全文获取类型
收费全文 | 18160篇 |
免费 | 969篇 |
国内免费 | 947篇 |
专业分类
林业 | 2675篇 |
农学 | 1621篇 |
基础科学 | 530篇 |
2392篇 | |
综合类 | 7607篇 |
农作物 | 1236篇 |
水产渔业 | 255篇 |
畜牧兽医 | 1149篇 |
园艺 | 710篇 |
植物保护 | 1901篇 |
出版年
2024年 | 88篇 |
2023年 | 289篇 |
2022年 | 485篇 |
2021年 | 560篇 |
2020年 | 604篇 |
2019年 | 680篇 |
2018年 | 487篇 |
2017年 | 686篇 |
2016年 | 842篇 |
2015年 | 625篇 |
2014年 | 827篇 |
2013年 | 1208篇 |
2012年 | 1511篇 |
2011年 | 1235篇 |
2010年 | 1014篇 |
2009年 | 1097篇 |
2008年 | 998篇 |
2007年 | 1087篇 |
2006年 | 919篇 |
2005年 | 737篇 |
2004年 | 630篇 |
2003年 | 495篇 |
2002年 | 391篇 |
2001年 | 357篇 |
2000年 | 303篇 |
1999年 | 267篇 |
1998年 | 215篇 |
1997年 | 175篇 |
1996年 | 156篇 |
1995年 | 180篇 |
1994年 | 136篇 |
1993年 | 143篇 |
1992年 | 127篇 |
1991年 | 128篇 |
1990年 | 111篇 |
1989年 | 85篇 |
1988年 | 60篇 |
1987年 | 58篇 |
1986年 | 25篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
901.
A method for mapping the distribution of willow at a catchment scale using bi-seasonal SPOT5 imagery 总被引:1,自引:0,他引:1
This study showed that seasonal imagery acquired at specific stages of phenology can be used to improve the mapping accuracy of invasive willow at a catchment scale. SPOT5 XI (10 m) satellite imagery was acquired for early autumn and winter to represent the phenological stages of leaf cover and leaf fall respectively. Four classification regimes were evaluated using single‐ and bi‐seasonal composite imagery to determine the most accurate method. Significant spectral noise was found in willow populations, especially in the winter image, due to the effects of undergrowth exposure, shadowing, topography and boundary‐mixed pixels. Two noise reduction techniques were applied to the bi‐seasonal composite image to improve the classification results. The noise‐reduced bi‐seasonal composite image was classified using the spectral angle mapper (SAM) algorithm before importation into a geographical information system. Aerial photography was used to reduce the errors of commission associated with misclassification of pastures. The class accuracy achieved for willow using the method described in this study was 77.5% (Kappa =0.87). The high cost of eradicating willow means that managers must establish priorities for control; this technique can provide a powerful tool for prioritizing control programmes and for monitoring results at a catchment scale. 相似文献
902.
以研磨法从中华真地鳖五龄若虫体内提取得到抗菌物质,分别采用比浊法和菌碟法测定了其对多种细菌和植物病原真菌的抑菌活性。结果表明,该抗菌物质对大肠杆菌、金黄色葡萄球菌、白菜软腐菌和枯草芽孢杆菌等4种细菌生长均有一定的抑制作用,对数期延迟1~4h,菌的浓度、繁殖速度减缓。对7种植物病原真菌的抑菌试验表明,该抗菌物质对玉米茎基腐病菌、小麦根腐病菌、棉花枯萎病菌抑制效果较好,抑制率分别为50.0%、49.2%、45.2%;而对白菜黑斑病菌和玉米大斑病菌活性较差,抑制率仅为28.3%和24.5%。 相似文献
903.
904.
卷叶螟绒茧蜂Apanteles derogatae Watanabe是棉大卷叶螟Sylepta derogata Fabricius幼虫期的一种重要内寄生蜂,对棉大卷叶螟种群有较好的控制作用。在实验室条件下,运用人工接虫观察法研究卷叶螟绒茧蜂对棉大卷叶螟幼虫的寄生行为。结果表明,该蜂搜寻寄主的时间平均为2.38 min,处理寄主的时间仅需0.86 min;寄生寄主的最适龄期为寄主2龄幼虫;对1龄幼虫的致死率达32%,但其寄生率仅为16%,其余16%的个体为穿刺致死。饲喂蜂蜜水能显著增加该蜂的产卵量,延长其产卵历期和增强其穿刺能力。寄生蜂的寄生能力与寄主密度的关系呈抛物线型:在寄生蜂均为16头/m~3雌蜂条件下,当寄主密度从80头/m~3增加到320头/m~3时,寄主的被寄生个体数随之增加,但当寄主密度超过320头/m~3后,寄主的被寄生个体数反而随寄主密度的上升呈下降趋势。 相似文献
905.
906.
Takato Nakayama Mitsuo Horita Tadayuki Shimanuki 《Journal of General Plant Pathology》2007,73(4):229-234
We investigated soil contamination by Spongospora subterranea f. sp. subterranea (Sss) and disease severity of powdery scab in 29 potato fields in Hokkaido, Japan, using a hydroponic culture method with
tomato seedlings as bait plants. The quantity of Sss infection on the roots of bait plants was evaluated using the polymerase
chain reaction (PCR) and expressed in terms of the infection potential in the soil. The infection potential was positively
correlated with the disease severity of harvested tubers, whereas the spore ball density determined using PCR had an indistinct
relationship with disease severity. The infection potential can be useful in evaluating soil contamination and in applying
countermeasures against powdery scab. 相似文献
907.
Management of nematodes and a stalk borer by increasing within-field sugarcane cultivar diversity 总被引:1,自引:0,他引:1
The aim of this study was to determine whether an increase in plant diversity could modify the structure of the nematode community in favour of less pathogenic species and whether it affected the lepidopteran stalk borer, Eldana saccharina . Four sugarcane cultivars were planted either singly or as a mixture in the furrow. Nematodes were enumerated when the crops were 6 months old for five successive 10 to 13 month old crop cycles. Results showed that the mixture of cultivars had little significant impact on the overall abundance of the plant-parasitic or free-living nematodes. However, the nematode community structure within the mixture was slightly different to that observed on any single cultivar planted on its own and there was a higher than expected proportion of Helicotylenchus dihystera during the first three crops. With successive crops and in all treatments, there was an overall trend of increasing proportions of Meloidogyne javanica in the nematode community. However, the way that the change in the species-balance occurred in the mixed cultivar plots suggested a resistance to the evolving dominance of M. javanica . These results showed that sugarcane cultivars interacted within the rhizosphere when planted in a mixture because the observed nematode community in the mixture differed from that expected. The level of damage caused by E. saccharina was slightly lower in the mixture than expected from the actual cultivar composition. 相似文献
908.
909.
枯草芽孢杆菌抗菌物质对镰刀菌抑制机理的镜下研究 总被引:18,自引:1,他引:17
将枯草芽孢杆菌B-903菌株液体培养72h后,将培养滤液高温灭菌,以不同比例将培养滤液加入镰刀菌孢子悬浮液中,置于扫描电镜和光学显微镜下定时观察。在处理12h后,镰刀菌孢子芽管顶端、菌丝末端及菌丝中央的多处细胞均可发生畸形的球状结构,这种畸变结构随处理时间延长而增加,致使菌丝变成捻珠状。处理36h后,畸变球形细胞及菌丝纷纷断裂离解,细胞内含物渗出,致使大部分细胞成为空泡。到处理48h后,镜下几乎无完整菌丝,均成单个的球状细胞空泡,这些空泡最终胞壁崩裂,不复存在。笔者认为正是枯草芽孢杆菌在生活过程中分泌到体外的抗高温代谢物对镰刀菌孢子和菌丝细胞的这种畸变作用造成拖子和菌丝失活,侵染能力丧失。 相似文献
910.
An antisense coat protein gene confers immunity to potato leafroll virus in a genetically engineered potato 总被引:2,自引:0,他引:2
Andrzej Pałucha Włodzimierz Zagórski Mirosława Chrzanowska Danuta Hulanicka 《European journal of plant pathology / European Foundation for Plant Pathology》1998,104(3):287-293
The Bzura commercial potato cultivar was transformed by sense or antisense constructs which included the coat protein gene of potato leafroll virus RNA. In the sense construct, the coat protein gene was preceded by a leader sequence shorter than that in the subgenomic RNA formed in infected cells. The antisense construct consisted of a sequence complementary to the first 2020 nucleotides of the subgenomic RNA. Selected transformants expressing viral RNA were resistant to virus challenge by viruliferous aphids. In one line, expression of the antisense RNA prevented virus infection even after grafting with scions from infected plants and therefore this transformant might be regarded as virus immune. 相似文献