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   33613篇
  免费   928篇
  国内免费   1788篇
林业   5412篇
农学   3524篇
基础科学   1447篇
  5370篇
综合类   6373篇
农作物   3196篇
水产渔业   2497篇
畜牧兽医   3750篇
园艺   1780篇
植物保护   2980篇
  2024年   79篇
  2023年   270篇
  2022年   664篇
  2021年   820篇
  2020年   785篇
  2019年   710篇
  2018年   3186篇
  2017年   3354篇
  2016年   1735篇
  2015年   841篇
  2014年   787篇
  2013年   846篇
  2012年   1867篇
  2011年   3331篇
  2010年   3108篇
  2009年   2272篇
  2008年   2202篇
  2007年   2417篇
  2006年   813篇
  2005年   818篇
  2004年   479篇
  2003年   454篇
  2002年   367篇
  2001年   299篇
  2000年   402篇
  1999年   477篇
  1998年   397篇
  1997年   352篇
  1996年   328篇
  1995年   274篇
  1994年   314篇
  1993年   260篇
  1992年   217篇
  1991年   182篇
  1990年   147篇
  1989年   145篇
  1988年   95篇
  1987年   66篇
  1986年   34篇
  1985年   24篇
  1984年   18篇
  1983年   14篇
  1982年   14篇
  1981年   10篇
  1980年   8篇
  1979年   7篇
  1977年   4篇
  1975年   4篇
  1968年   4篇
  1956年   7篇
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
941.
There is a great interest in the nutritional value of vegetables and fruits and how the habitat affects nutritive and biological properties. In vitro studies here reported were performed to evaluate the inhibitory activity of formulations from edible plant on pancreatic lipase. The aim of this study was also to evaluate the biovariability of L. comosa (L.) Parl. bulbs from Italy. The wild bulbs were compared with the same cultivated species that are commonly commercialized to identify samples with the best quality for a potential therapeutic application. Hydroalcoholic extract and polar fraction of wild bulbs showed a very important pancreatic lipase inhibitory activity, with IC50 values of 0.166 ± 0.005 and 0.153 ± 0.005 mg/mL, respectively. In order to characterize the extracts, gas chromatography associated with mass spectrometry (GC/MS) analysis was performed, revealing the predominance of palmitic acid. Phenolic and flavonoid composition was also evaluated. L. comosa extract obtained from wild bulbs demonstrated both antioxidant and anti-obesity activities that might be attributed to a wide range of present phenolic compounds.  相似文献   
942.
Studies on porous starch have been directed to explore different industrial applications as bio-adsorbents of a variety of compounds. However, the analysis of starch digestibility is essential for food application. The objective of this study was to determine the impact of porous structure on in vitro starch digestibility. Porous starches were obtained using a range of concentrations of amyloglucosidase (AMG), α-amylase (AM), cyclodextrin-glycosyltransferase (CGTase) or branching enzyme (BE). Porous starches exhibited major content of digestible starch (DS) that increased with the intensity of the enzymatic treatment, and very low amount of resistant starch (RS). Porous starches behaved differently during in vitro hydrolysis depending on their enzymatic treatment. AMG was the unique treatment that increased the digestive amylolysis and estimated glycemic index, whereas AM, CGTase and BE reduced them. A significant relationship was found between the pore size and the severity of the amylolysis, suggesting that a specific pore size is required for the accessibility of the digestive amylase. Therefore, pore size in the starch surface was a limiting factor for digestion of starch granules.  相似文献   
943.
利用田间试验结合生物化学分析研究了不同的栽培大豆品系对草甘膦的抗性,试验结果显示:在草甘膦有效剂量为0.31~0.92 kg·hm~(-2)时,不同栽培大豆品种对草甘膦抗性存在明显差异,在草甘膦剂量为0.92 kg·hm~(-2)时,石豆1号存活率为100%,表现了较高的抗性,8份材料的存活率为4%~48%,表现出一定的抗性;44份材料表现敏感,存活率为0。试验研究了抗性材料石豆1号、中抗材料Williams和敏感材料中黄35对草甘膦的生理生化反应的差异,结果表明:随着草甘膦处理剂量的增加和处理时间的延长,石豆1号莽草酸含量、叶绿素含量和超氧化物歧化酶(SOD)相对活性没有明显变化。而Williams和中黄35的莽草酸含量和SOD相对活性明显升高,叶绿素含量明显降低。结果表明草甘膦(浓度0.92 kg·hm~(-2))处理下,不同抗性栽培大豆叶片中莽草酸含量有明显差异,处理5~14 d时敏感材料叶片中莽草酸含量保持较高水平,因此栽培大豆叶片莽草酸含量可以作为判断其对草甘膦抗性高低的主要生理指标之一,而SOD的活性和叶绿素含量可以作为判断栽培大豆对草甘膦抗性的辅助生理指标。  相似文献   
944.
对分离自玉米植株的内生细菌R-4进行鉴定,测试其对玉米南方锈病(SCR)的防病增产效果。用盆栽方法测定菌株R-4对玉米植株的促生效果,根据16S r DNA序列对菌株R-4进行鉴定,采用自然发病方式测试其对SCR的防病增产效果。结果表明,菌株R-4处理玉米的发芽率、播种后18 d株高、播种后28 d株高、植株地上部干重及根干重比对照分别增加17.19%、32.03%、41.80%、92.53%和37.03%。通过16S r DNA序列分析,将菌株R-4鉴定为蜡样芽孢杆菌(Bacillus cereus)。菌株R-4在先玉335、银糯1号、浚单20上对SCR的防效达39.2%~92.1%(平均为65.7%),经该菌处理后玉米增产2.58%~9.02%(平均为6.66%)。  相似文献   
945.

Background

Developmental biology has made great strides in recent years towards the quantification of cellular properties during development. This requires tissues to be imaged and segmented to generate computerised versions that can be easily analysed. In this context, one of the principal technical challenges remains the faithful detection of cellular contours, principally due to variations in image intensity throughout the tissue. Watershed segmentation methods are especially vulnerable to these variations, generating multiple errors due notably to the incorrect detection of the outer surface of the tissue.

Results

We use the level set method (LSM) to improve the accuracy of the watershed segmentation in different ways. First, we detect the outer surface of the tissue, reducing the impact of low and variable contrast at the surface during imaging. Second, we demonstrate a new edge function for a level set, based on second order derivatives of the image, to segment individual cells. Finally, we also show that the LSM can be used to segment nuclei within the tissue.

Conclusion

The watershed segmentation of the outer cell layer is demonstrably improved when coupled with the LSM-based surface detection step. The tool can also be used to improve watershed segmentation at cell-scale, as well as to segment nuclei within a tissue. The improved segmentation increases the quality of analysis, and the surface detected by our algorithm may be used to calculate local curvature or adapted for other uses, such as mathematical simulations.
  相似文献   
946.
947.
Powdery scab caused by Spongospora subterranea f. sp. subterranea (Sss) causes extensive losses in potato production systems globally. Two pot experiments were established in the greenhouse in summer 2013 and winter 2014 to evaluate the effectiveness of different soil chemicals, fumigant, amendments and biological control agents (BCAs) against Sss in the rhizospheric soil, potato roots and tubers. The study used visual assessment methods to assess the effect of treatments on root galling and zoosporangia production, and qPCR to measure Sss concentration in the soil and in the potato roots and tubers. All six soil treatments, namely metam sodium, fluazinam, ZincMax, calcium cyanamide, Biocult and a combination of Bacillus subtilis and Trichoderma asperellum recorded significantly (P < 0.05) lower numbers of zoosporangia in the roots compared to the untreated control. The same effect was observed on the concentration of Sss DNA in the roots at tuber initiation. A more diverse picture was obtained when root gall scores at tuber initiation and Sss DNA in the rhizospheric soil at tuber initiation and harvesting were compared. Significant differences (P < 0.05) were also noted in disease severity, disease incidence, and tuber yield between metam sodium, fluazinam, ZincMax, calcium cyanamide and the untreated control. Calcium cyanamide gave the highest tuber yield. The study demonstrated the potential of soil treatments such as metam sodium, fluazinam, ZincMax and calcium cyanamide in managing Sss in potatoes by reducing the pathogen both in the rhizospheric soil and the roots of the potato plant.  相似文献   
948.
Three species of wireworms (Coleoptera: Elateridae) known to cause severe damage to potatoes in Europe and Asia have been introduced to Canada and are now well established as pests in the westernmost province of British Columbia (BC) (Agriotes obscurus and A. lineatus) and the eastern provinces of Nova Scotia and Prince Edward Island (A. obscurus, A. lineatus and A. sputator). Conventional insecticide-based efforts to control these invasive pests have had serious environmental impacts, or are failing to prevent severe economic damage from occurring to potatoes in some key potato production areas. Research toward developing an IPM programme for these exotic species has been completed or is underway in Canada, including the following: biological and ecological studies, development of monitoring and risk assessment programmes, and development of insecticidal and alternative control strategies. This research is summarized and implications for wireworm management in Europe are discussed. In addition to wireworms, one of the primary economic insect pests of potatoes in BC is the tuber flea beetle, Epitrix tuberis (Coleoptera: Chrysomelidae). The larvae of this beetle feed on developing or mature daughter tubers, producing surface channels or tracks and holes directly into the tuber. In the past, growers would apply 7–10 foliar sprays of broad-spectrum insecticides per growing season, which was not always successful in controlling this pest. In the EU, a newly identified flea beetle, Epitrix papa sp. n., as well as the North American species, E. cucumeris, have been identified as attacking potatoes initially in Portugal (2004) and later also in Spain. The potential spread of these emergent pests to various EU and non-EU countries is of concern, and if established would require the development of management strategies. The former development of a highly effective IPM programme developed in BC for E. tuberis is discussed, as is its potential for E. papa and E. cucumeris management in Europe.  相似文献   
949.
Precision agriculture is a farming management concept based on observing, measuring and responding to inter- and intra-field variability in crops. In this paper, we focus on responding to intra-field variability in potato crops and analyse variable rate applications (VRAs). We made an overview of potential VRAs in potato crop management in The Netherlands. We identified 13 potential VRAs in potato, ranging from soil tillage to planting to crop care to selective harvest. We ranked them on availability of ‘proof of concept’ and on-farm test results. For five VRAs, we found test results allowing to make a cost-benefit assessment. These five VRAs were as follows: planting, soil herbicide weed control, N side dress, late blight control and haulm killing. They use one of two types of spatial data: soil maps or biomass index maps. Data on costs and savings of the VRAs showed that the investments in VRAs will pay off under practical conditions in The Netherlands. Savings on pesticide use and N-fertilizer use with the VRAs were on average about 25%, which benefits the environment too. We foresee a slow but gradual adoption of VRAs in potato production. More VRAs will become available given ongoing R&D. The perspectives of VRAs in potatoes are discussed.  相似文献   
950.
This paper surveys issues and developments emerging from the keynotes and participants’ presentations during the 20th EAPR Triennial Conference, held in Versailles, France, in July 2017, and puts them into the perspective of the general theme of the meeting: potato facing global challenges. It reveals important and far-reaching innovations in many areas of potato research, including breeding (genomic selection, high-throughput phenotyping, genome editing), plant protection (biocontrol) and seed production, but also a general innovation approach that follows an incremental pathway (and maintains many existing lock-in situations), and favours substitution strategies over system re-conceptualisation and testing, however urgently needed these might be to achieve or improve the sustainability of potato production in the medium to long run. While the assets and importance of potato as both a wholesome food and an essential part of future worldwide food systems were once more evidenced and highlighted, the contributions also revealed (i) a growing dichotomy between research activities carried out in high-income vs in low- or moderate-income countries, respectively; (ii) some gaps in the assessment of the sustainability of potato production in the medium to long run (external inputs such as fertilisers or pesticides, land and energy use efficiency); and (iii) persistent weaknesses in the integration of individual solutions into complete, operational potato production systems. Some areas for future investigations are identified.  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