全文获取类型
收费全文 | 857篇 |
免费 | 77篇 |
国内免费 | 79篇 |
出版年
2023年 | 20篇 |
2022年 | 31篇 |
2021年 | 52篇 |
2020年 | 51篇 |
2019年 | 39篇 |
2018年 | 41篇 |
2017年 | 35篇 |
2016年 | 31篇 |
2015年 | 43篇 |
2014年 | 43篇 |
2013年 | 45篇 |
2012年 | 44篇 |
2011年 | 38篇 |
2010年 | 38篇 |
2009年 | 33篇 |
2008年 | 34篇 |
2007年 | 48篇 |
2006年 | 39篇 |
2005年 | 39篇 |
2004年 | 32篇 |
2003年 | 24篇 |
2002年 | 26篇 |
2001年 | 17篇 |
2000年 | 16篇 |
1999年 | 19篇 |
1998年 | 11篇 |
1997年 | 13篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1989年 | 7篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有1013条查询结果,搜索用时 31 毫秒
1.
Summary Amylase, dehydrogenase, arylsulphatase and phosphatases activities were measured in a clay-loam soil amended with seven different
crop residues. All enzyme activities, except phosphomonoesterase, were generally higher in the derived soil samples than in
the original soil. Addition of tobacco and sunflower residues caused an increase on most of the enzyme activities while tomato
residues increased only the amylase and phosphodiesterase activities.
As the enzyme activities were positively correlated to each other, a common source of the enzymes is suggested even though
the coefficients of correlation demonstrate that only a low percentage of the variability can be ascribed to the interactions
among enzyme activities. 相似文献
2.
Summary In comparison to cassava grown in monoculture the root infection of cassava with vesicular-arbuscular mycorrhiza was increased by crop rotation with grain legumes in the field. This was also found when cassava was intercropped with legumes and fertilized. A possible specificity of mycorrhizal fungi to increase the yield of one species more than the other when grown in association, is discussed. 相似文献
3.
Glycoalkaloids were used as evidence of the affinities of nine taxa of Solanum Series Megistacrolobum and related potato cultigens from western Bolivia. S. boliviense, S. sanctae-rosae and S. toralapanum contain the commertetraose sugar moiety and appear to represent a relatively wild group within the Series. S. megistacrolobum, S. sogarandinum and S. raphanifolium show anomolous glycoalkaloid profiles that probably reflect hybridization associated with human disturbance. Primitive forms of the S. χ ajanhuiri cultigen are indistinguishable chemicaliy from conspecific weeds that were previously classified as S. megistacrolobum. Variation in total glycoalkaloid content within Series Megistacrolobum likely reflects direct selection by humans for reduced glycoalkaloid levels during the domestication process. 相似文献
4.
《Current biology : CB》2020,30(22):4441-4453.e4
- Download : Download high-res image (186KB)
- Download : Download full-size image
5.
John Bower 《Evolutionary anthropology》1995,4(4):130-139
The study of early food production in sub-Saharan Africa is at least as challenging as it is rewarding. Problems arise in large degree from the scarcity of relevant archeological material, particularly the remains of domesticated plants from prehistoric sites. This is attributable to several factors, including poor preservation, difficulties in recovering such material, and the limited amount of work so far invested in obtaining it. But, problems notwithstanding, fresh data and new methodological approaches have revealed aspects of early African food production that are interesting in themselves, as well as in global perspective. For example, contrary to what occurred in most other parts of the world, livestock herding in Africa often predated the earliest evidence of cultivation of domesticated plants. Moreover, the initial spread of food production throughout much of sub-Saharan Africa was accompanied by iron, rather than lithic, technology. This overview of current knowledge about early African food production is aimed at highlighting developmental patterns while also exposing limitations in our understanding of these patterns. Because of Africa's vast size, uniform coverage in reasonable depth of all parts of the continent is not possible. Thus, for example, I will not explicitly cover the complex neolithic record from Africa's Mediterranean region. Instead, I will generally concentrate on bodies of data and lines of investigation that characterize distinctive features of the African version of initial steps in raising crops and animals. 相似文献
6.
Michael R. Dohm Jack P. Hayes Theodore Garland 《Evolution; international journal of organic evolution》1996,50(4):1688-1701
We tested the hypothesis that locomotor speed and endurance show a negative genetic correlation using a genetically variable laboratory strain of house mice (Hsd:ICR: Mus domesticus). A negative genetic correlation would qualify as an evolutionary “constraint,” because both aspects of locomotor performance are generally expected to be under positive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at testing, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaw's (1987) restricted maximum-likelihood programs, modified to account for our breeding design, which incorporated cross-fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We initially analyzed performances on each trial day and the maximal value. For endurance, the three estimates of narrow-sense heritabilities ranged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The heritability estimate for sprint speed measured on trial day 1 was 0.17, but negative for all other measures. Moreover, the additive genetic covariance between speeds measured on the two days was near zero, indicating that the two measures are to some extent different traits. The additive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistically significant. None of the measures of speed or endurance was significantly genetically correlated with body mass. Thus, we predict that artificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such experiments could lead to a better understanding of the physiological mechanisms leading to trade-offs in aspects of locomotor abilities. 相似文献
7.
Gavin R. Sills William Bridges Salah M. Al-Janabi Bruno W. S. Sobral 《Molecular breeding : new strategies in plant improvement》1995,1(4):355-363
Saccharum robustum Brandes & Jesw. ex Grassl has been suggested as the immediate progenitor species of cultivated sugarcane (S. officinarum L.) [4]. Chromosome pairing and assortment in these two species were previously studied by genetic analysis of single-dose DNA markers in parents in and 44 F1 progeny of a cross between euploid, meiotically regular 2n=80S. officinarum LA Purple andS. robustum Mol 5829 [2]. This same population was subsequently clonally propagated and evaluated in replicated trials for quantitative traits important to sugarcane breeders. Numbers of stalks, tasseled stalks, and stalks with smut, and the average diameter of two stalks were determined one day prior to harvest. At harvest, plant material from each plot was weighed and evaluated for pol (sucrose content) and fiber percentages. Clones were significantly different (P<0.01) for all traits analyzed. Associations of 83 single-dose arbitrarily primed PCR genetic markers with quantitative trait loci (QTL) of recorded traits was determined by single-factor ANOVA, and multiple regression. QTL analysis revealed markers significantly (P<0.05) associated with the expression of each trait analyzed. Markers associated with QTL after multiple regression were tested for digenic linear × linear epistatic interactions. The various multilocus models explained between 23% and 58% of the total phenotypic variation and 32% and 76% of the genotypic variation for the various traits. Digenic interactions were uncommon. Implications for marker-assisted selection in sugarcane and sugarcane domestication are discussed. 相似文献
8.
Genetic diversity in a seed production population vs. natural populations of Sitka Spruce 总被引:2,自引:0,他引:2
Isozyme analysis was applied to estimate the level of variation and the genetic structure of a seed-production population (i.e., seed orchard) and 10 range-wide natural populations of Sitka spruce (Picea sitchensis (Bong.) Carr.). Gene diversity and heterozygosity estimates were comparatively high in both the seed orchard and the natural populations studied. The seed orchard population showed a significantly higher number of alleles per locus and percentage of polymorphic loci. Though not significant, mean heterozygosity of the seed orchard was higher than that observed for all natural populations. Genetic distance analysis indicated that the seed-orchard population was genetically similar to three natural populations from which the parent trees were selected. Parent trees sampling breadth has been identified as the major cause for the observed increased level. The impact of recurrent selection and seed orchard biology and management on maintaining the genetic diversity is discussed. 相似文献
9.
Sonja K. Glasser Martín Hesajim de Santiago-Hernández Oliverio Delgado-Carrillo Luis Alberto Villanueva Espino Adonaji Cortés Pérez Antonio González-Rodríguez Rafael Lira-Saade Mauricio Quesada 《American journal of botany》2023,110(5):e16170
Premise
Domestication of plant species results in phenotypic modifications and changes in biotic interactions. Most studies have compared antagonistic plant-herbivore interactions of domesticated plants and their wild relatives, but little attention has been given to how domestication influences plant-pollinator interactions. Floral attributes and interactions of floral visitors were compared between sister taxa of the genus Cucurbita (Cucurbitaceae), the domesticated C. moschata, C. argyrosperma ssp. argyrosperma and its wild progenitor C. argyrosperma ssp. sororia in the place of origin.Methods
We conducted univariate and multivariate analyses to compare floral morphological traits and analyzed floral reward (nectar and pollen) quantity and quality between flowers of wild and domesticated Cucurbita taxa. Staminate and pistillate flowers of all three taxa were video recorded, and visitation and behavior of floral visitors were registered and analyzed.Results
Most floral morphological characteristics of flowers of domesticated taxa were larger in both staminate and pistillate flowers. Staminate and pistillate flowers presented distinct correlations between floral traits and integration indices between domesticated and wild species. Additionally, pollen quantity and protein to lipid ratio were greater in domesticated species. Cucurbit pollen specialists, Eucera spp., had the highest probability of visit for all Cucurbita taxa.Conclusions
We provide evidence that floral traits of domesticated and wild Cucurbita species experienced different selection pressures. Domesticated Cucurbita species may have more resources invested towards floral traits, thereby increasing attractiveness to pollinators and potentially plant reproductive success. Wild ancestor plant populations should be conserved in their centers of origin to preserve plant-pollinator interactions. 相似文献10.
Approaches to breeding for salinity tolerance - a case study on Porteresia coarctata 总被引:2,自引:0,他引:2
R LATHA C SRINIVAS RAO H M SR SUBRAMANIAM P EGANATHAN M S SWAMINATHAN 《The Annals of applied biology》2004,144(2):177-184
Cereals are the world's major source of food for human nutrition. Among these, rice (Oryza sativa) is the most prominent and represents the staple diet for more than two-fifths (2.4 billion) of the world's population, making it the most important food crop of the developing world (Anon., 2000a). Rice production in vast stretches of coastal areas is hampered due to high soil salinity. This is because rice is a glycophyte and it does not grow well under saline conditions. In order to increase rice production in these areas there is a need to develop rice varieties suited to saline environments. Research has shown that Porteresia coarctata, a highly salt tolerant wild relative of rice growing in estuarine soils, is an important material for transferring salt tolerant characteristics to rice. It is quite possible that Porteresia may be used as a parent for evolving better and truly salt resistant varieties. The inadequate results and the difficulties associated with conventional breeding techniques necessitate the use of the tools of crop biotechnology in unravelling some of the characteristics of Porteresia that have been highlighted in this report. In view of the limited resources available for increasing salinity tolerance to the breeders to wild rice germplasm, Porteresia is undoubtedly one of the key source species for elevating salinity tolerance in cultivated rice. 相似文献