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   155篇
  免费   5篇
  国内免费   1篇
林业   15篇
农学   3篇
  54篇
综合类   12篇
农作物   6篇
水产渔业   9篇
畜牧兽医   50篇
园艺   5篇
植物保护   7篇
  2022年   2篇
  2021年   3篇
  2020年   5篇
  2019年   5篇
  2018年   2篇
  2017年   9篇
  2016年   6篇
  2015年   1篇
  2014年   4篇
  2013年   7篇
  2012年   12篇
  2011年   12篇
  2010年   2篇
  2009年   8篇
  2008年   9篇
  2007年   16篇
  2006年   5篇
  2005年   13篇
  2004年   10篇
  2003年   7篇
  2002年   7篇
  2001年   1篇
  1999年   3篇
  1998年   3篇
  1996年   1篇
  1995年   1篇
  1994年   1篇
  1991年   4篇
  1990年   1篇
  1986年   1篇
排序方式: 共有161条查询结果,搜索用时 15 毫秒
1.
Background: Nitrogen losses is an economic problem for wheat production and a high risk to the environment. Therefore, improved N fertilizer management is a key to increasing the N efficiency and minimizing N losses. To increase N efficiency, enhanced fertilizers such as urea combined with urease inhibitor can be used. Aims: The aim of present study was to evaluate the effects of different N forms on grain storage protein subunits in winter wheat and to examine whether the observed changes correlate with parameters of baking quality. Methods: The investigation was performed over two consecutive years at two locations in Germany. Protein subunits were analyzed by SDS‐PAGE. Results: Protein concentrations were similarly increased after fertilization with ammonium nitrate and urea + urease inhibitor. Analysis of the individual storage protein fractions indicated that both fertilizers specifically enhanced ω‐gliadins and HMW glutenins, but the effect was more pronounced in the ammonium nitrate treatment. Application of urea + urease inhibitor had greater influence on the protein composition and resulted in higher specific baking volume as well as the best fresh keeping ability, in comparison with urea treatment. Conclusion: Considering that the urea + urease inhibitor treatment resulted in almost comparable improvements of NUE and baking quality, with the additional benefit of reduced N losses in combination with easy handling, urea + urease inhibitor can be recommended as a viable alternative to both urea alone and ammonium nitrate treatments. This opens up an opportunity for the reduction of N loss in wheat production when use of urea is preferred.  相似文献   
2.
There is a general understanding that university plays a crucial role in regional development under the current age of knowledge economy. At the same time, we have observed a fair number of counter‐evidentiary studies, demonstrating several regions that do not thrive despite strong research universities or other patterns of development in the absence of research universities. However, they are based on case studies of individual universities or regions, and the findings currently cannot be generalized. Another set of studies that have challenged the effect of universities at the nationwide scale and demonstrated its disproportionately small impact of universities, but their scope is limited to spin‐offs, patents, and license activities. In this article, we propose to investigate this subject more systematically by triangulating three methods: 1) national‐scale regression models with Business Dynamics Statistics and National Establishment Time Series data, 2) a regional survey of IT and life science firms in four so‐called “second tier regions,” and 3) a microeconomic firm‐level analysis based on interviews with founders of technology intensive firms. Furthermore, we test the roles of university beyond those explicit, direct measures of spin‐offs and patents by analyzing the firm formation rate and firm growth rate at the regional level, as well as the sources of growth at the company level. All results point in the same direction: The research function of the university has been overstated, and the teaching function of the university has been underappreciated. These findings provide profound implications for the policy of regional development.  相似文献   
3.
The influence of transplanting on height, root neck diameter, and grading was investigated for Carpinus betulus, Quercus robur, and Euonymus europaeus on three sites with different climate and soil properties. Additionally, plant growth was characterized by the shoot-to-root ratio, height-to-root neck diameter, and proportions of coarse and fine roots. The significance of the growth parameters as criteria for survival is discussed. Transplanting influenced shoot growth more than root growth. The growth of the plants was highly site-dependent. Transplanting increased growth most on the site with the best conditions for water and N supply. Transplanting had no effect on grading for the site with the worst water supply.  相似文献   
4.
OBJECTIVE: To evaluate effects of sedation on stability of resistance of the respiratory system (RRS) and measures of resting energy expenditure (REE) by use of open-flow indirect calorimetry (IC) and treatment with aerosolized albuterol on REE in horses with recurrent airway obstruction (RAO). ANIMALS: 9 clinically normal horses and 8 horses with RAO. PROCEDURE: In phase 1, RRS was measured by using forced oscillometry (FOT) in 5 clinically normal horses before and after sedation with xylazine. In phase 2, REE was measured in 4 clinically normal horses between 20 and 25 minutes and again 35 to 40 minutes after sedation with xylazine. In phase 3, IC was performed between 20 and 25 minutes and FOT was performed between 30 and 35 minutes after xylazine administration in 8 horses with RAO; after administration of 450 microg of albuterol, IC and FOT were repeated. RESULTS: In phase 1, RRS values were significantly lower 5 and 10 minutes after sedation. In phase 2, diminishing sedation did not significantly affect REE. In phase 3, there was a significant decrease in mean RRS (1.15 +/- 0.25 vs 0.84 +/- 0.14 cm H20/L/s) and REE (30.68 +/- 17.89 vs 27.46 = 16.54 kcal/kg/d) after albuterol administration. CONCLUSIONS AND CLINICAL RELEVANCE: FOT and IC are useful in obtaining repeatable measurements of RRS and REE, respectively, in sedated horses. Concurrent bronchodilation and decreased REE after albuterol administration suggest that increased work of breathing as a result of airway obstruction may contribute to increased energy demands in horses with RAO.  相似文献   
5.
6.
Habitat fragmentation is a major cause for species loss, but its effect on invertebrates with low active dispersal power, like terrestrial gastropods, has rarely been studied. Such species can not cross a hostile habitat matrix, for which the predictions of island theory, such as positive relations between species richness and patch size, should apply. In order to test this prediction, we studied gastropod species diversity by assessing gastropod assemblage characteristics from 35 sites in 19 fragments of deciduous old-growth forests in the Lower Rhine Embayment, Germany. Assemblages differed between larger (≥700 ha) and smaller forests (<400 ha), those of large forests held a higher percentage of forest species. Although α-diversity was similar between the two forest size classes, small forests often comprised matrix species, resulting in a higher β-diversity. Edge effects on the species richness of matrix species were noticeable up to 250 m into the forest. Hierarchical partitioning revealed that distance to disturbances (external edge, internal edges like roads) explained most assemblage variables, whereas forest size and woodland cover within a 1 km radius from the sites explained only a few assemblage variables. Densities of two forest-associated species, Discus rotundatus and Arion fuscus, decreased with forest size. Yet, forest size was positively correlated with richness of typical forest species and densities of Limax cinereoniger. The latter species seems to need forests of >1,000 ha, i.e., well above the size of most fragments. In conclusion, the prediction is valid only for forest species. The response to fragmentation is species specific and seems to depend on habitat specialization and macroclimatic conditions. Jean-Pierre Maelfait: Deceased.  相似文献   
7.
The non‐specific rust hyperparasite Sphaerellopsis filum occurs naturally on Melampsora rusts of many species of the genus Salix as well as on a large range of other rust genera worldwide. To study the genetic diversity of the hyperparasitic fungus 77 S. filum isolates collected from rusts on willow clones from plantations, clone collections and natural habitats of different sites were investigated using polymerase chain reaction ‐ restriction fragment length polymorphism (PCR‐RFLP) analysis of the rDNA internal transcribed spacer regions including 5.8S rDNA and sequence analysis. Additionally, strains from Melampsora poplar rusts (4) and strains of Puccinia abrupta from Parthenium hysterophorus (5) and of P. obscura from Bellis perennis (1) were used for comparisons. Results of genetic analysis demonstrated distinct variation within the S. filum isolates. Two main groups with more than 32% difference between their nucleotide sequences were distinguished, indicating two taxa within S. filum. Within the first main group three profiles (I, II, III) were detected. The differences between these profiles were about 12%. The variation within each profile was very low (less than 2%). The second main group comprised two profiles (IV, V), which differed in 12 to 16% of their nucleotide positions. The isolates of group IV possessed a higher variation (up to 5%) within the group than those of the first main group (I, II, III). Group V was only represented by a single isolate. Neither interrelations between the S. filum profiles and the Melampsora genotypes nor a spatial distribution could be detected. It is remarkable that the six strains of S. filum from Puccinia rusts belong to one subgroup.  相似文献   
8.
European Journal of Forest Research - In Germany, Douglas fir (Pseudotsuga menziesii Franco) is seen as a valuable species for future cultivation in times of climate change. Local seed production...  相似文献   
9.
Viral infections have been suggested to play a role in the decline of the panmictic population of the European eel (Anguilla anguilla). However, despite the importance of knowledge about pathogenic eel viruses, little is known about their spread in the wild European eel population and only a few eel pathogenic viruses have been described so far. In this study, we aimed to investigate the health status of the A. anguilla stock in North Rhine Westphalia (NRW) State of Germany. For this purpose, we examined tissue samples of 16 elvers, 100 yellow eels and 6 silver eels, sampled from the rivers Rhine, Lippe and Ems. Virus detection was performed via a combination of cell culture and PCR. Next to the detection of frequently encountered pathogenic eel viruses (anguillid herpesvirus 1 and eel virus European X (EVEX)), we isolated the eel picornavirus 1 (EPV-1) from tissue of yellow eels and elvers and demonstrate the distribution of EPV-1 in wild eel population in NRW.  相似文献   
10.
To prepare composite films from biopolymers with anti-listerial activity and moisture barrier properties, the antimicrobial efficiency of chitosan-hydroxy propyl methyl cellulose (HPMC) films, chitosan-HPMC films associated with lipid, and chitosan-HPMC films chemically modified by cross-linking were evaluated. In addition, the physicochemical properties of composite films were evaluated to determine their potential for food applications. The incorporation of stearic acid into the composite chitosan-HPMC film formulation decreased water sensitivity such as initial solubility in water and water drop angle. Thus, cross-linking of composite chitosan-HPMC, using citric acid as the cross-linking agent, led to a 40% reduction in solubility in water. The water vapor transfer rate of HPMC film, approximately 270 g x m(-2) x day(-1) x atm(-1), was improved by incorporating chitosan and was further reduced 40% by the addition of stearic acid and/or cross-linking. Anti-listerial activity of films was determined on solid medium by a numeration technique. Chitosan-HPMC-based films, with and without stearic acid, inhibited the growth of Listeria monocytogenes completely. On the other hand, a loss of antimicrobial activity after chemical cross-linking modification was observed. FTIR and 13C NMR analyses were then conducted in order to study a potential chemical modification of biopolymers such as a chemical reaction with the amino group of chitosan. To complete the study, the mechanical properties of composite films were determined from tensile strength assays.  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