-batch cultivation. 相似文献