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   377篇
  免费   12篇
  国内免费   4篇
林业   17篇
农学   22篇
基础科学   1篇
  101篇
综合类   44篇
农作物   33篇
水产渔业   42篇
畜牧兽医   96篇
园艺   7篇
植物保护   30篇
  2023年   2篇
  2022年   4篇
  2021年   8篇
  2020年   4篇
  2019年   8篇
  2018年   6篇
  2017年   5篇
  2016年   9篇
  2015年   9篇
  2014年   13篇
  2013年   23篇
  2012年   34篇
  2011年   32篇
  2010年   13篇
  2009年   11篇
  2008年   25篇
  2007年   39篇
  2006年   29篇
  2005年   12篇
  2004年   11篇
  2003年   7篇
  2002年   7篇
  2001年   9篇
  2000年   5篇
  1999年   3篇
  1998年   3篇
  1997年   3篇
  1994年   2篇
  1992年   5篇
  1991年   3篇
  1990年   2篇
  1989年   4篇
  1987年   2篇
  1985年   2篇
  1980年   3篇
  1979年   4篇
  1978年   2篇
  1974年   2篇
  1973年   2篇
  1972年   2篇
  1971年   3篇
  1969年   1篇
  1968年   1篇
  1966年   1篇
  1965年   1篇
  1964年   3篇
  1963年   2篇
  1962年   1篇
  1930年   2篇
  1929年   1篇
排序方式: 共有393条查询结果,搜索用时 656 毫秒
121.
High levels of ozone and acidic fog have been measured along the coast of Maine, U.S.A. Some of the ozone and nitrate are a consequence of direct transport from East Coast metropolitan centers, but we discuss evidence supporting N2O5, and perhaps ozone, generation at forested sites from NOx precursors. Fogs with pH below 3.3 occur with greatest frequency in the mid-coast region. Ozone is consistently highest along the southwest coast, but peak hourly averages are often highest in the mid-coast. Historical evidence suggests an increase in fog acidity and nitrate levels, but no change in sulphate within the past 48 years. Red spruce with decline symptoms can be observed from the south-coast to mid-coast region, with the greatest symptom development in the mid-coast. Trees in the mid-coast on soils with high base saturation and Ca/Al ratios develop symptoms of late winter injury (reddening and casting of current year needles in late winter and spring). Trees on poorer soils develop chlorosis on upper surfaces of older needles. Nutrient analysis of soils and foliage reveals differences between stressed and unstressed sites. Scanning electron microscopy of needle surfaces also reveals response differences between pollutant stressed and unstressed trees. Hypotheses and research directions are discussed.  相似文献   
122.
Using intumescent coatings on wood-based materials is an effective method for fire safety. Previous studies have demonstrated that the formulation of components strongly influences the performance of coatings. This study investigated the effect of intumescent formulation of vinyl acetate-acrylic coating on flame retardancy of plywood. The fire retardancy of materials was assessed by both heat release and CO and CO2 emissions. The CO and CO2 emissions have not been used frequently to rank materials; the highly toxic CO and CO2 may cause most fire fatalities. The fire retardancy of coatings on plywood was assessed by a cone calorimeter. Total heat release and time to peak heat release rate were the two primary parameters. The data show that low contents of binder resin (BR) and foam producing substance (FPS) decreased total heat release and lengthened time to peak heat release rate. Additionally, low BR and FPS content can form an ideal char layer. The ideal char layer significantly decreased the CO and CO2 emission. The mechanism to achieve better fire performance was verified by thermogravimetrical analysis exhibiting lower weight loss. Moreover, evaluated by 31P NMR, the low BR and FPS content can extend the survival duration of phosphor-carbonaceous chars. The results provide information for designing vinyl acetate-acrylic emulsion coating.  相似文献   
123.
Despite its medical use, little is known about the mechanisms underlying amikacin-induced embryotoxicity, including fin reduction, in zebrafish. In this study, we examined the expression of well-known autophagy markers mTOR (target of rapamycin), atg10 (autophagy-related gene), atg12 and LC3 (mammalian homolog of Atg8) in amikacin-treated zebrafish embryos. Our results indicated that the mRNA expression level of atg12 in the amikacin-treated group was significantly increased by 1.5-fold (p<0.05) compared with the corresponding mock control group, while the expression levels of atg10 and mTOR were significantly decreased by 0.74-fold (p<0.05) and 0.58-fold (p<0.05), respectively. Western blot analysis revealed that LC3 protein expression was induced by amikacin. Taken together, these data suggest that amikacin-induced fin reduction is mediated by fin cell autophagy.  相似文献   
124.
The aim of this study is to examine the efficacy of the coconut fiber on the sound absorption and thermal insulation performance towards the composite nonwoven fabrics. The 2D polyester fiber and 12D fire retardant three-dimensional hollow crimp polyester fiber are individually mixed with 4D low-melting point polyester fiber (4DLMf) to produce 2D polyester nonwoven fabric (2D-PETF) and 12D polyester nonwoven fabric (12D-PETF) respectively. Subsequently, the coconut fiber (CF) is then laminated with the 2D-PETF and 12D-PETF to fabricate two types of PET/CF composite boards through the multiple needle-punching techniques. Accordingly, the sound absorption, thermal insulation, Limiting Oxygen Index and relative mechanical properties of the PET/CF composite boards are evaluated properly. The experimental results reveal that both types of PET/CF composite boards possess excellent thermal insulation performance and fire resistance property. Also, for both types of PET/CF composite boards, the average sound absorption coefficient increases with the increased amount of CF.  相似文献   
125.
A new spatane diterpenoid, leptoclalin A (1), along with two previously reported known norcembranoid diterpenes (2 and 3), were isolated from a cultured soft coral Sinularia leptoclados. The structures were determined by extensive spectroscopic analyses and by comparison with the spectral data of related known compounds. Metabolite 1 is rarely found in spatane skeletons reported from soft corals. In addition, compound 1 exhibited weak cytotoxicity towards human tumor cell lines T-47 D and K-562.  相似文献   
126.
ObjectiveTo compare the postoperative analgesic effects of intravenous (IV) lidocaine, meloxicam, and their combination in dogs undergoing ovariohysterectomy.Study designProspective, randomized, double‐blind, controlled clinical trial.AnimalsTwenty‐seven dogs aged (mean ± SD) 16.1 ± 7.5 months and weighing 22.4 ± 17.9 kg scheduled for ovariohysterectomy.MethodsAnaesthesia was induced with propofol and maintained with isoflurane. Dogs (n = 9 in each group) were allocated to receive just prior to and during surgery one of the following regimens: M group, 0.2 mg kg?1 IV meloxicam then a continuous rate infusion (CRI) of lactated Ringer's at 10 mL kg?1 hour?1; L group, a bolus of lidocaine (1 mg kg?1 IV) then a CRI of lidocaine at 0.025 mg kg?1 minute?1; and M + L group, both the above meloxicam and lidocaine treatments. Pain and sedation were scored, and venous samples taken for serum cortisol and glucose measurement before and at intervals for 12 hours after anaesthesia. Pain scores were assessed using a multi‐parameter subjective scoring scale (cumulative scale 0–21) by three observers. The protocol stated that dogs with a total score exceeding 9 or a sub‐score above 3 in any one category would receive rescue analgesia. Sedation was scored on a scale of 0–4.ResultsThere were no significant differences in subjective pain scores, serum cortisol, and glucose concentrations between the three groups. The highest pain score at any time was 5, and no dog required rescue analgesia. None of the three regimens caused any observable side effects during or after anaesthesia. At 1 and 2 hours after extubation dogs in group L were significantly more sedated than in the other two groups.Conclusions and Clinical relevanceThis study suggests that, with the scoring system used, IV lidocaine and meloxicam provide similar and adequate post‐operative analgesia in healthy dogs undergoing ovariohysterectomy.  相似文献   
127.
128.
Renal protective effects of naringenin at 0.5, 1, and 2% of the diet in diabetic mice were examined. Naringenin supplemented at 1 and 2% increased its deposit in liver and kidney of diabetic mice. Compared with the diabetic control group, naringenin treatments at 1 and 2% lowered plasma levels of glucose and blood urea nitrogen, as well as increased insulin level and creatinine clearance (P < 0.05). Naringenin treatments dose-dependently reduced renal tumor necrosis factor-α level and expression (P < 0.05) but only at 1 and 2% significantly decreased production and expression of interleukin (IL)-1β, IL-6, and monocyte chemoattractant protein-1 (P < 0.05). Naringenin intake at 2% decreased renal formation and expression of type IV collagen, fibronectin, and transforming growth factor-β1 (P < 0.05). This compound at 1 and 2% lowered protein kinase C activity and suppressed nuclear factor κB (NF-κB) p65 activity, mRNA expression, and protein production in kidney. However, this agent only at 2% diminished NF-κB p50 activity, mRNA expression, and protein production (P < 0.05). These results indicate that naringenin could attenuate diabetic nephropathy via its anti-inflammatory and antifibrotic activities.  相似文献   
129.
Fish gelatin and its hydrolysates exhibit a variety of biological characteristics, which include antihypertensive and antioxidant properties. In this study, fish gelatins were extracted from extrusion-pretreated tilapia scales, and then subjected to analyses to determine the physicochemical properties and antioxidant activity of the extracted gelatins. Our findings indicate that TSG2 (preconditioned with 1.26% citric acid) possessed the greatest extraction yield, as well as higher antioxidant activities compared with the other extracted gelatins. Hence, TSG2 was subjected to further hydrolyzation using different proteases and ultrafiltration conditions, which yielded four gelatin hydrolysates: TSGH1, TSGH2, TSGH3, and TSGH4. The results showed that TSGH4 (Pepsin + Pancreatin and ultrafiltration < 3000 Da) had a higher yield and greater antioxidant activity in comparison with the other gelatin hydrolysates. As such, TSGH4 was subjected to further fractionation using a Superdex peptide column and two-stage reverse-phase column HPLC chromatography, yielding a subfraction TSGH4-6-2-b, which possessed the highest 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity compared with the other fractions. Further LC-ESI/MS/MS analysis of TSGH4-6-2-b suggested two novel peptides (GYDEY and EPGKSGEQGAPGEAGAP), which could have potential as naturally-occurring peptides with antioxidant properties. These promising results suggest that these antioxidant peptides could have applications in food products, nutraceuticals, and cosmetics.  相似文献   
130.
This study elucidates a vehicle routing problem with a single depot and multi-customers, in which a simulated annealing approach is used for transporting live aquaculture products. In contrast with previous models that focused on cost, transportation distance, or time, this study separately optimizes a sole depot with multi-customer flows by using a mathematical model that considers transportation, survival rates, and cost classifications of live aquaculture products. Specifically, the simulated annealing approach is adopted to facilitate the multi-customer transport vehicle routing problem on carrier selections and their interactions. The proposed model is applied to several relevant scenarios with cost-effectiveness. The computational results demonstrate the effectiveness of the model for solving the problem of transporting live aquaculture products.  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