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   800篇
  免费   39篇
林业   55篇
农学   17篇
基础科学   1篇
  238篇
综合类   43篇
农作物   44篇
水产渔业   106篇
畜牧兽医   284篇
园艺   16篇
植物保护   35篇
  2024年   2篇
  2023年   9篇
  2022年   10篇
  2021年   28篇
  2020年   31篇
  2019年   42篇
  2018年   28篇
  2017年   20篇
  2016年   35篇
  2015年   18篇
  2014年   21篇
  2013年   60篇
  2012年   76篇
  2011年   64篇
  2010年   32篇
  2009年   29篇
  2008年   43篇
  2007年   52篇
  2006年   46篇
  2005年   43篇
  2004年   33篇
  2003年   36篇
  2002年   33篇
  2001年   4篇
  2000年   2篇
  1999年   7篇
  1998年   3篇
  1996年   1篇
  1995年   2篇
  1992年   4篇
  1989年   3篇
  1988年   1篇
  1987年   3篇
  1986年   2篇
  1982年   2篇
  1981年   1篇
  1980年   1篇
  1979年   4篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
  1972年   2篇
  1914年   1篇
  1902年   1篇
排序方式: 共有839条查询结果,搜索用时 15 毫秒
61.
Summary

Oil contents and fatty acid compositions were determined in 17 almond cultivars growing under two different Mediterranean conditions, in northeast Spain and in central Morocco. The major differences were irrigation management in Spain and higher temperatures in Morocco. Although a cultivar effect was observed for some compounds, the general trend was for an increase in total oil content [58.65% vs. 55.58% (w/w)] and in the percentage of oleic acid [71.1% vs. 68.62% (w/w)] in those kernels harvested in Spain, probably due to having lower temperatures and better water status. As the total oil content and level of oleic acid in almond oil are the major parameters for higher quality almond kernels, these results emphasise the need to optimise the management of almond orchards to improve kernel quality, not only from the commercial and industrial points of view, but also because of their healthgiving properties.  相似文献   
62.
Although low water use landscaping is becoming common in arid regions, little is known about drought tolerance and drought responses of many ornamental plants, especially herbaceous perennials. Drought responses were assessed for six herbaceous ornamental landscape perennials in a 38 l pot-in-pot system in northern Utah over a 2-year period. The first year was an establishment period. During the second year, drought responses were evaluated for established Echinacea purpurea (L.) Moench, Gaillardia aristata Pursh, Lavandula angustifolia P. Mill., Leucanthemum × superbum (J.W. Ingram) Berg. ex Kent, ‘Alaska’, Penstemon barbatus Roth var. praecox nanus rondo, and Penstemon × mexicali Mitch. ‘Red Rocks’. Plants were irrigated at frequencies of 1 (control), 2, or 4 weeks between June and September, simulating well-watered conditions, moderate drought, or severe drought. Osmotic potential (Ψs), gas exchange, visual quality, leaf area, and dry weight were assessed. In a confined root zone, P. barbatus showed the greatest tolerance to all levels of drought, avoiding desiccation by increasing root:shoot ratio and decreasing stomatal conductance as water became limiting. L. angustifolia and P. × mexicali showed tolerance to moderate drought conditions, but died after exposure to the first episode of severe drought. Neither G. aristata nor L. superbum were able to regulate shoot water loss effectively. Instead, both species displayed drought avoidance mechanisms, dying back when water was limiting and showing new growth after they were watered. Compared to control plants, G. aristata shoot dry weight was reduced by 50% and 84%, and L. superbum shoot dry weight was reduced by 47% and 99% for the 2- and 4-week irrigation intervals, respectively. Root dry weights were affected similarly for both species. E. purpurea exhibited poor visual quality at all irrigation intervals, in particular wilting severely in both drought treatments, but regaining turgor when watered again. P. barbatus is recommended for ornamental landscapes that receive little or no supplemental irrigation, while E. purpurea is not recommended for low water landscapes because of low visual quality under even mild drought.  相似文献   
63.

Context

Amphibians are declining worldwide and land use change to agriculture is recognized as a leading cause. Argentina is undergoing an agriculturalization process with rapid changes in landscape structure.

Objectives

We evaluated anuran response to landscape composition and configuration in two landscapes of east-central Argentina with different degrees of agriculturalization. We identified sensitive species and evaluated landscape influence on communities and individual species at two spatial scales.

Methods

We compared anuran richness, frequency of occurrence, and activity between landscapes using call surveys data from 120 sampling points from 2007 to 2009. We evaluated anuran responses to landscape structure variables estimated within 250 and 500-m radius buffers using canonical correspondence analysis and multimodel inference from a set of candidate models.

Results

Anuran richness was lower in the landscape with greater level of agriculturalization with reduced amount of forest cover and stream length. This pattern was driven by the lower occurrence and calling activity of seven out of the sixteen recorded species. Four species responded positively to the amount of forest cover and stream habitat. Three species responded positively to forest cohesion and negatively to rural housing. Two responded negatively to crop area and diversity of cover classes.

Conclusions

Anurans within agricultural landscapes of east-central Argentina are responding to landscape structure. Responses varied depending on species and study scale. Life-history traits contribute to responses differences. Our study offers a better understanding of landscape effects on anurans and can be used for land management in other areas experiencing a similar agriculturalization process.
  相似文献   
64.
The stone-like otoliths from the ears of teleost fishes are involved in balance and hearing and consist of calcium carbonate crystallites embedded in a protein framework. We report that a previously unknown gene, starmaker, is required in zebrafish for otolith morphogenesis. Reduction of starmaker activity by injection of modified antisense oligonucleotides causes a change in the crystal lattice structure and thus a change in otolith morphology. The expression pattern of starmaker, along with the presence of the protein on the growing otolith, suggest that the expression levels of starmaker control the shape of the otoliths.  相似文献   
65.
The neuron-specific protein PGP 9.5 is a ubiquitin carboxyl-terminal hydrolase   总被引:42,自引:0,他引:42  
A complementary DNA (cDNA) for ubiquitin carboxyl-terminal hydrolase isozyme L3 was cloned from human B cells. The cDNA encodes a protein of 230 amino acids with a molecular mass of 26.182 daltons. The human protein is very similar to the bovine homolog, with only three amino acids differing in over 100 residues compared. The amino acid sequence deduced from the cDNA was 54% identical to that of the neuron-specific protein PGP 9.5. Purification of bovine PGP 9.5 confirmed that it is also a ubiquitin carboxyl-terminal hydrolase. These results suggest that a family of such related proteins exists and that their expression is tissue-specific.  相似文献   
66.
High-performance liquid chromatography with fluorescence detection was used for the analysis of polycyclic aromatic hydrocarbons (PAHs), in "Diavoletto" smoked cheese. Such cheese is typically produced in the Sorrento peninsula, and it is smoked commonly with different materials of vegetable origin. The importance of the smoking generation material is proven by the attention that the EU is paying in indicating the list of wood that may be used to produce smoking flavor agents. The PAHs considered are classified as "probable human carcinogens" by the U.S. Environmental Protection Agency for sufficient data from animal bioassays. The smoked samples contained high molecular mass PAHs with different levels ranging from 0.12 to 6.21 microg/kg. The determination was carried out also on liquid smoking flavor agents, smoke-flavored cheese, and nonsmoked cheese to measure the level of contamination before the treatment.  相似文献   
67.
A method was developed using high-performance liquid chromatography to assay 4,4'-dinitrocarbanilide (DNC), the active ingredient in Nicarbazin, in eggshells collected from Canada geese fed a formulated feed fortified with Nicarbazin at doses of 0, 125, 250, and 500 microg/g. The method was developed using chicken eggshells fortified with DNC. The method was used to quantify DNC in both the shell-associated membranes and the calcified shell extracellular matrix. These values were compared to those obtained for a composite sample consisting of both the membranes and the calcified shell extracellular matrix. The validated method was used to quantify DNC in eggshells from geese fed fortified feed to ascertain the effect of Nicarbazin feed concentration on shell DNC concentration. DNC levels in the eggshells were highly correlated with feed dose.  相似文献   
68.
Enzyme activities have been used as indicators of soil quality and changes in biogeochemical function due to management or perturbations. The objective of this study was to answer a number of methodological questions regarding sampling schemes, sample handling recommendations, and assay procedures to facilitate the use of enzyme assays in the tropical highlands of East Africa. We used para-nitrophenol (pNP) based substrates for five enzymes: β-glucosidase, cellobiohydrolase, chitinase, acid phosphatase, and alkaline phosphatase. In the first experiment, we examined sampling procedures and compared the results of determining enzyme activities on a plot using composite or discrete samples. Composite samples usually had higher activities than the means of individual cores (P<0.05), but relative ranking of sites was the same if analyses were based on composite or discrete samples. In the second experiment, we examined the effects of storage time and conditions on enzyme activity. Enzyme activity degraded rapidly in frozen samples, but was better maintained in samples stored at 4 °C. Phosphatase and cellobiohydrolase activity declined after 14 days of storage, while the activity of the other enzymes remained close to the values of fresh samples for 28 or more days. In the third experiment, we examined the effect of the addition of an antiseptic, toluene, to prevent bacterial growth during the assay. We found no consistent toluene effect (P>0.4), probably because the assays were of short duration and microbial growth was minimized. Finally, we looked at the incubation time necessary to produce reliable results. Phosphatases, with relatively high activities could reliably be determined in 2 h incubations, but the other enzymes had much lower activities and required longer incubation times for reliable determination. For the enzymes we looked at, 4 h was a good standard time for determining the activity of even the lowest activity enzymes. The results of this study provide practical guidelines for applying these enzyme assays in the degraded tropical soils.  相似文献   
69.
The influence of the provenance soil and vinification process on the wine multielemental composition was investigated. For this purpose, two different vineyards from the Douro wine district, Portugal, were selected. Monovarietal grapes from a 10 year old vineyard were used to produce a red table wine, in a very modern winery. Polyvarietal grapes from a 60-70 year old vineyard were used to produce a red fortified wine, similar to Port, through a traditional vinification process. The multielement compositions (Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Hf, Li, Mn, Mo, Nb, Ni, Pb, Rb, Sb, Sc, Sr, Ti, Th, Tl, U, V, W, Y, Zn, Zr, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) of soil, grape juices (prepared in the laboratory), and samples collected in the different steps of each winemaking process were measured. Inductively coupled plasma mass spectrometry was used, after suitable pretreatment of the samples (by UV irradiation for liquid samples and high-pressure microwave digestion for soil). Both vinification processes influenced the multielement composition of the wines. Most of the elements presented similar or even lower concentrations in the wine as compared to that observed in the respective grape juice, probably as a result of precipitation or coprecipitation with suspended particles during fermentation and/or wine aging. Evidence of effective contamination during grape pressing, fermentation, and/or fining of wines (depending on the element) was observed for Cd, Cr, Cu, Fe, Ni, Pb, V, and Zn in the fortified wine and Al, Cr, Fe, Ni, Pb, and V in the table wine. Nevertheless, significant correlations were obtained between the multielement composition of the wine and the respective grape juice (R = 0.997 and 0.979 for the fortified and table wines, respectively, n = 31, P < 0.01), as well as between that in the wine (median of the two studied wines) and the provenance soil (R = 0.994, n = 19, P < 0.01), for the set of elements determined in common in the different types of samples. These results are promising concerning the usefulness of the elemental patterns of both soil and wine as fingerprints of the origin of the studied wines. Nevertheless, more wines from the same and other wine districts must be studied in order to consolidate this conclusion. The multielement compositions of the studied wines were compared with those of wines of different characteristics and origins, as well as with the respective legal threshold limit values, when available. Relatively low metal levels, below their threshold limit values, were found in all cases.  相似文献   
70.
The purpose of the present study is to simulate concentrations of ozone and its precursors (NO and NO2) in an area under the influence of the Candiota coal-fired power station by applying numerical models. The photochemical simulations were conducted during two distinct periods: in summer (from 24 to 26 January 2003) and in winter (from 10 to 12 August, 2004). With the RAMS atmospheric model (Regional Atmospheric Modelling System) we obtained meteorological fields to initialize the CIT photochemical model. The emission data were based on the AP-42 and only compounds emitted during coal burning were considered. The simulated results revealed high ozone levels in warmer periods, being related to high temperatures and water vapor concentrations. The simulated concentrations are located far from the emitting source, favoring reaction and mixing time between precursors to form ozone. Simulated winds of small intensity favored ozone transport, mixing and accumulation in areas far from the emitting source.  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