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   1023篇
  免费   54篇
林业   29篇
农学   44篇
基础科学   3篇
  100篇
综合类   260篇
农作物   124篇
水产渔业   87篇
畜牧兽医   377篇
园艺   11篇
植物保护   42篇
  2023年   3篇
  2022年   15篇
  2021年   30篇
  2020年   8篇
  2019年   14篇
  2018年   20篇
  2017年   13篇
  2016年   24篇
  2015年   17篇
  2014年   26篇
  2013年   47篇
  2012年   68篇
  2011年   72篇
  2010年   40篇
  2009年   40篇
  2008年   83篇
  2007年   85篇
  2006年   85篇
  2005年   70篇
  2004年   49篇
  2003年   46篇
  2002年   48篇
  2001年   38篇
  2000年   26篇
  1999年   13篇
  1998年   4篇
  1997年   6篇
  1996年   3篇
  1995年   2篇
  1994年   2篇
  1993年   2篇
  1992年   4篇
  1990年   6篇
  1989年   2篇
  1988年   4篇
  1987年   4篇
  1986年   6篇
  1985年   8篇
  1984年   5篇
  1983年   6篇
  1980年   3篇
  1978年   4篇
  1977年   2篇
  1976年   2篇
  1975年   3篇
  1974年   3篇
  1973年   4篇
  1971年   2篇
  1970年   3篇
  1969年   2篇
排序方式: 共有1077条查询结果,搜索用时 19 毫秒
981.
The present study investigated the photoprotective properties of an ethanol extract derived from the red alga Bonnemaisonia hamifera against ultraviolet B (UVB)-induced cell damage in human HaCaT keratinocytes. The Bonnemaisonia hamifera ethanol extract (BHE) scavenged the superoxide anion generated by the xanthine/xanthine oxidase system and the hydroxyl radical generated by the Fenton reaction (FeSO4 + H2O2), both of which were detected by using electron spin resonance spectrometry. In addition, BHE exhibited scavenging activity against the 1,1-diphenyl-2-picrylhydrazyl radical and intracellular reactive oxygen species (ROS) that were induced by either hydrogen peroxide or UVB radiation. BHE reduced UVB-induced apoptosis, as shown by decreased apoptotic body formation and DNA fragmentation. BHE also attenuated DNA damage and the elevated levels of 8-isoprostane and protein carbonyls resulting from UVB-mediated oxidative stress. Furthermore, BHE absorbed electromagnetic radiation in the UVB range (280–320 nm). These results suggest that BHE protects human HaCaT keratinocytes against UVB-induced oxidative damage by scavenging ROS and absorbing UVB photons, thereby reducing injury to cellular components.  相似文献   
982.
γ-Oryzanol (steryl ferulates; SF) has been shown to be a major bioactive compound in rice. To determine the content of individual γ-oryzanols in brown rice by high performance liquid chromatography (HPLC), purification of individual SF for use as an external standard is required. Four main SF were isolated from a commercial γ-oryzanol mixture and identified as cycloartenyl ferulate (1), 24-methylenecycloartanyl ferulate (2), campesteryl ferulate (3), and sitosteryl ferulate (4) based on mass spectrometry and nuclear magnetic resonance spectroscopic data. The SF contents between conventional and organic brown rice were qualitatively determined by HPLC using SF isolated from a commercial γ-oryzanol mixture as the external standard. The total γ-oryzanol content (mg/100 g) in organic brown rice (65.6 ± 2.7) was slightly higher (P < 0.05) than that found in conventional brown rice (60.2 ± 1.8). The content (mg/100 g) of 1 (21.2 ± 0.9) and 4 (9.8 ± 0.4) in organic brown rice was higher (P < 0.05) than that observed in conventional brown rice (1, 18.2 ± 1.1; 4, 8.5 ± 0.3). However, the content of 2 and 3 in the conventional and organic brown rice samples did not differ significantly. These results indicate that the cultivation methods do significantly alter the γ-oryzanol content for conventional and organic brown rice.  相似文献   
983.
Textile sound absorbents are getting more and more popular on the market as noise reduction is a major requirement for human comfort today. In this paper we focus on a new textile material for sound absorption, by investigating the acoustic characteristics of nanofibers. Through impedance tube method we measured the sound absorption coefficients of multiple layers of nanofiber webs and compared those with microfiber materials per fabric weight. We also examined the effect of layers of nanofiber webs on regularfiber knitted fabric on sound absorption. The test results showed that the sound absorption coefficients of nanofiber layers were superior that of microfiber fabrics in the frequency range 1000–4000 Hz. In this range, the sound absorption of nanofiber webs improved with numbers of layers. Also, adding nanofiber web plies to regularfiber fleece increased the sound absorption coefficient with 85 % at 4000 Hz. From our results we could observe differences in the sound absorption coefficients between two test methods, which are higher absorbance coefficients through the reverberation room method than impedance tube method.  相似文献   
984.
The purpose of this study was to classify various fabrics into some meaningful groups and to predict the fabrics’ acoustic characteristics using their mechanical properties. Two hundred seventeen fabrics, fifty one knitted fabrics, fifty nine woven fabrics and one hundred and seven vapor permeable water repellent fabrics, were used as test specimen. Fabric frictional sounds of the specimen were measured with a MAFN(Measuring Apparatus for Fabric Noise, Patent: No, 2001-73360). Sound Pressure Level(SPL), psychoacoustic parameters such as Loudness(z) and Sharpness(z) of the specimen were obtained by the sound quality system. KES-FB system was used for mechanical property measurements. Cluster analysis was used to classify the specimen and discriminant analysis was used to predict the clusters. Linear regression analysis was used to suggest the equations to predict the acoustic properties using mechanical properties.  相似文献   
985.
Asia traditional herbal feed additives have often been shown to have beneficial effects on disease resistance, growth, feed conversion and protein conversion. There have been few studies on the use of these feed additives in olive flounder diets. The present study was designed to determine if there are benefits associated with the inclusion of any of these traditional herbal feed additives in olive flounder diets. One thousand two hundred sixty juvenile fish averaging 6.5 g were randomly distributed into 36, 180‐L flow‐through tanks. No additive was included into the control (Con) diet. The aqueous type of additives: 2.5% bamboo (BB), 0.25% Prunus mume (PM), 2.5% Artemisia capillaris (AC), 2.5% cinnamon (CC), 2.5% Cirtus junos (CJ), 0.25% Cudrania tricuspidata (CT) and 2.5% Ficus carica (FC), and the powder type of additives: 1% Houttuynia cordata (HC), 1% safflower (SF) and 1% sea tangle (ST) were included into the experimental diets. Finally, 0.1% emulsifying oil (EO) was included into the experimental diet. Feed efficiency ratio (FER) of fish fed the BB, CT and SF diets was higher than that of fish fed the FC and HC diets. Protein efficiency ratio (PER) of fish fed the BB and SF diets was higher than that of fish fed the AC, FC and HC diets. None of serum criteria of fish was affected by any of dietary additives. Dietary additives did not affect weight gain and serum chemistry of olive flounder, but 2.5% bamboo extract and 1% safflower powder was effective to improve FER and PER.  相似文献   
986.
In the study, we characterized 29 Streptococcus iniae isolates from diseased olive flounder Paralichthys olivaceus in Korea from 2000 to 2005. Biochemical characteristics of 29 isolates using API 20 strep were identical. Through analysis of repetitive sequence-based PCR (rep-PCR) using BoxA primer and random amplified polymorphic DNA using p14 primer, 29 isolates of S. iniae were divided into two genotypes. The isolates were divided into two clusters by comparison of genetic distance using a sequence of the capsular polysaccharide D gene that was consistent with genotyping by the rep-PCR. The isolates belonging to genotype 1 in rep-PCR analysis showed a high virulence in the flounder, while the isolates belonging to genotype 2 were relatively low in virulence. Therefore, a correlation between the genotype and the virulence of S. iniae isolates has been identified.  相似文献   
987.
The effects of herbicide dose on rice‐weed competition were investigated to develop a combined model, which can be utilised to estimate an optimum herbicide dose for a given weed density in paddy rice cultivation. Field studies were conducted in Suwon for rice‐Echinochloa crus‐galli competition and Iksan for rice‐Eleocharis kuroguwai during 2007. The competitive effect of the weeds E. crus‐galli and E. kuroguwai decreased with increasing doses of flucetosulfuron and azimsulfuron, respectively, in the same manner as the standard dose–response curve. The combination of the rectangular hyperbolic model and the standard dose–response curve adequately described the complex effects of herbicide dose and weed competition on rice yield. Parameter estimates were used with the model to predict rice yield and estimate the doses of flucetosulfuron and azimsulfuron required to restrict rice yield loss caused by E. crus‐galli and E. kuroguwai, respectively, to an acceptable level. For a rice yield of 5.0 t ha?1, the model recommended flucetosulfuron doses of 8.7, 13.4 and 20.1 g a.i. ha?1 when infested with E. crus‐galli at 12, 24 and 48 plants m?2 respectively. For a rice yield of 5.2 t ha?1, the model recommended azimsulfuron doses of 3.9, 7.5 and 12.6 g a.i. ha?1 when infested with E. kuroguwai at 24, 48 and 96 plants m?2 respectively. The theoretical outputs of the combined model appear robust and indicate there are opportunities for reduced herbicide use in the field. These now require evaluation under field conditions.  相似文献   
988.
This study was conducted in order to compare the nutrient and chemical contents of two Korean teas: traditional Chungtaejeon tea (CTJ) with that of green tea (GT). Main bioactive compounds and the antioxidant activities using four radical scavenging assays (ABTS, CUPRAC, FRAP and DPPH) in methanol and acetone extracts of both teas were determined. It was found that the contents of vitamin C, amino acids and total nitrogen in CTJ were lower than that of GT (p < 0.05). Caffeine, reducing sugar and chlorophyll contents in CTJ were similar to GT. Catechin (C), epicatechin (EC), and epigallocatechin (EGC) contents were lower in CTJ than in green tea (p < 0.05), but gallocatechin (GC), epicatechin gallate (ECG), epigallocatechin gallate (EGCG) and catechin gallate (CG) showed no significant differences between CTJ and GT (p ≥ 0.05). The contents of polyphenols, flavonoids, flavanols and tannins in CTJ methanol were 229.30 ± 11.3 mg GAE/g dry weight (DW), 15.24 ± 0.8 mg CE/g DW, 109.10 ± 5.1 mg CE/g DW and 25.68 ± 1.2 mg CE/g DW, respectively, and significantly higher than in acetone extracts (p < 0.05). Flavonoids (quercetin and kaempferol) were higher in GT than in CTJ and myricetin was higher in CTJ (p < 0.05). Threonine and aspartic acid was lower, and glutamic acid was higher in CTJ compared with GT (p < 0.05). Free amino acid content in CTJ and GT showed no significant difference. Potassium and magnesium in CTJ were lower compared to GT, but no significant difference was found for iron, manganese and calcium. Also, the level of the antioxidant activity by all four used assays was significantly higher in CTJ and in methanol was higher than in acetone extracts (p < 0.05). In conclusion, traditional fermented Korean tea Chungtaejeon contains high quantities of bioactive compounds and possesses high antioxidant activity. The contents of the bioactive compounds and the levels of antioxidant activities are significantly higher in methanol than in acetone extracts. 3-D fluorescence and FTIR- spectroscopy showed slight differences between the two investigated tea samples and can be used as additional tools for identification of polyphenols. Both studied teas can be recommended as a source of bioactive compounds.  相似文献   
989.
The methanol extract from the leaves of Petasites japonicus Maxim (PJ) was studied for its (anti-)mutagenic effect with the SOS chromotest and reverse mutation assay. The (anti-)carcinogenic effects were evaluated by the cytotoxicity on human cancer line cells and by the function and the expression of gap junctions in rat liver epithelial cell. PJ extracts significantly decreased spontaneous β-galactosidase activity and β-galactosidase activity induced by a mutagen, ICR, in Salmonella (S.) typhimurium TA 1535/pSK 1002. All doses of the extract (0.08-100 mg/plate) decreased the reversion frequency induced by benzo (α)pyrene (BaP) in S. typhimurium TA 98. It decreased not only the spontaneous reversion frequency but also that induced by BaP in S. typhimurium TA 100. PJ extract showed greater cytotoxic effects on human stomach, colon and uterus cancer cells than on other cancer cell types and normal rat liver epithelial cells. Dye transfers though gap junctions were significantly increased by PJ extracts at concentrations greater than 200 µg/mL and the inhibition of dye transfer by 12-O-tetradecanoylphorobol-13-acetate (TPA) was obstructed in all concentrations of PJ. PJ significantly increased the numbers of gap junction protein connexin 43, and increased the protein expression decreased by TPA in a dose-dependent manner. Based on these findings, PJ is suggested to contain antimutagenic and anticarcionogenic compounds.  相似文献   
990.
The effects of dietary substitution of fishmeal with the combined dry Nannochloropsis oceanica (NO) biomass residue and casein on growth performance and carcass composition of juvenile abalone was determined. One thousand two hundred and sixty juvenile abalone were randomly distributed into the 18 L plastic rectangular containers. Five experimental diets in triplicate were prepared. The 28% fishmeal was included into the control (Con) diet. The 25%, 50%, 75% and 100% fishmeal were substituted with the combination of graded levels of dry NO biomass residue and casein, referred to as the NO25, NO50, NO75 and NO100 diets respectively. Finally, the salted sea tangle (ST), Laminaria japonica was prepared to compare the effect of the experimental diets on performance of abalone. The essential amino acids, such as isoleucine, leucine, phenylalanine, threonine and valine tended to increase with dietary substitution of fishmeal with NO biomass residue in the experimental diets. Arginine tended to decrease with dietary substitution of fishmeal with NO biomass residue. Survival of abalone fed the experimental diets was higher than that of abalone fed the ST diet for 16 weeks. Weight gain and specific growth rate (SGR) of abalone fed the NO100 diet were higher than those of abalone fed the all other diets. Weight gain and SGR of abalone linearly increased with dietary substation of fishmeal with NO. Shell length of abalone tended to increase with dietary substitution of fishmeal with the combined dry NO biomass residue and casein. The chemical composition of the soft body of abalone was different among treatments except for moisture content. In conclusion, fishmeal in the diets for abalone could be completely replaced with the combined dry NO biomass residue and casein when the 28% fishmeal was included.  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