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   224篇
  免费   22篇
  国内免费   1篇
林业   10篇
农学   10篇
基础科学   2篇
  29篇
综合类   20篇
农作物   19篇
水产渔业   47篇
畜牧兽医   96篇
园艺   7篇
植物保护   7篇
  2024年   2篇
  2023年   3篇
  2022年   5篇
  2021年   4篇
  2020年   14篇
  2019年   9篇
  2018年   13篇
  2017年   11篇
  2016年   9篇
  2015年   6篇
  2014年   13篇
  2013年   32篇
  2012年   6篇
  2011年   9篇
  2010年   7篇
  2009年   8篇
  2008年   9篇
  2007年   8篇
  2006年   8篇
  2005年   3篇
  2004年   3篇
  2003年   6篇
  2002年   5篇
  2001年   2篇
  2000年   2篇
  1999年   3篇
  1998年   3篇
  1997年   5篇
  1996年   2篇
  1994年   1篇
  1993年   1篇
  1992年   4篇
  1991年   2篇
  1990年   4篇
  1989年   1篇
  1988年   3篇
  1986年   1篇
  1980年   1篇
  1979年   2篇
  1978年   7篇
  1976年   1篇
  1971年   1篇
  1970年   1篇
  1966年   1篇
  1963年   2篇
  1960年   3篇
  1951年   1篇
排序方式: 共有247条查询结果,搜索用时 15 毫秒
61.
The selection of a suture material, in the past, was based primarily on habit, guesswork and physical properties. This is understandable because there was a limited number of materials to select from and the interaction of suture material with tissues was not understood.  相似文献   
62.
Selected tropical Acacia species are used extensively for short-rotation plantation forestry in many parts of Asia and, to a limited degree, in Australia. We explored leaf-level photosynthetic activity and leaf water potential (Ψleaf) of three field-grown Acacia tree species (aged between 7 and 18 months) in contrasting wet–dry tropical plantations in southern Vietnam and northern Australia. Light-saturated photosynthetic rate (A1500) declined throughout the morning and early afternoon in the dry season; in the wet season, levels remained high and relatively constant throughout most of the day. Maximum daily A1500 at 09:00 ranged from 22.2 μmol?m?2?s?1 in the wet to 10.4 μmol?m?2?s?1 in the dry season. At both locations, trees were able to extract soil water such that pre-dawn leaf water potential (Ψpd) remained>?1.5?MPa even at the end of the dry season. Stomatal conductance to water vapour (gs) did not respond to decreasing Ψleaf during the wet season but was sensitive to changes in Ψleaf in the dry season. Species comparisons of the relationships between A1500 and Ψleaf revealed different strategies to balance carbon uptake and water loss in a wet–dry environment. Acacia crassicarpa and A. mangium regulated Ψleaf to a greater extent than the A. mangium×A. auriculiformis hybrid such that ?Ψleaf (determined as Ψpd?midday Ψleaf) was unaffected by season. This result suggests that the hydraulic regulation of tree water status varies amongst young tropical Acacia species. From a management perspective, for Acacia species that tend to strongly regulate water loss in environments with an extended dry season, overall productivity at the end of a rotation may be less than for species that prioritise carbon gain.  相似文献   
63.
Japanese oak wilt (JOW) has been prevalent in Japan since the late 1980s. Infections of the fungus, Raffaelea quercivora Kubono et Shin. Ito, which is transmitted by an ambrosia beetle [Platypus quercivorus (Murayama)], can cause JOW. Although R. quercivora, P. quercivorus, and oak trees are distributed in other Asia–Pacific countries, the incidence of JOW has not been reported outside Japan. In this study, we collected R. quercivora isolates from 5 Asian countries, including Japan, and compared their ability to induce sapwood discoloration by inoculating Q. serrata logs. The tangential widths of the discoloration in sapwoods inoculated with non-Japanese isolates were equivalent to or greater than those of the 2 Japanese isolates. This indicates that a lack of JOW incidence outside Japan is not because of the lowered ability of R. quercivora to spread discoloration compared with the Japanese isolates. Statistical analyses of the relationship between discoloration and phylogeny based on DNA sequences of actin and chitin synthase showed that the discoloration width was independent of phylogenetic relatedness among the isolates. To discuss why the occurrence of JOW has not been reported outside Japan, further studies (e.g., on host susceptibility and P. quercivorus aggression) throughout Asia are needed.  相似文献   
64.
65.
Pham TC  Kim HS  Yoon KB 《Science (New York, N.Y.)》2011,334(6062):1533-1538
Applications of zeolite films benefit from alignment of the integrated channels, but methods for film growth have nearly always introduced orientational randomization in the direction normal to the substrate. We now report facile methods to grow silicalite-1 films and pure silica beta zeolite films on substrates with straight or sinusoidal channels positioned uniformly upright at a thickness of up to 8 micrometers. Precise gel compositions and processing temperatures are critical to promote secondary growth on pre-formed oriented crystal monolayers while suppressing self-crystallization in the bulk medium. Preliminary results highlight the potential of these uniformly oriented films in the nonlinear optical response and separation of xylene isomers.  相似文献   
66.
Sweetpotato-pig production is an important system that generates income, utilizes unmarketable crops, and provides manure for soil fertility maintenance. This system is widely practiced from Asia to Africa, with many local variations. Within this system, pigs are generally fed a low nutrient-dense diet, yielding low growth rates and low economic efficiency. Our project in Vietnam went through a process of situation analysis, participatory technology development (PTD), and scaling up over a seven-year period to improve sweetpotato-pig production and to disseminate developed technologies. The situation analysis included a series of pig production assessments in several provinces in northern and southern Vietnam, and pig supply-market chain identification was conducted in 13 provinces. The analysis of these studies informed the project of the following: (1) appropriate locations for our activities; (2) seasonal available feedstuff and farmers feeding practices; (3) market fluctuation and requirements; and (4) feeding and management improvement needs based on which the subsequent phase of PTD was designed. The PTD involved a limited number of farmers participating in sweetpotato varietal selection, sweetpotato root and vine silage processing, seasonal feeding combination, and pig feeding with balanced crop-feed diet and silage. Six years of multi-location and multi-season sweetpotato selection resulted in a few promising varieties that yielded up to 75% more dry matter and have since been formally released. The most significant results of silage processing and feeding trials include improved growth, higher feeding efficiency, increased year-round local feedstuff, and considerable labor reduction from eliminated cooking and vine cutting. Once these technologies were developed, a farmer-to-farmer training model was designed for scaling up the adoption and impact. Farmer trainers from seven communes in seven provinces received training in these technologies. In turn, they undertook the responsibility of training other farmers on sweetpotato selection, processing, and feeding. An impact study was also administered to monitor and evaluate (M&E) the dissemination process and to document the impact of the new technologies and farmer-to-farmer training model on pig growth and farmer income generation. The results showed that both participating and non-participating farmers have taken up the technologies, although the former demonstrates higher rates of adoption than the latter. The participants also generated more income and saved more labor from the adoption of the technologies. While the scaling up and M&E activities are on-going, the project has since broadened from a sweetpotato-pig system perspective to a pig-cropfeed system perspective based on farmers needs. It has included other crop feeds such as cassava and peanut stems in the research portfolio. New technologies based on on-going PTD will continuously be incorporated into the future training curriculum.Dai Peters is currently a senior scientist with the International Center for Tropical Agriculture (CIAT) based in Hanoi, Vietnam. This publication is based on research conducted while she was a research scientist with the International Potato Center (CIP). She holds a PhD from North Carolina State University. Her research interests include on-farm participatory research methodologies, agroenterprise development, and sweetpotato-pig farming systems in Vietnam and Papua, Indonesia. Her recent publications include a manual on crop-based pig feed systems, post-harvest fermentation of sweetpotato roots and vines, and agroenterprise development.Nguyen Thi Tinh is currently a coordinator of the sweetpotato-pig improvement project at the International Potato Center in Vietnam. Ms. Tinh holds a Masters degree in animal nutrition from Wageningen University, Netherlands. She participated in the project on pig feeding trials for five years.Mai Thach Hoanh is a sweetpotato breeder with the Root Crop Research Center of Vietnam Agricultural Science Institute (VASI). He holds a PhD in sweetpotato breeding from the same institute. He participated in the project for seven years on sweetpotato selection. Nguyen The Yen is a crop scientist in the Food Crop Research Institute of Vietnam. He holds a PhD in sweetpotato breeding from VASI. He participated in the project for five years on sweetpotato selection.Pham Ngoc Thach is a lecturer with the Hanoi Agricultural University. He holds a PhD in veterinarian science from the same university. He participated in the project for five years in controlling pig diseases during the feeding trials.Keith Fuglie is a regional coordinator and research scientist in the International Potato Center based at Bogor, Indonesia. He holds a PhD in agriculture economics from the University of Wisconsin. He designed the impact study for the project.  相似文献   
67.
GADS is an adaptor protein implicated in CD3 signaling because of its ability to link SLP-76 to LAT. A GADS-deficient mouse was generated by gene targeting, and the function of GADS in T cell development and activation was examined. GADS- CD4-CD8- thymocytes exhibited a severe block in proliferation but still differentiated into mature T cells. GADS- thymocytes failed to respond to CD3 cross-linking in vivo and were impaired in positive and negative selection. Immunoprecipitation experiments revealed that the association between SLP-76 and LAT was uncoupled in GADS- thymocytes. These observations indicate that GADS is a critical adaptor for CD3 signaling.  相似文献   
68.
晚疫病是越南马铃薯上最重要的病害。各马铃薯产区均有发生。在江河三角洲地区晚疫病的发生通常比较晚,从12月下旬即第二个生长季后半期开始发病,在1月至2月气温较低、湿度较大时,晚疫病常常严重流行。在Sapa地区(北京高原),整个生长晚疫病对马铃薯作物都是主要威胁。在感病品种上病害的发生和严重度可达到100%。在大叨地区,在干旱季节(11-4月)晚疫病发生最轻,而在雨季(5-10月)严重流行。在越南,还未进行晚疫病菌生物学和流行学方面的基础研究。对晚疫病防治研究主要集中在筛选新的杀菌剂。大部分工作集中在抗病育种和抗病品种的筛选。目前需要进行晚疫综合治疗、流行学、晚疫病菌生物学和抗病育方面的深入研究和培训。  相似文献   
69.
Rice crop diversity hasdecreased dramatically in the recent past.Understanding the causes that underlie theevident genetic erosion is critical for thefood security of subsistence rice farmers andbiodiversity. Our study shows that farmers inthe northeastern Philippines had a markedreduction in rice diversity from 1996 to 1998.The ultimate causes were a drought resultingfrom the El Niño phenomenon in 1997 andflooding due to two successive typhoons in1998. The proximate causes, however, includedlocal water control factors, limitations in thehousehold and village-level seedinfrastructure, farm location in relation tothe goods and services necessary to obtainseeds, policies and programs of the Departmentof Agriculture, and the characteristics of therice varieties themselves. The implications ofour study are that genetic erosion is notalways the result of purposeful acts by farmersnor is it necessarily gradual. Improvingon-farm seed technology will stabilize the seedproduction, distribution, and use system andthereby enhance household food security.Ultimately, rice diversity will be improvedonly if diversity is a safe and viable optionfor farmers. Therefore, public policy thatsupports farmers who maintain a diverse set ofcultivars is critical for any on-farmconservation strategy.  相似文献   
70.
Endophytic bacteria were isolated from surface sterilized stems, root, and nodules of wild and cultivated soybean varieties. Various phenotypic traits that are expected to be involved in the persistence and functions of the bacteria were analyzed. Most of the isolates from soybean were motile and indoleacetic acid producers, and 70 and 33% of isolates excreted cellulase and pectinase, respectively. These traits may be involved in endophytic characteristics in soybean. Some isolates were resistant to Str100, formed capsules, and produced fluorescent pigments. Molecular characterization of selected 35 endophytic bacteria by 16S rDNA–polymerase chain reaction–restriction fragment length polymorphism showed two main clusters at 48 and 43% similarity coefficients in which most of the endophytes belonged. The genetic variation was more among endophytes isolated from Glycine max tissues than from G. soja.  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