全文获取类型
收费全文 | 607篇 |
免费 | 31篇 |
专业分类
林业 | 52篇 |
农学 | 10篇 |
基础科学 | 1篇 |
138篇 | |
综合类 | 66篇 |
农作物 | 22篇 |
水产渔业 | 51篇 |
畜牧兽医 | 285篇 |
园艺 | 3篇 |
植物保护 | 10篇 |
出版年
2022年 | 7篇 |
2021年 | 7篇 |
2020年 | 11篇 |
2019年 | 15篇 |
2018年 | 12篇 |
2017年 | 17篇 |
2016年 | 14篇 |
2015年 | 10篇 |
2014年 | 11篇 |
2013年 | 26篇 |
2012年 | 29篇 |
2011年 | 39篇 |
2010年 | 19篇 |
2009年 | 11篇 |
2008年 | 29篇 |
2007年 | 30篇 |
2006年 | 27篇 |
2005年 | 28篇 |
2004年 | 22篇 |
2003年 | 21篇 |
2002年 | 17篇 |
2001年 | 13篇 |
2000年 | 11篇 |
1999年 | 9篇 |
1998年 | 5篇 |
1997年 | 6篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1993年 | 7篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1985年 | 5篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1972年 | 7篇 |
1971年 | 6篇 |
1970年 | 9篇 |
1969年 | 10篇 |
1968年 | 13篇 |
1967年 | 6篇 |
1966年 | 4篇 |
排序方式: 共有638条查询结果,搜索用时 15 毫秒
1.
The dinoflagellate Prorocentrum cordatum, often called P. minimum, is a potentially toxic alga found in algal blooms. Volatile compounds released by the alga might carry important information, e.g., on its physiological state, and may act as chemical messengers. We report here the identification of volatile organic compounds emitted by two strains, xenic P. cordatum CCMP 1529 and axenic P. cordatum CCMP 1329. The volatiles released during culture were identified despite their low production rates, using sensitive methods such as open-system-stripping analysis (OSSA) on Tenax TA desorption tubes, thermodesorption, cryofocusing and GC/MS-analysis. The analyses revealed 16 compounds released from the xenic strain and 52 compounds from the axenic strain. The majority of compounds were apocarotenoids, aromatic compounds and small oxylipins, but new natural products such as 3,7-dimethyl-4-octanolide were also identified and synthesized. The large difference of compound composition between xenic and axenic algae will be discussed. 相似文献
2.
3.
4.
Adolfo Isla Mnica Saldarriaga‐Crdoba Derie E. Fuentes Romina Albornoz Denise Haussmann Jorge Mancilla‐Schulz Alexis Martínez Jaime Figueroa Ruben Avendao‐Herrera Alejandro Yez 《Journal of fish diseases》2019,42(5):721-737
Piscirickettsia salmonisis the causative bacterial pathogen of piscirickettsiosis, a salmonid disease that causes notable mortalities in the worldwide aquaculture industry. Published research describes the phenotypic traits, virulence factors, pathogenicity and antibiotic‐resistance potential for various P. salmonisstrains. However, evolutionary and genetic information is scarce for P. salmonis. The present study used multilocus sequence typing (MLST) to gain insight into the population structure and evolution of P. salmonis. Forty‐two Chilean P. salmonisisolates, as well as the type strain LF‐89T, were recovered from diseased Salmo salar, Oncorhynchus kisutchand Oncorhynchus mykissfrom two Chilean Regions. MLST assessed the loci sequences of dnaK, efp, fumC, glyA, murG, rpoD and trpB. Bioinformatics analyses established the genetic diversity among P. salmonis isolates (H = 0.5810). A total of 23 sequence types (ST) were identified, 53.48% of which were represented by ST1, ST5 and ST2. Population structure analysis through polymorphism patterns showed few polymorphic sites (218 nucleotides from 4,010 bp), while dN/dS ratio analysis indicated purifying selection for dnaK, epf, fumC, murG, and rpoD but neutral selection for the trpB loci. The standardized index of association indicated strong linkage disequilibrium, suggesting clonal population structure. However, recombination events were detected in a group of seven isolates. Findings included genogroups homologous to the LF‐89T and EM‐90 strains, as well as a seven‐isolate hybrid genogroup recovered from both assessed regions (three O. mykiss and four S. salar isolates). The presented MLST scheme has comparative potential, with promising applications in studying distinct P. salmonis isolates (e.g., from different hosts, farms, geographical areas) and in understanding the epidemiology of this pathogen. 相似文献
5.
The growth data of a commercial aquaculture recirculation system were analysed to investigate the growth performance of reared
turbot (Psetta maxima). Three common growth models (von Bertalanffy, Gompertz and Schnute) were fitted to the growth data documented over a time
period of 6 years. To determine the most suitable model, three different criteria were used: (1) the Akaike index criterion,
(2) the sum of squared residuals and (3) the average daily deviation between the estimated final weight and the observed final
weight. The evaluation of the growth models showed that the Schnute model had the lowest Akaike index, the lowest sum of squared
residuals and the lowest daily deviation between estimated and real weight of all tested growth models. The Schnute model
produced sigmoid growth curves. The estimated growth coefficients were the most realistic ones in regard to biological interpretation.
In contrast, the von Bertalanffy growth model and the Gompertz model estimated inaccurate exponential growth curves and are
therefore unable to simulate the growth data as well as the Schnute model. The results indicate that the von Bertalanffy growth
model is not the optimal model to simulate the present growth data and that the growth potential of reared turbot has probably
not yet been fully exploited in the aquaculture system(s) examined (so far). 相似文献
6.
Spermatogenesis and its endocrine regulation 总被引:2,自引:2,他引:2
Three major phases compose spermatogenesis: mitotic proliferation of spermatogonia, meiosis of spermatocytes, and spermiogenesis,
the restructuring of spermatids into flagellated spermatozoa. The process is fuelled by stem cells that, when dividing, either
self-renew or produce spermatogonia that are committed to proliferation, meiosis, and spermiogenesis. During all phases, germ
cells are in close contact with and require the structural and functional support of Sertoli cells. In contrast to germ cells,
these somatic cells express receptors for sex steroids and follicle-stimulating hormone (FSH), the most important hormones
that regulate spermatogenesis. A typical Sertoli cell response to an endocrine stimulus would be to change the release of
a growth factor that would then mediate the hormone's effect to the germ cells. Recent studies in the Japanese eel have shown,
for example, that in the absence of gonadotropin Sertoli cells produce a growth factor (an orthologue of anti-Müllerian hormone)
that restricts stem cell divisions to the self-renewal pathway; also estrogens stimulate stem cell renewal divisions but not
spermatogonial proliferation. Gonadotropin or 11-ketotestosterone (11-KT) stimulation, however, induces spermatogonial proliferation,
which is in part mimicked by another Sertoli cell-derived growth factor (activin B). Since FSH (besides luteinizing hormone,
LH) stimulates steroidogenesis in fish, and since FSH is the only gonadotropin detected in the plasma of sexually immature
salmonids, increased FSH signalling may be sufficient to initiate spermatogenesis by activating both Sertoli cell functions
and 11-KT production. Another important androgen is testosterone (T), which seems to act via feedback mechanisms that can
compromise FSH-dependent signalling or steroidogenesis. The testicular production of T and 11-KT therefore needs to be balanced
adequately. Further research is required to elucidate in what way(s) 11-KT stimulates later stages of development, such as
entry into meiosis and spermiogenesis. At this period, LH becomes increasingly important for the regulation of androgen production.
Results from mammalian models suggest that during the later phases, the control of germ cell apoptosis via Sertoli cell factors
is an important regulatory mechanism. In many species, sperm cells cannot fertilize eggs until having passed a maturation
process known as capacitation, which includes the acquisition of motility. Progestins that are produced under the influence
of LH appear to play an important role in this context, which involves the control of the composition of the seminal plasma
(e.g., pH values).
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
Relation between soil organic matter and yield levels of nonlegume crops in organic and conventional farming systems 总被引:2,自引:0,他引:2
Christopher Brock Andreas Fließbach Hans‐Rudolf Oberholzer Franz Schulz Klaus Wiesinger Frank Reinicke Wernfried Koch Bernhard Pallutt Bärbel Dittman Jörg Zimmer Kurt‐Jürgen Hülsbergen Günter Leithold 《植物养料与土壤学杂志》2011,174(4):568-575
The aim of this study was to evaluate the interaction between yield levels of nonleguminous crops and soil organic matter (SOM) under the specific conditions of organic and conventional farming, respectively, and to identify implications for SOM management in arable farming considering the farming system (organic vs. conventional). For that purpose, correlations between yield levels of nonlegume crops and actual SOM level (Corg, Nt, Chwe, Nhwe) as well as SOM‐level development were examined including primary data from selected treatments of seven long‐term field experiments in Germany and Switzerland. Yield levels of nonlegume crops were positively correlated with SOM levels, but the correlation was significant only under conditions of organic farming, and not with conventional farming treatments. While absolute SOM levels had a positive impact on yield levels of nonlegumes, the yield levels of nonlegumes and SOM‐level development over time correlated negatively. Due to an increased demand of N from SOM mineralization, higher yield levels of nonlegumes obviously indicate an increased demand for OM supply to maintain SOM levels. Since this observation is highly significant for farming without mineral‐N fertilization but not for farming with such fertilization, we conclude that the demand of SOM‐level maintenance or enhancement and thus adequate SOM management is highly relevant for crop production in organic farming both from an agronomical and ecological point of view. Under conventional management, the agronomic relevance of SOM with regard to nutrient supply is much lower than under organic management. However, it has to be considered that we excluded other possible benefits of SOM in our survey that may be highly relevant for conventional farming as well. 相似文献
8.
Effects of biochar compared to organic and inorganic fertilizers on soil quality and plant growth in a greenhouse experiment 总被引:5,自引:0,他引:5
Our contemporary society is struggling with soil degradation due to overuse and climate change. Pre‐Columbian people left behind sustainably fertile soils rich in organic matter and nutrients well known as terra preta (de Indio) by adding charred residues (biochar) together with organic and inorganic wastes such as excrements and household garbage being a model for sustainable agriculture today. This is the reason why new studies on biochar effects on ecosystem services rapidly emerge. Beneficial effects of biochar amendment on plant growth, soil nutrient content, and C storage were repeatedly observed although a number of negative effects were reported, too. In addition, there is no consensus on benefits of biochar when combined with fertilizers. Therefore, the objective of this study was to test whether biochar effects on soil quality and plant growth could be improved by addition of mineral and organic fertilizers. For this purpose, two growth periods of oat (Avena sativa L.) were studied under tropical conditions (26°C and 2600 mm annual rainfall) on an infertile sandy soil in the greenhouse in fivefold replication. Treatments comprised control (only water), mineral fertilizer (111.5 kg N ha–1, 111.5 kg P ha–1, and 82.9 kg K ha–1), compost (5% by weight), biochar (5% by weight), and combinations of biochar (5% by weight) plus mineral fertilizer (111.5 kg N ha–1, 111.5 kg P ha–1, and 82.9 kg K ha–1), and biochar (2.5% by weight) plus compost (2.5% by weight). Pure compost application showed highest yield during the two growth periods, followed by the biochar + compost mixture. biochar addition to mineral fertilizer significantly increased plant growth compared to mineral fertilizer alone. During the second growth period, plant yields were significantly smaller compared to the first growth period. biochar and compost additions significantly increased total organic C content during the two growth periods. Cation‐exchange capacity (CEC) could not be increased upon biochar addition while base saturation (BS) was significantly increased due to ash addition with biochar. On the other hand, compost addition significantly increased CEC. Biochar addition significantly increased soil pH but pH value was generally lower during the second growth period probably due to leaching of base cations. Biochar addition did not reduce ammonium, nitrate, and phosphate leaching during the experiment but it reduced nitrification. The overall plant growth and soil fertility decreased in the order compost > biochar + compost > mineral fertilizer + biochar > mineral fertilizer > control. Further experiments should optimize biochar–organic fertilizer systems. 相似文献
9.
Species composition of arbuscular mycorrhizal fungi in two mountain meadows with differing management types and levels of plant biodiversity 总被引:1,自引:0,他引:1
Boris Börstler Carsten Renker Ansgar Kahmen François Buscot 《Biology and Fertility of Soils》2006,42(4):286-298
Species composition of arbuscular mycorrhizal fungi (AMF) was analysed in two differently managed mountain grasslands in Thuringia
(Germany). Arbuscular mycorrhizal fungi were studied in the roots of 18 dominant plant species from a total of 56 (32%). Additionally,
spores of AMF were isolated from soil samples. Arbuscular mycorrhizal fungi species composition was analysed based on 96 sequences
of the internal transcribed spacer of the nuclear ribosomal DNA, 72 originated from mycorrhizal roots, and 24 originated from
AMF spores. Phylogenetic analyses revealed a total of 19 AMF species representing all genera of the Glomeromycota except Scutellospora and Pacispora. Despite a different farming intensity, resulting in remarkable differences concerning their plant species diversity (27
against 43 plant species), the diversity of AMF was found to be similar with 11 species on the intensively farmed meadow and
ten species on the extensively farmed one. Nevertheless, species composition between both sites was clearly different. It
thus seems likely that the AMF species composition, but not necessarily the species number, is related to above ground plant
biodiversity in the system under study. 相似文献
10.