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   272篇
  免费   7篇
林业   14篇
农学   14篇
  63篇
综合类   23篇
农作物   9篇
水产渔业   16篇
畜牧兽医   90篇
园艺   4篇
植物保护   46篇
  2022年   1篇
  2021年   6篇
  2020年   3篇
  2019年   5篇
  2018年   5篇
  2017年   5篇
  2016年   6篇
  2015年   10篇
  2014年   13篇
  2013年   18篇
  2012年   13篇
  2011年   22篇
  2010年   3篇
  2009年   21篇
  2008年   20篇
  2007年   22篇
  2006年   20篇
  2005年   25篇
  2004年   14篇
  2003年   11篇
  2002年   7篇
  2001年   8篇
  2000年   3篇
  1999年   5篇
  1997年   3篇
  1996年   1篇
  1990年   2篇
  1989年   1篇
  1984年   1篇
  1983年   2篇
  1971年   1篇
  1970年   1篇
  1968年   1篇
排序方式: 共有279条查询结果,搜索用时 31 毫秒
91.
Interspecific hybrids were efficiently produced in the cross-incompatible combination between Alstroemeria pelegrina L. var. rosea and A. magenta Bayer by culturing immature ovules with placenta 7–14 days after pollination on 2 g/l Gelrite-solidified MS medium containing 3% (w/v)sucrose. The plants showed intermediate characteristics between the parents and their hybridity was confirmed by karyotype and DNA analyses. The mean number of chromosome association per PMC at metaphase I was 2.60I+6.70II, pollen stainability was20.8%, and they produced viable seeds after self-pollination. Furthermore, mature plants were obtained when the hybrids were backcrossed as male parents with both the parents. The backcross-progeny from A. pelegrina var. rosea × hybrids exhibited 3.8 to 79.7% pollen stainability and that from A. magenta × hybrids 78.8 to 98.3%. Almost all of these plants produced viable seeds after self-pollination, which implies that they can beutilized for breeding of novel cultivars of Alstroemeria. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
92.
93.

Purpose

Understanding the transport behaviour of fine cohesive sediment is fundamental to the sustainable management of aquatic environments. Sediment tracing techniques are widely used for measuring the transport pathways of sand-sized material in the field. However, the development of tracers, including geochemically labelled clays, for fine, cohesive sediment is more problematic. Such tracers should have chemical signatures that can be easily detected following significant dilution in the field and should remain constant for the duration of the tracer study.

Materials and methods

We have examined the potential of rare earth element (REE)-labelled phlogopite and hydrobiotite as cohesive sediment tracers. Clays were first treated with sodium tetraphenylborate to extract interlayer potassium and enhance their cation exchange capacity. Ho, La and Sm were then sorbed to the clays in batch experiments. Desorption of the chemical signature in both fresh and saline conditions was examined after 1 and 10 days.

Results and discussion

Potassium extraction enhanced REE sorption, resulting in REE concentrations in excess of 40,000 mg kg?1 in the labelled clays, and these signatures should be easily detected following dilution in the aquatic environment. In both fresh and saline conditions, over 90% of the tracer signature was retained over a 1-day period. However, over longer time scales, there was considerable loss of the REE signature.

Conclusions

Over short time scales, there is considerable potential to use these materials as cohesive sediment tracers. Over longer time scales, although much of the label is lost, the tracers could still provide qualitative information identifying net sediment transport pathways.  相似文献   
94.
Cocoa powder is rich in polyphenols, such as catechins and procyanidins, and has been shown to inhibit low-density lipoprotein (LDL) oxidation and atherogenesis in a variety of models. Human studies have also shown daily intake of cocoa increases plasma high-density lipoprotein (HDL) and decreases LDL levels. However, the mechanisms responsible for these effects of cocoa on cholesterol metabolism have yet to be fully elucidated. The present study investigated the effects of cacao polyphenols on the production of apolipoproteins A1 and B in human hepatoma HepG2 and intestinal Caco2 cell lines. The cultured HepG2 cells or Caco2 cells were incubated for 24 h in the presence of cacao polyphenols such as (-)-epicatechin, (+)-catechin, procyanidin B2, procyanidin C1, and cinnamtannin A2. The concentration of apolipoproteins in the cell culture media was quantified using an enzyme-linked immunoassay, and the mRNA expression was quantified by RT-PCR. Cacao polyphenols increased apolipoprotein A1 protein levels and mRNA expression, even though apolipoprotein B protein and the mRNA expression were slightly decreased in both HepG2 cells and Caco2 cells. In addition, cacao polyphenols increased sterol regulatory element binding proteins (SREBPs) and activated LDL receptors in HepG2 cells. These results suggest that cacao polyphenols may increase the production of mature form SREBPs and LDL receptor activity, thereby increasing ApoA1 and decreasing ApoB levels. These results elucidate a novel mechanism by which HDL cholesterol levels become elevated with daily cocoa intake.  相似文献   
95.
Staphylococcus intermedius isolates (n=106), including 44 dog isolates and 62 pigeon isolates, were examined for their susceptibility to ampicillin, cephalexin, erythromycin, gentamicin, kanamycin, lincomycin, norfloxacin, oxacillin, tetracycline, and vancomycin by standard disk-diffusion test. The frequencies of resistance to ampicillin, kanamycin, and tetracycline were significantly higher in dog isolates than pigeon isolates (95.5% vs. 0%, 31.8% vs. 0%, and 45.5% vs. 9.7%, respectively; P<0.01). Antimicrobial resistance patterns of dog isolates and pigeon isolates were categorized respectively into nine and five distinct profiles. Significantly higher occurrence of resistance to two or more antimicrobials was observed in dog isolates than pigeon isolates (54.5% vs. 12.9%; P<0.01) and also in domesticated pigeon isolates than non-domesticated pigeon isolates (53.3% vs. 0%; P<0.01).  相似文献   
96.
It has been suspected that in comparison with glucose or fatty acids, the levels of amino acids may readily change with different forms of exercise. In the present study, we measured the concentrations of amino acids, glucose, triglycerides, total protein and total cholesterol in the blood and/or cerebrospinal fluid (CSF) of rats subjected to forced running exercise on a treadmill, and voluntary running exercise using a wheel, with a constant running distance of 440 m. Rats that performed no running and rats subjected to immobilization stress were used as controls. We observed a few significant changes in the levels of plasma glucose, triglycerides, total protein and total cholesterol in all groups. Whereas, plasma amino acid levels were significantly changed by exercise and stress, especially during the light period. The plasma levels of many amino acids were specifically increased by forced running; some were decreased by immobilization stress. Few amino acids showed similar changes in their levels as a result of voluntary running. In addition, there was a significant difference in the degree of amino acid imbalance between blood and CSF. These results provide the first information on changes in levels of amino acids in plasma and CSF resulting from forced and voluntary exercises.  相似文献   
97.
The contamination of hazardous metal(loid) is one of the serious environmental and human health risks. This study isolated a total of 40 cadmium (Cd)- and arsenic (As)-resistant bacterial isolates from coastal sediments by pour plate technique using tryptic soy agar supplemented with Cd or As (50 mg l?1) for use as metal(loid) bioremediation agents. Out of 40, 4 isolates, RCd3, RCd6, RAs7, and RAs10, showed a relatively higher growth rate in Cd- or As-supplemented culture media which were selected for further study. The selected isolates showed a high minimum inhibitory concentration (60–400 mg l?1 for Cd and 400–2200 mg l?1 for As), which demonstrated their remarkable Cd and As resistance capabilities. The metal(loid) removal efficiencies (0.032–0.268 μg Cd h?1 mg?1 and 0.0003–0.0172 μg As h?1 mg?1 [wet weight cell]) of selected isolates indicated their greater magnitude in absorbing Cd compared to As from water. Phylogenetic analysis of the 16S rDNA sequences revealed that isolates RCd3, RCd6, RAs7, and RAs10 were closely related to Acinetobacter brisouii, Pseudomonas abietaniphila, Exiguobacterium aestuarii, and Planococcus rifietoensis, respectively. Because of high Cd and As resistance and removal efficiency, the selected isolates can survive in a high metal(loid)-contaminated environment and could be a potential tool for bioremediation of high metal(loid)-contaminated effluents to protect the aquatic environment.  相似文献   
98.
99.
100.
Sorption of ionizable compounds of pyridines and aromatic carboxylic acids into low-density polyethylene (LDPE) film was investigated as a function of pH ranging from 4 to 7. The sorptions for pyridines were increased with increasing pH. Within the range examined, pH 7 was observed to promote the highest degree of sorption. When the pH increased by one unit from 6 to 7, the sorptions for 2-propylpyridine and 2,4,6-trimethylpyridine were both increased approximately 3.8 and 10.5 times, respectively. At pH 4, the sorption for the pyridines entirely disappeared. In contrast, the sorptions for aromatic carboxylic acids increased with decreasing pH. Within the range examined, pH 4 was observed to promote the highest degree of sorption. The magnitude of sorption for pyridines and carboxylic acids apparently depended on the affinity (delta(c)) of these compounds for LDPE film. Another factor affecting the sorption at various pHs was the pK(a) of these compounds; the sorption greatly decreased with the ionization degree of these compounds. To elucidate the phenomena, the following thermodynamic sorption equation was applied: S = S(0)gamma exp[V(v){(delta(w) - delta(v))(2) - delta(c)(2))/RT}. The plots of ln S for pyridines vs the term of the equation gave an insufficient relationship (r = 0.519). In contrast, taking into account the ratio (chi(i)()) of concentration of un-ionized compound to total concentration, the improvement for the equation was made: S' = S/chi(i)() = S(0)gamma exp[V(v){(delta(w) - delta(v))(2) - delta(c)(2))/RT}. The plots of ln S' vs the term of the equation gave a better relationship (r = 0.884). Furthermore, the equation was also applicable for the sorption behavior of carboxylic acids into LDPE (r = 0.769).  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