全文获取类型
收费全文 | 5392篇 |
免费 | 329篇 |
国内免费 | 827篇 |
专业分类
林业 | 301篇 |
农学 | 411篇 |
基础科学 | 1619篇 |
1673篇 | |
综合类 | 1485篇 |
农作物 | 82篇 |
水产渔业 | 288篇 |
畜牧兽医 | 372篇 |
园艺 | 61篇 |
植物保护 | 256篇 |
出版年
2024年 | 45篇 |
2023年 | 126篇 |
2022年 | 212篇 |
2021年 | 228篇 |
2020年 | 221篇 |
2019年 | 256篇 |
2018年 | 179篇 |
2017年 | 263篇 |
2016年 | 338篇 |
2015年 | 240篇 |
2014年 | 289篇 |
2013年 | 314篇 |
2012年 | 406篇 |
2011年 | 400篇 |
2010年 | 323篇 |
2009年 | 287篇 |
2008年 | 287篇 |
2007年 | 300篇 |
2006年 | 283篇 |
2005年 | 214篇 |
2004年 | 188篇 |
2003年 | 158篇 |
2002年 | 146篇 |
2001年 | 132篇 |
2000年 | 118篇 |
1999年 | 95篇 |
1998年 | 68篇 |
1997年 | 75篇 |
1996年 | 60篇 |
1995年 | 63篇 |
1994年 | 60篇 |
1993年 | 42篇 |
1992年 | 36篇 |
1991年 | 36篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 16篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1974年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有6548条查询结果,搜索用时 31 毫秒
981.
转基因植物在批准商业化之前通常要求在控制条件下开展实验。我国规定转基因油菜田间试验的隔离距离为1 000 m,该距离大于世界上绝大多数国家和地区的规定。转基因油菜基因流研究的数据显示,自然条件下甘蓝型油菜品种间的天然异交率一般不超过3%,多数在1%以下,隔离距离和保护行可以显著降低异交率,隔离距离400 m处异交率低于0.01%。白菜型油菜、自交不亲和系和雄性不育系的异交率则要高得多。建议对转基因油菜的安全监管采取分类管理和阈值管理的原则,在0.1%允许阈值下,普通甘蓝型油菜的隔离距离可设为400 m,而以异花授粉为主的其他油菜(包括白菜型油菜与其他类型油菜的自交不亲和系和雄性不育系)的隔离距离可设为1 000 m。 相似文献
982.
983.
An assumption of Atlantic salmon, Salmo salar L., management is that river flow influences their upstream migration; but the quantitative relationships between flow and movements are variable and inconsistent. A consensus has emerged that standard rules on salmon flow needs for water management across all rivers are unattainable. However, this does not preclude an underlying conceptual framework that should guide a consistent scientific basis to flow management for protecting salmon. This article examines the potential for a conceptual framework from a British Isles perspective, assuming that upstream migration is an emergent property from selection to optimise lifetime fitness. The template for this adaptation is the diversity of river types and hydro‐morphology in British rivers coupled with life‐history and migration adaptations that may be influenced by flow‐related factors. Models of adult salmon migration through estuaries and rivers are outlined that may provide a basis for developing generic guidance. 相似文献
984.
985.
C. P. MAINSTONE R. THOMAS C. W. BEAN T. WATERMAN 《Fisheries Management and Ecology》2012,19(6):557-569
Abstract An outline is given of the scale and nature of special wildlife designations in the UK river network, the general approach of the UK conservation agencies to their evaluation and management, and the specific way in which impacts of river flows are handled. The need for a holistic ecological and biodiversity view of water resource impacts on river systems is stressed, within which the specific needs of individual species, such as Atlantic salmon, Salmo salar L., can be framed. An approach founded on the provision of a natural flow regime within a natural physical channel not only provides the most suitable conditions for characteristic riverine wildlife to flourish but also provides the best local defence against global climate change. Arguments are made for a future focus on the generic evidence base for flow targets and its strengthening through strategic and demonstrably fit‐for‐purpose research. The practical constraints to an approach based on protection of the natural flow regime are discussed. Ultimately, transparent separation and consideration of what river ecosystems really need, and what can realistically be provided, is the key to shared ownership of the water resource dilemma. 相似文献
986.
Understanding the potential for the evolution of herbicide tolerance in weeds following gene flow from transgenic crops is important for their management. The inter-specific hybrids formed between four Brassica juncea populations with different glyphosate-susceptibility and transgenic oilseed rape was studied, emphasizing on the possible population difference. Compared with both parents, the fitness-related components and photosynthetic capacity of hybrids decreased dramatically, but significant differences were observed among different hybrids. All F1 hybrids exhibited highly enhanced but similar herbicide tolerance level regardless of the wild B. juncea parent, indicating that genetically engineered herbicide tolerance may over-dominate phenotype ones by wild-crop hybridization. The analysis of parental loci transmission revealed a higher transfer ratio of male-specific loci detected in F1 hybrids, suggesting that oilseed rape genetic markers can be transferred at relatively high frequencies to the next generation. Therefore, higher transfer ratio of oilseed rape-specific loci, coupled with variation of populations in fitness-related parameters in F1 hybrids, could complicate environmental risk assessment of transgenic oilseed rape, especially in current agroecosystems with increasing application of glyphosate. 相似文献
987.
988.
流式细胞术鉴定植物染色体倍性的方法广泛应用于果树多倍体育种中,然而相关流式细胞术鉴定梨染色体倍性的方法还没有系统的研究。以梨树叶片为试材系统优化了各个流式细胞术关键实验节点,结果表明:实验材料选取当年春季新梢第3~5 片梨树嫩叶,尤其是大棚新梢嫩叶最好;叶片前处理时用蒸馏水、去离子水依次洗净叶片表面后,再用去离子水浸泡10 min;使用WPB解离液对梨叶片细胞核进行提取后,用500 目滤膜过滤2 次,离心1 次后用流式细胞仪进行检测可得到理想的实验结果。该方法步骤简单、结果可靠,是一种快速、高效的梨染色体倍性鉴定的方法,可广泛用于梨树多倍体鉴定。 相似文献
989.
纵轴流玉米脱粒分离装置喂入量与滚筒转速试验 总被引:2,自引:0,他引:2
在玉米籽粒直收过程中,脱粒滚筒转速与联合收获机的额定喂入量相匹配才能发挥出最佳的作业效果。为了获得不同喂入量时玉米联合收获机最优的滚筒转速范围,设计了一种零部件可更换、结构参数和工作参数均可调的纵轴流玉米脱粒分离装置,并在自主研制的试验台上以脱粒滚筒转速、喂入量为影响因素,以籽粒破碎率、未脱净率为性能指标进行玉米脱粒试验。通过台架试验、回归分析和单变量求解,最终确定了不同喂入量的最优滚筒转速范围:喂入量为8 kg/s时,最优的滚筒转速为254~486 r/min;喂入量为10 kg/s时,最优的滚筒转速为278~466 r/min;喂入量为12 kg/s时,最优的滚筒转速为313~445 r/min。在以上条件下籽粒破碎率均小于5%,未脱净率小于2%,达到了国家和相关标准的要求。 相似文献
990.