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   349篇
  免费   8篇
  国内免费   1篇
林业   27篇
农学   18篇
基础科学   5篇
  201篇
综合类   31篇
农作物   15篇
水产渔业   16篇
畜牧兽医   21篇
园艺   6篇
植物保护   18篇
  2024年   3篇
  2023年   9篇
  2022年   4篇
  2021年   4篇
  2020年   18篇
  2019年   10篇
  2018年   5篇
  2017年   9篇
  2016年   16篇
  2015年   7篇
  2014年   8篇
  2013年   30篇
  2012年   4篇
  2011年   8篇
  2010年   9篇
  2009年   7篇
  2008年   25篇
  2007年   9篇
  2006年   7篇
  2005年   16篇
  2004年   5篇
  2003年   8篇
  2002年   10篇
  2001年   10篇
  2000年   8篇
  1999年   4篇
  1997年   5篇
  1996年   5篇
  1995年   2篇
  1994年   3篇
  1993年   3篇
  1991年   8篇
  1990年   2篇
  1989年   15篇
  1988年   2篇
  1987年   5篇
  1986年   3篇
  1985年   4篇
  1984年   5篇
  1983年   3篇
  1982年   5篇
  1981年   2篇
  1980年   2篇
  1979年   2篇
  1978年   3篇
  1977年   4篇
  1976年   5篇
  1971年   4篇
  1970年   3篇
  1966年   2篇
排序方式: 共有358条查询结果,搜索用时 399 毫秒
111.
  总被引:10,自引:0,他引:10  
Plasmodium vivax is one of the four malaria parasites that cause disease in humans. The structure of the immunodominant repeating peptide of the circumsporozoite (CS) protein of P. vivax was determined. A fragment of P. vivax DNA that encodes this tandemly repeating epitope was isolated by use of an oligonucleotide probe whose sequence is thought to be conserved in CS protein genes. DNA sequence analysis of the P. vivax clone indicates that the CS repeat is nine amino acids in length (Gly-Asp-Arg-Ala-Asp-Gly-Gln-Pro-Ala). The structure of the repeating region was confirmed with synthetic peptides and monoclonal antibodies directed against P. vivax sporozoites. This information should allow synthesis of a vaccine for P. vivax that is similar to the one being tested for P. falciparum.  相似文献   
112.
    
Calcareous particles present in Pacific waters at depths of 50 to 3500 meters were collected by filtering seawater through spongin matrix. The specific activity of carbon-14 could be measured in two of these collections from depths of 2300 and 3500 meters. The ratios of carbon-14 to carbon-12 correspond to values observed in surface waters in recent years as a result of the addition of manmade carbon-14, thus indicating that the calcareous particles resulted from recent biological productivity. The results are related to the mean settling rates and the sizes and dissolution rates of biogenic calcareous particles in transit through a seawater column.  相似文献   
113.
Investigations were undertaken to explore the possibility of improving setting, retention and weight of fruits in ‘Early Seedless’ and ‘Calcuttia’ cultivars of lichi (Litchi chinensis) by means of growth regulators. Indole acetic acid (IAA) at 20, 40 and 80 mg l?1, 2,4-dichlorophenoxy acetic acid (2,4-D) at 2,4 and 8 mg l?1 and gibberellic acid (GA3) at 50, 100 and 150 mg l?1 were sprayed on panicles in the first fortnight of April, when 50–100% flowers had opened. All 3 growth regulators caused a favourable effect on fruit setting, fruit retention and weight of individual fruits, but IAA at 20 mg l?1 proved the best for enhancing setting, GA3 at 50 mg l?1 for increasing retention and GA3 at 100 mg l?1 for improving fruit weight. IAA and GA3 should, therefore, be used in combination. Between the 2 cultivars tested, ‘Calcuttia’ proved superior to ‘Early Seedless’ in fruit setting, fruit retention and weight of individual fruits.  相似文献   
114.
    
The marine fisheries of Bangladesh have experienced an uncontrolled expansion of fishing over the last few decades. Consequently, most fish stocks became vulnerable to overfishing. Similar to many other fisheries, catch of white sardine (Escualosa thoracata) is rapidly declining. Therefore, three data-limited approaches (length-based Bayesian biomass analysis, LBB; length-based spawning potential ratio, LB-SPR; and Froese's length-based indicators, LBIs) were used to estimate growth, mortality, and fisheries reference points to evaluate stock status of the white sardine fishery. Results revealed that the fishery is being overfished: F/M = 1.32 years−1 in LBB and 6.67 years−1 in LB-SPR. However, the stock biomass is still good with B/B0 = 0.43 and B/BMSY = 1.20. Stock indicators revealed that the fishery was mainly targeting mature fishes, most of which were mega-spawners (older, larger fish), which suggested stocks were recruitment overfished. Harvesting an optimum size range (6.57 to 8.03 cm) and reducing fishing mortality to a sustainable level (F = M) would sustain a healthy stock biomass.  相似文献   
115.
Tropical Animal Health and Production - Mutation studies in different prolific sheep breeds have shown that the transforming growth factor beta super family ligands viz. the growth differentiation...  相似文献   
116.
    
  相似文献   
117.
118.
Hibernation of the beetles, Altica caerulescens Baly and Macrima armata Baly in the Kula Valley, Himachal Pradesh The flea beetle,Altica caerulescens Baly, is a pest of bringjal, tomato, spinach, strawberry and the weedsRumex sp. andImpatiens sp. in India. The beetle,Macrima armata Baly attacks the trees ofAlnus nitida and is also found as a minor pest of radish in Kulu Valley. Both of the beetles are found only in summer. Concerning the overwintering formerly it was supposed thatA. caerulescens hibernates in pupal stage. The autor describes for the first time about the overwintering of the two species in Kulu Valley based on a survey conducted in 1972 to 1975. Altica caerulescens hibernates in adult stage under the bark of willow (Salix alba).Macrima armata adults were found in the winter in cracks of Kosh trees (Alnus nitida).  相似文献   
119.
To compare the effect of methods (foliar and soil) and rate of application of zinc sulphate on zinc and phosphorus uptake, tree size, yield and fruit quality of mango (Mangifera indica L.) cv. Dusheri, zinc sulphate was applied as a foliar spray application (0.25, 0.50, 1.0%) and soil (0.5, 1.0, 2.0 kg tree‐1) treatments during the second week of October (during flower bud differentiation period). All the zinc sulphate treatments of soil and foliar spray were effective in increasing the leaf zinc concentrations above recommended adequate level of (>20 mg kg‐1) whereas control trees maintained low leaf zinc concentrations (13.8 to 13.3 mg kg‐1). The uptake of foliar‐applied zinc was more rapid than that of soil applied zinc. All the treatments of zinc sulphate except the foliar spray treatment of zinc sulphate (0.25%) significantly increased zinc concentrations in the fruit pulp as compared with those in the control trees. The percent increase in the stem girth of trees was highest with the soil application of zinc sulphate (0.5 kg tree1) followed by foliar application of zinc sulphate (1.0%) as compared with all other treatments. The percent increase in the tree canopy volume was highest with the foliar application of zinc sulphate (1.0%) followed by soil application of zinc sulphate (1.0 kg tree1) as compared with control and all other treatments. There was no significant (P<0.05) increase in yield, fruit size and weight, pulp or stone weight with any treatment of zinc sulphate. Total soluble solid (TSS) in the fruit was significantly higher (18.6%) with the treatment of soil application of zinc sulphate (0.5 kg tree1) as compared with all other treatments of zinc sulphate and the control. Acid and sugar content of the fruit was not significantly affected by the foliar or soil application of zinc sulphate.  相似文献   
120.
Ilan Stavi  Rattan Lal   《CATENA》2011,84(3):148-155
Physical degradation of the soil increases its susceptibility to erosion by water action. However, relatively few studies have evaluated the opposite, i.e., the impact of water erosion on soil erodibility. This study was conducted in a corn field in Ohio. Some sites within the field have experienced water-induced soil erosion following heavy rainstorms. Physical characteristics of the soil were compared between eroded (ER) and un-eroded sites (UN). Compared with ER, the soil in UN had lower penetration resistance (4.87 vs. 4.53 MPa), bulk density (1.45 vs. 1.33 Mg m?3), and sand content (17.4 vs. 14.2%), and higher shear strength (80.1 vs. 125.3 KPa), hydraulic conductivity (3.0 vs. 3.4 cm h?1), intrinsic permeability (31.9 vs. 36.4 × 10?10 cm2), and contents of soil organic carbon (36.1 vs. 32.1 g kg?1), total nitrogen (3.3 vs. 3.1 g kg?1), clay (25.2 vs. 24.2%), silt (60.5 vs. 58.4%), and very fine sand (3.4 vs. 1.1%). Also Munsell's variables differed between ER and UN (1.24 vs. 0.54 for hue, 4.59 vs. 4.35 for value, and 1.99 vs. 1.79 for chroma, respectively). The erodibility factor (K) was lower in UN than in ER (0.00327 vs. 0.00354 Mg ha h ha?1 MJ?1 mm?1, respectively). Hence, it is suggested the ER sites within the corn field agroecosystem are more susceptible to accelerated erosion as compared with UN sites.  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