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   899篇
  免费   43篇
  国内免费   2篇
林业   76篇
农学   70篇
基础科学   15篇
  258篇
综合类   51篇
农作物   104篇
水产渔业   70篇
畜牧兽医   216篇
园艺   19篇
植物保护   65篇
  2023年   16篇
  2022年   25篇
  2021年   29篇
  2020年   47篇
  2019年   50篇
  2018年   53篇
  2017年   61篇
  2016年   53篇
  2015年   23篇
  2014年   38篇
  2013年   99篇
  2012年   57篇
  2011年   64篇
  2010年   40篇
  2009年   36篇
  2008年   34篇
  2007年   22篇
  2006年   16篇
  2005年   15篇
  2004年   11篇
  2003年   13篇
  2002年   15篇
  2001年   15篇
  2000年   15篇
  1999年   8篇
  1998年   6篇
  1997年   4篇
  1996年   4篇
  1995年   3篇
  1994年   5篇
  1993年   5篇
  1992年   5篇
  1991年   7篇
  1990年   8篇
  1989年   5篇
  1988年   4篇
  1987年   3篇
  1986年   3篇
  1985年   4篇
  1980年   5篇
  1978年   2篇
  1977年   2篇
  1975年   1篇
  1973年   1篇
  1971年   2篇
  1970年   1篇
  1968年   1篇
  1960年   1篇
  1959年   3篇
  1957年   2篇
排序方式: 共有944条查询结果,搜索用时 46 毫秒
921.
Understanding the effect of boron (B) on plant physiology will help to refine the diagnosis of B deficiency and improvement in B fertilizer recommendations for cotton (Gossypium hirsutum L.) growing areas. This study shows the testing of hypotheses “that application of B-fertilizer improves net photosynthetic rate (PN) and water use efficiency (WUE) for cotton plant on a B-deficient soil [< 0.50 mg B kg?1 hydrochloric acid (HCl)-extractable] in an arid environment”. Thus, a permanent layout [two-year field experiment (2004 and 2005)] was conducted to study the impact of B fertilizer at 0, 1.0, 1.5, 2.0, 2.5, and 3.0 kg ha?1 on gas exchange and electrolyte leakage (EL) characteristics of cotton crop (cv. ‘CIM-473’). The soil at experimental site was alkaline (pH 8.1), calcareous [calcium carbonate (CaCO3 5.6%)], and silt loam (Typic Haplocambid). Boron use decreased EL of plant membrane (P ≤ 0.05), and increased PN, transpiration rate (E) and stomatal conductance (gs), while intercellular concentration of carbon dioxide (CO2; Ci) significantly decreased (P ≤ 0.05) during both experimental years. There was a positive, but non-significant effect of B concentration on chlorophyll content in plant leaves. Application of 3.0 kg B ha?1 improved WUE up to 9.7% [4.62 μmol (CO2) mmol?1 water (H2O)] compared to control plants (4.21 [μmol (CO2) mmol?1 (H2O)]. Principal component analysis (PCA) of data indicates positive correlations between leaf B concentration and PN, E, gs, and WUE, while a negative relationship existed between leaf B concentration and intercellular CO2 (Ci). This study showed that addition of B fertilizer in the B-deficient calcareous soil proved beneficial for growth and development for cotton crop by enhancing its WUE and gas exchange characteristics.  相似文献   
922.
OBJECTIVE: To determine the correlates of central adiposity. DESIGN: Population-based cross-sectional study. SUBJECTS: A total of 926 women (aged 40-60 years) from all districts of Tehran. METHODS: Demographic data were collected and anthropometric indices were measured according to standard protocols. Dietary intakes were assessed by means of a semi-quantitative food-frequency questionnaire. The suggested cut-off point for waist-to-hip ratio (WHR>or=0.84) for Tehrani people, adjusted for their age group, was used to determine central adiposity. Logistic regression analysis was used to determine the correlates of WHR, which were adjusted for age, taking medications and body mass index (BMI). The components of dietary intake were determined by factor analysis. Pearson correlation was used to determine the association between the dietary components and WHR. Analysis of covariance was employed to compare the mean values of WHR in different lifestyle groups, with adjustment for BMI and age. RESULTS: Mean WHR was 0.82 +/- 0.06. The possibility of being centrally obese was higher in women with light physical activity (odds ratio: 2.11; 95% confidence interval: 1.40-2.53), depressed women (1.36; 1.02-1.93), smokers (1.21; 1.02-1.56) and unemployed women (1.41; 1.13-1.72). Marriage (1.31; 1.10-1.82), menopause (1.22; 1.02-1.61), low vitamin C intake (2.31; 1.25-4.25) and low calcium intake (1.30; 1.07-3.78) were associated with central fat accumulation. Dairy consumption was inversely correlated with central fat accumulation (r = -0.2, P < 0.05). CONCLUSION: Central adiposity is associated with poor lifestyle factors including low physical activity, depression, smoking, low intake of vitamin C, low intake of calcium and dairy products and high fat consumption. Thus lifestyle modifications should be encouraged to achieve a healthier body shape.  相似文献   
923.
The aqueous leaf extract of Euphorbia hirta decreased the gastrointestinal motility in normal rats and decreased the effect of castor oil-induced diarrhoea in mice.  相似文献   
924.
This study examined (1) the effects of seasonal and annual environmental characteristics (temperature, chlorophyll content, salinity, microbial water quality, and algal lipid composition) in four aquaculture locations on the quality (meat content, shell size, and microbial and lipid compositions) of cultivated blue mussels and (2) the optimum harvest time for premium-quality mussels. Seasonal and annual temperature fluctuation of the seawater in the aquaculture sites significantly (P < 0.05) affected their salinity, microbial content, and algal fatty acid compositions, which in turn affected the quality attributes of cultivated mussels. The optimum growth period in terms of meat content (4-5 g) and shell size (50-60 mm) can be achieved within 1 year of cultivation. Because the cultivated mussels examined in this study never reached the maximum microbial load limits (10(5) or log 5 colony-forming units/g of meat), they can be harvested throughout the year. Meanwhile, no significant (P > 0.05) annual changes were observed in total omega-3 polyunsaturated fatty acids content of cultivated mussels. Nonetheless, the optimum harvest time may be April-June for eicosapentaenoic acid (EPA) rich mussels and September-October for docosahexaenoic acid (DHA) rich mussels in terms of fatty acid proportions; however, June may be the best month for the highest concentrations of both EPA and DHA.  相似文献   
925.
Woody biochars derived by pyrolyzing Gliricidia sepium at 300°C and 500°C and a waste byproduct of same biomass from a bioenergy industry (BC700) were tested for their effect on soil enzymes activities and available form of heavy metals in multi-metals contaminated soil. Pot experiments were conducted during 6 weeks with tomato (Lycopersicon esculentum L.) at biochar application rates, 1, 2.5, and 5% (w/w). A reduction in polyphenol oxidase with biochars produced at increasing pyrolysis temperature compared to the control whereas the maximum activity of dehydrogenase and catalase was observed in 1% BC500 and 2.5% BC300, respectively. Soil available form of Ni, Mn, and Cr were reduced by 55, 70% and 80% in 5% BC700 amended soil, respectively. The highest geometric mean of enzyme activities was observed in 2.5% BC300 treatment. Overall the application of high dosages of high temperature derived biochar masks/deteriorates soil enzyme activities but immobilizes bioavailable heavy metals and reduces toxicity.  相似文献   
926.
In this work, the natural dyeing behavior of woollen yarn with madder (Rubia tinctorum L.) root extract was studied. The effects of different tannin-rich plants (Rhus coriaria, Eucalyptus, Terminalia chebula, Quercus castaneifolia, Pomegranate) extract as biomordants and alum (as a chemical mordant) with two mordanting procedures (pre- and metamordanting) on color characteristics of the dyed samples were also investigated. The CIEDE2000 values, color strength (K/S), washing fastness and tensile property of the mordanted and dyed samples were assessed. Visually, a range of hues from orange to brownish-red were obtained. In general, pre-biomordanted samples with Rhus coriaria (10 %owf), Eucalyptus (10 %owf), Terminalia chebula (5 %owf), Quercus castaneifolia (5 %owf) and Pomegranate (5 %owf) showed almost the same color difference (ΔE00) and wash fastness values compared to those treated with 3 %owf alum. Finally, it was concluded from the comparative studies that the biomordants have good potential to be considered as alternatives to the common chemical mordants.  相似文献   
927.
Microwave-absorptive polymer composite materials provide protection against interference to communication systems caused by microwave-inducing devices. Microwave-absorptive polymer composites were prepared from polylactic acid (PLA) biocomposite blended with oil palm empty fruit bunch (OPEFB) fiber and commercial Iron oxide (Fe2O3) as filler using the melt-blending method. The composites characterization was carried out using the scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The coefficient of reflection S11 and coefficient of transmission S21 of the composites for various Fe2O3 filler percentages were determined using a rectangular waveguide in connection with microwave vector network analyser (HP/Agilent model PNA N5227). These coefficients were then used to calculate microwave-absorption properties (in decibels). XRD analysis showed that increasing amounts of reinforced material (Fe2O3) reduces the crystallinity of the composites. SEM data indicated that Fe2O3 filler ratio increased in the composites, and adhesion to the cellulose fiber grew gradually until the highest percentage of filler was added. The complex relative permittivity and relative permeability were obtained within the broad frequency range of 8–12 GHz at room temperature for various percentages of filler and were measured by the transmission/reflection method using a vector network analyser. Fe2O3 embedment in OPEFB/PLA was observed to have resulted in enhancing the dielectric and magnetic properties. The values of permittivity and permeability increased with increasing Fe2O3 filler content. Theoretical simulation studied the relation between ε′ and ε″ of the relative complex permittivity in terms of Cole-Cole dispersion law. The result indicated that the processes of Debye relaxation in Fe2O3/OPEFB/PLA, the unique dielectric characteristics of Fe2O3 cannot be accounted for by both the Debye dipolar relaxation and natural resonance. Results further showed that the material transmission, reflection, and absorption properties could be controlled by changing the percentage of Fe2O3 filler in the composites.  相似文献   
928.
International Aquatic Research - This work aimed to appraise growth performance, immune responsiveness, bacterial disease resistance and mRNA expression of two antimicrobial peptides in Labeo...  相似文献   
929.

Purpose

This study was aimed to investigate the potential of biochar (BC), a waste byproduct of a bioenegy industry, Sri Lanka, as a soil amendment to immobilize and reduce the phytotoxicity of Cr in tannery waste-polluted soil (TWS).

Materials and methods

The TWS and bioenergy waste BC were characterized for physio-chemical parameters. A pot experiment was conducted by adding three BC application rates, 1, 2.5, and 5 % (w/w) to investigate the immobilizing capacity and bioaccumulation of chromium (Cr) in tomato plants (Lycopersicon esculentum L.). Soils and plants were digested via microwave digestion and analyzed for total Cr. Further, sequential extraction was conducted to assess the fractionation of Cr before and after the application of bioenergy waste BC on TWS.

Results and discussion

The total Cr concentration in TWS was 12,285 mg/kg. The biomass of tomato plants grown in the 5 % BC amendment doubled compared to the biomass in BC-unamended soil. Bioaccumulation of Cr in plants grown in 5 % BC-amended TWS showed a decrease by 97 % compared to that of the BC-unamended soil. The CaCl2 extractability of Cr indicated that the bioavailability of Cr in the 5 % BC amendment has decreased by 68 % compared to the control. Sequentially extracted Cr in the exchangeable fraction decreased by 98 % in the 5 % BC amendment.

Conclusions

Pore diffusion, and adsorption via π-π electron donor-acceptor interactions were the primary mechanisms to be involved in the Cr retention in BC. Results suggested that the addition of BC to TWS reduces the mobility, bioavailability, and phytotoxicity of Cr in tomato plants.
  相似文献   
930.
以甘蔗苗为材料研究了甘蔗CBF1转录激活因子ScCBF1在水杨酸(SA)、茉莉酸甲酯(Me JA)以及重金属镉(Cd Cl_2)和铜(Cu Cl_2)胁迫下的表达情况;将ScCBF1的原核表达载体p GEX-6P-1-ScCBF1转化大肠杆菌(Escherichia coli)Rosetta(DE3),检测其在NaCl、PEG8000胁迫下的生长情况.结果表明:ScCBF1在Me JA、SA、Cd Cl_2以及Cu Cl_2逆境胁迫下表达下调;在500 mmol·L-1NaCl胁迫下,含有重组质粒的细胞生长速度显著减缓,不同浓度NaCl胁迫下的平板胁迫结果进一步说明了ScCBF1基因不具耐盐性;在15%PEG8000胁迫下,含有重组质粒细胞的生长速度明显高于对照,不同浓度PEG8000胁迫下的平板胁迫结果说明了ScCBF1基因在应答干旱胁迫中具有抗逆性.构建了ScCBF1的植物表达载体p CAMBIA1301-ScCBF1并通过农杆菌侵染在本氏烟叶片中瞬时表达,在4℃低温胁迫下,T0代烟草植株长势显著优于对照.  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