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   6883篇
  免费   421篇
  国内免费   839篇
林业   863篇
农学   1069篇
基础科学   500篇
  1065篇
综合类   2084篇
农作物   413篇
水产渔业   312篇
畜牧兽医   1140篇
园艺   243篇
植物保护   454篇
  2024年   28篇
  2023年   114篇
  2022年   241篇
  2021年   319篇
  2020年   326篇
  2019年   304篇
  2018年   197篇
  2017年   287篇
  2016年   257篇
  2015年   313篇
  2014年   350篇
  2013年   407篇
  2012年   435篇
  2011年   471篇
  2010年   465篇
  2009年   422篇
  2008年   366篇
  2007年   382篇
  2006年   352篇
  2005年   281篇
  2004年   151篇
  2003年   154篇
  2002年   145篇
  2001年   173篇
  2000年   146篇
  1999年   158篇
  1998年   137篇
  1997年   109篇
  1996年   111篇
  1995年   102篇
  1994年   89篇
  1993年   71篇
  1992年   57篇
  1991年   59篇
  1990年   42篇
  1989年   43篇
  1988年   31篇
  1987年   13篇
  1986年   10篇
  1985年   4篇
  1984年   4篇
  1983年   1篇
  1982年   4篇
  1981年   7篇
  1979年   1篇
  1965年   2篇
  1964年   1篇
  1956年   1篇
排序方式: 共有8143条查询结果,搜索用时 15 毫秒
991.
通过对浙江省安吉县毛竹林业园区的现状调查,发现安吉县毛竹林业园区存在基础设施损坏严重、组织化程度低、产品结构单一、经济效益低等问题。因此,提出落实经营主体、引进专业人才或机构、出台扶持政策等有关建议,以期为安吉县毛竹林业园区可持续健康发展提供参考。  相似文献   
992.
Rapid growth and high quality have always been important goals pursued for timber forests. Therefore, growth traits and wood properties are the basis of superior materials to select and breed new cultivars. In this study, 42 half-sib families of 31-year-old P. koraiensis trees were used as materials. Nine growth traits (tree height, diameter at breast height, volume, basal diameter, average annual growth of the tree height, under branch height, stem straightness degree, form quotient, and branching angle) and eight wood properties (cellulose contents, hemicellulose contents, holocellulose contents, lignin contents, ash contents, wood density, fiber length and fiber width) were measured and analyzed. The results of an analysis of variance showed that there were extremely significant differences among the families in their growth and wood properties with the exception of the form quotient and wood density; that there were significant differences among block by family for growth traits except for the tree height, basal diameter, under branch height and branching angle; and that there was no significant difference for each growth trait among the blocks. The phenotypic coefficient of variation and heritability of the growth traits ranged from 7.17 to 42.35% and from 0.13 to 0.62, respectively. The phenotypic coefficient of variation and heritability of the wood properties ranged from 10.21 to 50.26% and from 0.67 to 0.92, respectively. There were extremely significantly positive correlations between the tree height, basal diameter, diameter at breast height, volume, under branch height, average annual growth of tree height and form quotient. However, there was no significant correlation between growth traits and wood properties. The result of the principal component analysis indicated that the tree height, diameter at breast height, volume, basal diameter, and cellulose contents, holocellulose contents, and lignin contents could be selected as comprehensive evaluation indices of growth and wood properties, respectively. According to a comprehensive evaluation, when the selection rate was 10%, four families (PK 40, PK 80, PK 42, and PK 71) were selected as elite families based on growth performance; another four families (PK 70, PK 62, PK 52, and PK 44) were selected as elite families based on their wood quality; and finally, four elite families (PK 70, PK 62, PK 61, and PK 40) were selected due to a combination of growth traits and wood properties. This study will provide a theoretical basis for the genetic improvement of fast-growing and high-quality P. koraiensis families.  相似文献   
993.
戴林建  陈武  周田  钟军 《作物杂志》2018,34(1):66-107
首先在接种有不同浓度黑胫病菌发酵液活性组分的选择培养基上,获得高钾烤烟GK2的花药培养苗,并采用离体叶片法对这些花药培养苗进行黑胫病抗性鉴定,结果花药培养植株表现出不同程度的抗性;然后选其中6株抗性较高且农艺性状与GK2相近的单株,经秋水仙素加倍后收获种子,在黑胫病病圃及大田条件下于2016年对其R-1代、2017年对3个抗病性表现较好的R-2代植株进行黑胫病抗性及农艺性状测试,2年试验结果一致表明:在染色体加倍以后,所选植株的后代在基因型上已完全纯合,黑胫病抗性较GK2有较大提高且均可稳定遗传,是不同于原GK2的烤烟新品系,在生产上具有广泛应用前景。  相似文献   
994.
北部冬麦区冬小麦区试品种(系)的稳定性和适应性分析   总被引:4,自引:1,他引:3  
为客观准确地评价区域试验中北部冬麦区冬小麦新品种(系)的丰产性和稳产性,以2014-2015年度国家北部冬麦区水地组冬小麦品种(系)区域试验产量数据为资料,应用AMMI模型对小区产量的基因型、环境和基因型与环境(G×E)互作进行了研究。结果表明,稳定性最好的为农大3486、京农12-79,较好的有科遗4174、长6794、中麦93,较差的为航麦109、晋太102、众信7198;试点以天津武清、山西榆次、新疆阿拉尔、河北遵化、山西屯玉分辨力较高,河北固安、河北滦县、北京顺义、北京昌平分辨力较低。在对区域试验中品种(系)稳定性和试点的分辨力进行判定时,综合使用双标图和稳定性参数两种方法,既直观又准确。  相似文献   
995.
The purpose of this study was to explore the mechanism of by which docosahexaenoic acid (DHA) inhibit the accumulation of adipose tissue lipid in grass carp (Ctenopharyngodon idella). We therefore designed two semi-purified diets, namely DHA-free (control) and DHA-supplemented, and fed them to grass carp (22.19 ± 1.76 g) for 3 and 6 weeks. DHA supplementation led to a significantly lower intraperitoneal fat index (IPFI) than that in the control group by reducing the number of adipocytes but significantly higher adipocyte size (P < 0.05). In the intraperitoneal adipose tissue, the DHA-fed group showed significantly higher peroxisome proliferator-activated receptor (PPAR)γ, CCAAT enhancer-binding protein (C/EBP)α, and sterol regulatory element-binding protein (SREBP)1c mRNA expression levels at both 3 and 6 weeks (P < 0.05). However, the ratio of the expression levels of B cell leukemia 2 (Bcl-2) and Bcl-2-associated X protein (Bax) was significantly lower in the DHA-fed group than in the control group (P < 0.05), and the protein expression levels of the apoptosis-related proteins caspase 3, caspase 8, and caspase 9 were also significantly higher (P < 0.05). Overall, although DHA promotes lipid synthesis, it is more likely that DHA could suppress the lipid accumulation in adipocytes of grass carp by inducing adipocyte apoptosis.  相似文献   
996.
Saroma Lake is the southernmost lagoon exhibiting seasonal sea ice coverage in the Northern hemisphere and one of the most important aquaculture areas for Japanese scallops (Mizuhopecten yessoensis). Under conditions of adequate food and appropriate temperature, scallops grow well from the time of ice melting in April to harvesting starting in July. However, over the past decade, Saroma Lake frequently has not been completely covered by ice in winter, and the production of Japanese scallops has shown significant changes. Therefore, this study integrated data from satellite remote sensing, buoys, and in situ observations with climatic events [the winter East Asian Monsoon (EAM) and El Niño/La Niña–Southern Oscillation events] to investigate the impact of ice coverage variations on scallop growth in Saroma Lake between 2007 and 2015. Daily ice conditions were detected using Moderate Resolution Imaging Spectroradiometer false colour images and an unsupervised classification method. The results indicated that EAM strongly influenced the ice coverage variation in Saroma Lake through their effects on temperature during winter. Ice coverage variations show a strong correlation with water temperature and spring phytoplankton blooms, which are the two most important environmental factors for scallop growth. In addition, extreme climate events could cause water temperature anomalies (as in 2015) which are unfavourable for scallop growth. Monitoring ice conditions should be considered when developing plans and management strategies for scallop aquaculture in Saroma Lake.  相似文献   
997.
998.
Due to the ambient temperature fluctuation during the process of storage, transportation, and sale, frozen fillets may go through frozen-thawed cycles that result in quality changes. In order to detect the frozen-thawed cycles in frozen tilapia fillets, near infrared spectroscopy was applied. The thawing loss, cooking loss, moisture content, total volatile basic nitrogen, and texture profile of fillets in seven frozen-thawed cycles were analyzed to reflect the change of quality. A total of 60 fillets were used to collect the spectra on both dorsal and belly flesh in frozen and thawed state from the first to the seventh cycle. The results showed that there is a better discrimination among the once and the repeated frozen-thawed samples in the frozen state than the thawed state. Frozen samples were still conducive to the differentiation and had higher classification accuracy, ranging from 80% to 93.33%. Dorsal flesh was more beneficial for the differentiation than belly flesh and had the highest accuracy, ranging from 86.67% to 93.33% in the frozen state. These results showed that the nondestructive and fast detection of repeated frozen-thawed cycles in frozen tilapia fillets can be accomplished by near infrared spectroscopy, which has enormous potential for practical application.  相似文献   
999.
Effects of rhythmic temperature change on the growth, body composition and energy budget of hybrid grouper (Epinephelus lanceolatus ♂ × E. fuscoguttatus ♀) were investigated. Nine groups of fish received repeating cycles of low‐temperature manipulation (22°C) for 1, 2 and 4 days followed by recovery temperature (28°C) for (3, 7 and 11 days), (6, 14 and 22 days) and (12, 28 and 44 days) respectively, designated as L1R3, L1R7, L1R11, L2R6, L2R14, L2R22, L4R12, L4R28 and L4R44 respectively. In the control group (C), fish were reared at 28°C throughout the whole experiment. After 96‐day feeding trail, the final weight, relative weight gain rate, specific growth rate, food conversion efficiency and apparent digestibility coefficient of fish in L4R28 were significantly higher than those of control (< .05). The crude protein contents of fish in L2R14, L2R22, L4R12, L4R28 and L4R44 were significantly higher compared to the control (< .05). According to energy budget, fish in L4R28 exhibited significantly higher proportion of food energy assimilated into growth and lower proportion consumed for excretion than the control fish (< .05). These results indicated the occurrence of complete or over growth compensation in hybrid grouper, and the use of repeated cycles of low temperature (22°C) for 4 days followed by recovery temperature (28°C) for 28 days could obtain enhanced growth, that is overcompensation, with improved apparent digestibility coefficient, food conversion efficiency and energetic efficiency into growth. This technique would be more practically applicable in indoor culture for this species.  相似文献   
1000.
Zea mays has been historically imported to Japan via two independent geographical routes: into southern Japan by trading with Europe in the 16th century and into northern Japan by import from North America in the 19th century. Breeding to genetically improve on quality traits and high yields has led to the current domestic landraces in each region. In a survey of 82 domestic landraces, nine out of 38 landraces originating from southern Japan showed complete immunity to cucumber mosaic virus yellow strain (CMV(Y)) without the formation of necrotic local lesions (NLLs). In contrast, three out of 44 landraces originating from northern Japan developed NLLs, but revealed no systemic spread of the virus. Due to the absence of good documentation on NLL formation in Z. mays, the response of domestic landraces Aso‐1 and Aso‐3, originating from Ibaraki in northern Japan, to a challenge with CMV(Y) and CMV(Ma‐1) was further analysed. Aso‐3 only formed NLL in response to CMV(Y) but not to CMV(Ma‐1). Moreover, in CMV(Y)‐inoculated Aso‐3, virus spread was restricted to the primary infection site and the expression of defence‐related genes was up‐regulated, whereas Aso‐1 became systemically infected with either CMV(Y) or CMV(Ma‐1). The response of Aso‐3 to CMV(Y) was inherited as a single dominant trait. Together, these results pointed towards the induction of hypersensitive response (HR)‐mediated resistance to CMV(Y) in Aso‐3. Although HR‐mediated resistance to viruses has been studied mainly in dicots, the pathosystem CMV–Z. mays may provide a model to investigate HR‐mediated resistance to viruses in monocot plants.  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