全文获取类型
收费全文 | 10998篇 |
免费 | 607篇 |
国内免费 | 1156篇 |
专业分类
林业 | 1230篇 |
农学 | 1729篇 |
基础科学 | 938篇 |
1879篇 | |
综合类 | 3194篇 |
农作物 | 546篇 |
水产渔业 | 429篇 |
畜牧兽医 | 1724篇 |
园艺 | 376篇 |
植物保护 | 716篇 |
出版年
2024年 | 13篇 |
2023年 | 146篇 |
2022年 | 300篇 |
2021年 | 459篇 |
2020年 | 385篇 |
2019年 | 419篇 |
2018年 | 283篇 |
2017年 | 399篇 |
2016年 | 407篇 |
2015年 | 534篇 |
2014年 | 504篇 |
2013年 | 612篇 |
2012年 | 669篇 |
2011年 | 791篇 |
2010年 | 741篇 |
2009年 | 637篇 |
2008年 | 659篇 |
2007年 | 586篇 |
2006年 | 668篇 |
2005年 | 526篇 |
2004年 | 273篇 |
2003年 | 221篇 |
2002年 | 188篇 |
2001年 | 221篇 |
2000年 | 222篇 |
1999年 | 270篇 |
1998年 | 203篇 |
1997年 | 204篇 |
1996年 | 185篇 |
1995年 | 171篇 |
1994年 | 161篇 |
1993年 | 135篇 |
1992年 | 116篇 |
1991年 | 101篇 |
1990年 | 76篇 |
1989年 | 68篇 |
1988年 | 45篇 |
1987年 | 43篇 |
1986年 | 25篇 |
1985年 | 11篇 |
1984年 | 19篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1975年 | 3篇 |
1972年 | 6篇 |
1971年 | 3篇 |
1970年 | 6篇 |
1965年 | 3篇 |
1964年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
51.
Zhicong Dai Chuncan Si Deli Zhai Ping Huang Shanshan Qi Ying Lin Ruiping Wang Qiongxin Zhong Daolin Du 《林业研究》2018,(2)
The endangered Vatica mangachapoi,a longlived,tropical tree with economic and ecological importance found in Hainan,China,was used to assess the hypothesis that historical human activities in Hainan's tropical rain forest could have negative effects on the genetic diversity of V.mangachapoi.Three hundred and twenty individuals from 11 natural populations—which were classified into three groups according to levels of disturbance—were sampled and analyzed with ISSRmarkers.Although genetic diversity of V.mangachapoi is high at the species level,it is relatively low within populations.A significant genetic differentiation occurs among different disturbance levels.Significant isolation-by-distance indicated relevant historical anthropogenic changes.Our findings showed that historical human disturbances significantly increase the genetic differentiation and slightly decrease the genetic diversity of long-lived tree V.mangachapoi.Relevant targeting conservation actions were recommended. 相似文献
52.
陆地棉脱水素基因GhDHN1启动子序列分析 总被引:1,自引:1,他引:0
为了筛选棉花抗逆转基因育种的候选启动子,根据陆地棉脱水素基因GhDHN1的cDNA序列和陆地棉基因组序列,利用生物信息学分析方法,获得棉花陆地棉脱水素基因GhDHN1启动子序列。结果表明,该启动子上多个位点含有启动子的基本元件TATA-box和CAAT-box,并含有非生物逆境胁迫响应元件、响应植物激素顺式作用元件、多种光调控相关的顺式作用元件以及胚乳表达顺式调控元件等。推测GhDHN1基因的启动子受光诱导,同时受低温和干旱胁迫等非生物逆境的诱导,参与棉花的生长发育。 相似文献
53.
A 56‐day feeding trial was conducted to elucidate the effects and mechanism action of dietary α‐linolenic acid (ALA, 18:3n‐3) on lipid accumulation and fatty acid profile of muscle, hepatopancreas and intraperitoneal fat (IPF) in juvenile grass carp using three isonitrogenous and isoenergetic semi‐purified diets containing 0.0% (control group), 1.0% and 2.0% ALA, respectively. The lowest intraperitoneal fat (IPF) ratio was found in 2.0% group. In the muscle, hepatopancreas and IPF, docosahexaenoic acid (DHA, 22:6n‐3) and eicosapentaenoic acid (EPA, 20:5n‐3) contents increased with the increase in dietary ALA. In the IPF, caspase 3, caspase 8 and caspase 9 showed the highest activities in 2.0% group, while the value of Bcl‐2/Bax (B‐cell leukaemia 2/Bcl‐2‐associated X protein) reached the lowest. Meanwhile, swelling of the IPF mitochondria was observed in 2.0% group. The gene expressions of fatty acid desaturase (FAD) and fatty acid elongase (ELO) in the hepatopancreas and muscle showed significantly higher levels in the treatment groups, whereas an opposite trend was existed in the IPF. Fatty acid synthase (FAS), sterol regulatory element binding protein‐1c (SREBP‐1c) in the IPF and hepatopancreas reached the lowest in 2.0% group. Overall, dietary ALA could promote n‐3 highly unsaturated fatty acids (HUFAs) synthesis and suppress the accumulation of lipid by decreasing the expression of related genes and promoting the apoptosis in IPF. 相似文献
54.
Impact of Dietary L‐Malic Acid Supplementation on Growth,Feed Utilization,Ash Deposition,and Hepatic Lipid Metabolism of Juvenile Genetically Improved Farmed Tilapia,Oreochromis niloticus 下载免费PDF全文
55.
Du Hwan Chun 《Fibers and Polymers》2017,18(9):1790-1795
The numerical analysis was performed to predict the potential problem, often occurring during the manufacturing process of the disposable medical device of a great volume. The cavity filling analyses were performed for the new design of the 3cc Syringe Barrel using polypropylene(PP1), and a new nucleated polypropylene(PP2) material for better clarity. These analyses have been performed for different processing conditions as well as various wall thickness designs for both materials. With the nucleated material, only the original wall thickness design has been studied at two different processing conditions for comparison purpose. This study was conducted to investigate the optimum part design and processing condition for two different materials. The most desirable design was selected with Design 3 for material utilization and reduced flow stresses by comparing the field results. The new nucleated polypropylene provided slightly better product quality and processing. 相似文献
56.
Zhongyi Sun Xiufeng Wang Haruhiko Yamamoto Jiquan Zhang Hiroshi Tani Guosheng Zhong Shuai Yin 《Paddy and Water Environment》2017,15(1):181-191
Rice is the second largest staple crop in the world and therefore plays an important role in food security. As a thermophilic crop, rice is sensitive to temperature changes. Thus, research on the chilling damage of rice is essential. The Sanjiang Plain is an emerging rice production area and is located at high latitudes in China, the world’s largest rice-producing country. Landsat data were used to extract rice-planting area from 1985 to 2015. MODIS 13Q1, which was uniformly distributed during the growing period of rice, was used to obtain NDVI values of paddies during 2002–2015. Dynamic Identification Index of sterile-type chilling damage and monitoring standard of delayed-type chilling damage were the proposed methods used in this paper, which were used to judge the chilling damage of rice. The results show that in the study region, the rice-planting area in 2015 is nearly 12 times larger than that in 1985. Delayed-type chilling damage occurred in 2002 and 2009, while sterile-type chilling damage occurred in 2005, 2006, 2009, 2010, 2014, and 2015. Comparing with the prevalent meteorological standards, the results indicate that the index and standards proposed in this paper are precise, applicable, and more sensitive than them. The method is a macroscopic and accurate method to identify chilling damage in rice and can also provide a scientific basis to ensuring the stability of rice yield. 相似文献
57.
58.
为了帮助广大研究者在短时间内了解DNA折纸术的最新研究现状,掌握DNA折纸术相关软件的应用,本论文将DNA折纸术的相关内容进行了归纳。首先介绍了DNA折纸术的起源与基本原理;然后综述了近几年国内外的研究进展与取得的成果;接下来对DNA折纸术相关软件-Tiamat和Cadnano的应用和功能进行了简单的介绍,并对不同的软件进行了比较;最后对DNA折纸术的应用前景进行了展望。DNA折纸术虽然取得了一定的研究进展,但其未来的发展和应用前景将更广阔。 相似文献
59.
Cotton fabric was modified with β-cyclodextrin (β-CD) forming inclusion complex to yield color strength, pattern sharpness, and color fastness for ink-jet printing. The modified cotton fabric was confirmed with the presence of new strong absorption peaks around 1713 cm-1 and 1243 cm-1 in FT-IR. β-CD had been covalently grafted on cotton fabric via the esterification reaction of citric acid (CTR) with cellulose and β-CD. The results indicated that printing performances of the ink-jet printed fabric were enhanced through β-CD modification. The K/S value was enhanced from 4.21 to 6.72, the width of printed line was decreased from 1.48 mm to 1.25 mm, and the color fastness was improved to 3-4 level. These improvements were due to the truncated cone structure of β-CD, which can form inclusions with water-based pigment. Meanwhile, the crease recovery performance was also improved with the aid of CTR. A comparison between the unmodified and modified cotton fabric suggested that the crease recovery angle of β-CD modified cotton fabric was increased by 25.0 % in the warp direction. Therefore, printing performance and crease recovery performance of β-CD modified and water-based pigment printed cotton fabric were enhanced remarkably. 相似文献
60.
N.‐N. Zhang Q.‐Q. Ma W.‐J. Fan Q. Xing Y.‐L. Zhao L.‐Q. Chen J.‐Y. Ye M.‐L. Zhang Z.‐Y. Du 《Aquaculture Nutrition》2017,23(2):313-321
Oriental river prawn (Macrobrachium nipponense) has been widely cultured in Asian countries. However, its nutritional studies are very limited. In the present 8‐week study, we investigated the effects of dietary protein to energy ratio (P/E ratio) on the growth, feed utilization and body composition in juvenile M. nipponense (initial weight 0.302 ±0.03 g). Two‐factor experiment was designed and nine semi‐purified diets were formulated to contain three lipid levels (20, 80 and 140 g kg?1) and three protein levels (330, 380 and 430 g kg?1), producing P/E ratios from 16.5 to 23.4 mg KJ?1 protein. The results indicated that the growth, survival rate and protein efficiency were dose dependently improved by the increased dietary lipid, but not dietary protein content. Increased dietary lipid content and/or protein content increased lipid accumulation in whole body, hepatopancreas and muscle, but did not change the feed intake and hepatopancreas weight. In conclusion, our present study indicated that M. nipponense is a species with relatively high‐energy requirement. It could utilize dietary lipid content up to 140 g kg?1, while the dietary protein with more than 330 g kg?1 would not promote growth and protein efficiency. Taken together, 330 g kg?1 dietary protein and 140 g kg?1 dietary lipid level with P/E ratio 16.49 could be optimum for M. nipponense. 相似文献