首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1283篇
  免费   97篇
  国内免费   167篇
  2023年   28篇
  2022年   17篇
  2021年   41篇
  2020年   46篇
  2019年   32篇
  2018年   42篇
  2017年   40篇
  2016年   42篇
  2015年   29篇
  2014年   51篇
  2013年   65篇
  2012年   45篇
  2011年   48篇
  2010年   37篇
  2009年   54篇
  2008年   61篇
  2007年   65篇
  2006年   61篇
  2005年   66篇
  2004年   48篇
  2003年   56篇
  2002年   41篇
  2001年   38篇
  2000年   52篇
  1999年   30篇
  1998年   32篇
  1997年   27篇
  1996年   22篇
  1995年   26篇
  1994年   16篇
  1993年   21篇
  1992年   18篇
  1991年   16篇
  1990年   18篇
  1989年   13篇
  1988年   11篇
  1987年   9篇
  1985年   18篇
  1984年   17篇
  1983年   14篇
  1982年   8篇
  1981年   11篇
  1980年   9篇
  1977年   12篇
  1976年   14篇
  1974年   9篇
  1973年   12篇
  1972年   16篇
  1971年   8篇
  1970年   8篇
排序方式: 共有1547条查询结果,搜索用时 15 毫秒
61.
In a previous study, carnitine supplementation to piglets during the suckling period resulted in an increased total muscle fibre number at weaning in piglets of low birth weight. The objective of the present study was to investigate whether this effect is maintained until market age and whether this would attenuate the negative consequences of low birth weight on carcass and meat quality. Using a split-plot design with litter as block, sex as whole plot and treatment as subplot, the effects of early-postnatal l-carnitine supplementation on female and castrated male piglets of low birth weight were investigated on a total of 56 German Landrace piglets from 14 litters. From days 7 to 27 of age piglets were orally supplemented once daily with 400 mg of l-carnitine dissolved in 1 ml of water or received an equal volume of water without carnitine. From weaning (day 28) until slaughter (day 166 of age) all pigs were fed standard diets. At weaning, carnitine-supplemented piglets had a twofold increased concentration of free carnitine (P < 0.001) and a lower concentration of non-esterified fatty acids (P < 0.05) in blood plasma indicating that carnitine became bioavailable and increased fatty acid utilization during the period of supplementation. Growth performance was not influenced by treatment in any growth period. Dual-energy X-ray absorptiometry revealed no differences in body composition between groups in weeks 12, 16 and 20 of age. LW at slaughter, carcass weight, measures of meat yield and fat accretion, as well as body composition by chemical analyses and dissection of primal cuts did not differ between treatments. No differences between control and carnitine-treated pigs in total fibre number (P = 0.85) and fibre cross-sectional area (P = 0.68) in m. semitendinosus (ST) measured at slaughter could be observed. The carnitine group tended to exhibit a smaller proportion of slow-twitch oxidative fibres (P = 0.08), a greater proportion of fast-twitch glycolytic fibres (P = 0.11), and increased specific lactate dehydrogenase activity (P = 0.09) in ST indicating a more glycolytic muscle metabolism. Compared with the controls, a lower pH24 value was observed (P = 0.05) in ST muscle of carnitine-supplemented pigs, which – in castrates only – was associated with an increased drip loss (P < 0.01). Meat quality traits in m. longissimus were not influenced by treatment. In conclusion, our hypothesis that early-postnatal carnitine supplementation to piglets of low birth weight permanently increases myofibre number and improves later carcass and meat quality could not be confirmed by this experiment.  相似文献   
62.
Roff et al. (Ecological Management and Restoration, 17 , 2016, 000) provide a discussion of the criteria expected for the best approach to validation of mapping programs and uses Hunter (Ecological Management & Restoration 17 , 2016, 40) to highlight issues involved. While we support the general principles outlined, we note that the review does not apply the same standards to Sivertsen et al. (Greater Hunter Native Vegetation Mapping Geodatabase Guide (Version 4.0). Office of Environment and Heritage, Department of the Premier and Cabinet, Sydney, Australia, 2011), the original document critiqued by Hunter (Ecological Management & Restoration 17 , 2016, 40). The Hunter (Ecological Management & Restoration 17 , 2016, 40) validation was based on a larger sample size, greater sampling within mapping units and greater representation of landscapes than Sivertsen et al. (Greater Hunter Native Vegetation Mapping Geodatabase Guide (Version 4.0). Office of Environment and Heritage, Department of the Premier and Cabinet, Sydney, Australia, 2011). Survey and validation sites being placed along public roads and lands are common to both the general Office of Environment and Heritage (OEH) and Hunter (Ecological Management & Restoration 17 , 2016, 40) validation methodologies. Thus, the criticisms of Roff et al. (Ecological Management and Restoration, 17 , 2016, 000) of the Hunter (Ecological Management & Restoration 17 , 2016, 40) approach apply equally, if not more, to Sivertsen et al. (Greater Hunter Native Vegetation Mapping Geodatabase Guide (Version 4.0). Office of Environment and Heritage, Department of the Premier and Cabinet, Sydney, Australia, 2011). We outline in the article how the Roff et al. (Ecological Management and Restoration, 17 , 2016, 000) critique was selective and in some cases incorrect in its analysis of issues presented in Hunter (Ecological Management & Restoration 17 , 2016, 40) and did not apply the same criteria to their own work. We conclude by discussing future directions for validating and mapping vegetation communities.  相似文献   
63.
Plant communities are often assumed to be stable on a year-to-year basis. We present evidence that species composition in permanent quadrats changes considerably between years at a range of Environmental Change Network sites across the UK. The extent of this variability varies with habitat type. Communities associated with low disturbance levels and low agricultural inputs, particularly moorland (upland grass and heath) and bog communities, are most stable.Inter-annual variability should, therefore, be considered in designing monitoring schemes to ensure that frequency of recording is sufficient to avoid short-term fluctuations obscuring long-term trends.More diverse communities were more stable, with less species turnover between years. However, diverse communities also tended to be dominated by slow-growing, slow-reproducing plants, adapted to low nutrient conditions, identified as ‘stress tolerators’ in the Grime CSR scheme and low Ellenberg N values. Species compositional stability was more strongly correlated with these indices of plant functional types than species richness. Nevertheless, a significant effect of species richness could be identified, even after other causes of variation were accounted for.More stable communities in our study are likely to be resilient to low levels of environmental change, although they may still change, and possibly change dramatically if critical ‘tipping points’ are reached.  相似文献   
64.
65.
66.
67.
Fiber-type distribution is known to vary widely within and between muscles according to differences in muscle functions. 2-DE and MALDI-MS were used to investigate the molecular basis of muscle fiber type-related variability. We compared four lamb skeletal muscles with heterogeneous fiber-type composition that are relatively rich in fast-twitch fiber types, i.e., the semimembranosus, vastus medialis, longissimus dorsi, and tensor fasciae latae (TL). Our results clearly showed that none of the glycolytic metabolism enzymes detected, including TL which was most strongly glycolytic, made intermuscular differentiation possible. Muscle differentiation was based on the differential expression of proteins involved in oxidative metabolism, including not only citric acid cycle enzymes but also other classes of proteins with functions related to oxidative metabolism, oxidative stress, and probably to higher protein turnover. Detected proteins were involved in transport (carbonate dehydratase, myoglobin, fatty acid-binding protein), repair of misfolding damage (heat shock protein (HSP) 60 kDa, HSP-27 kDa, alpha-crystallin beta subunit, DJ1, stress-induced phosphoprotein), detoxification or degradation of impaired proteins (GST-Pi, aldehyde dehydrogenase, peroxiredoxin, ubiquitin), and protein synthesis (tRNA-synthetase). The fractionating method led to the detection of proteins involved in different functions related to oxidative metabolism that have not previously been shown concomitancy.  相似文献   
68.
Purple basil is among the most important basil varieties and its essential oil is used for several purposes including medicinal and aromatic uses. Soil types may impact the plant growth, development, and essential oil composition. Hence, it is important to find the most suitable soil type which may produce basil plants having essential oil with the best composition and concentration. For this reason, plant samples of purple basil that were grown in areas with clay, loamy sand, and sandy-clay loam soil types were collected and evaluated to determine the changes in the yield and essential oil components. Essential oil contents were determined with the Clevenger Device, and essential oil compositions were determined by using GC and GC/MS analysis. The highest essential oil yield according to soil types was obtained from the plant samples that were grown in the loamy sand soil. It was also found that the main compounds present in Arapgir town purple basil were methylcinnamate and linalool that was also present in all Turkish purple basil under all types of soil. According to the soil types, the highest concentration (46.03%) of methylcinnamate was observed in loamy sand soils, and the lowest (42.33%) was obtained from sandy-clay loam soils and found to be significantly different. Data regarding correlations between soil types and essential oil ratios showed that organic matter and P2O5 had a significant negative correlation with methylcinnamate. The present study will help researchers and farmers to choose the most suitable soil type to achieve maximum essential oil production from purple basil.  相似文献   
69.
Background: The psychological problems of Shidu Parents (SDP) under the China’s One-Child Policy have been documented. The purpose of this study was to investigate the relationships among personality types, social support, and post-traumatic stress disorder (PTSD) in SDP. Methods: The PTSD Checklist-Civilian Version (PCL-C), The Big Five Personality Traits (NEO), and Social Support Revalued Scale (SSRS) were administered to the sample of 149 SDP who were over 50 years old and had lost their only child more than one year ago. Results: Among SDP, mothers were more likely to develop PTSD than fathers (χ2 = 11.16, p < 0.01). Parents who were extraverted had a lower risk of developing PTSD-related symptoms (χ2 = 8.58, p < 0.01), and the effect of neuroticism was significant (χ2 = 23.73, p < 0.01). The more social support parents utilized, the lower the incidence of PTSD (t = 4.56, p < 0.01). The result of multilevel linear regression showed that sex, neuroticism, and objective social support remained significantly different after combining all personality types and social support systems in the same model. Social support partially mediated the relationship between neuroticism and PTSD. Meanwhile, it was a complete mediator between extraversion and PTSD. Conclusions: Female sex/gender, neuroticism, and introversion were risk factors of developing PTSD, while receiving social support protected SDP from developing PTSD symptoms. Losing an only child is undoubtedly an enormous disaster for the family, which has become a huge, unavoidable social problem that must be addressed in China.  相似文献   
70.
The purpose of this study was to develop a new micropropagation system for Tylophora indica, an important medicinal plant in India, using root explants as starting material. Root explants cultured on MS medium supplemented with 6-benzyladenine (BA) or 2-isopentyladenine (2iP) produced organogenic nodular meristemoids (NMs) within 4 weeks. NMs induced from the cut ends of root segments showed two types of organogenic response—direct shoot bud formation and somatic embryogenesis—when maintained on induction medium. In 42% of the explants, NMs developed shoot buds directly in the presence of 10.72–26.80 M BA. On average, 18.5±0.7 shoots per gram of NM tissue were obtained after each 4-week subculture. Elongation of microshoots and root initiation were correlated with the auxin used, with the optimal response occurring in the presence of 28.54 M indole-3-acetic acid. In 39% of the explants, NMs dedifferentiated into friable embryogenic callus (FEC) in the presence of BA or 2iP after 12 weeks of culture. Of the different treatments, MS medium supplemented with 10.72 M BA was the most effective in inducing FEC and somatic embryogenesis: at this concentration 64% of the cultured NMs developed FEC and, on the same medium, 89% of the FEC produced globular somatic embryos (SEs). FEC biomass increased nearly five-fold with every 4-week subculture, and about 30 SEs were recovered per gram of FEC during this period. The best conversion of mature SEs to complete plantlets was obtained on basal MS medium–42%. Plantlets derived via somatic embryogenesis and shoot organogenesis were successfully hardened (88–96%) and transferred to the field.Abbreviations BA: 6-Benzyladenine - 2,4-D: 2,4-Dichlorophenoxyacetic acid - FEC: Friable embryogenic callus - GRS: Green root segment - IAA: Indole-3-acetic acid - IBA: Indole-3-butyric acid - 2ip: 2-Isopentyladenine - Kin: Kinetin - NAA: -Naphthaleneacetic acid - NM: Nodular meristemoid - SE: Somatic embryo - WRS: White root segmentCommunicated by P. Lakshmanan  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号