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   215篇
  免费   29篇
林业   41篇
农学   12篇
基础科学   1篇
  56篇
综合类   2篇
农作物   18篇
水产渔业   21篇
畜牧兽医   47篇
园艺   15篇
植物保护   31篇
  2023年   2篇
  2022年   1篇
  2021年   10篇
  2020年   10篇
  2019年   7篇
  2018年   18篇
  2017年   23篇
  2016年   27篇
  2015年   11篇
  2014年   12篇
  2013年   7篇
  2012年   13篇
  2011年   16篇
  2010年   8篇
  2009年   21篇
  2008年   7篇
  2007年   6篇
  2006年   7篇
  2005年   1篇
  2004年   2篇
  2003年   2篇
  2002年   2篇
  2001年   5篇
  2000年   3篇
  1998年   2篇
  1997年   4篇
  1994年   1篇
  1993年   3篇
  1991年   1篇
  1990年   1篇
  1989年   1篇
  1988年   2篇
  1987年   1篇
  1986年   1篇
  1985年   2篇
  1982年   1篇
  1975年   1篇
  1968年   1篇
  1943年   1篇
排序方式: 共有244条查询结果,搜索用时 15 毫秒
91.
We compared two different strategies to increase the catches of Ips typographus (L.), particularly males, in pheromone-baited traps. The first of these strategies, the barrier approach, used alternating pheromone blends, targeting males and females respectively, in closely-spaced traps forming a barrier around forest stands. The second strategy, the single trap approach, used widely-spaced traps that were all baited with the same lure and intended to trap the highest possible numbers of males without compromising trapping of females. In the blend used for the barrier traps targeting primarily males, with a lower percentage of (4S)-cis-verbenol (cV), the (−)-α-pinene was replaced step wise with (+)-limonene at rates of 0%, 1%, 10%, 35%, 60% and 90%. This replacement had no significant effect on the numbers of responding I. typographus males, but there was a slight effect on the percentage of males caught. In the attractant blend for the barrier traps targeting females, with a higher percentage of cV, the 2-methyl-3-buten-2-ol (MB) was replaced with 1-methoxy-2-propanol (MP) in a similar fashion as for the male-specific blends. The replacement did not significantly affect the catch of females. Thus, it is possible to use the MP in the blend with cV and ipsdienol without significant change in catch efficacy. In the blends for single traps, the (−)-α- pinene was replaced with (+)-limonene and MB with MP. The replacement of (−)-α-pinene had only a slight effect on the percentage of males, but the results suggest that replacing MB with MP in the blend will not significantly reduce trapping efficacy. Foundation project: The research was supported by Slovak Research and Development Agency (APVV-51-P06005 and APVV-27-P05205) and by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences (2/6153/26)  相似文献   
92.
  • ? The secondary succession of wet grasslands to communities of alder carr dominated by Alnus glutinosa was recorded in different parts of Europe during the 20th century. However, knowledge of such development of alder carr remains insufficient.
  • ? The development of alder carr was reconstructed at five sites in the Czech Republic, using historical aerial photographs and methods of dendrochronology. The aims were to investigate the succession from wet grasslands to alder carr at sites previously intensively managed for agriculture and to find out the role of fluctuations in the groundwater table, caused by artificial drainage channels, in the observed stand dynamics and tree growth.
  • ? The spread of forest (i.e., an increase in forest cover) predominated until the 1970s at all sites. This trend was disrupted by a large-scale dieback of forest stands in four of the five sites after the 1970s, followed by an increase in patch heterogeneity, as indicated by landscape metrics. The radial growth increment in Alnus glutinosa has been affected predominately by local environmental factors, probably including the changing degree of waterlogging. Forest dieback was presumably connected with a lesser extent of drainage channels.
  • ? Our results indicate that observed successional pathways at sites of alder carr were probably caused by local changes in the groundwater table.
  •   相似文献   
    93.
    Degree of the association between line per se performance (LP) and testcross performance (TP) is important in breeding programs and simultaneous improvement of commercial hybrids and their parental lines. This experiment was designed to study genetic variability and genetic correlation for several agronomic traits in two maize (Zea mays L.) broad-based populations (NS12-SG and NS14-SG). Independent trials with 80 entries of S1 progenies as well as their testcrosses were arranged according to an incomplete block design with replicates in sets. Grain yield, stay green, anthesis-silking interval, stalk water content and grain moisture were evaluated in four environments. The anthesis-silking interval had the highest genetic variation, followed by stay green. High heritability estimates (>0.50) for all traits, pointed out that further selection would be possible. Genetic correlations between line per se and testcross performance were lowest for grain yield (0.396** and 0.592**, for NS12-SG and NS14-SG, respectively), and highest for grain moisture (0.937** and 0.821**, respectively). High correlations between line per se and their testcrosses for stay green, anthesis-silking interval, stalk water content and grain moisture indicated that additive gene action might be more important than dominance in controlling the expression of these traits.  相似文献   
    94.
    This study examines the impact of taurine on the viability, morphology and acrosome integrity of rabbit spermatozoa in vitro. Semen samples, obtained from four to five sexually mature and healthy New Zealand White rabbits, were pooled in heterospermic semen sample. This was divided and treated with taurine in a concentration of 0 (control), 1.5, 7, 12.5, 50 mM to a final concentration of 108 spermatozoa/ml. The samples were then incubated at 37°C for 4 hr. A combination of fluorescent probes SYBR‐14/propidium iodide/PNA‐Alexa Fluor 647 was used to assess spermatozoa viability and acrosome integrity on a flow cytometer. The sperm morphology was evaluated under a light microscope following fixation in 1.5% paraformaldehyde. The experiment was repeated three times. According to the obtained results, the spermatozoa neither could have benefit from immediate taurine treatment, nor had they after 4‐hr incubation with respect to viability and acrosome integrity. Taurine did not initially alter the total and acrosome morphology of treated spermatozoa nor has it by 4 hr upon treatment. In conclusion, taurine may have no protective effect on the viability, morphology and acrosome integrity of short‐term stored rabbit spermatozoa.  相似文献   
    95.
    Leaves with typical symptoms of downy mildew were found on common agrimony in the Czech Republic in 2014 and 2015 and at several locations in Germany from 2010 to 2014. The causal agent of downy mildew of agrimony was often reported as Peronospora agrimoniae, but sometimes also as P. sparsa. Morphological characteristics of the pathogens found in both countries are in the range of previous works for P. agrimoniae, but also other downy mildews parasitic on Rosaceae, rendering their discrimination based on published observations difficult. For molecular identification sequencing of several loci (nrITS rDNA, cox1 and cox2) was performed. Phylogenetic analyses based on nrITS rDNA clearly separated P. agrimoniae from other Peronospora species infecting Rosaceae. Thus, considering P. agrimoniae as separate species seems justified. Two German specimens were identical to two Czech samples in both nrITS rDNA and cox1 mtDNA sequences, but differed in a single nucleotide substitution in cox2 region. To our knowledge, this is the first verified record of P. agrimoniae on common agrimony in the Czech Republic.  相似文献   
    96.
    In this study, chromosomal imbalances in tumor tissues (lymphomas) and nucleotide changes in tumor suppressor TP53 were studied in a Bernese Mountain dog bitch and a cross breed bitch. Using comparative genomic hybridization, numerous chromosomal rearrangements were detected, which indicated the heterogeneity in tumor growth: in the cross breed bitch, a deletion on the chromosome 9, and duplications on chromosomes 5, 8 and 17 have been found. In the Bernese Mountain Dog bitch, losses on chromosomes 1, 5, 8, 12, 18, 22, 27, 29 and gains on chromosomes 1, 2, 9, 11, 15, 16, 18, 20, 23, 24, 25, 28, 29, 30, 34, 36, 37 and 38 were identified. With the sequencing of the TP53 gene, one silent mutation, transition A/G at position 138 in exon 5 was detected, without changing the amino acid.  相似文献   
    97.
    98.
    A previous study indicated decreased DNA content of chromosome 4A in the wheat (Triticum aestivum L. cv. Tähti) compared to cvs. Chinese Spring and Rennan. Here we show that the lower 4A DNA content is associated with a specific haplotype in the distal part of 4AL. In 41 cultivars of bread wheat (T. aestivum L.), including cv. Tähti, a common haplotype was identified in the linkage disequilibrium region on the long arm of chromosome 4A (4AL). The haplotype (haplotype A) is characterized by 7 SSR and 5 EST marker alleles, including five zero-alleles. Haplotype A was found in 46 % of the Swedish/Finnish/Estonian spring wheat genotypes, while only one of the modern wheat accessions from Germany carried the same haplotype. Fluorescent cytometry analysis linked haplotype A to diminished DNA content of chromosome 4A. The haplotype was introduced into the Canadian and US breeding programs at the beginning of the twentieth century (cvs. Marquis, Thatcher, Ruby) from the common progenitor, the Polish landrace Fife, and it is still found in modern wheat germplasm in these countries. Zero-alleles characteristic for haplotype A were also detected in several accessions of European spelt (Triticum spelta L.), and in two accessions of tetraploid Triticum timopheevii Zhuk. The presence of haplotype A in European spelt indicates the considerable antiquity of the haplotype, as it must have been inherited from the hexaploid or tetraploid parent of spelt in at least one hybridization event.  相似文献   
    99.
    Aboveground and belowground biomass of 15-year-old under-planted European beech seedlings (Fagus sylvatica L.) in Norway spruce stand were studied along a light gradient in three plots, in the northern part of Slovenia. Differences in soil water content, aboveground and fine root biomass distribution were confirmed between studied plots. Light had significant effect on the total biomass, root-shoot ratio (0.388 ± 0.076 under canopy, 0.549 ± 0.042 in the edge, 0.656 ± 0.047 in the open), specific root length (SRL) of fine beech roots (561.9 ± 42.2 under canopy, 664.3 ± 51.2 in the edge, 618.2 ± 72.8 in the open) and specific leaf area in beech, indicating morphological adjustment to shade. However, SRL of beech fine roots indicated no change between plots. The correlation between total aboveground and root biomass and light below the mature stand canopy was higher in the case of diffuse light intensity. Most fine roots of spruce were concentrated in the top (0–20 cm) soil layer. Beech fine roots under canopy and edge conditions were also concentrated in top (0–20 cm) soil layer and exhibited shift downwards to deeper soil horizons in open plot. Root proportion between beech and spruce changed with light toward beech with increasing light intensity for both fine and coarse roots.  相似文献   
    100.
    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