全文获取类型
收费全文 | 1065篇 |
免费 | 67篇 |
专业分类
林业 | 117篇 |
农学 | 34篇 |
基础科学 | 9篇 |
249篇 | |
综合类 | 44篇 |
农作物 | 49篇 |
水产渔业 | 173篇 |
畜牧兽医 | 343篇 |
园艺 | 24篇 |
植物保护 | 90篇 |
出版年
2024年 | 1篇 |
2023年 | 10篇 |
2022年 | 16篇 |
2021年 | 46篇 |
2020年 | 34篇 |
2019年 | 42篇 |
2018年 | 40篇 |
2017年 | 37篇 |
2016年 | 39篇 |
2015年 | 28篇 |
2014年 | 57篇 |
2013年 | 72篇 |
2012年 | 95篇 |
2011年 | 99篇 |
2010年 | 51篇 |
2009年 | 52篇 |
2008年 | 75篇 |
2007年 | 66篇 |
2006年 | 50篇 |
2005年 | 44篇 |
2004年 | 40篇 |
2003年 | 36篇 |
2002年 | 32篇 |
2001年 | 10篇 |
2000年 | 12篇 |
1999年 | 5篇 |
1998年 | 5篇 |
1997年 | 8篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1986年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1132条查询结果,搜索用时 234 毫秒
61.
Javier Fernández Laura Marín Raquel álvarez-Alonso Saúl Redondo Juan Carvajal Germán Villamizar Claudio J. Villar Felipe Lombó 《Marine drugs》2014,12(5):2668-2699
Diverse actinomycetes produce a family of structurally and biosynthetically related non-ribosomal peptide compounds which belong to the chromodepsipeptide family. These compounds act as bisintercalators into the DNA helix. They give rise to antitumor, antiparasitic, antibacterial and antiviral bioactivities. These compounds show a high degree of conserved modularity (chromophores, number and type of amino acids). This modularity and their high sequence similarities at the genetic level imply a common biosynthetic origin for these pathways. Here, we describe insights about rules governing this modular biosynthesis, taking advantage of the fact that nowadays five of these gene clusters have been made public (thiocoraline, triostin, SW-163 and echinomycin/quinomycin). This modularity has potential application for designing and producing novel genetic engineered derivatives, as well as for developing new chemical synthesis strategies. These would facilitate their clinical development. 相似文献
62.
M. N. Jiménez J. R. Pinto M. A. Ripoll A. Sánchez-Miranda F. B. Navarro 《Agroforestry Systems》2014,88(5):935-946
In Southwestern Spain, multifunctional silvopastoral systems consisting of pastureland and open oak woodlands are known as Dehesas. These, and other similar systems of the Mediterranean basin, are currently threatened by increasing intensive land use. As a consequence, oak regeneration is declining and is in need of adequate management and active restoration. Traditional restoration practices outplant one-year-old, nursery-produced oak seedlings grown in 250–350 cm3 containers, but establishment and growth results are typically poor. This work examines holm oak (Quercus ilex L. subsp. ballota (Desf.) Samp.) grown in a non-conventional container size (24 l) and age (6–7 years) with three mulch treatments (control, stone, and straw). In an open Dehesa of SE Spain, 106 oak saplings were planted in March 2010, and survival, diameter at breast height (DBH), and leaf-nutrient concentrations were analyzed. Forty months after planting, all treatments showed high survival (81 %) but only straw-mulched saplings differed significantly (94 %) from control (74 %). DBH increased over time but showed no significant differences among mulch treatments. Saplings with high initial DBH showed the greatest growth and change in DBH at the end of the study period. Leaf-nutrient concentrations changed significantly in the year following outplanting. Bi-monthly foliar nutrient concentration data show decreases in P, K, Zn, and B and sharp increases in Ca and Fe. In this work, we provide some evidence concerning the viability of non-conventional oak size for restoring, regenerating, or building up new agroforestry or silvopastoral systems. A combination of saplings with more than 10 mm of DBH and straw mulch is recommended. 相似文献
63.
Andrés de la Cruz Fernando Ramos Gabriel Navarro Andrés Cózar Juan Bécares Gonzalo Muñoz Arroyo 《水产资源保护:海洋与淡水生态系统》2021,31(7):1700-1714
- Spatial modelling is an important research tool to improve our knowledge about the distribution of wildlife in the ocean. Using different modelling techniques (MaxEnt and a generalized linear mixed model), a predictive habitat suitability model was developed for one of the most threatened seabirds in the world: the Balearic shearwater, Puffinus mauretanicus.
- Models were developed using a 10-year dataset from the Gulf of Cádiz (on the south-western Iberian Peninsula), a key foraging area for Balearic shearwaters during migration and the non-breeding season.
- Predictive habitat maps strongly matched the observed distribution patterns, pointing to bathymetric features as the main modelling drivers. The species was concentrated on shallow areas (up to approximately 100 m in depth) of the continental shelf, very close to the mouth of the Guadalquivir River. In contrast with previous studies, Balearic shearwater distribution in the highly dynamic Gulf of Cádiz was not correlated with areas of high chlorophyll a concentration.
- This lack of spatial correlation probably arises from the delay between the phytoplankton bloom and the response of the zooplankton and small fish that are preyed upon by Balearic shearwaters, which may result in important displacements of this trophic chain across the Gulf of Cádiz.
- The analysis presented contributes to a better understanding of the spatial distribution and ecology of the critically endangered top predator in the Gulf of Cádiz and offers important information to improve management plans.
64.
How well does a botanical garden collection of a rare palm capture the genetic variation in a wild population? 总被引:1,自引:0,他引:1
Sandra Namoff Javier Francisco-Ortega Larry R. Noblick M. Patrick Griffith 《Biological conservation》2010,143(5):1110-1117
Conservation is increasingly central to the botanic garden mission. Living plant collections are important components of conservation. Critical evaluation of living conservation collections with population genetic analysis can directly inform ex situ conservation strategy. Here, we quantify the degree of genetic variation captured through a population-based collection protocol, and explore optimal sampling for ex situ conservation. An extensive living collection derived from one population of Leucothrinax morrisii (Arecaceae) provided a model system. We compared 58 specimens from the ex situ collection with 100 individuals from throughout the parent population via 6 ISSR loci. Random bootstrapped resamples of the data were made to model differently structured ex situ collections. Mean diversity (He) differed little between the collection (0.204) and the population (0.216), and genetic distance (D) was very close (0.036). Very few private alleles were found between the collection and the population. Allelic capture, as measured by percent of private alleles, was greater than 94%. Resampled collections of different sizes captured from 48% to 94% of alleles. Pairwise comparison of bootstrapped resamples suggests that increasing the representation of half-sibling groups does not significantly increase allele capture. Increase in allele capture with increasing sample size is greatest at low resample sizes, and showed diminishing returns as resample size increased. No appreciable increase in allele capture was gained through maintaining different half-sibling groups. These data inform sampling for ex situ conservation purposes, and recommend sample sizes of at least 15 individuals, with the upper limit based on resources. 相似文献
65.
Claudia S. Benimeli Analía Medina Carolina M. Navarro Roxana B. Medina María J. Amoroso María I. Gómez 《Water, air, and soil pollution》2010,210(1-4):365-370
In the present study, the growth and the Cu2+accumulation by roots, shoots and leaves of Zea mays were examined using copper sulphate in the range of 10?4 to 10?2 M. Plants of Z. mays did not show inhibition of growth in the presence of 10?4 to 10?2 M Cu2+; however, it was observed growth effects on root when different Cu2+ solution concentrations were used. Only the seedlings exposed to 10?2 M exhibited substantial root growth reduction, yielding only 56% of length with respect to the control. Seedlings exposed to 10?4 M Cu2+ exhibited 16% and 42% growth increase in shoots and leaves, respectively, when compared with the controls. The seedlings treated with 10?3 and 10?2 M Cu2+ were inhibited in shoot and leaf growth. The fresh weights in roots, shoots and leaves significantly decreased at 10?2 M Cu2+. The tolerance index, based on root length, was not significantly different for the three different treatments with copper. However, the total accumulation rate was very low at 10?4 and 10?3 M compared to 10?2 Cu treatments. The capacity of copper accumulation by roots, shoots and leaves of Z. mays plants increased concomitant to the copper concentration, arriving to 382 times more in roots, 157 in shoots and only 16 in leaves, all compared to the controls. Cu could be accumulated by roots, shoots and leaves when the initial concentrations were 10?3 and 10?4 M. However, when it was 10?2 M, the metal could not be accumulated by leaf and shoot levels; the roots could increase their copper accumulation capacity three times compared to the control. Z. mays has potential ability to accumulate Cu without being overly sensitive to Cu toxicity. 相似文献
66.
F. T. Conceição D. S. Sardinha A. D. G. Souza G. R. B. Navarro 《Water, air, and soil pollution》2010,205(1-4):79-91
The chemical dynamics at Meio Stream Basin, São Paulo State, Brazil were evaluated using major elements as natural tracers. The surface water samples from Meio Stream were collected near the mouth of Meio Stream at the confluence with the Mogi-Guaçu River on February 25, 2005, April 20, 2005, and July 8, 2005. Rainwater samples were collected (using a “bulk” collector) for 1 year at one sampling point located about 4 km from downtown Leme city and other possible sources of contamination. The analyses were performed by pH, temperature, dissolved oxygen, electrical conductivity, total solids, sulfate, nitrate, phosphate, alkalinity, chloride, sodium, calcium, potassium, and magnesium. This basin has serious environmental problems in terms of rainwater and surface water quality, which result in the negative annual flux of cations and anions at Meio Stream Basin, with the exception of chlorine. The Meio Stream, downstream from Leme city, receives several elements/compounds through anthropogenic activities, mainly related to the discharge of domestic effluents. Anthropogenic inputs (mining, fossil fuel burning, and agricultural activities) are responsible for the higher concentrations of cations and anions in the rainwater from this basin. 相似文献
67.
L. R. Salado‐Navarro T. R. Sinclair M. Morandini 《Journal of Agronomy and Crop Science》2013,199(1):57-65
Abstract Estimates of soil evaporation and available soil water of no‐tillage fields under farm conditions are important to assess soil water status at sowing of rainfed grain crops. The objective of this study was to predict stored soil water of no‐tillage fields during the fallow periods following soybean (Glycine max (L.) Merr.) and maize (Zea mays L.) crops by accounting for decreased soil evaporation as a result of the residues left on the soil surface. Three simple phenomenological models were used to simulate stored soil water under field conditions at seven locations in Argentina. Two models calculated decreased soil evaporation based on crop residue mass, and the third assumed a constant fractional decrease in bare soil evaporation. All models gave good estimates of soil water content during the fallow periods following a soybean crop. In cases with large quantities of maize residue, however, the models resulted in more water retention in the soil than observed as a consequence of underprediction of soil evaporation. These results indicate that full benefit of crop residue was not being achieved in these fields, probably due to a failure to finely chop and uniformly distribute the crop material on the soil surface. 相似文献
68.
José Javier Peguero-Pina José María Alquézar-Alquézar Stefan Mayr Hervé Cochard Eustaquio Gil-Pelegrín 《Annals of Forest Science》2011,68(3):565-574
Introduction
Scots pine populations in the SE “Sistema Ibérico” range suffered a severe defoliation in the S face of the crown in isolated trees of thinned stands. This process was detected at the end of the winter 2001–2002. 相似文献69.
Estimating balanced structure areas in multi-species forests on the Sierra Madre Occidental,Mexico 总被引:1,自引:0,他引:1
Christian Wehenkel José Javier Corral-Rivas José Ciro Hernández-Díaz Klaus von Gadow 《Annals of Forest Science》2011,68(2):385-394
• Introduction
This study presents a method for estimating the minimum area which exhibits a balanced diameter distribution, and the corresponding number of trees, for different tree species and forest types in the Santiago Papasquiaro region in the State of Durango, Mexico. The balanced structure area is defined as the minimum contiguous area that is required for sustainable management of a multi-sized selection forest. A multi-sized forest represents a balanced structure unit if the relationship between harvest and growth can be maintained, using a defined target diameter distribution and disregarding major natural disturbances. The study is based on 17,577 sample plots in uneven-aged forests, which are selectively harvested by local communities. 相似文献70.
Díaz-Maroto MC Díaz-Maroto Hidalgo IJ Sánchez-Palomo E Pérez-Coello MS 《Journal of agricultural and food chemistry》2005,53(13):5385-5389
Volatile oil extracts of fennel seeds (Foeniculum vulgare Mill.) and thyme leaves (Thymus vulgaris L.) were obtained by simultaneous distillation-extraction (SDE) and supercritical fluid extraction (SFE) and analyzed by gas chromatography-mass spectrometry (GC-MS). In general, fennel oil extracted by SDE and SFE showed similar compositions, with trans-anethole, estragole, and fenchone as the main components. In contrast, thymol and p-cymene, the most abundant compounds in thyme leaves, showed big differences, with generally higher amounts of monoterpenes obtained by SDE. However, in this case, the differences between the extracts were higher. Key odorants of fennel seeds determined by gas chromatography-olfactometry (GC-O) showed similar patterns when applying SDE and SFE. trans-Anethole (anise, licorice), estragole (anise, licorice, sweet), fenchone (mint, camphor, warm), and 1-octen-3-ol (mushroom) were the most intense odor compounds detected in fennel extracts. Thymol and carvacrol, with oregano, thyme, and spicy notes, were identified as key compounds contributing to the aroma of thyme leaves. 相似文献