全文获取类型
收费全文 | 47834篇 |
免费 | 2200篇 |
国内免费 | 5186篇 |
专业分类
林业 | 2791篇 |
农学 | 4602篇 |
基础科学 | 638篇 |
4812篇 | |
综合类 | 20370篇 |
农作物 | 3340篇 |
水产渔业 | 3228篇 |
畜牧兽医 | 9903篇 |
园艺 | 2043篇 |
植物保护 | 3493篇 |
出版年
2024年 | 131篇 |
2023年 | 501篇 |
2022年 | 1102篇 |
2021年 | 1325篇 |
2020年 | 1410篇 |
2019年 | 1659篇 |
2018年 | 1099篇 |
2017年 | 1831篇 |
2016年 | 2422篇 |
2015年 | 2103篇 |
2014年 | 1840篇 |
2013年 | 2512篇 |
2012年 | 3680篇 |
2011年 | 3624篇 |
2010年 | 2977篇 |
2009年 | 2691篇 |
2008年 | 2877篇 |
2007年 | 3452篇 |
2006年 | 3066篇 |
2005年 | 2331篇 |
2004年 | 1896篇 |
2003年 | 1416篇 |
2002年 | 1116篇 |
2001年 | 1053篇 |
2000年 | 992篇 |
1999年 | 832篇 |
1998年 | 646篇 |
1997年 | 572篇 |
1996年 | 518篇 |
1995年 | 536篇 |
1994年 | 455篇 |
1993年 | 443篇 |
1992年 | 429篇 |
1991年 | 367篇 |
1990年 | 338篇 |
1989年 | 243篇 |
1988年 | 196篇 |
1987年 | 141篇 |
1986年 | 128篇 |
1985年 | 54篇 |
1984年 | 35篇 |
1983年 | 23篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 23篇 |
1979年 | 30篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1956年 | 17篇 |
1955年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Relationship between stable carbon isotope discrimination and water use efficiency under regulated deficit irrigation of pear-jujube tree 总被引:2,自引:0,他引:2
Ningbo Cui Taisheng Du Shaozhong Kang Fusheng Li Xiaotao Hu Mixia Wang Zhijun Li 《Agricultural Water Management》2009,96(11):1615-1622
To investigate the relationship between stable carbon isotope discrimination (Δ) of different organs and water use efficiency (WUE) under different water deficit levels, severe, moderate and low water deficit levels were treated at bud burst to leafing, flowering to fruit set, fruit growth and fruit maturation stages of field grown pear-jujube tree, and leaf stable carbon isotope discrimination (ΔL) at different growth stages and fruit stable carbon isotope discrimination (ΔF) at fruit maturation stage were measured. The results indicated that water deficit had significant effect on ΔL at different growth stages and ΔF at fruit maturation stage. As compared with full irrigation, the average ΔL at different growth stages and ΔF at fruit maturation stage were decreased by 1.23% and 2.67% for different water deficit levels, respectively. ΔL and ΔF among different water deficit treatments had significant difference at the same growth stage (P < 0.05). Under different water deficit conditions, significant relationships between the ΔL and WUEi (photosynthesis rate/transpiration rate, Pn/Tr), WUEn (photosynthesis rate/stomatal conductance of CO2, Pn/gs), WUEy (yield/crop water consumption, Y/ETc) and yield, or between the ΔF and WUEy and yield were found, respectively. There were significantly negative correlations of ΔL with WUEi, WUEn, WUEy and yield (P < 0.01) at the fruit maturation stage, or ΔL with WUEi and WUEn (P < 0.01) over whole growth stage, respectively. ΔF was negatively correlated with WUEy, WUEn and yield (P < 0.05), but positively correlated with ETc (P < 0.01) over the whole growth stage. Thus ΔL or ΔF can compare WUEn and WUEy, so the stable carbon isotope discrimination method can be applied to evaluate the water use efficiency of pear-jujube tree under the regulated deficit irrigation. 相似文献
994.
Temporary water trading is an established and growing phenomenon in the Australian irrigation sector. However, decision support and planning tools that incorporate economic and biophysical factors associated with temporary water trading are lacking. In this paper the integration of an economic trading model with a hydrologic water allocation model is discussed. The integrated model is used to estimate the impacts of temporary water trading and physical water transfers. The model can incorporate economic and biophysical drivers of water trading. The economic model incorporates the key trade drivers of commodity prices, seasonal water allocations and irrigation deliveries. The hydrologic model is based on the Resource Allocation Model (REALM) framework, which facilitates hydrologic network simulation modelling. It incorporates water delivery system properties and operating rules for the main irrigation and urban centres in a study area.The proposed integration method has been applied to a case study area in northern Victoria, Australia. Simulations were conducted for wet and dry spells, a range of commodity prices and different irrigation distribution system configurations. Some example analyses of scenarios incorporating water trading were undertaken. From these analyses potential bottlenecks to trade that constrain the economic benefits from temporary water trading were identified. Furthermore, it was found that in certain areas of the system, trading can make impacts of long drought spells worse for water users, e.g. irrigators. Thus, the integrated model can be used to quantify short-term and long-term third party impacts arising from temporary water trading. These findings also highlight the need to link “paper trades” (estimated by economic models) to physical water transfers (estimated by biophysical models). 相似文献
995.
朱双华 《拖拉机与农用运输车》2009,36(5):129-130,132
控制器局域网中网关工作的好坏决定了不同的总线、模块和网络相互间通信的好坏。本文详细阐述了网关的特点和功能,并对网关在奥迪A4-B6车用控制器局域网(CAN)中的作用作了详细说明。 相似文献
996.
依据Griffing方法配制种植不同类型的复合双列杂交组合,分别设水、旱二种水分条件。在不同生育时期测定株高、株穗数、株粒重、穗粒数、黄叶片数、SOD、POD、MDA等农艺、生理生化性状,研究不同组合类型对杂交后代的遗传力影响及其与抗旱性的关系及遗传规律。结果表明,可采用抗旱指数(DRI)作为评价小麦品种抗旱性强弱的指标。旱地株粒重、旱地穗下节长、旱地黄叶片、籽粒饱满度、落黄、旱地成穗数、旱地株高等7个质量性状和数量性状与抗旱性关系密切且遗传力较强,可作为高产种质杂种后代的早期抗旱性鉴定指标。SOD活性和MDA含量由于广义遗传力和狭义遗传力均较高,可以在杂种后代早期世代进行选择,OA能力和POD活性广义遗传力较高但狭义遗传力低,适宜在杂种后代的晚期世代进行选择。 相似文献
997.
降水、灌溉和品种对玉米产量和水分利用效率的影响 总被引:4,自引:0,他引:4
8年田间试验结果表明,玉米产量明显受降水和灌溉的影响。玉米苗期灌溉最为关键,是保证高产的基础。在只灌溉苗水而没有其他灌溉条件下,夏玉米产量与7、8月份降水密切相关,而与生长季节的降水总量关系不明显。除了极干旱年份(季节降雨量小于200 mm)外,随着灌溉量增加,玉米水分利用效率(WUE)降低。玉米最优灌溉制度为:干旱年份,除了苗期的1次灌溉外,在拔节和扬花期分别实施2次灌溉;常年和湿润年份,除了出苗水外,在抽穗到扬花期的8月初实施1次灌溉。3年的田间试验结果表明,当地种植的不同玉米品种之间产量最大相差20%,水分利用效率差异在12%~19%。因此,将适宜的品种与优化的灌溉制度相结合,是提高华北平原夏玉米产量和水分利用效率的有效途径。 相似文献
998.
针对大型矩形渡槽的动力特性和动力响应问题,建立了流固耦合(FSI)系统的位移-压力(ui,p)有限元格式,给出流固耦合系统的动力特性方程;基于FSI系统的(ui,p)格式建立槽体-水体-槽墩-基础-地基系统的力学模型,采用非对称模态提取法求解了4种水深下渡槽的动力特性,并用隐式-显式积分算法计算了4种水深、2种强震作用下大型渡槽的动力响应。计算结果表明,基于FSI的(ui,p)格式,考虑到槽体与水体的相互作用,简化了计算模型,提高了计算精度,同时也得出不同水深、不同激励下渡槽结构动力特性和动力响应的变化规律等结论,可为大型渡槽及同类工程动力设计提供参考。 相似文献
999.
A flow cytometric virus-binding assay that directly visualizes the binding and entry of infectious pancreatic necrosis virus (IPNV), infectious haematopoietic necrosis virus (IHNV) and virus haemorrhagic septicaemia virus (VHSV) to several cell lines was established. The highest efficiency of binding was shown by the BF-2 cell line and this was used to study, at the attachment level, the interactions of these cells with salmonid fish viruses in coinfections, and to further determine if the earliest stage of the viral growth cycle could explain the previously described loss of infectivity of IHNV when IPNV is present. Our results demonstrated that IPNV binds to around 88% of cells either in single or dual infections, whereas IHNV attachment always decreased in the presence of any of the other viruses. VHSV binding was not affected by IPNV, but coinfection with IHNV reduced the percentage of virus-binding cells, which suggests competition for viral receptors or co-receptors. Internalization of the adsorbed IHNV was not decreased by coinfection with IPNV, so the hypothetical competence could be restricted to the binding step. Treatment of the cells with antiviral agents, such as amantadine or chloroquine, did not affect the binding of IPNV and VHSV, but reduced IHNV binding by more than 30%. Tributylamine affected viral binding of the three viruses to different degrees and inhibited IPNV or IHNV entry in a large percentage of cells treated for 30 min. Tributylamine also inhibited IHNV cytopathic effects in a dose-dependent manner, decreasing the virus yield by 4 log of the 50% endpoint titre, at 10 mm concentration. IPNV was also inhibited, but at a lower level. The results of this study support the hypothesis that IHNV, in contrast to VHSV or IPNV, is less efficient at completing its growth cycle in cells with a simultaneous infection with IPNV. It can be affected at several stages of viral infection and is more sensitive to the action of antiviral compounds. 相似文献
1000.
Laddawan Krongpong Kunihiko Futami Takayuki Katagiri Makoto Endo Masashi Maita 《Fisheries Science》2008,74(5):1055-1061
Furazolidone, an antibacterial drug that was once widely used in the livestock industry and aquaculture, is now prohibited
in numerous countries. It is difficult to detect residual furazolidone because it is readily metabolized in animal tissues
but, by using and liquid chromatography coupled with tandem mass spectrometry, its metabolite, 3-amino-2-oxazolidinone (AOZ)
can be detected. Here we describe the validity of an enzyme-linked immunosorbent assay (ELISA) kit to detect AOZ in Japanese
eel Anguilla japonica tissue. ELISA is capable of detecting AOZ at 1.0 μg/kg in an eel sample with excellent accuracy and precision. Our results
show that ELISA is suitable for regulatory purposes and for studying the fate of AOZ residues in eel treated with furazolidone.
To measure the persistence of AOZ in eel tissues, eels (1.4–6.5g) were immersed in tanks containing 2 and 10 mg furazolidone/L
for 3 h, and then maintained in a tank supplying well water for the next 160 days. The half-lives of AOZ, calculated from
the linear terminal part of the excretion curve, were 25.0 days in muscle and 21.6 days in liver from fish exposed to 2 mg/L
furazolidone. In the eels treated with 10 mg/L furazolidone, by contrast, high levels of AOZ were detected in liver and muscle,
but the half-lives of AOZ were similar to those in fish treated with 2 mg/L furazolidone. The half-lives of AOZ in eel tissues
were prolonged by the condition of low water temperature. 相似文献