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   16699篇
  免费   12篇
林业   3634篇
农学   1297篇
基础科学   137篇
  2766篇
综合类   717篇
农作物   2102篇
水产渔业   1858篇
畜牧兽医   1182篇
园艺   1112篇
植物保护   1906篇
  2022年   6篇
  2021年   9篇
  2020年   4篇
  2019年   8篇
  2018年   2748篇
  2017年   2706篇
  2016年   1188篇
  2015年   79篇
  2014年   31篇
  2013年   42篇
  2012年   803篇
  2011年   2133篇
  2010年   2109篇
  2009年   1262篇
  2008年   1331篇
  2007年   1587篇
  2006年   48篇
  2005年   133篇
  2004年   123篇
  2003年   170篇
  2002年   73篇
  2001年   12篇
  2000年   49篇
  1998年   2篇
  1995年   1篇
  1993年   15篇
  1992年   9篇
  1990年   1篇
  1989年   5篇
  1988年   11篇
  1987年   1篇
  1977年   4篇
  1972年   1篇
  1969年   1篇
  1968年   4篇
  1967年   1篇
  1959年   1篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
More than 7000 red algae species have been classified. Although most of them are underused, they are a protein-rich marine resource. The hydrolysates of red algal proteins are good candidates for the inhibition of the angiotensin-I-converting enzyme (ACE). The ACE is one of the key factors for cardiovascular disease, and the inhibition of ACE activity is related to the prevention of high blood pressure. To better understand the relationship between the hydrolysates of red algal proteins and the inhibition of ACE activity, we attempted to identify novel ACE inhibitory peptides from Pyropia pseudolinearis. We prepared water soluble proteins (WSP) containing phycoerythrin, phycocyanin, allophycocyanin, and ribulose 1,5-bisphosphate carboxylase/oxygenase. In vitro analysis showed that the thermolysin hydrolysate of the WSP had high ACE inhibitory activity compared to that of WSP. We then identified 42 peptides in the hydrolysate by high-performance liquid chromatography and mass spectrometry. Among 42 peptides, 23 peptides were found in chloroplast proteins. We then synthesized the uncharacterized peptides ARY, YLR, and LRM and measured the ACE inhibitory activity. LRM showed a low IC50 value (0.15 μmol) compared to ARY and YLR (1.3 and 5.8 μmol). In silico analysis revealed that the LRM sequence was conserved in cpcA from Bangiales and Florideophyceae, indicating that the novel ACE inhibitory peptide LRM was highly conserved in red algae.  相似文献   
2.
Semiaquatic walking has resulted in the evolution of functional and morphological changes in various hoofed mammals, such as hippopotamus and Brazilian tapir. The biomechanics of skilful walking in wetlands or at the bottom of a waterbody involve the medio‐lateral opening and closing of the feet to effectively support and stabilize the body on soft ground and to reduce the water resistance during recovery stroke, respectively. We demonstrate that the opening and closing of the feet in hippopotamus and Brazilian tapir are mediated by the adduction and abduction of the most medial and lateral phalanges from the CT examination. The axial toes, metacarpals and metatarsals do not contribute to changes in the width and shape of the feet, unlike the medial and lateral toes. We suggest that this semiaquatic walking motion is derived from the original terrestrial mode of locomotion, in contrast to the highly functional swimming motion using webs or fins in morphologically modified feet and tail. From the present data we demonstrate that semiaquatic locomotion evolved due to the acquisition of adductor–abductor mobility in the phalanges of the most medial and lateral digits, as shown in hippopotamus and Brazilian tapir.  相似文献   
3.
In Western Kenya, smallholder dairy production is becoming incrementally commercialized through the commodification and sale of milk through formal market channels. While commercialization is often construed as a way to boost rural livelihoods through increased income from milk, emerging evidence suggests that married women are not directly benefiting from formal milk market participation. This critical issue of gender power imbalance has been framed by development interventions in economic efficiency and social justice perspectives, but thus far interventions in the sector have not addressed how underlying social-market mechanisms embedded in gendered ideology influence smallholder engagement in dairy commercialization. Drawing on feminist theories of power and social embeddedness, this study investigates how gendered power relationships materialize and influence formal milk marketing engagement and practices in Western Kenya. Facilitated discussion groups with smallholder farmers revealed the gendered ideologies and norms that ascribe masculinized meaning to cattle, milk, and commercial enterprise. Key informant interviews with commercial dairy management and farmers were used to identify current practices for increasing women’s formal market participation—namely, direct payments to women for milk deliveries. Findings from this study indicate that cattle and formal dairy market participation are imbued with gendered meaning that create legitimacy around men’s privilege over dairy proceeds. Interventions in the sector aimed at addressing gender power imbalances must acknowledge this dynamic, and accept the social trade-offs and gendered costs of dairy commercialization.  相似文献   
4.
5.
Extremely low post-settlement survival is one of the largest barriers for artificial rehabilitation of Acropora corals. However, little data have been found for interspecific difference of the post-settlement survival probably because the observation of coral juvenile is difficult in the field. Here, we analyzed the survival of three dominant species of Acropora corals (A. digitifera, A. tenuis, A. yongei), with different colony morphologies and habitat preferences, for 2 years after settlement under the same environmental conditions. The post-settlement survival was significantly higher for A. tenuis than for A. digitifera 3 months after settlement. Two years later, the survival rate of A. tenuis was approximately 15 times higher than A. digitifera. In a separate analysis of three bottle-brush species (A. awi, A. echinata, A. subglabra) and A. tenuis, post-settlement survival was always higher for A. awi than for other two bottle-brush species, suggesting that the initial survival was different among morphologically sister species. Low survival was possibly associated with slow growth rates during the first 7 months. Thus, species selection is important for successful artificial coral rehabilitation, with A. tenuis being the most viable option. Alternatively, new techniques are required to improve post-settlement survival of slow growing coral species.  相似文献   
6.
The aim of this experiment was to evaluate the symptoms, variation, and correlation of hematological factors and some parameters of the innate immune response of the infected rainbow trout (Oncorhynchus mykiss) fish with Lactococcus garvieae bacteria. A total of 270 fish were divided into three groups including infected fish with a high and low concentration of L. garvieae and a control group without infection. The blood and tissue (brain, head, kidney, and spleen) samples were collected from each group (n?=?6 fish) at 0, 3, 14, and 21 days after treatment. The most observed symptoms during infection were lethargy, exophthalmia, and ascites. The mortality rates for the high and low dose-infected fish were defined as 60 and 25%, respectively. The infected groups had significantly (p?<?0.05) higher activity of serum lysozyme, myeloperoxidase, classical and alternative pathways of complement, serum bactericidal effects, and the specific antibody titer. Also, a significant (p?<?0.05) lower hematocrit and hemoglobin levels and higher white blood cell numbers were observed in the infected groups. A significant correlation was observed between the CRP levels and some of the hematological and immunological indices as bactericidal effects, classical complement pathways, lysozyme activity myeloperoxidase activity, white blood cell numbers, and hematocrit levels. The clinical symptoms, immune responses, and hematological indices variation among L. garvieae-infected fish are dependent on the duration and bacterial dose of the infection.  相似文献   
7.
Brucellosis is a zoonotic disease worldwide distributed and having the economic as well as public health importance. The prevalence of brucellosis among sheep flock having history of abortions was studied. A total of 229 samples comprising of 157 blood and 72 clinical samples (vaginal swabs) were collected from 157 animals. Clinical samples were processed for the isolation of Brucella melitensis. Serum samples (n = 157) were tested by Rose Bengal plate test (RBPT) and i-ELISA. A total of 68 (43.31%) and 104 (66.24%) samples were positive by RBPT and ELISA, respectively. Brucella isolates (n = 2) were recovered from clinical samples. Both isolates demonstrated amplification for bcsp 31 and IS711 genes. On AMOS PCR, both the isolates amplified at 731 bp, i.e., belongs to B. melitensis species. The incidence of B. melitensis in a migratory flock warns the thorough testing and culling of Brucella-infected sheep from the flock on a continuous basis; otherwise, such incidence will be routine and poor farmers will be at a loss.  相似文献   
8.
In rice, totally one hundred and three genotypes were evaluated for resistance against blast disease in three hot spot locations of Guyana. The experiments were conducted in spring and autumn seasons of 2015, 2016 and 2017. The modified upland blast nursery (UBN) method was adopted for screening blast resistance. The Additive Main effects and Multiplicative Interaction (AMMI) analysis was applied to understand the interaction between genotype (G) and environment (E). The results of all seasons at three locations showed the consistent expression of high resistance by genotype FL 127 followed by FG12–08 and FG12–273. The AMMI analysis of variance (ANOVA) revealed that, resistance was dominated by the genotype main effect. The magnitude of genotypes by environment (G x E) sum of square (SS) was 3.64 times smaller than genotype SS, and 6.74 times larger than environment SS. The analysis has indicated that the differences of the genotypes across the environment were not substantial. All of the seven environment vectors appeared to be positively correlated. The screening at Onverwagt Back, autumn 2016 (E7) and Black Bush Polder, spring 2015 (E5) were observed to be the most representative environment. This was clearly revealed by smaller average - environment axis (AEA) angle. Likewise, E4 (Canje, autumn 2015) was observed closer to the “ideal test environment”, thus suggested as an ideal environment for blast resistant screening. Also, genotypes clustered near the origin have showed stable blast disease resistant status and contributed little to both G and G x E. These genotypes proposed as useful ones for breeding disease resistant varieties and hybrids in rice.  相似文献   
9.
Metarhizium guizhouense PSUM02 treated males of Bactrocera latifrons were investigated for the mating competition among males and mating choice by female flies to develop an auto-dissemination for the control of B. latifrons. In the present study, on day 1–4 of experiment, M. guizhouense–treated male flies were equally competitive with the normal male flies as we did not observe any differences in mating by treated and normal male flies of B. latifrons. Further, mating competitiveness were found low in treated adult male B. latifrons than normal male B. latifrons from 5th days of treatment until death. Kaplan-Meier survival analysis of treated male flies gave average survival times (AST) of 4.3?±?0.1 days, while the healthy female and male flies in the same cage showed AST of 9.3?±?0.3 and 8.3?±?0.4 days, respectively. The AST of untreated flies in control experiment ranged from 14.2–14.5 days. In mating preference experiment, M. guizhouense–treated male flies were chosen by virgin female than gravid female flies for mating. The treated male flies caused mortality in both virgin and gravid female flies in the same cage with AST of 4.4?±?0.1, 5.6?±?0.1 and 7.4?±?0.2 days, respectively, while untreated flies showed AST ranged from 13.9–14.3 days in control. The treated male flies could transmit the fungus infection to both untreated female and male flies as well as in virgin and gravid female flies by mating and contact. Our experiments showed the potentiality of M. guizhouense PSUM02 in management of B. latifrons by auto-dissemination with treated male flies, which transmit the fungus to a healthy population to reduce insect pest infestations.  相似文献   
10.

Context

The study of ecosystem services has extended its influence into spatial planning and landscape ecology, the integration of which can offer an opportunity to enhance the saliency, credibility, and legitimacy of landscape ecology in spatial planning issues.

Objectives

This paper presents a conceptual framework suitable for spatial planning in human dominated environments supported by landscape ecological thinking. It seeks to facilitate the integration of ecosystem services into current practice, including landscape metrics as suitable indicators.

Methods

A literature review supported the revision of existing open questions pertaining to ecosystem services as well as their integration into landscape ecology and spatial planning. A posterior reflection of the current state-of-the-art was then used as a basis for developing the spatial planning conceptual framework.

Results and conclusion

The framework is articulated around four phases (characterisation, assessment, design, and monitoring) and three concepts (character, service, and value). It advocates integration of public participation, consideration of “landscape services”, the inclusion of ecosystem disservices, and the use of landscape metrics for qualitative assessment of services. As a result, the framework looks to enhance spatial planning practice by providing: (i) a better consideration of landscape configuration in the supply of services (ii) the integration of anthropogenic services with ecosystem services; (iii) the consideration of costs derived from ecosystems (e.g. disservices); and (iv) an aid to the understanding of ecosystem services terminology for spatial planning professionals and decision makers.
  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