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   210篇
  免费   8篇
林业   96篇
农学   3篇
基础科学   4篇
  43篇
综合类   37篇
农作物   5篇
水产渔业   2篇
畜牧兽医   20篇
园艺   2篇
植物保护   6篇
  2023年   2篇
  2021年   2篇
  2018年   3篇
  2016年   6篇
  2014年   5篇
  2013年   14篇
  2012年   4篇
  2011年   18篇
  2010年   4篇
  2009年   7篇
  2008年   14篇
  2007年   9篇
  2006年   9篇
  2005年   10篇
  2004年   8篇
  2003年   5篇
  2002年   6篇
  2000年   3篇
  1999年   2篇
  1998年   3篇
  1995年   2篇
  1994年   2篇
  1991年   3篇
  1989年   3篇
  1980年   2篇
  1979年   5篇
  1978年   3篇
  1974年   2篇
  1973年   6篇
  1972年   2篇
  1971年   4篇
  1970年   4篇
  1967年   3篇
  1966年   3篇
  1963年   2篇
  1956年   3篇
  1954年   2篇
  1953年   2篇
  1946年   1篇
  1941年   1篇
  1940年   1篇
  1938年   1篇
  1937年   2篇
  1936年   1篇
  1935年   1篇
  1934年   3篇
  1931年   1篇
  1930年   4篇
  1929年   3篇
  1928年   1篇
排序方式: 共有218条查询结果,搜索用时 31 毫秒
11.
Ohne Zusammenfassung  相似文献   
12.
Allgemeines     
Ohne Zusammenfassung  相似文献   
13.
Responses to successive drought and re-watering cycles (1-3 cycles) were compared in greenhouse-grown cuttings of Populus x canadensis (Moench) clones, 'Luisa_ Avanzo' and 'Dorskamp.' Total leaf number increment rate, duration of leaf expansion, total and individual leaf area expansion rates and stomatal conductance were recorded periodically during the experiment. Soil water content (SWC) and predawn leaf water potential (Psi(WP)) were measured four times during each drought cycle. In parallel, relative leaf water content (RWC) and specific leaf area (SLA) were estimated on leaves collected from the top to bottom of each cutting. Under well-watered conditions, 'Luisa_Avanzo' and 'Dorskamp' differed in their patterns of leaf area expansion. Although duration of leaf expansion was similar between clones, 'Luisa_ Avanzo' exhibited higher total leaf number increment rates and individual leaf area increases than 'Dorskamp.' As a result, 'Luisa_Avanzo' cuttings reached larger individual and total leaf areas than 'Dorskamp.' 'Dorskamp' leaves had lower SLA than 'Luisa_Avanzo' leaves. In response to successive drought cycles, both clones underwent decreases in total leaf number increment rates and in total leaf area expansion rates, but both whole-plant and individual leaf areas were drastically reduced only in 'Luisa_Avanzo.' 'Dorskamp' maintained a constant leaf area as a result of an increase in the duration of leaf expansion during drought and a significant stimulation of individual leaf area expansion rate and total leaf number increment rate in response to re-watering. Drought caused a greater decrease in SLA in 'Luisa_Avanzo' than in 'Dorskamp.' Expanded leaves of 'Dorskamp' were constitutively dense or thick, or both, whereas leaves of 'Luisa_Avanzo' became dense or thick, or both, only in response to drought. In both clones, re-watering caused partial recovery of SLA to control values. Our data confirm previous field and greenhouse observations that 'Dorskamp' is more tolerant to successive drought and re-watering cycles than 'Luisa_Avanzo.' We conclude that this difference is mainly attributable to the plasticity of 'Dorskamp' after re-watering, rather than to clonal differences in drought responses.  相似文献   
14.
Temperature effects on photosynthesis were studied in seedlings of evergreen Mediterranean cork oak (Quercus suber L.). Responses to changes in temperature and the temperature optima of maximal carboxylation rate (V(cmax)) and maximal light-driven electron flux (J(max)) were estimated from gas exchange measurements and a leaf-level photosynthesis model. The estimated temperature optima were approximately 34 and 33 degrees C for V(cmax) and J(max), respectively, which fall within the lower range of temperature optima previously observed in deciduous tree species. The thermostability of the photosynthetic apparatus was estimated according to the temperature at which basal chlorophyll a fluorescence begins to increase (T(c)). The T(c) was highly variable, increasing from 42 to 51 degrees C when ambient temperature rose from 10 to 40 degrees C, and increasing from 44 to 54 degrees C with decreasing soil water availability while net CO(2) assimilation rate dropped to almost zero. When a heat shock was imposed, an additional small increase in T(c) was observed in drought-stressed and control seedlings. Maximal T(c) values following heat shock were about 56 degrees C, which, to our knowledge, are the highest values that have been observed in tree species. In conclusion, the intrinsic temperature responses of cork oak did not differ from those of other species (similar T(c) under ambient temperature and water availability, and relatively low thermal optima for photosynthetic capacity in seedlings grown at cool temperatures). However, the large ability of cork oak to acclimate to drought and elevated temperature may be an important factor in the tolerance of this evergreen Mediterranean species to summer drought and high temperatures.  相似文献   
15.
16.
Rundschau     
Ohne Zusammenfassung  相似文献   
17.
18.
Ohne Zusammenfassung  相似文献   
19.
Journal of Pest Science - On insects found in excavations from the Roman Age. A review is given of the results of excavations made in several regions of Europe with regard to the insect fauna...  相似文献   
20.
Three-year-old saplings of Pinus sylvestris L. were labeled with 13CO2 prior to inoculating the trunk with Ophiostoma brunneo ciliatum, a blue-staining fungus usually associated to Ips sexdentatus. During incubation, half the trees were submitted to a severe drought that decreased photosynthesis and natural 13C content in non-labeled saplings. A large 13C-excess was obtained in wood and phloem, especially in the fractions of soluble proteins, starch and soluble sugars of labeled saplings. Drought increased 13C-excess, due to reduced photosynthesis and smaller dilution of 13C by the addition of newly assimilated 12C. The induced-reaction zones in inoculated saplings displayed large total C (58 g 100 g?1) because of the accumulation of secondary metabolites. They also showed much larger 13C-excess than any other compartment: the contribution of stored C to the reaction zones was much higher than that of currently assimilated C. Moreover, drought lowered the contribution of the latter, as shown by the increase of 13C in the reaction zones. We conclude that stored C was readily mobilized for the construction of reaction tissues, and that the contribution of currently assimilated C was only minor.  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