全文获取类型
收费全文 | 1598篇 |
免费 | 87篇 |
国内免费 | 2篇 |
专业分类
林业 | 109篇 |
农学 | 27篇 |
基础科学 | 4篇 |
251篇 | |
综合类 | 175篇 |
农作物 | 76篇 |
水产渔业 | 86篇 |
畜牧兽医 | 815篇 |
园艺 | 33篇 |
植物保护 | 111篇 |
出版年
2023年 | 17篇 |
2022年 | 9篇 |
2021年 | 26篇 |
2020年 | 39篇 |
2019年 | 46篇 |
2018年 | 38篇 |
2017年 | 40篇 |
2016年 | 44篇 |
2015年 | 43篇 |
2014年 | 42篇 |
2013年 | 82篇 |
2012年 | 102篇 |
2011年 | 138篇 |
2010年 | 40篇 |
2009年 | 68篇 |
2008年 | 104篇 |
2007年 | 107篇 |
2006年 | 88篇 |
2005年 | 95篇 |
2004年 | 82篇 |
2003年 | 84篇 |
2002年 | 62篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 21篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1976年 | 7篇 |
1975年 | 18篇 |
1974年 | 4篇 |
1970年 | 4篇 |
排序方式: 共有1687条查询结果,搜索用时 15 毫秒
861.
Assessing genetic diversity in the cassava (Manihot esculenta Crantz) germplasm collection in Brazil using PCR-based markers 总被引:2,自引:0,他引:2
Knowledge of the origin, organization and nature of the cassava (Manihot esculenta Crantz) germplasm collection in Brazil is incomplete due to lack of critical information on several aspects of the collection. This study verifies the utility of SSR-primed PCR markers for germplasm assessment and then utilizes these markers as well as RAPD's to characterize the Brazilian collection. We specifically address the following questions: 1)what is the relationship of morphologically closely related species to cultivated cassava? 2) What is the genetic diversity of cultivars within and between different habitats in Brazil? 3) Do agronomic traits and molecular markers reveal the same relationship among cassava accessions? 4) How complete is the Brazilian cassava collection and how well is it represented in the Word Core Collection of cassava, maintained by CIAT? Results of the interspecific studies of cassava and its wild relatives confirms the close relationship of cassava, Manihot esculenta ssp. esculenta to Manihot esculenta ssp. flabellifolia as well identifying several other closely related wild species. Next, PCR-based markers indicate a strong grouping of varieties related to the region of cultivation in Brazil. Moreover, important regions such as Cerrados and Amazon are relatively poorly represented in germplasm collections. Interestingly, the relationships of accessions based on agronomic traits are not fully congruent with relationships revealed with RAPD markers. Finally, the genetic diversity of the Brazilian cassava collection is not fully represented in the Core of the Word Core Collection of CIAT. 相似文献
862.
Barillari J Iori R Broccoli M Pozzetti L Canistro D Sapone A Bonamassa B Biagi GL Paolini M 《Journal of agricultural and food chemistry》2007,55(14):5505-5511
Brassica vegetables are an important dietary source of glucosinolates (GLs), whose breakdown products exhibit anticancer activity. The protective properties of Brassicaceae are believed to be due to the inhibition of Phase-I or induction of Phase-II xenobiotic metabolizing enzymes (XMEs), thus enhancing carcinogen clearance. To study whether GLs affect XMEs and the role of their chemical structure, we focused on two alkylthio GLs differing in the oxidation degree of the side chain sulfur. Male Sprague-Dawley rats were supplemented (per oral somministration by gavage) with either glucoraphasatin (4-methylthio-3-butenyl GL; GRH) or glucoraphenin (4-methylsulfinyl-3-butenyl GL; GRE), at 24 or 120 mg/kg body weight in a single or repeated fashion (daily for four consecutive days), and hepatic microsomes were prepared for XME analyses. Both GLs were able to induce XMEs, showing different induction profiles. While the inductive effect was stronger after multiple administration of the higher GRH dosage, the single lower GRE dose was the most effective in boosting cytochrome P-450 (CYP)-associated monooxygenases and the postoxidative metabolism. CYP3A1/2 were the most affected isoforms by GRH treatment, whereas GRE induced mainly CYP1A2 supported oxidase. Glutathione S-transferase increased up to approximately 3.2-fold after a single (lower) GRE dose and UDP-glucuronosyl transferase up to approximately 2-fold after four consecutive (higher) GRH doses. In conclusion, the induction profile of these GLs we found is not in line with the chemopreventive hypothesis. Furthermore, the oxidation degree of the side chain sulfur of GLs seems to exert a crucial role on XME modulation. 相似文献
863.
864.
The toxicity of deltamethrin to larvae of the pine beauty moth, Panolis flammea Schiff., and its two parasitoid species, Barichneumon bilunulatus Grav. and Rictichneumon pachymerus Katz., was evaluated. Two methods, exposure of insects on treated filter paper and topical application of insecticide, were used. Results showed that larvae of the pest were many times more susceptible to deltamethrin than imagines of either of the two parasitoid species. 相似文献
865.
Belowground interactions for water between trees and grasses in a temperate semiarid agroforestry system 总被引:1,自引:0,他引:1
María Elena Fernández Javier Gyenge Julian Licata Tomás Schlichter Barbara J. Bond 《Agroforestry Systems》2008,74(2):185-197
A fundamental hypothesis of agroforestry is the complementary use of soil resources. However, productivity of many agroforestry
systems has been lower than expected due to net competition for water, highlighting the need for a mechanistic understanding
of belowground interactions. The goal of this study was to examine root–root interactions for water in a temperate semiarid
agroforestry system, based on ponderosa pines and a Patagonian grass. The hypotheses were: (a) A greater proportion of water
uptake by pines is from deeper soil layers when they are growing with grasses than when they are growing alone; (b) Growth
of grasses is improved by the use of water hydraulically lifted by pines. We used stable isotopes of O to analyze water sources
of plants, and we measured sapflow direction in pine roots and continuous soil water content with a very sensitive system.
We also installed barriers to isolate the roots of a set of grasses from pine roots, in which we measured water status, relative
growth and water sources, comparing to control plants. The results indicated that pines and grasses show some complementary
in the use of soil water, and that pines in agroforestry systems use less shallow water than pines in monoculture. We found
evidence of hydraulic lift, but contradicting results were obtained comparing growth and isotope results of the root isolation
experiment. Therefore, we could not reject nor accept that grasses use water that is hydraulically lifted by the pines, or
that this results in a positive effect on grass growth. This information may contribute to understand the complex and variable
belowground interactions in temperate agroforestry. 相似文献
866.
Guillermo Gea-Izquierdo Barbara Allen-Díaz Alfonso San Miguel Isabel Cañellas 《Annals of Forest Science》2010,67(1):112-112
867.
Christoph Straub Holger Weinacker Barbara Koch 《European Journal of Forest Research》2010,129(6):1069-1080
This article compares three methods for forest resource estimation based on remote sensing features extracted from Airborne
laser scanning and CIR orthophotos. The estimation was made exemplarily for the total stem volume of trees for a given area,
measured in cubic metres per hectare [m3 ha−1] (as one of the most important quantitative parameters to characterise a forest stand). The following methods were compared:
Regression Analysis (RA), k-NN (nearest neighbour) method and a method that utilises regional yield tables, referred to as the yield table method (YT-method).
The estimation of stem volume was examined in a mixed forest in Southern Germany using 300 circular inventory plots, each
with a size of 452 m2. Remote sensing features relating to vegetation height and structures were extracted and used as input
variables in the different approaches. The accuracy of the estimation was analysed using scatter plots and quantified using
absolute and relative root mean square errors (RMSE). The comparison was made for all plots, as well as for averaged plot
values located within forest stands that have the same age class. On “plot level” the RMSE yielded 79.79 m3 ha−1 (RA), 81.93 m3 ha−1 (k-NN) and 81.78 m3 ha−1 (YT-method) and for the averaged values 35.75 m3 ha−1 (RA), 35.06 m3 ha−1 (k-NN) and 42.98 m3 ha−1 (YT-method). Advantages and disadvantages, as well as requirements, of the methods are discussed. 相似文献
868.
869.
Age-related trends in red spruce foliar plasticity in relation to declining productivity 总被引:2,自引:0,他引:2
Phenotypic plasticity in needle morphology with increasing tree size and age was investigated by comparing four age classes of red spruce (Picea rubens Sarg.) ranging from juvenile (3-12 years old) to mature (over 100 years old). With increase in tree age there were significant increases in leaf mass per unit area (LMA), mesophyll and vascular bundle area as a percentage of total needle cross-sectional area, and stomatal density. Within the vascular bundle, both xylem cross-sectional area and tracheid lumen area increased significantly, whereas air space as a percentage of total cross-sectional area decreased. These morphological changes were associated with a significant decrease in photosynthetic capacity and stomatal conductance, and an increase in (13)C enrichment. Although both photosynthetic capacity and whole-tree conductance decreased significantly between age classes 3 and 12 years, they did not differ between age classes 53 and 127 years, even though needle (13)C enrichment was significantly greater in the 127-year age class. Thus there appear to be compensatory mechanisms that maintain photosynthetic capacity as trees increase in size and vascular complexity, which in red spruce and other species, may affect leaf hydraulic conductance. Although increased LMA may contribute to reduced photosynthetic capacity in red spruce, similar relationships are not seen in other conifers. 相似文献
870.