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   1686篇
  免费   87篇
  国内免费   3篇
  2023年   12篇
  2022年   40篇
  2021年   50篇
  2020年   48篇
  2019年   82篇
  2018年   68篇
  2017年   37篇
  2016年   60篇
  2015年   56篇
  2014年   83篇
  2013年   114篇
  2012年   133篇
  2011年   114篇
  2010年   62篇
  2009年   55篇
  2008年   90篇
  2007年   65篇
  2006年   60篇
  2005年   38篇
  2004年   45篇
  2003年   60篇
  2002年   23篇
  2001年   40篇
  2000年   25篇
  1999年   14篇
  1998年   11篇
  1997年   8篇
  1995年   6篇
  1994年   7篇
  1992年   15篇
  1991年   12篇
  1990年   11篇
  1989年   15篇
  1988年   21篇
  1987年   12篇
  1986年   13篇
  1985年   13篇
  1984年   10篇
  1983年   11篇
  1982年   8篇
  1981年   6篇
  1980年   10篇
  1979年   9篇
  1978年   10篇
  1977年   6篇
  1976年   12篇
  1975年   8篇
  1974年   7篇
  1973年   6篇
  1964年   6篇
排序方式: 共有1776条查询结果,搜索用时 31 毫秒
241.
The understanding of host specificity of predacious insects is one of the most important problems in the investigation of biological control. Prey preference and the potential for prey switching in Anthocoris minki Dohrn (Hem.: Anthocoridae) was assessed in two- choice experiments under environmentally controlled conditions. Manly's preference index was used to compare A. minki predation on Agonoscena pistaciae Burckhardt and Lauterer (Hem.: Psyllidae) versus Psyllopsis repens Loginova (Hem.: Psyllidae) but showed no statistically significant evidence for preference. Anthocoris minki did prefer psyllids (Agonoscena pistaciae and Psyllopsis repens) over aphids (Forda hirsuta Mordvilko (Hem.: Pemphigidae) and Aphis craccivora Koch (Hem.: Aphididae)). The predator also consumed significantly more 2nd instar than 4th instar nymphs of P. repens. Switching behavior experiments showed that A. minki did not preferentially attack the most abundant prey when the prey combination was either biased toward A. pistaciae or toward F. hirsuta. Linear regression between preference indices and prey ratios indicated an overall preference for psyllids over aphids. The results suggest that a psyllid-specialized feeder such as A. minki is more likely to be effective biocontrol agent against psyllid species and less effective at reducing the numbers of other pest species that might co-occur with psyllids.  相似文献   
242.
Circadian and stress systems have a crucial role in adaptation of organisms to environmental challenges. This study investigates the ability of Oscillatoria brevis (O. brevis) β-carotene extract (βC) in modulating the circadian alternations of locomotor activity (LA) and serum melatonin (M) rhythms under chronic unpredictable stress (CUS) in rats. Twenty rats (5 rats/group) were used in monitoring LA using running wheels. Eighty rats (20 rats/group) were used in observing circadian serum M profile. Rats were randomly divided into four groups, viz. control, βC-treated, CUS-exposed, and “βC-treated&CUS-exposed” groups. CUS-exposure was applied for 21 days. One hour before exposure, βC was daily administered (10 mg/kg), intraperitoneally (IP). Blood was sampled at 6-h intervals for 5 rats/time point at Zeitgeber (ZT) 3, 9, 15, and 21. Results demonstrated that unstressed rats exhibited circadian M pattern and nocturnal LA rhythm with acrophase around ZT 21 and ZT 15, respectively. CUS-exposure revealed a disturbance in these patterns. Phase shifting of M and LA profiles was recorded. A decrease M acrophase and a significant decrease in LA (p < 0.05) were recorded at ZT 9. Daily βC administration in stressed rats modulates the CRs alternation induced by CUS. It may be concluded that βC ameliorated the induced alternations in circadian rhythms.  相似文献   
243.
Alternaria alternata is the most common fungal pathogen of tomatoes in Upper Egypt. Morphological identification of this fungus is challenging; therefore, this study searched for new classification tools based on molecular techniques. Using a dilution plating method, 67 strains of A. alternata were isolated from 34 samples of rotten tomato fruits representing the Giza 80 and Edkawy cultivars. The collected strains were identified using the amplification products of the internal transcribed spacer (ITS) region, glyceraldehyde 3‐phosphate dehydrogenase (Gpd) and Alt a1, which is a gene involved in the production of most of the allergens produced by A. alternata. The screening revealed that A. alternata constituted more than half of the total fungi recovered from rotten tomatoes in this study. According to the phylogenetic analysis using these three loci, the collected strains clustered in accordance with the host cultivar type from which they had been isolated. Specific gene random primer polymerase chain reaction (SGRP‐PCR) techniques indicated that the A. alternata population in the tested region has a high genetic diversity. The pathogenicity test showed that most of the A. alternata isolates (67.2%) were highly pathogenic, and no correlation was found between the phylogenetic analysis and pathogenicity. In addition, the influence of the fungicide Disan 80% on the collected strains showed significant differences that were attributed to the source of isolation.  相似文献   
244.
Lead (Pb) is a ubiquitous and toxic heavy metal and it can damage the immune system in humans and animals. Many researchers have reported that Selenium (Se) could possess various pharmacological effects in mammals. However, few studies have been carried out to investigate the protective role of Se in birds, especially in chickens. In this study, we investigated the protective effects of Se against Pb-induced inflammatory responses and the expression of heat shock proteins (HSPs) in peripheral blood neutrophils. One hundred eighty Hy-Line brown chickens were randomly divided into the control group (Con group), Se supplementation group (+Se group), Pb supplementation group (+Pb group), and the Se and Pb compound group (Se+Pb group). On the 90th day of the experiment, the peripheral blood was collected to extract neutrophils, and then, the levels of HSPs and cytokines were examined. The results showed that, after Pb treatment, the levels of IL-(1β, 1R, 4, 8, 10, and 12β), TGF-β4, and HSP (27, 40, 60, 70, and 90) mRNA were significantly increased and levels of IL-2 and IFN-γ mRNA were decreased compared with those in the control group. Compared with the control group, the protein levels of HSP60 and HSP70 were also increased in the Pb treatment group. Co-administration of Se (1 mg/kg/day) and Pb resulted in a reversal of the Pb-induced cytokine changes in neutrophils accompanied by a significant decrease in HSPs. Our study demonstrated that Pb could decrease the immune function via changing the expression of cytokines and HSPs in chicken neutrophils, but Se could relieve the toxic effect induced by Pb.  相似文献   
245.
High salinity is one of the major environmental factors limiting the productivity of crop species worldwide. Improving the stress tolerance of cultivated plants and thus increasing crop yields in an environmentally friendly way is a crucial task in agriculture. In the present work the ability of a new derivative, S-methylmethionine-salicylate (MMS), to improve the salt tolerance of wheat plants was tested parallel with its related compounds salicylic acid and S-methylmethionine. The results show that while these compounds are harmful at relatively high concentration (0.5 mM), they may provide protection against high salinity at lower (0.1 mM) concentration. This was confirmed by gas exchange, chlorophyll content and chlorophyll-a fluorescence induction measurements. While osmotic adjustment probably plays a critical role in the improved salt tolerance, neither Na or K transport from the roots to the shoots nor proline synthesis are the main factors in the tolerance induced by the compounds tested. MMS, S-methylmethionine and Na-salicylate had different effects on flavonol biosynthesis. It was also shown that salt treatment had a substantial influence on the SA metabolism in wheat roots and leaves. Present results suggest that the investigated compounds can be used to improve salt tolerance in plants.  相似文献   
246.
Fusarium subglutinans (Wollenweber and Reinking) Nelson et al. was found to produce intracellular milk-clotting enzyme (MCE) with good milk-clotting activity (MCA). The crude activity of the produced enzyme was recorded as optimum at 55 degrees C and pH 4.5. The highest yield i.e. 78.43 SU/mg dry biomass was obtained after 4 days of rotary shaking at 30 degrees C when the fermentation medium containes wheat flour 2%, glucose 1% and (NH4)2SO4 0.1% with an initial pH value 6.0. Under these conditions, the maximum ratio of MCA to proteolytic activity (PA) amounting to 603.31 SU/PU mg(-1) was also achieved. Production of intracellular MCE by F. subglutinans was assumed to be active growth-associated type. This enzyme preparation was less active than the calf rennet, but was superior to those of Meito's and Pfizer's rennets.  相似文献   
247.
The synthesis of macrocyclic lanthanide complexes via the reaction of pyridine-2,6-dicarboxaldehyde with 1,2-diaminoethane, 1,2-diaminopropane and 1,3-diaminopropane in the presence of lanthanide nitrates as templating agents is discussed together with the use of the lanthanum derivatives in transmetallation reactions with copper(II).  相似文献   
248.
249.
Isoenzymes of sphingomyelinase have been resolved by isoelectric focusing. The two major species (I and II) in human liver have distinct isoelectric points, pH optima and Km values. Liver from Niemann-Pick disease Type C contained isoenzyme I (pI 4.6) while isoenzyme II (pI 5.2) was absent. The absence of isoenzyme II likely constitutes the genetic defect in this disease.  相似文献   
250.
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