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   58782篇
  免费   3355篇
  国内免费   37篇
林业   3057篇
农学   2216篇
基础科学   351篇
  7441篇
综合类   6822篇
农作物   2105篇
水产渔业   3210篇
畜牧兽医   32933篇
园艺   725篇
植物保护   3314篇
  2019年   500篇
  2018年   959篇
  2017年   1035篇
  2016年   1010篇
  2015年   867篇
  2014年   1017篇
  2013年   2327篇
  2012年   1815篇
  2011年   2209篇
  2010年   1443篇
  2009年   1374篇
  2008年   2035篇
  2007年   2050篇
  2006年   1936篇
  2005年   1793篇
  2004年   1692篇
  2003年   1673篇
  2002年   1543篇
  2001年   2335篇
  2000年   2187篇
  1999年   1724篇
  1998年   685篇
  1997年   623篇
  1996年   601篇
  1995年   672篇
  1994年   572篇
  1993年   558篇
  1992年   1262篇
  1991年   1398篇
  1990年   1450篇
  1989年   1331篇
  1988年   1243篇
  1987年   1230篇
  1986年   1167篇
  1985年   1104篇
  1984年   918篇
  1983年   723篇
  1982年   524篇
  1979年   792篇
  1978年   616篇
  1977年   469篇
  1976年   524篇
  1975年   502篇
  1974年   624篇
  1973年   605篇
  1972年   633篇
  1971年   601篇
  1970年   588篇
  1969年   537篇
  1967年   497篇
排序方式: 共有10000条查询结果,搜索用时 13 毫秒
911.
Plant Foods for Human Nutrition - The effects of cocoa-derived polyphenols on cognitive functions have been analyzed through numerous studies using different interventions (doses, vehicles, time...  相似文献   
912.
While nitrogen and carbon stable isotope values can reflect ecological segregation, prey choice and spatial distribution in seabirds, the interpretation of bulk stable isotope values is frequently hampered by a lack of isotopic baseline data. In this study, we used compound-specific isotope analyses of amino acids (CSIA-AA) to overcome this constraint and to study interspecific differences, seasonal and historical changes in trophic positions of five seabird species, three penguins and two petrels, from a sub-Antarctic seabird community. CSIA-AA allowed comparing trophic positions of seabirds with temperate and polar distributions. Gentoo and Magellanic penguins had the highest trophic positions during the breeding season (3.7 and 3.9), but decreased these (2.9 and 3.3) during the feed-up for moult. Intra-specific differences were also detected in Thin-billed prions, where carbon isotope values clearly separated individuals with polar and temperate distributions, both in the breeding and interbreeding periods. Thin-billed prions that foraged in polar waters had lower trophic positions (3.2) than conspecifics foraging in temperate waters (3.8). We further investigated historical changes by comparing museum samples with samples collected recently. Our pilot study suggests that Rockhopper penguins, Magellanic penguins and Thin-billed prions with temperate non-breeding distributions had retained their trophic levels over a 90–100 year period, while Gentoo penguins and Thin-billed prions with polar non-breeding distributions had decreased trophic levels compared to historical samples. In contrast, Wilson’s storm-petrels had slightly increased trophic levels compared to samples taken in 1924–1930. We applied compound-specific stable isotope analyses across a range of contexts, from intra-specific comparisons between stages of the breeding cycle to inter-specific seabird community analysis that would not have been possible using bulk stable isotope analyses alone due to differences in isotopic baselines.  相似文献   
913.
Paddy and Water Environment - Most of the lands of the southern coastal zone of Bangladesh are protected from tidal flooding and storm surges by embankments constructed during the 1960s and 1970s,...  相似文献   
914.
915.
916.
Landscape Ecology - The majority of remaining tropical forests exist as fragments embedded in a matrix of agricultural production. Understanding the effects of these agricultural landscapes on...  相似文献   
917.
Spot blotch (SB), caused by Bipolaris sorokiniana, is a devastating disease of wheat globally, especially in South Asia and South America. Understanding the genetics of resistance to SB is important for developing breeding strategies to improve resistance. A panel of 301 genotypes from Afghanistan was phenotyped over two crop seasons using a mixture of virulent B. sorokiniana isolates and genotyped using DArTSeq to obtain genome-wide markers. Fifty genotypes (16.6%) showed disease scores less than the resistant control. Principal component analysis using the genotypic data clustered the genotypes into five different groups. Among models used for genome-wide association mapping, the multilocus mixed model, and fixed and random model circulating probability unification algorithms were most effective in identifying significant marker-trait associations (MTA). Twenty-five MTAs at p ≤ .001 were identified on chromosomes 1A, 1B, 1D, 2B, 2D, 3A, 3B, 4A, 5A, 5B, 6A, 7A, and 7D, indicating the quantitative nature of resistance to SB. Phenotypic variation explained by these markers ranged from 2.0% to 17.7%, and genomic regions on the chromosomes 1D, 2D, 3A, 3B, 4A, 5A, and 5B coincided with loci identified in previous studies. Three single nucleotide polymorphism (SNP) markers on chromosomes 1B (SNP 1113207) and 5A (SNPs 5411867 and 998276) were significant in both crop seasons as well as in the combined analysis across seasons. Marker 5411867 is close to Vrn-A1, shown to be associated with SB in previous studies. Furthermore, among known SB resistance genes, Sb2 on chromosome 5B was predicted to be significant in this panel.  相似文献   
918.
919.
Phaeomoniella chlamydospora and wood-rotting basidiomycete fungi, namely Fomitiporia spp., are known esca pathogens. However, the effect of their mixed infections and the sequence of infection on disease development is unclear. To determine the effects of single and co-inoculations on symptoms, potted Vitis vinifera ‘Crimson Seedless’ was inoculated with P. chlamydospora either alone or in combination with one of four basidiomycetes: Coprinellus radians, Fomitiporia langloisii, F. polymorpha and the novel species Tropicoporus texanus. Basidiomycetes were isolated from vines with foliar symptoms of esca in California and Texas. In sequential co-inoculations, the effects of different sequences of infection (P. chlamydospora first, basidiomycete 6 months later; and vice versa) were tested, compared to simultaneous co-inoculations. Plants inoculated with P. chlamydospora either alone or in combination with a basidiomycete (in any sequence) did not differ significantly in the length of black-streaking lesions. In plants inoculated only with a basidiomycete, the appearance of large brown lesions, coupled with the absence of this wood symptom from control plants, suggests that C. radians, F. langloisii and T. texanus are pathogenic. Foliar symptoms resembling those of esca in the field (marginal and/or interveinal scorching, combined with red and/or yellow discoloration) were statistically more frequent among plants inoculated with F. polymorpha or T. texanus, either simultaneously or following P. chlamydospora, compared to single inoculations. Sequential co-inoculations of a basidiomycete before or after P. chlamydospora were associated with similar lesion lengths, suggesting that basidiomycetes may not require infection by P. chlamydospora in order to extensively colonize the wood.  相似文献   
920.
The purpose of this study was to examine the genotypic variation in maize doubled haploid (DH) lines response to brassinosteroid and gibberellin inhibitors. Plant responses to hormone inhibitors were determined in growth chamber experiments using germination paper for three different seedling treatments: application of propiconazole (Pcz), uniconazole (Ucz) or water (control). Mesocotyl length (ML) was more sensitive to hormone inhibitors, especially to the Ucz treatment, than other seedling traits. ML was significantly correlated with other traits in the Ucz treatment. All the seedling traits showed moderate-to-high broad sense heritability values, ranging from 0.39 to 0.82. The Euclidian genetic distances of inbred line pairs ranged from 1.27 to 19.94, indicating there was a high level of variability across the maize DH lines used in this study. DH lines with extreme MLs were identified, which can provide valuable breeding resources for improving abiotic stress tolerance, and for further genetic studies.  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