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   2066篇
  免费   230篇
  国内免费   164篇
  2023年   28篇
  2022年   41篇
  2021年   53篇
  2020年   48篇
  2019年   61篇
  2018年   62篇
  2017年   65篇
  2016年   59篇
  2015年   51篇
  2014年   92篇
  2013年   130篇
  2012年   65篇
  2011年   99篇
  2010年   72篇
  2009年   107篇
  2008年   89篇
  2007年   112篇
  2006年   109篇
  2005年   100篇
  2004年   77篇
  2003年   76篇
  2002年   70篇
  2001年   59篇
  2000年   48篇
  1999年   43篇
  1998年   30篇
  1997年   45篇
  1996年   35篇
  1995年   45篇
  1994年   33篇
  1993年   38篇
  1992年   42篇
  1991年   28篇
  1990年   22篇
  1989年   21篇
  1988年   21篇
  1987年   18篇
  1986年   32篇
  1985年   27篇
  1984年   30篇
  1983年   19篇
  1982年   28篇
  1981年   17篇
  1980年   27篇
  1979年   20篇
  1978年   14篇
  1977年   12篇
  1976年   14篇
  1975年   6篇
  1973年   7篇
排序方式: 共有2460条查询结果,搜索用时 15 毫秒
1.
R. Rouger  A. S. Jump 《Molecular ecology》2014,23(13):3158-3170
Little is known about the processes shaping population structure in saltmarshes. It is expected that the sea should act as a powerful agent of dispersal. Yet, in contrast, import of external propagules into a saltmarsh is thought to be small. To determine the level of connectivity between saltmarsh ecosystems at a macro‐geographical scale, we characterized and compared the population structure of two polyploid saltmarsh species, Puccinellia maritima and Triglochin maritima based on a seascape genetics approach. A discriminant analysis of principal components highlighted a genetic structure for both species arranged according to a regional pattern. Subsequent analysis based on isolation‐by‐distance and isolation‐by‐resistance frameworks indicated a strong role of coastal sediment transport processes in delimiting regional structure in P. maritima, while additional overland propagule dispersal was indicated for T. maritima. The identification and comparison of regional genetic structure and likely determining factors presented here allows us to understand the biogeographical units along the UK coast, between which barriers to connectivity occur not only at the species level but at the ecosystem scale. This information is valuable in plant conservation and community ecology and in the management and restoration of saltmarsh ecosystems.  相似文献   
2.
The distribution of excreted flavonoid aglycones within the familyLabiatae was studied and differences were found, especially in the A-ring substitution patterns. Thus, 5,7-dihydroxy-6-methoxyflavones with substituted B-rings are characteristic of species ofSalvia (sect.Salvia),Rosmarinus andOcimum; 5,7-dihydroxy-6,8-dimethoxyflavones occur only inOcimum and 5,6-dihydroxy-7,8-dimethoxyflavones inThymus and related species. Members of the two subfamiliesLamioideae andNepetoideae produce exudate flavonoids, but some genera are devoid of these compounds. There is a correlation between the habitat where the plant grows and production of these compounds, the species from (semi-)arid habitats being those which generally accumulate external flavonoids.  相似文献   
3.
To discuss the cold resistance performance of different Herba Rhodiolae and successfully transplant Herba Rhodiolae to the Gansu plateau area for nursing, domestication and planting, this paper systematically studies six physiological and biochemical features of Rhodiola kirilowii, Rhodiola algida, Rhodiola crenulata and Herba Rhodiolae that are closely associated with cold resistance features and concludes with the cold resistance capability of Rhodiola kirilowii. In the selected six main indexes of the Herba Rhodiolae, the POD, SOD and CAT activity and MDA and Pro content in the leaf are the main physiological and biochemical indexes to indicate the cold resistance performance of four Herba Rhodiolae seedlings and can be regarded as the preliminary indexes to assess the winter performance of Herba Rhodiolae. The research work will provide the theoretical basis for the wild variants of Herba Rhodiolae and GAPJ base construction.  相似文献   
4.
甘草次酸(glycyrrhetinic acid,GA)是甘草主要活性组分,可诱导肿瘤细胞凋亡,抑制肿瘤细胞生长.然而,其对BLM解旋酶的抑制作用尚未见报道.本文注视甘草次酸对BLM解旋酶构象、二级结构和生化活性的影响.圆二色光谱和紫外光谱分析显示,GA可破坏BLM642-1290解旋酶α-螺旋结构,改变其构象,并具有2个结合位点.采用荧光偏振技术和自由磷检测证明,GA以浓度依赖的方式抑制BLM642-1290解旋酶与底物dsDNA及ssDNA的结合,抑制BLM642-1290解旋酶活性及ATP酶活性,且抑制类型为混合抑制.综上所述,本文证明GA可通过结合BLM解旋酶,改变BLM解旋酶构象,抑制BLM解旋酶与DNA的结合,从而抑制BLM解旋酶的生化活性.我们的发现将对深入认识GA的抗肿瘤作用有新的启示.  相似文献   
5.
In this study, distribution of metal accumulation and their biological changes of Indian mustard plants (Brassica nigra L.) grown in soil irrigated with different concentration of rayon grade paper effluent (RGPE, 25%, 50%, 75%, 100%, v/v) were studied. A pronounced effect was recorded at 50% (v/v) RGPE on germination of seeds, amylase activity and other growth parameters in Indian mustard plants. An increase in the chlorophyll and protein contents was also recorded at <50% (v/v) RGPE followed by a decrease at higher concentrations of RGPE (>75%). A significant increase lipid peroxidation was recorded, which was evidenced by the increased malondialdehyde (MDA) content in shoot, leaves and seeds in tested plant at all the concentrations of RGPE. This Indian mustard plants (Brassica nigra L.) are well adapted for tolerance of significant amount of heavy metals due to increased level of antioxidants (cysteine and ascorbic acid) in root shoot and leaves of treated plants at all concentration of RGPE. Moreover, it is also important that RGPE should be treated to bring down the metal concentration well within the prescribed limit prior to use in agricultural soil for ferti-irrigation.  相似文献   
6.
1. For solitary bees that specialise on select pollen types (oligoleges), larval development depends on the availability of forage pollen from appropriate host plants and the naturally occurring microbiota present therein. While access to host pollen may be critical for the development of oligolectic bees, the extent to which pollen microbiota contribute to their brood success is unknown. 2. To investigate, we used a diet manipulation experiment to rear larvae of the oligolege, Osmia ribifloris, under in-vitro conditions. Larvae were reared either on host pollen provisioned by their mother or on non-host pollen collected by honey bees, in the presence or absence of the respective pollen-associated microbiota. We assessed impacts on components of larval fitness: developmental time, biomass, and survivorship. 3. Our results revealed a significant interaction between pollen type and pollen-associated microbes. The relative effect of microbes on larval performance was substantially greater than that of pollen type. Host pollen substrate produced the fittest larvae but only when combined with its full complement of naturally occurring microbiota. In contrast, host pollen without microbes resulted in a marked decline in fitness components. Larvae consuming non-host pollen showed intermediate fitness, regardless of whether microbes were present or not. 4. These findings imply that the microbiota associated with maternally provisioned host pollen perform critical functions in larval nutrition and survival. For oligoleges in particular, the ability to develop on poorer quality host pollen likely derives from this sustained symbiosis with their microbial exosymbionts, rather than the biochemical characteristics of pollen type alone.  相似文献   
7.
Plasma lipidome is now increasingly recognized as a potentially important marker of chronic diseases, but the exact extent of its contribution to the interindividual phenotypic variability in family studies is unknown. Here, we used the rich data from the ongoing San Antonio Family Heart Study (SAFHS) and developed a novel statistical approach to quantify the independent and additive value of the plasma lipidome in explaining metabolic syndrome (MS) variability in Mexican American families recruited in the SAFHS. Our analytical approach included two preprocessing steps: principal components analysis of the high-resolution plasma lipidomics data and construction of a subject-subject lipidomic similarity matrix. We then used the Sequential Oligogenic Linkage Analysis Routines software to model the complex family relationships, lipidomic similarities, and other important covariates in a variance components framework. Our results suggested that even after accounting for the shared genetic influences, indicators of lipemic status (total serum cholesterol, TGs, and HDL cholesterol), and obesity, the plasma lipidome independently explained 22% of variability in the homeostatic model of assessment-insulin resistance trait and 16% to 22% variability in glucose, insulin, and waist circumference. Our results demonstrate that plasma lipidomic studies can additively contribute to an understanding of the interindividual variability in MS.  相似文献   
8.
In order to enhance in terms of accuracy and predict the modeling of the potential distribution of species, the integration of using principal components of environmental variables as input of maximum entropy (MaxEnt) has been proposed in this study. Principal components selected previously from the principal component analysis results performed in ArcGIS in the environmental variables was used as an input data of MaxEnt instead of raw data to model the potential distribution of red spiny lobster from the year 1997 to 2015 and for three different future scenarios 2020, 2050, and 2070. One set of six original environmental variables pertaining to the years 1997–2015 and one set of four variables for future scenarios were transformed independently into a single multiband raster in ArcGIS in order to select the variables whose eigenvalues explains more than 5% of the total variance with the purpose to use in the modeling prediction in MaxEnt. The years 1997 and 1998 were chosen to compare the accuracy of the model, showing better results using principal components instead of raw data in terms of area under the curve and partial receiver operating characteristic as well as better predictions of suitable areas. Using principal components as input of MaxEnt enhances the prediction of good habitat suitability for red spiny lobster; however, future scenarios suggest an adequate management by researches to elaborate appropriate guidelines for the conservation of the habitat for this valuable specie with face to the climate change.  相似文献   
9.
《植物生态学报》2015,39(8):785
Aims The objective of the study was to evaluate the dynamics of recalcitrant components during foliar litter decomposition at different forest gap size in Pinus massoniana plantation in the low hilly land, Sichuan basin. Methods The experiment was set up by thinning P. massoniana plantation to establish seven different gap sizes (G1: 100 m2, G2: 225 m2, G3: 400 m2, G4: 625 m2, G5: 900 m2, G6: 1225 m2, G7: 1600 m2). The contents of four recalcitrant components (condensed tannins, total phenol, lignin, cellulose) in foliar litter of two native species (Cinnamomum camphora and Toona ciliata) placed in litterbags at different locations in the forest gaps were evaluated. The litterbags placed under closed canopy were used as the control. Litterbags with air-dried leaves of C. camphora and T. ciliata were placed at center, edge of the gap and under the closed canopy in November 2013, and collected in December 2013, February 2014, May 2014 and August of 2014 for lab analysis. Important findings The results showed that: 1) Forest gap size had significant effect on the content of condensed tannins, total phenol and lignin for T. ciliata in gap center. However, the forest gap size had no significant effect on the content of recalcitrant components in the litters of C. camphora and cellulose content of T. ciliata. With the increase of gap size, except for cellulose content, the other three recalcitrant components content in small and medium sized gaps (G1-G5) were significant lower than in large gaps (G6, G7). 2) The condensed tannis content of T. ciliata at the gap center were significant lower than at the gap edge. The lignin contents at gap center of G3 was significant reduced in the C. camphora litter. The condensed tannins, total phenol, and lignin contents of T. ciliata litter in small and medium gaps significantly decreased. 3) The contents of the four recalcitrant components in both species’ foliar litter changed with time. The contents of condensed tannins and cellulose decreased and the content of lignin increased significantly with time, however, the total phenol content increased initially, and then decreased. Therefore, small and medium sized gaps (100-900 m2) could be the optimal gap sizes to promote the degradation of litter recalcitrant components for two native species in P. massoniana plantations.  相似文献   
10.
利用赣江上游流域1990、1995、2000、2005和2010年5期的TM影像以及2010年的SPOT影像资料,通过目视解译方法及GIS软件的空间叠加分析功能,获取了研究区土地利用覆被变化数据。在此基础上,结合1990—2010年赣江上游流域的统计资料利用灰色关联分析和主成分分析法进行分析,找出引起土地利用变化的主导因素。结果表明:研究区20年来林地和建设用地总量增加,耕地、草地、水域和未利用地总量减少。相同时期不同类型的土地利用变化的驱动因子不相同,不同时期同一类型土地利用变化的驱动因子也不相同。再利用GM(1,1)灰色预测模型预测了2015—2030年间研究区驱动因素和土地利用情况,研究表明主导因素对土地利用变化的影响与1990—2010年间变化趋势一致。  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