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   586篇
  免费   51篇
  国内免费   5篇
  2024年   1篇
  2023年   21篇
  2022年   13篇
  2021年   15篇
  2020年   25篇
  2019年   32篇
  2018年   29篇
  2017年   16篇
  2016年   30篇
  2015年   21篇
  2014年   52篇
  2013年   41篇
  2012年   29篇
  2011年   28篇
  2010年   12篇
  2009年   20篇
  2008年   18篇
  2007年   25篇
  2006年   19篇
  2005年   21篇
  2004年   13篇
  2003年   17篇
  2002年   9篇
  2001年   15篇
  2000年   7篇
  1999年   20篇
  1998年   4篇
  1997年   6篇
  1996年   3篇
  1995年   5篇
  1994年   12篇
  1993年   4篇
  1992年   2篇
  1991年   8篇
  1990年   4篇
  1989年   6篇
  1988年   7篇
  1987年   4篇
  1986年   6篇
  1985年   4篇
  1984年   1篇
  1983年   3篇
  1982年   2篇
  1981年   1篇
  1977年   1篇
  1975年   3篇
  1974年   1篇
  1973年   2篇
  1972年   3篇
  1971年   1篇
排序方式: 共有642条查询结果,搜索用时 15 毫秒
81.
82.
Manipulating freezing tolerance in transgenic plants   总被引:5,自引:0,他引:5  
Winterhardiness is a composite of tolerances to freezing, desiccation, ice-encasement, flooding and diseases. From one point of view, winterhardiness may not be easily manipulated by genetic engineering technology because many different genes are involved in the tolerance of these diverse stresses. However, these various stresses have similarities. They promote formation of activated forms of oxygen, promote membrane lipid and protein degradation, cause similar biophysical changes in membrane structure, and culminate with increased leakage of cytoplasmic solutes and loss of cellular membrane functions. These similarities led to the hypothesis that winter injury might be reduced in crop plants if their tolerance of oxidative stress was increased. Towards that objective we created transgenic alfalfa (Medicago sativa L.) plants that overexpress either Mn-SOD or Fe-SOD cDNA (provided by Dirk Inzé, Universiteit Gent). Petiole explants were transformed using Agrobacterium tumefaciens and plants were regenerated by somatic embryogenesis. The primary transgenic plants were screened using PCR (polymerase chain reaction), Southern hybridization and native PAGE for SOD activity. Greenhouse and laboratory studies showed a minimal difference in stress tolerance between the primary transgenic and non-transgenic plants. In the first field trial, four primary transgenic plants expressing two forms of the Mn-SOD cDNA had greater survival after two winters than the non-transgenic RA3. Similar results were obtained in a second field trial, comparing 18 independent transformants with Mn-SOD targeted to the mitochondria, 11 independent transformants with Mn-SOD targeted to the chloroplast and 39 independent transformants with Fe-SOD targeted to the chloroplast, expressed in three different non-transgenic plants. The transgenic plants averaged over 25% higher survival than the non-transgenic controls after one winter. There was no effect of subcellular targeting or SOD type on field survival, but there was variation among independent transformants containing the same SOD construct. Activated oxygen therefore appears to be one of the possible causes of winter injury, and it should be possible to reduce winter injury in transgenic plants by constitutive overexpression of SOD.  相似文献   
83.
84.
85.
The effect of leaf species (willow, Salix fragilis L., and white gum, Eucalyptus viminalis Labill.) and leaf state (senescent or green) on the feeding selectivity and growth rates of three species of macroinvertebrate Notalina sp. Mosely (Trichoptera: Leptoceridae), Koorrnonga sp. Campbell and Suter (Ephemeroptera: Leptophlebiidae) and Physastra gibbosa (Gould) (Mollusca: Planorbidae) were tested in the laboratory. All three species of macroinvertebrate selected green willow most strongly over the other leaf types (senescent willow, green eucalypt and senescent eucalypt). Growth rates of P. gibbosa and Notalina sp. were significantly greater on green willow than on the other leaf types. We were unable to measure the growth of Koorrnonga sp. Invertebrates had access to softer internal tissues of leaf material during preference trials, therefore we do not think that leaf structure was the main influence on selection between these materials. Green willow material may have been a better food source because of the noticeably thicker biofilm that it supported, and this material may also retain higher levels of nutrients than abscissed leaves. We speculate that willow leaves may provide a preferred source of food but will be available for less time than native eucalypt detritus.  相似文献   
86.
Summary Transgenic plants of Osteospermum ecklonis were produced by cocultivation of leaf fragments with Agrobacterium tumefaciens harboring rol genes from A. rhizogenes. The phenotypic alterations caused by the different transgenes were evaluated in field trials. The genetic manipulation produced transgenic plants characterized by the following features: 1) increased number of flowers (e.g., 35SrolC and rolABC); 2) early flowering (e.g., 35SrolC); 3) change of plant growth habit: erect (rolAB, rolABC and 35SrolC) with an increased number of branches (e.g., rolABC). The color of leaves was pale green in 35SrolC and dark green in rolAB transgenic plants. In conclusion this work reports: 1) genetic engineering of the ornamental species O. ecklonis, 2) modification of the main ornamental traits of this species by rol genes, and 3) segregation of the transgenes in the progeny.  相似文献   
87.
For designs with longitudinal observations of ordered categorical data, a nonparametric model is considered where treatment effects and interactions are defined by means of the marginal distributions. These treatment effects are estimated consistently by ranking methods. The hypotheses in this nonparametric setup are formulated by means of the distribution functions. The asymptotic distribution of the estimators for the nonparametric effects are given under the hypotheses. For small samples, a rather accurate approximation is suggested. A clinical trial with ordered categorical data is used to motivate the ideas and to explain the procedures which are extensions of the Wilcoxon‐Mann‐Whitney test to factorial designs with longitudinal observations. The application of the procedures requires only some trivial regularity assumptions.  相似文献   
88.
Drought is a major and constantly increasing abiotic stress factor, thus limiting chickpea production. Like other crops, Kabuli Chickpea genotypes are screened for drought stress through Multi-environment trials (METs). Although, METs analysis is generally executed taking into account only one trait, which provides less significant reliability for the recommendation of genotypes as compared to multi trait-based analysis. Multi trait-based analysis could be used to recommend genotypes across diverse environments. Hence, current research was conducted for selection of superior genotypes through multi-trait stability index (MTSI) by using mixed and fixed effect models under six diverse environments. The genotypic stability was computed for all traits individually using the weighted average of absolute scores from the singular value decomposition of the matrix of best linear unbiased predictions for the genotype vs environment interaction (GEI) effects produced by a linear mixed-effect model index. A superiority index, WAASBY was measured to reflect the MPS (Mean performance and stability). The selection differential for the WAASBY index was 11.2%, 18.49% and 23.30% for grain yield (GY), primary branches per plant (PBP) and Stomatal Conductance (STOMA) respectively. Positive selection differential (0.80% ≤ selection differential ≤ 13.00%) were examined for traits averaged desired to be increased and negative (-0.57% ≤ selection differential ≤ -0.23%) for those traits desired to be reduced. The MTSI may be valuable to the plant breeders for the selection of genotypes based on many characters as being strong and simple selection process. Analysis of MTSI for multiple environments revealed that, the genotypes G20, G86, G31, G28, G116, G12, G105, G45, G50, G10, G30, G117, G81, G48, G85, G17, G32, G4, and G37 were the most stable and high yielding out of 120 chickpea genotypes, probably due to high MPS of selected traits under various environments. It is concluded that identified traits can be utilized as genitors in hybridization programs for the development of drought tolerant Kabuli Chickpea breeding material.  相似文献   
89.
Longini , Datta , and Halloran (1996) proposed to design HIV vaccine trials in a way that will permit the simultaneous estimation of the vaccine effects on susceptibility to infection and on infectiousness of vaccine brak-throughs. The main feature of their design is the inclusion of steady partners of trial participants. They estimate four parameters from the vaccine trial: the probability that a susceptible person will become infected from his/her steady partner, the probability of becoming infected from outside the partnership, the vaccine efficacy for susceptibility and the vaccine efficacy for infectiousness. We show how the estimates of these parameters can be used to predict the attack rate in a given population during a specified period following mass-vaccination. This is an iterative method, as the attack rate depends on the HIV prevalence which, in turn, depends on the number of new cases during that period. The same method is also used to estimate the attack rate in that population during the same period in the absence of vaccination. The estimated attack rates allow us to estimate the population vaccination effectiveness, defined as the fraction HIV cases prevented by a vaccination program.  相似文献   
90.
The classical group sequential test procedures that were proposed by Pocock (1977) and O'Brien and Fleming (1979) rest on the assumption of equal sample sizes between the interim analyses. Regarding this it is well known that for most situations there is not a great amount of additional Type I error if monitoring is performed for unequal sample sizes between the stages. In some cases, however, problems can arise resulting in an unacceptable liberal behavior of the test procedure. In this article worst case scenarios in sample size imbalancements between the inspection times are considered. Exact critical values for the Pocock and the O'Brien and Fleming group sequential designs are derived for arbitrary and for varying but bounded sample sizes. The approach represents a reasonable alternative to the flexible method that is based on the Type I error rate spending function. The SAS syntax for performing the calculations is provided. Using these procedures, the inspection times or the sample sizes in the consecutive stages need to be chosen independently of the data observed so far.  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