全文获取类型
收费全文 | 153778篇 |
免费 | 5376篇 |
国内免费 | 5980篇 |
专业分类
165134篇 |
出版年
2024年 | 177篇 |
2023年 | 917篇 |
2022年 | 2118篇 |
2021年 | 3385篇 |
2020年 | 2342篇 |
2019年 | 2876篇 |
2018年 | 13814篇 |
2017年 | 12089篇 |
2016年 | 9869篇 |
2015年 | 4599篇 |
2014年 | 5125篇 |
2013年 | 5214篇 |
2012年 | 9852篇 |
2011年 | 17589篇 |
2010年 | 14772篇 |
2009年 | 10793篇 |
2008年 | 12761篇 |
2007年 | 13991篇 |
2006年 | 2765篇 |
2005年 | 2568篇 |
2004年 | 2671篇 |
2003年 | 2587篇 |
2002年 | 1993篇 |
2001年 | 1312篇 |
2000年 | 1153篇 |
1999年 | 923篇 |
1998年 | 573篇 |
1997年 | 527篇 |
1996年 | 562篇 |
1995年 | 482篇 |
1994年 | 468篇 |
1993年 | 380篇 |
1992年 | 507篇 |
1991年 | 392篇 |
1990年 | 320篇 |
1989年 | 286篇 |
1988年 | 249篇 |
1987年 | 222篇 |
1986年 | 188篇 |
1985年 | 171篇 |
1984年 | 128篇 |
1983年 | 146篇 |
1982年 | 88篇 |
1981年 | 49篇 |
1980年 | 52篇 |
1979年 | 63篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1972年 | 299篇 |
1971年 | 300篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Kazuki Fujita Takashi Kunito Hitoshi Moro Hideshige Toda Shigeto Otsuka Kazunari Nagaoka 《Biogeochemistry》2017,132(3):325-342
According to the resource allocation model for extracellular enzyme synthesis, microorganisms should preferentially allocate their resources to phosphorus (P)-acquiring enzyme synthesis when P availability is low in soils. However, the validity of this model across different soil types and soils differing in their microbial community composition has not been well demonstrated. Here we investigated whether the resource allocation model for phosphatase synthesis is applicable across different soil types (Andosols, Acrisols, Cambisols, and Fluvisols) and land uses (arable and forest), and we examined which soil test P and/or P fraction microorganisms responded to when investing their resources in phosphatase synthesis in the soils. The ratio of alkaline phosphatase (ALP) to β-d-glucosidase (BG) activities in the arable soils and the ratio of acid phosphatase (ACP) to BG activities in the forest soils were significantly negatively related with the available inorganic P concentration. We also observed significant effects of available inorganic P, pH, soil types, and land uses on the (ACP + ALP)/BG ratio when the data for the arable and forest soils were combined and used in a stepwise multiple regression analysis. These results suggest that microbial resource allocation for phosphatase synthesis is primarily controlled by available inorganic P concentration and soil pH, but the effects of soil types and land uses are also significant. 相似文献
992.
993.
Cheryl Baldwin Nana Wilberforce Amit Kapur 《The International Journal of Life Cycle Assessment》2011,16(1):40-49
Purpose
There is no clear guidance for responsible food service operations to reduce their environmental footprint, so the efforts put forth by a restaurant may not have the environmental impact intended. As a result, Green Seal conducted life cycle assessment research on restaurants and food service operations to define priorities for environmental improvement. This information was then used to develop a sustainability standard and certification (i.e., ecolabel) program. 相似文献994.
Manco G Carrea G Giosuè E Ottolina G Adamo G Rossi M 《Extremophiles : life under extreme conditions》2002,6(4):325-331
The esterase genes est2 from Alicyclobacillus acidocaldarius and AF1716 from Archaeoglobus fulgidus were subjected to error-prone PCR in an effort to increase the low enantioselectivity of the corresponding enzymes EST2 and AFEST, respectively. The model substrate ( RS)- p-nitrophenyl-2-chloropropionate was chosen to produce ( S)-2-chloropropionic acid, an important intermediate in the synthesis of some optically pure compounds, such as the herbicide mecoprop. In the case of EST2, a single mutant, Leu212Pro, was obtained showing a slightly enhanced preference toward the ( S) substrate; in the case of AFEST, a double mutant, Leu101Ile/Asp117Gly, was obtained showing an increased preference in the opposite direction. The 3-D structures of the EST2 and AFEST enzymes were analyzed by molecular modeling to determine the effects of the mutations. Mutations were positioned differently in the structures, but in both cases caused small modifications around the active site and in the oxyanion loop. 相似文献
995.
Rasoul Ghasemi Hoorieh Share Roza Sharifi Robert S. Boyd Nishanta Rajakaruna 《Ecological Research》2018,33(4):737-747
The importance of calcium in nickel tolerance was studied in the nickel hyperaccumulator plant Alyssum inflatum by gene transformation of CAX1, a vacuolar membrane transporter that reduces cytosolic calcium. CAX1 from Arabidopsis thaliana with a CaMV35S promoter accompanying a kanamycin resistance gene was transferred into A. inflatum using Agrobacterium tumefaciens. Transformed calli were sub-cultured three times on kanamycin-rich media and transformation was confirmed by PCR using a specific primer for CAX1. At least 10 callus lines were used as a pool of transformed material. Both transformed and untransformed calli were treated with varying concentrations of either calcium (1–15 mM) or nickel (0–500 µM) to compare their responses to those ions. Increased external calcium generally led to increased callus biomass, however, the increase was greater for untransformed callus. Further, increased external calcium led to increased callus calcium concentrations. Transformed callus was less nickel tolerant than untransformed callus: under increasing nickel concentrations callus relative growth rate was significantly less for transformed callus. Transformed callus also contained significantly less nickel than untransformed callus when exposed to the highest external nickel concentration (200 µM). We suggest that transformation with CAX1 decreased cytosolic calcium and resulted in decreased nickel tolerance. This in turn suggests that, at low cytosolic calcium concentrations, other nickel tolerance mechanisms (e.g., complexation and vacuolar sequestration) are insufficient for nickel tolerance. We propose that high cytosolic calcium is an important mechanism that results in nickel tolerance by nickel hyperaccumulator plants. 相似文献
996.
Zhang Hao Qing Zhao Xue Qiang Chen Yi Ling Zhang Ling Yu Shen Ren Fang 《Plant and Soil》2019,439(1-2):293-308
Plant and Soil - Biocrusts are important functional units in dryland ecosystems. Regarded as ecosystem engineers, cyanobacteria in biocrusts contribute several major physico-chemical and biological... 相似文献
997.
In insect societies, worker vs. queen development (reproductive caste) is typically governed by environmental factors, but many Pogonomyrmex seed-harvester ants exhibit strict genetic caste determination, resulting in an obligate mutualism between two reproductively isolated lineages. Same-lineage matings produce fertile queens while alternate-lineage matings produce sterile workers. Because new virgin queens mate randomly with multiple males of each lineage type, and both worker and queen phenotypes are required for colony growth and future reproduction, fitness is influenced by the relative frequency of each lineage involved in the mutualistic breeding system. While models based solely on frequency-dependent selection predict the convergence of lineage frequencies towards equal (0.5/0.5), we surveyed the lineage ratios of 49 systems across the range of the mutualism and found that the global lineage frequency differed significantly from equal. Multiple regression analysis of our system survey data revealed that the density and relative frequency of one lineage decreases at lower elevations, while the frequency of the alternate lineage increases with total colony density. While the production of the first worker cohort is largely frequency dependent, relying on the random acquisition of worker-biased sperm stores, subsequent colony growth is independent of lineage frequency. We provide a simulation model showing that a net ecological advantage held by one lineage can lead to the maintenance of stable but asymmetric lineage frequencies. Collectively, these findings suggest that a combination of frequency-dependent and frequency-independent mechanisms can generate many different localized and independently evolving system equilibria. 相似文献
998.
Amira Sfaxi-Bousbih Abdelilah Chaoui Ezzedine El Ferjani 《Biological trace element research》2010,137(1):110-116
Seeds of bean (Phaseolus vulgaris L.) were germinated by soaking in distilled water or copper chloride solution. The relationships among copper excess treatment,
germination rate, dry weight, sugar contents, and carbohydrase activities in cotyledon were investigated. Heavy metal stress
provoked a diminution in germination rate and biomass mobilization, as compared with the control. A drastic disorder in soluble
sugars export, especially glucose and fructose liberation, was also imposed after exposure to excess copper. This restricted
the starch and sucrose breakdown in reserve tissue, as evidenced by the inhibition in the activities of α-amylase and invertase
isoenzymes (soluble acid, soluble neutral, cell wall-bound acid). 相似文献
999.
Tao Tan Jun Wu Chenyang Si Shaoxing Dai Youyue Zhang Nianqin Sun E Zhang Honglian Shao Wei Si Pengpeng Yang Hong Wang Zhenzhen Chen Ran Zhu Yu Kang Reyna Hernandez-Benitez Llanos Martinez Martinez Estrella Nuñez Delicado W. Travis Berggren Juan Carlos Izpisua Belmonte 《Cell》2021,184(8):2020-2032.e14
1000.
Jakub Horák Christopher Grefen Kenneth W Berendzen Achim Hahn York-Dieter Stierhof Bettina Stadelhofer Mark Stahl Csaba Koncz Klaus Harter 《BMC plant biology》2008,8(1):77