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   3408篇
  免费   282篇
  国内免费   203篇
林业   202篇
农学   193篇
基础科学   35篇
  122篇
综合类   1067篇
农作物   155篇
水产渔业   344篇
畜牧兽医   1149篇
园艺   505篇
植物保护   121篇
  2024年   8篇
  2023年   73篇
  2022年   101篇
  2021年   150篇
  2020年   156篇
  2019年   181篇
  2018年   119篇
  2017年   202篇
  2016年   207篇
  2015年   166篇
  2014年   217篇
  2013年   237篇
  2012年   238篇
  2011年   241篇
  2010年   197篇
  2009年   158篇
  2008年   170篇
  2007年   161篇
  2006年   117篇
  2005年   102篇
  2004年   84篇
  2003年   81篇
  2002年   78篇
  2001年   53篇
  2000年   58篇
  1999年   34篇
  1998年   27篇
  1997年   38篇
  1996年   28篇
  1995年   30篇
  1994年   29篇
  1993年   23篇
  1992年   22篇
  1991年   19篇
  1990年   20篇
  1989年   16篇
  1988年   8篇
  1987年   9篇
  1986年   7篇
  1985年   8篇
  1984年   4篇
  1983年   2篇
  1981年   2篇
  1980年   3篇
  1977年   1篇
  1975年   3篇
  1974年   2篇
  1973年   1篇
  1956年   2篇
排序方式: 共有3893条查询结果,搜索用时 437 毫秒
871.
872.
We tested if salicylic acid (SA) can alleviate chilling injury in anthurium flowers (Anthurium andraeanum). Cut flowers of five cultivars, individually placed in water, were held at 4 °C and 12 °C. Symptoms of chilling injury (CI) were found in the flowers stored at 4 °C. These symptoms included desiccation of the spadix (the compound floral stalk) and a colour change of the spathe (the large floral bract) to pink and then to brown. The time to the CI symptoms depended on the cultivar. CI symptoms were accompanied by an increase in electrolyte leakage, by loss of fresh weight, and by an increase in catalase (CAT) and superoxide dismutase (SOD) activity. SA at 2.0 mM in water was applied as a 15 min dip. It delayed the CI symptoms, as well as the loss of fresh weight, the increase in electrolyte leakage, and the increase CAT and SOD activity. The data suggest that CI in this system is related to an increase in the concentrations of active oxygen species.  相似文献   
873.
树木注射伤害分入针、注射及退针时的物理损伤,由药物—树木本身的生物化学反应造成的伤害、注射技术使用不当所引发的伤害5种类型。树木注射伤害是可以控制的,防止发生树木注射伤害必须选用正确的树木注射技术路线、合理控制高压注射的速度,需特别注意高压注射装置的针头密封以及进退针的方式,并选用安全、有效的药物。  相似文献   
874.
Reasons for performing study: No large scale epidemiological studies have previously quantified the occurrence of carpal, metacarpo‐ and metatarsophalangeal (MCP/MTP) joint injuries in Thoroughbred racehorses. Objectives: To develop an objective classification system for carpal and MCP/MTP joint injuries and estimate the incidence of these injuries in young Thoroughbreds in flat race training. Methods: In a prospective cohort study, data on daily exercise and veterinary‐diagnosed carpal and MCP/MTP joint injuries were collected from Thoroughbreds monitored since starting training as yearlings, for up to 2 years. Cases were classified in one of 4 categories: 1) localised to a carpal or MCP/MTP joint based on clinical examination and/or diagnostic analgesia; no diagnostic imaging performed; 2) localised to a carpal or MCP/MTP joint based on clinical examination and/or diagnostic analgesia; radiographs taken but no abnormalities detected; 3) evidence of abnormality of subchondral bone and/or articular margin(s) on diagnostic imaging and 4) evidence of discontinuity of the articular surface on diagnostic imaging. Incidence rates and rate ratios were estimated using Poisson regression, adjusting for trainer‐level clustering. Results: A total of 647 horses from 13 trainers throughout England contributed 7785 months at risk of joint injury. One‐hundred‐and‐eighty‐four cases of carpal (n = 82) or MCP/MTP (n = 102) joint injury were reported in 165 horses and classified in Category 1 (n = 21), Category 2 (n = 21), Category 3 (n = 72) or Category 4 (n = 70). The overall joint injury rate was 1.8 per 100 horse months (95% CI = 1.2, 2.8); rates did not differ significantly between 2‐ and 3‐year‐olds but females sustained Category 1 injuries at triple the rate of males (P = 0.03). Joint injury rates differed significantly between trainers (P<0.001) and there was trainer variation in anatomical site and severity of injury. Conclusions and potential relevance: Carpal and MCP/MTP joint injuries are an important cause of morbidity in Thoroughbred racehorses. Identification of modifiable risk factors for these injuries may reduce their incidence.  相似文献   
875.
Reasons for performing study: Traumatic injuries are a major cause of morbidity and mortality in the horse and consequently pose a serious threat to horses' wellbeing. To date, there have been no published studies assessing the frequency of injuries in the general horse population of the UK. Objectives: To obtain information regarding husbandry management strategies and injury prevalence in horses aged ≤15 years, with the aim of identifying predisposing risk factors for injury. Methods: A postal questionnaire was distributed to a randomly selected sample of horse owners across north‐west England, Midlands and north Wales. Factors associated with injury were assessed using univariable and multivariable logistic regression analysis performed with the binary outcome variable defined as whether or not the horse had sustained an injury within the previous 12 months. Results: A usable questionnaire response rate of 68% (652/953) was achieved. Forty percent of horses had sustained a traumatic injury within the past year, of which 62% occurred in the field and 13% during ridden exercise. Factors identified as being associated with an increased risk of traumatic injury included the following: breed other than cob or pony (P = 0.001), shorter duration of ownership (P = 0.002), being turned out with an increasing number of horses (P = 0.001), being used for competitive (P = 0.001) or Parelli (P = 0.006) purposes. Stabling at all times during the spring (P = 0.005), the use of wood fencing in paddocks (P = 0.05) and being prone to becoming distressed if left alone in a field (P = 0.04) were also found to be associated with an increased risk of injury. Stabling at all times during winter was associated with a decreased risk of injury (P = 0.006). Conclusions and potential relevance: Risk factors for sustaining injuries have been identified in association with management practices. This information may be used to educate owners regarding management of their horse(s) in order to prevent injury.  相似文献   
876.
Two novel homogeneous polysaccharides, APS-1a and APS-3a were successfully isolated from the root of Angelica sinensis. APS-1a was composed of galactose, arabinose and glucose in a relatively molar percentage of 57.34%, 27.67% and 14.98%, and had a molecular weight of 49.0 kDa, whereas APS-3a was composed of galactose, arabinose and glucose in a relatively molar percentage of 84.54%, 6.50%, and 8.96%, and had a molecular weight of 65.4 kDa. APS-1a and APS-3a mainly consisted of 1,4-linked galactose, 1,3,6-linked galactose, T-galactose and T-arabinose, and the molar ratio of each linkage was different between APS-1a and APS-3a. The bioactivity analysis showed that APS-1a and APS-3a increased the thymus and spleen index, the number of red blood cell (RBC) and white blood cell (WBC) in peripheral blood and the cellularity of bone marrow cell numbers in irradiated mice, protected mice against radiation-induced micronucleus formation in bone marrow, suggesting that polysaccharides could be used as radioprotective agents, especially for promoting bone marrow hematopoiesis.  相似文献   
877.
Akt/protein kinase B is a well-known cell survival factor and activated by many stimuli including mechanical stretching. Therefore, we evaluated the cardioprotective effect of a brief mechanical stretching of rat hearts and determined whether activation of Akt through phosphatidylinositol 3-kinase (PI3K) is involved in stretch-induced cardioprotection (SIC). Stretch preconditioning reduced infarct size and improved post-ischemic cardiac function compared to the control group. Phosphorylation of Akt and its downstream substrate, GSK-3β, was increased by mechanical stretching and completely blocked by wortmannin, a PI3K inhibitor. Treatment with lithium or SB216763 (GSK-3β inhibitors) before ischemia induction mimicked the protective effects of SIC on rat heart. Gadolinium (Gd3+), a blocker of stretch-activated ion channels (SACs), inhibited the stretch-induced phosphorylation of Akt and GSK-3β. Furthermore, SIC was abrogated by wortmannin and Gd3+. In vivo stretching induced by an aorto-caval shunt increased Akt phosphorylation and reduced myocardial infarction; these effects were diminished by wortmannin and Gd3+ pretreatment. Our results showed that mechanical stretching can provide cardioprotection against ischemia-reperfusion injury. Additionally, the activation of Akt, which might be regulated by SACs and the PI3K pathway, plays an important role in SIC.  相似文献   
878.
The aims of this study were to establish a normal reference range (mean ± 2 SD) and assess reliability of renal dimensions obtained using transabdominal ultrasonography in Thoroughbred horses (n = 7). A minimum of three ultrasonographic cineloops were obtained from each intercostal space and the left paralumbar fossa by two observers daily for three consecutive days. Renal length, width, and thickness and cortex, medulla, and pelvic dimensions were obtained. Measurements were undertaken by both observers, who were unaware of prior measurements, to assess reproducibility and measured on three separate occasions to evaluate short-term measurement repeatability. Measurements from images obtained by both operators were compared to evaluate image repeatability. The left kidney was consistently identified in the left 15th-17th intercostal space and the paralumbar fossa with maximal length in the 16th intercostal space (12.7 ± 2.0 cm) and maximal width in the paralumbar fossa (7.9 ± 1.1 cm). The right kidney was consistently identified in the right 15th-17th intercostal space with maximal length and maximal width in the 15th intercostal space (16.0 ± 0.7 cm and 7.9 ± 1.0 cm). Reproducibility, image repeatability, measurement repeatability were good to excellent, although were less good for the smaller structures. There were no differences in renal dimensions between horses. Overall renal ultrasonography was reliable and a normal reference range for Thoroughbred horses was established. Renal dimensions vary between rib spaces. As repeatability and reproducibility were excellent for renal length and width, it may be prudent to use those measurements in rib spaces where parameters were maximal.  相似文献   
879.
为探讨血管内皮生长因子(VEGF)在小鼠亚硝酸盐中毒组织损伤中的作用,将20只小鼠随机分为对照组和亚硝酸盐中毒组,分别注射0.3mL的生理盐水和50g/L亚硝酸钠。中毒组小鼠死亡后,处死对照组小鼠,采取心、肝、脾、肺、肾、脑等组织,制作石蜡切片,进行HE染色和VEGF免疫组化染色,镜下观察组织病变以及VEGF蛋白的分布特点。结果显示,亚硝酸盐中毒可引起小鼠机体产生明显的缺氧症状,心、肝、脾、肺、肾等组织都有不同程度的病理损伤,主要表现为水肿和出血;各组织均观察到VEGF阳性信号,在出血病变区域阳性信号最为明显。说明亚硝酸盐中毒可造成机体各器官不同程度的病理损伤,并引起小鼠体内VEGF表达量的增加,VEGF对器官的病理变化有明显的促进作用。  相似文献   
880.
Vertebral fractures in horses are relatively common and often occur due to trauma. Clinical signs may range from neck pain and mild neurological dysfunction to tetraplegia and death. Severity of signs and prognosis depends on extent of damage to the spinal cord. In this Case Report, serial radiographs, which track the bony changes associated with fracture healing, were performed over one year. Although surgical techniques have been described to repair fractures to the vertebrae, patients with minimal neurological deficit may have a favourable outcome with conservative medical management, as in this case.  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