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   1585篇
  免费   91篇
  国内免费   144篇
林业   132篇
农学   179篇
基础科学   55篇
  213篇
综合类   452篇
农作物   105篇
水产渔业   157篇
畜牧兽医   367篇
园艺   53篇
植物保护   107篇
  2023年   8篇
  2022年   22篇
  2021年   46篇
  2020年   45篇
  2019年   32篇
  2018年   42篇
  2017年   54篇
  2016年   50篇
  2015年   58篇
  2014年   54篇
  2013年   105篇
  2012年   76篇
  2011年   116篇
  2010年   75篇
  2009年   60篇
  2008年   65篇
  2007年   56篇
  2006年   63篇
  2005年   89篇
  2004年   66篇
  2003年   83篇
  2002年   131篇
  2001年   113篇
  2000年   75篇
  1999年   48篇
  1998年   25篇
  1997年   23篇
  1996年   13篇
  1995年   8篇
  1994年   13篇
  1993年   12篇
  1992年   17篇
  1991年   14篇
  1990年   7篇
  1989年   6篇
  1988年   5篇
  1987年   2篇
  1986年   4篇
  1985年   7篇
  1984年   3篇
  1982年   3篇
  1979年   4篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1975年   3篇
  1974年   3篇
  1973年   2篇
  1970年   1篇
  1966年   1篇
排序方式: 共有1820条查询结果,搜索用时 15 毫秒
991.
浙江安吉“优雅竹城”建设中竹景观的应用及其优化   总被引:1,自引:0,他引:1  
竹清秀淡雅、枝干挺拔、虚心有节,具有深厚的文化底蕴。竹因其独特的生物学特性和观赏特性成为城市园林中不可或缺的植物造景材料。安吉竹资源丰富,是著名的"中国竹乡"。近年来,安吉实施了"优雅竹城"建设工程,竹子被大量应用于景观营造中。文章通过资料查阅、人物访谈、实地调研等方法,调查了竹子在安吉"优雅竹城"建设中的应用情况,分析了竹子在景观设计中的应用形式、存在的问题,提出了优化竹景观营造的方法,为竹子更广泛地应用于城市园林景观建设提供借鉴。  相似文献   
992.
以‘凤丹’牡丹组织培养获得的增殖幼苗为材料,在含有0~5.0 mg·L-1硝酸银的初代培养基中进行培养后,通过测量总酚含量、单体酚种类及其含量的变化和相关酶活性变化,确定不同浓度的硝酸银对抑制愈伤组织褐化的作用。结果表明,培养基中加入硝酸银能明显减轻褐变,其中2.0 mg·L-1的综合效果最好且对组培苗正常生长无影响;硝酸银对组培苗褐变过程中产生影响的单体酚有绿原酸、芦丁、香豆酸、对香豆酸、表儿茶素、二氢槲皮素。愈伤组织中总酚含量与褐变等级呈现极显著相关性(P<0.01),过氧化物酶(POD)活性与褐变程度呈现显著负相关(P<0.05),多酚氧化酶(PPO)活性与褐变程度呈现显著正相关(P<0.05)。  相似文献   
993.
基于生态位和"反规划"思想的城市土地开发适宜性评价   总被引:3,自引:2,他引:1  
为深入了解城市土地开发适宜性情况并为城市土地合理利用提供依据,该研究以生态位理论为基础,根据城市土地现实生态位与最适宜生态位的贴近程度,构建出城市土地生态位适宜度模型,借鉴反规划思想构建出该模型的评价因子体系。以北京市房山区为研究区,根据城市土地生态适宜度对房山区土地综合开发适宜性进行分区,从平原区、浅山区和深山区3方面进行评价分析。结果表明:研究区城市土地生态适宜度整体呈阶梯状,土地开发适宜性由西到东逐渐升高;平原区城市土地开发最适区最多,深山区和浅山区多为不适宜区和特别不适宜区;深山区和浅山区受生态管控、采空塌陷等限制因素影响较大。利用生态位适宜度模型和反规划思想进行城市土地综合开发适宜性评价,丰富了反规划和生态位理论的应用领域。  相似文献   
994.
TTC染色指数配合Logistic方程鉴定山葡萄种质抗寒性   总被引:1,自引:0,他引:1  
为了能简便、准确地鉴定山葡萄种质抗寒性,该研究以40份山葡萄种质1 a生休眠期枝条为试材,利用连有图像分析软件的扫描仪对各低温处理下枝段染色图像进行可视化评估,配合Logistic方程建立回归方程,确定其低温半致死温度。结果表明,枝段染色指数随温度降低而下降,染色指数变化规律符合Logistic方程;40份山葡萄种质低温半致死温度在-39.41~-29.75℃范围内,拟合度在0.956 4~0.999 3范围内,种质抗寒性差异明显;各种质低温半致死温度与其平均隶属度及田间萌芽率均呈极显著负相关,相关系数分别为-0.883 03和-0.876 70。研究结果证明TTC染色图像可视化评估配合Logistic方程鉴定山葡萄种质抗寒性可行,同时为山葡萄种质抗寒性评价及抗寒亲本选择提供了简单、可靠的鉴定方法。  相似文献   
995.
This study was conducted to compare lipid deposition pattern of three fish species among fish size, Large yellow croaker (Larmichthys crocea), Japanese seabass (Lateolabrax japonicus) and Turbot (Scophthalmus maximus L.), Using magnetic resonance imaging technology for adult fish, results showed that lipid of large yellow croaker mainly deposits in abdominal cavity wall, while for Japanese seabass mainly deposit in visceral adipose tissue and for turbot lipid mainly distribute subcutaneous tissue. Three sizes for each species were selected: S1 (small size), S2 (intermediate size) and S3 (big size), to examine chemical analysis. Results of chemical analysis indicated that whole body lipid content of large yellow croaker significantly increased with the increase in body weight, but Japanese seabass and turbot significantly decreased (< .05). Lipid content of muscle and intestinal tract in large yellow croaker significantly increased with the increase in body weight (< .05), but lipid content of adipose tissue, kidney, heart and skin in S2 group were higher than S1 and S3 groups (< .05). Lipid content of liver, eye, kidney and brain in Japanese seabass significantly increased with the increase in body weight (< .05), but lipid content of stomach and heart showed an opposite trend. Lipid content of liver, adipose tissue, skin and eye in turbot significantly decreased (< .05), but lipid content of brain significantly increased with the increase in body weight (< .05). The results indicated that lipid content of different tissues in fish presented different trends, which was species‐dependent.  相似文献   
996.
为探索棉铃虫Helicoverpa armigera(Hübner)类肌钙蛋白基因HaCal的生理功能及其所编码蛋白的生物特性,采用PCR方法克隆了HaCal的开放阅读框(open reading frame,ORF)序列,借助在线软件对其进行生物信息学分析,通过原核表达技术诱导表达和纯化其编码的蛋白,并通过实时荧光定量PCR技术分析其在棉铃虫不同发育龄期和组织中的表达量。结果显示,HaCal的ORF序列长度为564 bp,编码187个氨基酸,理论分子量为20.52 kD,理论等电点为7.64。其保守结构域与Calponin家族一致,但不含跨膜区和信号肽,系亲水性蛋白。原核表达结果显示,融合蛋白大小与理论值一致,纯化获得了纯度较高的目的蛋白。HaCal在棉铃虫幼虫不同发育阶段和不同组织中均有表达,在6龄幼虫体内表达量最高,是1龄幼虫的2.34倍;在5龄幼虫中肠中表达量最高,是头部的257.14倍。表明棉铃虫类肌钙蛋白基因HaCal可能与棉铃虫的生长发育过程密切相关。  相似文献   
997.
The effects of dietary taurine on growth performance, liver and intestine morphology, serum physiological and antioxidant parameters, serum thyroid hormone level, muscle taurine content and fatty acid composition of turbot were first evaluated, for the safe utilization in marine fish feed and for human food safety. Four experimental diets were formulated to contain 0, 10, 50 and 100 g/kg taurine. Each diet was randomly assigned to six replicates of 30 juvenile turbot (initial mean weight of 7.46 g). The feeding trial lasted for 10 weeks. The growth performance of fish was significantly enhanced by 10 g/kg dietary taurine. The integrity of the distal intestine was impaired and the absorptive surface was found to be significantly reduced by 100 g/kg dietary taurine. The obvious pathological changes in liver were observed in fish fed 100 g/kg taurine. Dietary taurine with 10 and 50 g/kg significantly increased the activities of serum superoxide dismutase, lysozyme and thyroid hormone. The taurine content in muscle was found to be significantly increased by dietary taurine; however, no significant differences were observed among taurine‐supplemented treatments. This study suggested that 10 g/kg taurine was safe in turbot feed, and fivefold of safety margin was obtained.  相似文献   
998.
Five semi‐purified diets were formulated to contain graded levels of dietary protein (340, 390, 440, 490 and 540 g/kg diet), and each diet was assigned to triplicate groups of eight juvenile Dabry's sturgeon for 50 days. Results showed that specific growth rate increased linearly with increasing dietary protein levels from 340 to 440 g/kg diet, then levelled off at higher protein levels. Higher dietary protein (440 and 540 g/kg) significantly increased the feed intake and feed efficiency ratio of fish compared to lower protein diets (340 to 490 g/kg diet) (< .05). Fish fed a 440 g protein kg?1 diet had higher protein efficiency ratio and serum lysozyme activity than other treatments. Serum ammonia content and activities of liver aminotransferase were positively correlated with dietary protein levels. No significant difference (> .05) among groups was observed in glycogen content. As dietary protein level increased, protein and ash content of fish dorsal muscle were greatly enhanced, whereas lipid content was significantly reduced (< .05). Based on broken‐line regression analysis of SGR against dietary protein levels, the optimum dietary protein level for maximal growth of juvenile Dabry's sturgeon is 446.68 g/kg diet.  相似文献   
999.
几种冷地型草坪草在江淮地区前期适应性初探   总被引:4,自引:3,他引:1  
对4种冷地型草坪草品种在江淮地区的前期生长进行了观测评价,同时对其若干生理指标做了测定与分析.结果表明:各品种草坪草前期生长表现出的质量优劣依次为:多年生黑麦草Lolium perenne>凌志>美洲虎3号>猎狗5号;抗旱性强弱依次为:凌志Festuca arundinacea cv. Barlexas> 美洲虎3号Festuca arundinacea cv. Houndolog5>猎狗5号Festuca arundinacea cv. Jarguar3>多年生黑麦草;越夏和越冬表现较好的为美洲虎3号和凌志,猎狗5号和多年生黑麦草次之.  相似文献   
1000.
The flavor-enhancing effects of the volatile constituents in celery were investigated. The test samples were prepared by adding celery fractions to chicken broth at a concentration that distinct odors of them were not detected, and the samples were sensorially evaluated for the perceived intensities of 8 terms such as "thick," "impactful," "mild," "lasting," "satisfied," "complex," "refined," and "clarified," which are considered to be the elements of the complex flavor and for 3 terms such as "sweet," "salty," and "umami" taste. A comparison of effects between the volatile and nonvolatile fractions of celery revealed that the volatile compounds in celery enhanced the complex flavor of chicken broth more than the nonvolatile compounds. Among the characteristic odorants of celery, three phthalides, namely, sedanenolide, 3- n-butylphthalide, and sedanolide, were shown to contribute to the complex flavor of chicken broth, and sedanenolide was most effective. The three phthalides enhanced perceived intensities of "umami" and "sweet" despite their no taste properties in addition to the complex flavor.  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