全文获取类型
收费全文 | 16011篇 |
免费 | 1650篇 |
国内免费 | 2016篇 |
专业分类
19677篇 |
出版年
2024年 | 69篇 |
2023年 | 270篇 |
2022年 | 543篇 |
2021年 | 863篇 |
2020年 | 627篇 |
2019年 | 798篇 |
2018年 | 665篇 |
2017年 | 543篇 |
2016年 | 712篇 |
2015年 | 1049篇 |
2014年 | 1226篇 |
2013年 | 1273篇 |
2012年 | 1504篇 |
2011年 | 1382篇 |
2010年 | 889篇 |
2009年 | 800篇 |
2008年 | 919篇 |
2007年 | 815篇 |
2006年 | 746篇 |
2005年 | 660篇 |
2004年 | 590篇 |
2003年 | 603篇 |
2002年 | 518篇 |
2001年 | 300篇 |
2000年 | 250篇 |
1999年 | 194篇 |
1998年 | 123篇 |
1997年 | 107篇 |
1996年 | 95篇 |
1995年 | 63篇 |
1994年 | 97篇 |
1993年 | 53篇 |
1992年 | 53篇 |
1991年 | 40篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 31篇 |
1987年 | 22篇 |
1986年 | 18篇 |
1985年 | 33篇 |
1984年 | 13篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 3篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1973年 | 2篇 |
1971年 | 2篇 |
1961年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
61.
在过量铜或锌胁迫下,独立培养的外生菌根菌牛乳牛肝菌(Suillus 相似文献
62.
BM,并实现了融合基因在大肠杆菌BL21(DE3)pLysS中的表达.研究结果表明,融合酶Umcel5N-CBM与结晶纤维素(avicel)以及滤纸粉末的结合能力比原始酶Umcel5N提高了约一倍,但未显示出降解结晶纤维素的新活性,说明在结晶纤维素的降解过程中,纤维素酶的催化功能域起到关键作用. 相似文献
63.
DNA methylation changes in photoperiod-thermo-sensitive male sterile rice PA64S under two different conditions 总被引:1,自引:0,他引:1
Epigenetic modification can occur at a high frequency in crop plants and might generate phenotypic variation without changes in DNA sequences. DNA methylation is an important epigenetic modification that may contribute to environmentally-induced phenotypic variations by regulating gene expression. Rice Photoperiod-Thermo-Sensitive Genic Male Sterile (PTGMS) lines can transform from sterility to fertility under lower temperatures and short-day (SD) conditions during anther development. So far, little is known about the DNA methylation variation of PTGMS throughout the genome in rice. In this study, we investigated DNA cytosine methylation alterations in the young panicles of PTGMS line PA64S under two different conditions using methylation sensitive amplified polymorphism (MSAP) method. Compared with the DNA methylation level of PA64S under lower temperatures and SD conditions (fertility), higher methylation was observed in PA64S (sterility). The sequences of 25 differentially amplified fragments were successfully obtained and annotated. Three methylated fragments, which are homologous to D2, NAD7 and psaA, were confirmed by bisulfite sequencing and their expression levels were also evaluated by qPCR. Real time quantitative PCR analysis revealed that five of the six selected methylated genes were downregulated in PA64S (sterility). These results suggested that DNA methylation may be involved in the sterility–fertility transition of PA64S under two different environmental conditions. 相似文献
64.
65.
66.
Revegetation is a traditional practice widely used for soil and water conservation on the Loess Plateau in China. However, there has been a lack of reports on soil microbial–biochemical indices required for a comprehensive evaluation of the success of revegetation systems. In this study, we examined the effects of revegetation on major soil nutrients and microbial–biochemical properties in an artificial alfalfa grassland, an enclosed natural grassland, and an artificial shrubland (Caragana korshinskii), with an abandoned cropland as control. Results showed that at 0–5, 5–20, and 20–40 cm depths, soil organic carbon, alkaline extractable nitrogen and available potassium were higher in natural grassland and artificial shrubland compared with artificial grassland and abandoned cropland. Soil microbial biomass C (Cmic) and phosphorous (Pmic) substantially decreased with depth at all sites, and in abandoned cropland was significantly lower than those of natural grassland, artificial grassland, and artificial shrubland at the depth of 0–5 cm. Soil microbial biomass N (Nmic) was higher in artificial shrubland and abandoned cropland compared with that in natural and artificial grasslands. Both Cmic and Pmic were significantly different between the 23‐year‐old and the 13‐year‐old artificial shrublands at the 0–5 cm depth. The activities of soil invertase, urease, and alkaline phosphatase in natural grassland and artificial shrubland were higher than those in artificial grassland and abandoned cropland. This study demonstrated that the regeneration of both natural grassland and artificial shrubland effectively preserved and enhanced soil microbial biomass and major nutrient cycling, thus is an ecologically beneficial practice for recovery of degraded soils on the Loess Plateau. 相似文献
67.
68.
生物资源利用相关的传统知识是对生物资源进行识别和利用的传统知识系统。随着现代生物技术的发展, 这类传统知识显示出其在科学、经济、文化乃至粮食安全战略方面的价值。本研究根据《生物多样性相关传统知识分类、调查与编目技术规定(试行)》, 对我国青海省土族聚集区的土族生物资源利用相关的传统知识进行了系统调查与编目, 并借鉴生物多样性测度方法, 创建了传统知识多样性指数计算方法, 对土族生物资源利用相关传统知识多样性进行分析。结果如下: (1)编目现存的土族生物资源利用相关的传统知识词条共424条; (2)土族传统知识的α多样性指数DTK = 0.67, 表明其传统知识多样性较高; 土族传统知识的βwtk多样性指数在不同的县域差异较大, 表明其在县域之间存在差异, 在空间上分布不连续、不均匀。本研究利用生物多样性指数验证了传统知识的定量研究方法, 说明传统知识多样性指数不仅可用于定量研究表征区域传统知识的多样性, 揭示不同空间区域内的分布特征, 还可为未来构建传统知识的定量研究体系提供重要的参考依据。 相似文献
69.
70.
The function of NS4B is incompletely understood. The aim of the study is to understand the influence of NS4B on anti-viral response. After cell line stably expressing NS4B established, the influence of IFN-alpha of different concentration on VSV was studied using plaque assay; cell expression profiling caused by NS4B was studied using DNA microarray, and the IFNGR1 fluorescence intensity was analyzed. Our data showed that HCV-NS4B could suppress immuno-associated gene expression, in particular, IFN-gamma receptor signal transduction-related genes. Taken together, NS4B could play some roles in HCV resistance to IFN therapy. 相似文献