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   829篇
  免费   53篇
林业   52篇
农学   19篇
基础科学   1篇
  192篇
综合类   70篇
农作物   78篇
水产渔业   59篇
畜牧兽医   314篇
园艺   30篇
植物保护   67篇
  2024年   3篇
  2023年   23篇
  2022年   27篇
  2021年   41篇
  2020年   33篇
  2019年   46篇
  2018年   38篇
  2017年   37篇
  2016年   30篇
  2015年   31篇
  2014年   26篇
  2013年   41篇
  2012年   66篇
  2011年   99篇
  2010年   54篇
  2009年   33篇
  2008年   37篇
  2007年   38篇
  2006年   32篇
  2005年   28篇
  2004年   25篇
  2003年   19篇
  2002年   17篇
  2001年   6篇
  2000年   4篇
  1999年   2篇
  1998年   1篇
  1997年   1篇
  1996年   5篇
  1995年   1篇
  1991年   1篇
  1990年   5篇
  1988年   1篇
  1987年   4篇
  1986年   2篇
  1985年   2篇
  1984年   5篇
  1983年   2篇
  1982年   2篇
  1981年   3篇
  1980年   2篇
  1979年   1篇
  1976年   1篇
  1974年   2篇
  1970年   2篇
  1967年   1篇
  1963年   1篇
  1945年   1篇
排序方式: 共有882条查询结果,搜索用时 31 毫秒
871.
872.
In this study, the effect of ultrasonic sound (at 20- and 40-kHz frequency for 15 min) and drying (at 60 °C for 15 min) pretreatments on the moisture content and oil uptake of potato slices (cultivar Satina) during deep frying were studied. The potato slices were fried at temperatures of 170 and 190 °C for 5, 7 and 10 min. Samples pretreated with ultrasound at 20 kHz had higher moisture contents than those receiving the 40-kHz pretreatment. The highest moisture content was obtained at 20 kHz and frying at 170 °C for 5 min. On the other hand, samples pretreated with ultrasound at 40 kHz had higher effective water diffusivity coefficient and activation energy than those pretreated at 20 kHz. The highest effective water diffusivity coefficient was obtained at 40 kHz and frying at 190 °C for 5 min, whereas the highest activation energy belonged to pretreatment with 40 kHz and frying at 170 °C for 10 min. The range for effective water diffusivity coefficient was 6.95?×?10?8 to 8.80?×?10?8 m2 s?1 and that for activation energy was 13.161 to 16.307 kJ mol?1. In addition, samples pretreated at 40 kHz had a lower oil uptake than those pretreated at 20 kHz. The highest oil uptake took place at 20 kHz, with frying at 170 °C for 10 min. Regression analysis revealed that an exponential model was best for predicting variations in moisture content and oil uptake of potato slices.  相似文献   
873.
Water, Air, & Soil Pollution - Phosphorus (P) is an essential element for all living organisms, and plays a major role in many physiological processes. However, in recent years, excessive...  相似文献   
874.
Following the near-obliteration of American chestnut (Castanea dentata [Marsh.] Borkh.) by the chestnut blight early in the last century, interest in its restoration has been revived by efforts to develop a blight-resistant form of the species. We summarize progress and outline future steps in two approaches: (1) a system of hybridizing with a blight-resistant chestnut species and then backcrossing repeatedly to recover the American type and (2) transformation of American chestnut with a resistance-conferring transgene followed by propagation and conventional breeding. Several decades of effort have been invested in each approach. More work remains, but results indicate that success is within practical reach. The restoration of C. dentata to its native habitat now appears to be less a matter of time and conjecture than ever before in 90 years of work by public and private entities. The difficult and protracted task of incorporating extraspecific genes for resistance into a tree species with lethal susceptibility to a naturalized pathogen represents perhaps the most extreme of restoration challenges. Its pursuit by a small non-governmental organization supported primarily by philanthropy and volunteers may serve as a model for other species threatened by exotic pathogens or insects.  相似文献   
875.
Root water uptake is a component of water balance that has not been clearly understood. This study was carried out in a completely randomized design with three replications under the greenhouse condition at Shahrekord University, Shahrekord, Iran. In this study, the root water uptake (RWU) by pepper plant under various irrigation water levels was investigated. Irrigation treatments included control (full irrigation level, FI) and three deficit irrigation levels, 80%, 60% and 40% of the plant's water requirement called DI80, DI60 and DI40, respectively. A no-plant cover treatment with three replications was also used to measure evaporation from the soil surface. Daily measurements of volumetric soil moisture (VSM) were made at 10 cm intervals of the soil column. The differences between the measured VSM and the VSM in the next day, and evaporation rate at the soil surface at the same layer of the no-plant cover treatment were calculated and, eventually, the RWU in each layer per day was estimated. The results showed that the maximum and minimum RWUs were found in the FI and DI40 treatments, respectively. The averages of root water uptakes in the DI80, DI60, and DI40 treatments were reduced by 17.08%, 48.72% and 68.25%, respectively. Furthermore, in the DI80 treatment, the reduced rate of water uptake was less than the reduced rate of water applied to the plants.  相似文献   
876.
The natural environment represents an important source of drugs that originates from the terrestrial and, in minority, marine organisms. Indeed, the marine environment represents a largely untapped source in the process of drug discovery. Among all marine organisms, sponges with algae represent the richest source of compounds showing anticancer activity. In this study, the two secondary metabolites pelorol (PEL) and 5-epi-ilimaquinone (EPI), purified from Dactylospongia elegans were investigated for their anti-melanoma activity. PEL and EPI induced cell growth repression of 501Mel melanoma cells in a concentration- and time-dependent manner. A cell cycle block in the G1 phase by PEL and EPI was also observed. Furthermore, PEL and EPI induced significant accumulation of DNA histone fragments in the cytoplasmic fraction, indicating a pro-apoptotic effect of both compounds. At the molecular level, PEL and EPI induced apoptosis through the increase in pro-apoptotic BAX expression, confirmed by the decrease in its silencing miR-214-3p and the decrease in the anti-apoptotic BCL-2, MCL1, and BIRC-5 mRNA expression, attested by the increase in their silencing miRNAs, i.e., miR-193a-3p and miR-16-5p. In conclusion, our data indicate that PEL and EPI exert cytotoxicity activity against 501Mel melanoma cells promoting apoptotic signaling and inducing changes in miRNA expression and their downstream effectors. For these reasons could represent promising lead compounds in the anti-melanoma drug research.  相似文献   
877.
Few studies of land-use/land-cover change provide an integrated assessment of the driving forces and consequences of that change, particularly in Africa. Our objectives were to determine how driving forces at different scales change over time, how these forces affect the dynamics and patterns of land use/land cover, and how land-use/land-cover change affects ecological properties at the landscape scale. To accomplish these objectives, we first developed a way to identify the causes and consequences of change at a landscape scale by integrating tools from ecology and the social sciences and then applied these methods to a case study in Ghibe Valley, southwestern Ethiopia. Maps of land-use/land-cover change were created from aerial photography and Landsat TM imagery for the period, 1957–1993. A method called `ecological time lines' was developed to elicit landscape-scale explanations for changes from long-term residents. Cropland expanded at twice the speed recently (1987–1993) than two decades ago (1957–1973), but also contracted rapidly between 1973–1987. Rapid land-use/land cover change was caused by the combined effects of drought and migration, changes in settlement and land tenure policy, and changes in the severity of the livestock disease, trypanosomosis, which is transmitted by the tsetse fly. The scale of the causes and consequences of land-use/land-cover change varied from local to sub-national (regional) to international and the links between causes and consequences crossed scales. At the landscape scale, each cause affected the location and pattern of land use/land cover differently. The contraction of cropland increased grass biomass and cover, woody plant cover, the frequency and extent of savanna burning, and the abundance of wildlife. With recent control of the tsetse fly, these ecological changes are being reversed. These complex patterns are discussed in the context of scaling issues and current conceptual models of land-use/land-cover change.  相似文献   
878.
The comparative toxicity of two non-steroidal ecdysteroid agonists, RH-2485 and RH-5992 (tebufenozide), on development stages, fecundity and egg viability of a susceptible laboratory strain and a pyrethroid-resistant field strain ofSpodoptera littoralis (Boisduval) was evaluated. Taking the LC50s as the criterion, RH-2485 was 3–7-fold more potent than RH-5992 against the susceptible and 7–14-fold more against the field strain. The LC50 of RH-2485 in the 1st and 6th instars of the susceptible strain was 0.32 and 0.57 mg a.i./l, respectively. The field strain showed a mild cross-resistance of about threefold to both compounds in 1st instars and to a lesser extent in 6th instars. A considerable increase in fecundity (~3-fold) and no effect on egg viability was observed when 6th instars were fed on cotton leaves treated with 0.25 mg a.i./l RH-2485 (~LC40). Our results indicate that both compounds are potentially potent insecticides for controllingS. littoralis larvae, being 10-60-fold more potent than a previous ecdysteroid agonist, RH-5849.  相似文献   
879.
Background:Flavonoids are a large group of phenolic compounds possessing anti-inflammatory and antioxidant effects. NAR is a flavonoid with various pharmacological properties. Using pharmaceutical compounds on skin is one of the routes of administration to achieve local and systemic effects. The aim of this study was to develop a topical formulation of NAR by the preparation of a NAR ME, which was further tested its skin permeability in rats. Methods:Eight 0.5% NAR MEs were prepared by mixing appropriate amounts of surfactant (Tween 80 and Labrasol), cosurfactant (Capryol 90) and the oil phase (oleic acid-Transcutol P in a ratio of 1:10). The drug was dissolved in the oil phase. The physicochemical properties of MEs such as droplet size, viscosity, release, and skin permeability were assessed using Franz Cells diffusion. Results:Based on the results, the droplet size of MEs ranged between 5.07 and 35.15 nm, and their viscosity was 164-291 cps. Independent factors exhibited a strong relationship with both permeability and drop size. The permeability findings revealed that the diffusion coefficient of NAR by the ME carrier increased compared to the drug saturation solution. Conclusion: The most validated results were obtained for Jss and particle size. Optimal formulations containing MEs with Jss and particle sizes varying between minimum and maximum amounts are suitable for topical formulations of NAR. Key Words: Naringenin-Loaded Microemulsion, Topical Application, Treatment  相似文献   
880.
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