全文获取类型
收费全文 | 5781篇 |
免费 | 274篇 |
国内免费 | 753篇 |
专业分类
林业 | 714篇 |
农学 | 944篇 |
基础科学 | 464篇 |
1113篇 | |
综合类 | 1694篇 |
农作物 | 435篇 |
水产渔业 | 231篇 |
畜牧兽医 | 672篇 |
园艺 | 160篇 |
植物保护 | 381篇 |
出版年
2024年 | 24篇 |
2023年 | 91篇 |
2022年 | 164篇 |
2021年 | 247篇 |
2020年 | 192篇 |
2019年 | 209篇 |
2018年 | 168篇 |
2017年 | 224篇 |
2016年 | 168篇 |
2015年 | 238篇 |
2014年 | 231篇 |
2013年 | 318篇 |
2012年 | 312篇 |
2011年 | 385篇 |
2010年 | 361篇 |
2009年 | 302篇 |
2008年 | 348篇 |
2007年 | 339篇 |
2006年 | 339篇 |
2005年 | 284篇 |
2004年 | 133篇 |
2003年 | 129篇 |
2002年 | 140篇 |
2001年 | 141篇 |
2000年 | 163篇 |
1999年 | 173篇 |
1998年 | 154篇 |
1997年 | 112篇 |
1996年 | 110篇 |
1995年 | 91篇 |
1994年 | 91篇 |
1993年 | 93篇 |
1992年 | 97篇 |
1991年 | 56篇 |
1990年 | 47篇 |
1989年 | 39篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 7篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 6篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1964年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有6808条查询结果,搜索用时 0 毫秒
941.
In this study, we subjected the root systems of eight Eucalyptus hybrids currently cultivated in southern China to heterogeneous phosphorus stress to provide a scientific basis for the selection of a highly phosphorus-efficient Eucalyptus variety. When the ability of these hybrids to locate phosphorus under different experimental conditions (phosphorus supply in a homogeneous or heterogeneous manner vs. no phosphorus supply) was compared, the main growth characteristics of Eucalyptus, such as plant height, diameter, dry mass, and phosphorus content, significantly improved when the phosphorus supply was increased from no phosphorus or heterogeneous phosphorus (half of the phosphorus amount) to homogeneous phosphorus. Across these three conditions, the growth traits of different Eucalyptus hybrids differed significantly, indicating different adaptabilities of the hybrids to various phosphorus conditions. The growth traits of the aboveground tissues of Eucalyptus under different phosphorus conditions were largely influenced by the morphology of the underground root system. In addition, the root morphology of Eucalyptus under heterogeneous phosphorus treatment suggested that there were two mechanisms for locating nutrients. Eucalyptus hybrids such as Urophylla 3229, Grandis 9, Guanglin 3, 201-2, and Dunn produced more roots proximal to the phosphorus supply; the other hybrids, Urophylla 3216, Grandis 5, and Guanglin 9, relied mainly on the growth of roots opposite the phosphorus supply to obtain adequate nutrients for growth. With these two strategies, a wide range of nutrients was obtained, root distribution was greater, more soil volume was covered, the contact area of the roots with soil phosphorus was increased, and the uptake of phosphorus by the root system was increased. These results demonstrate that Eucalyptus relies on changes to morphological characteristics of the root system to increase accessibility to phosphorus resources. 相似文献
942.
基于低空无人机遥感技术的油菜机械直播苗期株数识别 总被引:3,自引:3,他引:0
植株数量识别是油菜机械直播效果和质量评估的关键.该文针对油菜机械直播田间植株数量检测中人工统计耗时、费工、效率低下的现实,通过自主搭建的低空无人机遥感平台采集油菜机械直播区域的遥感影像,基于超高分辨率(0.18 cm/pixel)遥感影像计算的颜色植被指数进行油菜目标识别及其形态特征信息提取.结合田间调查数据,采用逐步回归分析方法,建立了机械直播油菜在苗期的株数与遥感特征信息之间的关系.结果显示,油菜目标的株数与其外接矩形的长宽比、像素分布密度和周长栅格数具有较好的线性关系,回归模型的决定系数R2为0.803,并通过显著性检验,其标准估计误差为0.699.模型检验结果显示,观测值与预测值之间的R2为0.809,均方根误差RMSE为0.728.研究结果表明,利用集成超高分辨率传感器的低空无人机遥感平台,通过计算颜色植被指数并分析油菜目标数量与形态特征的相关性,能有效识别油菜机械直播的出苗株数,可为后续油菜机械直播效果的快速、准确评估提供技术支持. 相似文献
943.
森林作为陆地的最大生态系统,是游憩娱乐场所、宜人生态环境的提供者及可持续发展的重要保障,对缓解城市环境危机,实现资源的最大化利用具有重要影响。笔者从森林的生态、视觉等研究出发,阐述了森林和景观的密切联系,使森林的生态价值、美学价值以及可持续价值得到最大发挥并形成统一整体。 相似文献
944.
945.
基于遥感方法监测植被变化以及分析植被对气候变化的响应与反馈机制已经成为全球变化研究的一个重要领域(马国明等,2006;Mark et al.,2001).尽管归一化植被指数(NDVI)在高植被区的饱和性使其与一些植被生理参数之间的关系往往是非线性的(Carlson et al.,1990;Gillies et al.,1997),并且容易受到土壤背景的影响(Huete,1997),但由于NDVI与叶面积指数、光合有效辐射吸收率、生物量、初级生产力和植被盖度等植被生物物理特征的高度相关性(马国明等,2006;Veroustraete et al.,2002),使其成为研究大尺度植被变化的重要指标. 相似文献
946.
A novel chitinase gene (PjCHI‐1) isolated from Paecilomyces javanicus, a non‐nematophagous fungus, and driven by a CaMV35S promoter, was delivered into CLN2468D, a heat‐tolerant cultivar of tomato (Solanum lycopersicum). T1 tomato plants exhibited high endochitinase activity and reduced numbers of eggs and egg masses when infected with the root‐knot nematode (RKN) Meloidogyne incognita. The eggs found in transgenic tomato had lower shell chitin contents than eggs collected from control plants. Egg masses from transgenic plants exhibited higher chitinase activity than those from control plants. Moreover, only 30% of eggs from transgenic plants were able to develop to the multi‐cell/J1 stage, compared with more than 96% from control plants. The present study demonstrated that the expression of the PjCHI‐1 chitinase gene can effectively reduce the production of egg masses and repress the embryonic development of M. incognita, presenting the possibility of a novel agro‐biotechnological strategy for preventing crop damage by RKN. 相似文献
947.
948.
949.
950.
Based on a on site wind turbine’s output active power vs. generator speed curve, the shortage of conventional power control strategies of doubly fed wind turbine generation system are analyzed. Considering the generator speed restriction and active power fast track control requirement, the power decoupling control strategy in the rotor side converter and variable pitch control strategy are improved, that is, an output mechanical torque of wind turbine and the generator speed of DFIG are used in the rotor side active power control loop in order to achieve an accurate optimal speed control, as well as a direct active power control method is also adopted in the variable pitch control strategy. Finally, the overall operational performances of DFIG wind turbine generation system using the proposed control strategies are simulated, and also compared with the conventional control strategies. The results show that the improved control strategy is more valid and correct. 相似文献