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   502篇
  免费   28篇
林业   34篇
农学   9篇
基础科学   1篇
  77篇
综合类   103篇
农作物   15篇
水产渔业   41篇
畜牧兽医   181篇
园艺   13篇
植物保护   56篇
  2023年   4篇
  2021年   12篇
  2020年   10篇
  2019年   11篇
  2018年   14篇
  2017年   8篇
  2016年   11篇
  2015年   9篇
  2014年   14篇
  2013年   24篇
  2012年   26篇
  2011年   29篇
  2010年   14篇
  2009年   11篇
  2008年   39篇
  2007年   31篇
  2006年   19篇
  2005年   32篇
  2004年   27篇
  2003年   30篇
  2002年   37篇
  2001年   11篇
  2000年   9篇
  1999年   3篇
  1998年   7篇
  1995年   4篇
  1994年   3篇
  1993年   6篇
  1992年   4篇
  1990年   7篇
  1989年   2篇
  1988年   7篇
  1987年   7篇
  1986年   2篇
  1985年   2篇
  1984年   4篇
  1983年   5篇
  1982年   4篇
  1979年   3篇
  1977年   1篇
  1976年   3篇
  1975年   3篇
  1973年   2篇
  1971年   3篇
  1969年   2篇
  1967年   4篇
  1966年   2篇
  1964年   1篇
  1947年   1篇
  1945年   1篇
排序方式: 共有530条查询结果,搜索用时 15 毫秒
521.
Schizophrenia is a devastating neurodevelopmental disorder whose genetic influences remain elusive. We hypothesize that individually rare structural variants contribute to the illness. Microdeletions and microduplications >100 kilobases were identified by microarray comparative genomic hybridization of genomic DNA from 150 individuals with schizophrenia and 268 ancestry-matched controls. All variants were validated by high-resolution platforms. Novel deletions and duplications of genes were present in 5% of controls versus 15% of cases and 20% of young-onset cases, both highly significant differences. The association was independently replicated in patients with childhood-onset schizophrenia as compared with their parents. Mutations in cases disrupted genes disproportionately from signaling networks controlling neurodevelopment, including neuregulin and glutamate pathways. These results suggest that multiple, individually rare mutations altering genes in neurodevelopmental pathways contribute to schizophrenia.  相似文献   
522.
523.
As monkeys perform schedules containing several trials with a visual cue indicating reward proximity, their error rates decrease as the number of remaining trials decreases, suggesting that their motivation and/or reward expectancy increases as the reward approaches. About one-third of single neurons recorded in the anterior cingulate cortex of monkeys during these reward schedules had responses that progressively changed strength with reward expectancy, an effect that disappeared when the cue was random. Alterations of this progression could be the basis for the changes from normal that are reported in anterior cingulate population activity for obsessive-compulsive disorder and drug abuse, conditions characterized by disturbances in reward expectancy.  相似文献   
524.
The creation of enzymes capable of catalyzing any desired chemical reaction is a grand challenge for computational protein design. Using new algorithms that rely on hashing techniques to construct active sites for multistep reactions, we designed retro-aldolases that use four different catalytic motifs to catalyze the breaking of a carbon-carbon bond in a nonnatural substrate. Of the 72 designs that were experimentally characterized, 32, spanning a range of protein folds, had detectable retro-aldolase activity. Designs that used an explicit water molecule to mediate proton shuffling were significantly more successful, with rate accelerations of up to four orders of magnitude and multiple turnovers, than those involving charged side-chain networks. The atomic accuracy of the design process was confirmed by the x-ray crystal structure of active designs embedded in two protein scaffolds, both of which were nearly superimposable on the design model.  相似文献   
525.
526.
527.
528.
Spotted wilt, caused by tomato spotted wilt virus (TSWV), is a major disease of peanut (Arachis hypogaea ) in the south‐eastern United States. Cultivar resistance is the most important factor in disease control. However, spotted wilt resistance in current cultivars still carries risk in the absence of other practices when disease is severe. In contrast, a newly developed cultivar, Florida‐EP? “113,” has demonstrated excellent resistance even when spotted wilt is severe. Information on heritability of this resistance can help breeders better utilize it in breeding. F2‐derived populations from the cross Florida‐EP? “113”/Georgia Valencia were developed and tested in field experiments in Florida from 2012 to 2014. Disease symptoms were evaluated visually, and the frequency of TSWV infection was measured by ImmunoStrip®. Heritability estimated from ImmunoStrip® was higher (0.66) compared to visual ratings (0.48). Genetic correlations among evaluation methods (r A = 0.92–0.99) and environments (r B = 0.86–0.99) were high. These results indicate that resistance in Florida‐EP? “113” is highly heritable and that selection in a high disease risk environment is feasible without significant erosion of genetic gain.  相似文献   
529.
A number of agroforestry models have been developed to simulate growth outcomes based on the interactions between components of agroforestry systems. A major component of this interaction is the impact of shade from trees on crop growth and yield. Capability in the agricultural production systems simulator (APSIM) model to simulate the impacts of shading on crop performance could be particularly useful, as the model is already widely used to simulate agricultural crop production. To quantify and simulate the impacts of shading on maize performance without trees, a field experiment was conducted at Melkassa Agricultural Research Centre, Ethiopia. The treatments contained three levels of shading intensity that reduced incident radiation by 0 (control), 50 and 75% using shade cloth. Data from a similar field experiment at Machakos Research Station, Kenya, with 0, 25 and 50% shading were also used for simulation. APSIM adequately simulated maize grain yield (r2 = 0.97) and total above-ground biomass (r2 = 0.95) in the control and in the 50% treatments at Melkassa, and likewise in the control (r2 = 0.99), 25% (r2 = 0.90) and 50% (r2 = 0.98) treatments at Machakos. Similarly, APSIM effectively predicted Leaf Area Index attained at the flowering (r2 = 0.90) and maturity (r2 = 0.94) stages. However, APSIM under-estimated maize biomass and yield at 75% shading. In conclusion, the model can be reliably employed to simulate maize productivity in agroforestry systems with up to 50% shading, but caution is required at higher levels of shading.  相似文献   
530.
Trissolcus japonicus is an egg parasitoid of Halyomorpha halys, brown marmorated stink bug, a severe agricultural pest in the USA. T. japonicus is being evaluated in quarantine as a classical biological control agent to manage H. halys populations in the USA. To determine T. japonicus’ potential for successful management of the pest, we performed a series of no-choice and paired-host-range tests, evaluating parasitism and host recognition in ten nontarget insects. In laboratory no-choice tests, T. japonicus successfully parasitized egg masses of seven Pentatomidae native to Oregon in addition to H. halys. Mean parasitism proportions of egg masses were greater than 40% in two species, B. dimidiata and H. abbreviatus, and were statistically similar to parasitism of H. halys. However, paired-host tests identified higher proportions of parasitized H. halys egg masses compared to four other pentatomids. T. japonicus was equally attracted to volatiles produced by H. halys and other pentatomids but demonstrated significantly longer arrestment response time on surfaces with H. halys contact kairomones. Although host acceptance patterns were similar between stink bug species, our results suggest a greater potential for parasitoid development in H. halys eggs compared to the native pentatomids. During host-range testing, we detected field populations of T. japonicus at 11 sites in Portland, OR, indicating an unintentional introduction and establishment. Further work is needed to characterize its nontarget activity and dispersal patterns in areas where H. halys causes economic damage in Oregon.  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