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   435篇
  免费   20篇
  国内免费   2篇
林业   49篇
农学   16篇
基础科学   1篇
  149篇
综合类   29篇
农作物   24篇
水产渔业   14篇
畜牧兽医   142篇
园艺   6篇
植物保护   27篇
  2023年   6篇
  2022年   2篇
  2021年   12篇
  2020年   12篇
  2019年   15篇
  2018年   15篇
  2017年   10篇
  2016年   9篇
  2015年   10篇
  2014年   22篇
  2013年   32篇
  2012年   26篇
  2011年   43篇
  2010年   12篇
  2009年   21篇
  2008年   24篇
  2007年   24篇
  2006年   25篇
  2005年   27篇
  2004年   23篇
  2003年   26篇
  2002年   21篇
  2001年   7篇
  2000年   2篇
  1999年   4篇
  1998年   4篇
  1997年   1篇
  1996年   4篇
  1993年   4篇
  1992年   2篇
  1991年   1篇
  1990年   3篇
  1989年   1篇
  1987年   1篇
  1983年   1篇
  1980年   1篇
  1978年   1篇
  1977年   2篇
  1970年   1篇
排序方式: 共有457条查询结果,搜索用时 0 毫秒
391.
An overview is given of decision making frameworks for Ecological Risk Assessment (ERA) used for sediment in a number of European countries. These frameworks fall into two categories:
•  Biological Effects-Based Assessment of in situ risks (referred to as in situ BEBA);
•  Biological Effects-Based Assessment of the ex situ quality of dredged sediments (referred to as ex situ BEBA).
The first approach is usually part of an evaluation of whether remediation is needed in order to control or reduce the ecological risks of sediment pollution in a given location. The purpose of the second approach is to evaluate the risks of possible (unconfined) disposal options for dredged sediment (including sediment that is dredged for navigational reasons). Important aspects are:
•  Objectives for sediment management;
•  The level of integration of BEBA in legal frameworks;
•  The use of chemical (numeric) SQG’s in BEBA and their integration with biological information;
•  The criteria used to infer effects and to classify sediment quality.
  相似文献   
392.
Different food production methods may result in differences in the content of secondary metabolites such as polyphenolic compounds. The present study compared conventionally (CPD) and organically produced (OPD) diets in a human crossover intervention study (n = 16) with respect to the intake and excretion of five selected flavonoids and effect on markers of oxidative defense. The urinary excretion of quercetin and kaempferol was higher after 22 days of intake of the OPD when compared to the CPD (P < 0.05). The excretions of flavonoids in urine as a percentage of intake (0.6-4%) were similar after both interventions. Most markers of antioxidative defense did not differ between the diets, but intake of OPD resulted in an increased protein oxidation and a decreased total plasma antioxidant capacity compared to baseline (P < 0.05). Some varietal difference was seen in the study, and because selection of more resistant varieties is of central importance to organic farming, it cannot be excluded that the observed effects originate from these differences. The food production method affected the content of the major flavonoid, quercetin, in foods and also affected urinary flavonoids and markers of oxidation in humans.  相似文献   
393.
-  Dedicated to Prof. Dr. Ulrich Förstner on his 65th birthdayBackground, Aims and Scope   The main challenge in the application of biotests for hazard assessment of sediments is a transparent interpretation, especially if combinations of biotests are applied and an integrated assessment of all results is wanted. For decision making, assigning results of biotesting into different classes that indicate increasing hazards can be a useful tool. In this paper we suggest an approach on how to set up a site-independent classification system for sediment toxicity tests.Methods   About 250 sediment samples were collected from two rivers and evaluated. The bioassays applied are used in a standardized procedure for all samples over years. The test battery include toxicological endpoints like nematode growth and reproduction, algae growth, bacteria activity, and luminescence inhibition.Results and Discussion   The classification system described emphasizes the following steps: the assessment of test-specific response spans for each applied biotest and the subsequent interpretation of inhibition values in terms of toxicity, estimation of accuracy and uncertainty of different test systems, and the use of cluster- and k-means analyses in order to identify occurring pattern within a large biotest data base, followed by a ranking. The outcome of the procedure were 5 effect classes with an increasing potential of hazard. Conclusion   The effect classes represent the result of a logical procedure of deduction. The procedure is transparent giving an increased degree of certainty. The site independent ranking of 5 classes could be used for sediment monitoring and facilitate fast interpretation. Recommendation and Perspective   A large database comprising differently contaminated sediments is the precondition for setting up effect classes, because it is essential to know the range of effects and the variability of the test methods. The proposed procedure could then provide the basic rules for designing an expert system.  相似文献   
394.
The relationship between the physical structure of espresso coffee foam, called crema, and the above-the-cup aroma release was studied. Espresso coffee samples were produced using the Nespresso extraction system. The samples were extracted with water with different levels of mineral content, which resulted in liquid phases with similar volatile profiles but foams with different structure properties. The structure parameters foam volume, foam drainage, and lamella film thickness at the foam surface were quantified using computer-assisted microscopic image analysis and a digital caliper. The above-the-cup volatile concentration was measured online by using PTR-MS and headspace sampling. A correlation study was done between crema structure parameters and above-the-cup volatile concentration. In the first 2.5 min after the start of the coffee extraction, the presence of foam induced an increase of concentration of selected volatile markers, independently if the crema was of high or low stability. At times longer than 2.5 min, the aroma marker concentration depends on both the stability of the crema and the volatility of the specific aroma compounds. Mechanisms of above-the-cup volatile release involved gas bubble stability, evaporation, and diffusion. It was concluded that after the initial aroma burst (during the first 2-3 min after the beginning of extraction), for the present sample space a crema of high stability provides a stronger aroma barrier over several minutes.  相似文献   
395.
Endothelial anti-inflammatory effects of ac?ai? (Ac) and red muscadine grape (Gp) polyphenolics have not been extensively investigated. It was hypothesized that polyphenolics from Ac and Gp exert comparable protective effects in human vascular endothelial cells (HUVEC) upon inflammatory stress. Furthermore, this study investigated whether microRNAs relevant to endothelial function might be regulated by Ac and Gp. Results showed that Ac and Gp (5-20 mg gallic acid equivalent/L) protected HUVEC against glucose-induced oxidative stress and inflammation. Glucose-induced expression of interleukin-6 and -8 was down-regulated by Ac and Gp at mRNA and protein levels. Upon lipopolysaccharide (LPS; 1 μg/L)-induced inflammation, Ac and Gp inhibited gene expression of adhesion molecules and NF-κB activation to similar extents, although Gp was more effective in decreasing PECAM-1 and ICAM-1 protein. Of the screened microRNAs, only microRNA-126 expression was found to be modulated by Ac and Gp as the underlying mechanism to inhibit gene and protein expression of VCAM-1.  相似文献   
396.
In the present study, we addressed the question whether cyanidin-3-glucoside (C3G) or complex C3G-rich blackberry extracts affect human topoisomerases with special emphasis on the contribution of the potential degradation products phloroglucinol aldehyde (PGA) and protocatechuic acid (PCA). In HT29 colon carcinoma cells a C3G-rich blackberry extract suppressed camptothecin- (CPT-) or doxorubicin- (DOX-) induced stabilization of the covalent DNA-topoisomerase intermediate, thus antagonizing the effects of these classical topoisomerase poisons on DNA integrity. As a single compound, C3G (100 μM) decreased the DNA-damaging effects of CPT as well, but did not significantly affect those induced by DOX. At the highest applied concentration (100 μM), cyanidin protected DNA from CPT- and DOX-induced damage. Earlier reports on DNA-damaging properties of cyanidin were found to result most likely from the formation of hydrogen peroxide as an artifact in the cell culture medium when the incubation was performed in the absence of catalase. The suppression of hydrogen peroxide accumulation, achieved by the addition of catalase, demonstrated that cyanidin does not exhibit DNA-damaging properties in HT29 cells (up to 100 μM). The observed effects on topoisomerase interference and DNA protection against CPT or DOX were clearly limited to the parent compound and were not observed for the potential cyanidin degradation products PGA and PCA.  相似文献   
397.
The objective of this study was to examine the effect of an anaesthesia using 70% carbon dioxide and 30% oxygen on endocrine stress reaction, behaviour and clinical parameters of male suckling piglets during castration. One hundred and seventy one male piglets, three to five days of age, were allocated to two experiments. They were assigned either to the procedures control handling, control castration, handling under anaesthesia or castration under anaesthesia in each experiment. In Experiment 1, adrenaline and noradrenaline plasma concentrations were measured in blood samples taken before (-15 min) and after (immediately, 2 min) handling/castration. In Experiment 2, behavioural observations and clinical parameters such as heart and respiratory rate, oxygen saturation, reflexes and recovery time were assessed at several sampling times. Measurement of adrenaline and noradrenaline concentrations revealed an increase in all groups after handling/castration (p < 0.0167), but higher concentrations were seen in the anaesthetized groups (25 to 93 times) than in control groups (two to four times). The excessive endocrine reaction suggests that carbon dioxide inhalation causes a more stressful situation in piglets compared to castration without anaesthesia. Behavioural abnormalities, significant decreases in the heart rate, the respiratory rate and the oxygen saturation (p < or = 0.001) including a cardiac arrhythmia (extrasystoles) underline the impression that CO2 inhalation anaesthesia negatively affects animal welfare. Based on the results of this study, this anaesthetic method is unsuitable to reduce stress induced by castration. Further research on alternatives is necessary to ensure the well-being of the piglets during castration.  相似文献   
398.
399.
With the aim to extend the present knowledge on possible systemic spreading of Pasteurella multocida in pigs with bronchopneumonia, the occurrence and associated lesions of P. multocida were described by comparing cultural detection, pathological evaluation and in situ hybridization of P. multocida in lungs, hearts and kidneys from cases of porcine bronchopneumonia. P. multocida was cultivated from the lung lesions in 114 out of a total of 148 cases of porcine bronchopneumonia. Among the 114 cases, P. multocida was also cultivated from the pericardial sacs of 40 pigs and the kidneys of seven pigs. Gross lesions and histological findings included a variety of type and stages of bronchopneumonia in connection to the isolation of P. multocida. Furthermore, chronic fibrous pericarditis, interstitial nephritis and a high proportion of lympho-histocytic nephritis were observed. In situ hybridization identified P. multocida in the majority of the lungs, none of the hearts and in half of the kidneys examined. The results show a possible low rate of systemic spreading of P. multocida from lung lesions in pigs with bronchopneumonia.  相似文献   
400.
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