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   1371篇
  免费   80篇
  国内免费   2篇
林业   21篇
农学   38篇
基础科学   9篇
  182篇
综合类   181篇
农作物   45篇
水产渔业   49篇
畜牧兽医   845篇
园艺   22篇
植物保护   61篇
  2022年   11篇
  2021年   13篇
  2020年   13篇
  2019年   16篇
  2018年   18篇
  2017年   25篇
  2016年   24篇
  2015年   26篇
  2014年   35篇
  2013年   65篇
  2012年   74篇
  2011年   58篇
  2010年   45篇
  2009年   34篇
  2008年   50篇
  2007年   54篇
  2006年   45篇
  2005年   55篇
  2004年   52篇
  2003年   50篇
  2002年   36篇
  2001年   40篇
  2000年   31篇
  1999年   21篇
  1998年   23篇
  1997年   13篇
  1995年   14篇
  1994年   11篇
  1993年   19篇
  1992年   22篇
  1991年   29篇
  1990年   19篇
  1989年   21篇
  1988年   31篇
  1987年   17篇
  1986年   17篇
  1985年   21篇
  1984年   23篇
  1983年   15篇
  1979年   16篇
  1978年   14篇
  1977年   21篇
  1975年   14篇
  1974年   13篇
  1973年   14篇
  1972年   11篇
  1971年   11篇
  1970年   13篇
  1969年   19篇
  1966年   11篇
排序方式: 共有1453条查询结果,搜索用时 229 毫秒
91.
Abstract: Stakeholders are an integral part of environmental conflict. Analysing the positions and interests of stakeholders is vital in effective environmental conflict management. This paper reviews the literature related to stakeholders and their dynamics in environmental conflict. It also presents the results of a study relating to a New Zealand transport infrastructure project where the positions and interests of stakeholders have kept on changing during the life of the project. This study plots the changing positions of some of the key stakeholders, towards this project. These stakeholders include a policy manager, a community stakeholder, an environmental stakeholder and a political stakeholder. The paper also analyses the changing positions by identifying and classifying the key drivers for these changes.  相似文献   
92.
Phillip Jackson 《Euphytica》1994,79(1-2):101-108
Summary Saccharum spontaneum is being used in sugarcane breeding programs in attempt to improve characteristics such as ratooning ability and stress tolerance. A population of F1 (Saccharum officinarum or commercial variety x S. spontaneum) and F1 x F1 sugarcane clones was evaluated for sugar yield and a range of yield components in a plant and two ratoon crops. The aim was to determine genetic correlations between attributes in clones with a large component of S. spontaneum, that could be used to help derive appropriate selection indices in such populations.There were close associations between the same attributes measured in different crop-years and the associations between different attributes were generally similar across crop-years. Stalk number and fibre content were positively correlated, as were stalk weight and CCS. The latter two attributes (which are low in S. spontaneum but high in S. officinarum) were negatively correlated with the former two (high in S. spontaneum, low in S. officinarum). Sugar yield was more closely associated with stalk weight and CCS than with stalk number but became more closely associated with stalk number with successive ratoon crops.CCS was positively correlated (rg=0.55) with cane yield in the plant crop but showed a small negative correlation with cane yield (rg=–0.20) in the second ratoon crop. CCS (measured in any crop) also had a negative correlation with cane yield in the ratoon crops expressed as a percentage of plant cane yield. This suggests that CCS is negatively correlated with levels of traits contributing to ratooning ability. Intensive selection among such populations for CCS without consideration of ratooning performance may reduce the frequency of favourable specific ratooning characteristics.Abbreviations CCS commercial cane sugar  相似文献   
93.
利用AFLP进行“甘蔗属复合体”系统演化和亲缘关系研究   总被引:8,自引:0,他引:8  
采用AFLP分子标记技术,对“甘蔗属复合体”中4个属16个种的69份来自中国和澳大利亚的甘蔗种质资源材料进行系统演化和亲缘关系分析。2对AFLP引物共检测到173个标记,其中172个为多态标记,多态率达99.4%;通过计算Jaccard相似性系数,用UPGMA和PCA法构建了分子系统树和效应图。结果表明:(1)属间亲缘关系中,甘蔗属与芒属较  相似文献   
94.
95.
Summary The formation of triploids in 4x×2x crosses was studied in primitive potato varieties from the Andes of Peru and Bolivia. Triploid frequencies were as high as 80–100% in some crosses, although the actual number of seeds was low. The triploid block varied according to the tetraploid parent, and was never entirely absent. The likelihood of triploid formation was studied in 4x×2x crosses where pollen from both tetraploids and diploids of known genotype was used. Even though the diploids were chosen for their non-production of 2n gametes, as detected in pollen grain size, 66% of the hybrids were tetraploid. The amount and probability of gene-flow between the tetraploid and diploid gene-pools, either directly through 2n gametes or through triploid bridges was examined. Most of the 20 cultivars of S. x chaucha produced seed in crosses with tetraploids and diploids, although the fertility of the latter crosses was four times higher. The female fertility of some of the triploids thus allows them to exchange genes with the tetraploids and diploids, although gene-flow occurs more rapidly through direct transfer from diploids to tetraploids via 2n gametes.  相似文献   
96.
Summary The variation in Solanum × chaucha Juz. et Buk. (2n=3x=36) was studied using 114 accessions from the Potato Collection maintained by the International Potato Center. These accession represented less than 5% of the collection originally screened for ploidy. Twenty morphological characters of the tubers, inflorescences and leaves were scored. The accessions were grouped into 20 morphotypes on the basis of the tuber characters. Two-qualitative characters, red-anther tip and sky-blue flowers were diagnostic of 4 morphotypes, but the variation of the quantitative characters overlapped among morphotypes, and were of limited value. Electrophoretic separation of tuber proteins in 7.5% polyacrylamide gels generally confirmed the morphotype groupings. It is concluded that each of the morphotypes represents a single genotype, and these are named according to the International Code of Nomenclature for Cultivated Plants. The variation within S. × chaucha apparently has been restricted by its hybrid origin.  相似文献   
97.
Screening for resistance to the potato cyst nematode, Globodera pallida, in potatoes from. Bolivia, was carried out in 1983 and 1984, using a mixture of four nematode populations representing pathotypes Pa1, Pa2 and Pa3 From the 66 accessions of 17 species and subspecies evaluated, highly resistant genotypes were identified in 21 accessions from seven species. All had Pf/Pi values of 2 or less, whereas the susceptible control, Solanum tuberosum cv. ‘Disiree’ had Pf/Pi values of more than 2G in both tests. Two diploid wild species, S. brevicaule and S. leptophyes, showed the best resistant. The geographical distributional of resistant populations and the evolution of resistance in wild potato populations are discussed.  相似文献   
98.
99.
Abstract. Under a UK Government consultation procedure announced in 2001, it was proposed that measures agreed within already designated Nitrate Vulnerable Zones (NVZ 's) would be extended to include a considerably increased area of England, Wales and Scotland. Since existing NVZ 's in the UK have included relatively little grassland, it is important to examine how nitrate losses from grassland areas, especially from animal manures, one of the major potential sources of nitrate loss, can be minimized. Experiments were carried out on freely draining grassland soils at four sites (Devon, Hampshire, Shropshire and N Yorkshire) representative of a wide range of climatic and farming conditions across lowland England, over a four year period, 1990/91 to 1993/94. Slurry was applied to experimental plots over a range of times (including June and then monthly, from September to January) at a target rate of 200 kg N ha–1. Nitrogen leaching over the four years ranged from 0 to >50% of applied slurry N, with the largest losses occurring following applications in the September to November period. The use of a nitrification inhibitor with slurry applied in November failed to provide consistent reduction in nitrate leaching.
A strategy to reduce the risk of N leaching from manures applied to freely draining grassland soils must take account of the characteristics of the manure, in particular its N content, the application rate and the amount of excess rainfall following application. The experimental results suggest that slurry applications to freely draining grassland, in September, October and November should generally be avoided, the rationale for this being dependent on the amount of excess rainfall subsequent to application. Farmyard manure represents a lower risk and does not justify the restrictions on application timing that appear to be necessary with slurry.  相似文献   
100.
The purpose of this research was to evaluate the antibiotic resistance in two water reclamation systems developed from space missions. The first system is a small-scale water reclamation system operated at Johnson Space Center designed to reclaim wastewater during long-term space missions. The second system was a scaled-down version of the Johnson Space Center system operated at Texas Tech University. Antibiotic resistance patterns to 10 antibiotics were investigated before and after controlled doses of amoxicillin were added to the water reclamation systems. The results of this study indicate that bacteria in all systems were resistant to many antibiotics including beta-lactam antibiotics and a beta-lactam, beta-lactamase inhibitor combination, amoxicillin with clavulanic acid.  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