全文获取类型
收费全文 | 2266篇 |
免费 | 67篇 |
国内免费 | 63篇 |
出版年
2023年 | 11篇 |
2022年 | 10篇 |
2021年 | 24篇 |
2020年 | 25篇 |
2019年 | 30篇 |
2018年 | 19篇 |
2017年 | 24篇 |
2016年 | 23篇 |
2015年 | 33篇 |
2014年 | 37篇 |
2013年 | 67篇 |
2012年 | 33篇 |
2011年 | 36篇 |
2010年 | 38篇 |
2009年 | 75篇 |
2008年 | 78篇 |
2007年 | 91篇 |
2006年 | 79篇 |
2005年 | 88篇 |
2004年 | 71篇 |
2003年 | 74篇 |
2002年 | 66篇 |
2001年 | 63篇 |
2000年 | 79篇 |
1999年 | 91篇 |
1998年 | 100篇 |
1997年 | 88篇 |
1996年 | 84篇 |
1995年 | 91篇 |
1994年 | 70篇 |
1993年 | 86篇 |
1992年 | 76篇 |
1991年 | 79篇 |
1990年 | 72篇 |
1989年 | 53篇 |
1988年 | 64篇 |
1987年 | 43篇 |
1986年 | 45篇 |
1985年 | 49篇 |
1984年 | 30篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 25篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1972年 | 4篇 |
排序方式: 共有2396条查询结果,搜索用时 218 毫秒
121.
Geraldine H. Fleming Catherine M. Kramer Trang Le Raymond D. Shillito 《Plant science》1995,110(2):187-192
During eukaryotic cell transformation, the transforming DNA must enter the host cell, traverse the cytoplasm and enter the nucleus before becoming stably integrated into the genome. The limiting step for plant protoplast transformation may lie at the cell membrane, the nuclear membrane, or at the integration step. We show here that the size of the DNA fragment containing the selectable marker used to monitor transformation can directly affect the efficiency of stable transformation. In both tobacco and maize protoplasts, the smallest DNA fragments gave the highest stable transformation frequencies. 相似文献
122.
J. Svetek V. Furtula M. Nemec E. A. Nothnagel M. Schara 《The Journal of membrane biology》1995,143(1):19-28
Translational diffusion of a fluorescent sterol probe was measured in the plasma membranes of protoplasts isolated from cortical cells of the primary root of maize seedlings. The apparent lateral diffusion coefficient was typically observed to be nearly insensitive to temperature, while the mobile fraction increased with increasing temperature. These fluorescence photobleaching recovery (FPR) measurements were compared with the electron paramagnetic resonance (EPR) spectra of the methyl ester of 13-doxyl palmitic acid in membranes of corn root tissue in situ. The complex spectra observed with this probe were analyzed as weighted sums of simpler spectra of various order parameters and rotational correlation times. The reconstituted spectra calculated from the model show that EPR also detects a mobile (less ordered, fluid) fraction, distinguished by the order parameter S=0.1 to 0.2, which becomes more abundant as temperature increases and is qualitatively comparable to the mobile fraction determined by the FPR method. The observed results on the mobile fractions and the diffusion rates for translational (FPR) as well as rotational (EPR) motions are interpreted in terms of membrane organization, thus providing information on the population and structural patterns of the coexisting domains with a special emphasis on the response of the membrane to temperature changes.This work was supported in part by grants from the Ministry of Science and Technology of the Republic of Slovenia and the International Research Program of the U.S. Department of Agriculture (USDA-JF 814-51) to M.S., and by grants from the Competitive Grants Program of the U.S. Department of Agriculture (88-37264-3807 and 90-37264-5471) to E.A.N. 相似文献
123.
Induced single fertilization in maize 总被引:1,自引:1,他引:0
Akio Kato 《Sexual plant reproduction》1997,10(2):96-100
Bicellular pollen with one vegetative nucleus and one diploid arrested generative cell (”monospermic” pollen) was induced
by trifluralin treatment of diploid maize plants at 7–9 days before flowering. The arrested generative cell (seemingly a diploid
sperm cell) fused with the central cell of diploid plants and produced shriveled endosperm resembling that of a 2n×4n cross
in maize. Dual pollination experiments with a purple embryo marker revealed single fertilization events in which the union
of one sperm cell with the egg occurs but there is no union of a second sperm cell with the central cell. Singly fertilized
ovules survived at least 4 days. Furthermore, many viable triploid plants were obtained. This technique therefore appears
to have the potential for manipulating ploidy level in crops and may become useful in investigating fertilization mechanisms
of angiosperms.
Received: 1 October 1996 / Revision accepted: 8 January 1997 相似文献
124.
NADH specific glutamate dehydrogenase (GDH) activity was examined in roots and shoots of maize seedlings grown in half-strength
Hoagland’s solution containing NH4NO3 as sole nitrogen source under irradiance of 60 W m−2 and temperature of 25±2°C. When 5,5′-dithio-bis (2-nitrobenzoic acid) (DTNB) was supplied to the assay mixture, it inhibited
NADH-GDH activity in both roots and shoots, irrespective of whether the enzymes were extracted from light- or dark-treated
roots and shoots. In each case the inhibition increased with the increase in DTNB concentration. At the maximum concentration
of DTNB used (20 μM) the inhibition of shoot NADH-GDH was more pronounced than inhibition of root enzyme. This indicated differences
in shoot and root NADH-GDH. 相似文献
125.
126.
Tomomi Nakamoto 《Journal of plant research》1995,108(1):71-75
The growth of the primary seminal root of maize (Zea mays L.) is characterized by an initial negative gravitropic reaction and a later positive one that attains a plagiotropic liminal
angle. The effects of temperature and water potential of the surrounding soil on these gravitropic reactions were studied.
Temperatures of 32, 25, and 18C and soil water potentials of −5,−38, and −67 kPa were imposed and the direction of growth
was measured for every 1 cm length of the root. The initial negative gravitropic reaction extended to a distance of about
10cm from the graln. Higher temperatures reduced the initial negative gravitropic reaction. Lower soil water potential induced
a downward growth at root emergence. A mathematical model, in which it was assumed that the rate of the directional change
of root growth was a sum of a time-dependent negative gravitropic reaction and an establishment of the liminal angle, adequately
fitted the distance-angle relations. It was suggested that higher temperatures and/or a lower water potential accelerated
the diminution of the intitial negative gravitropic reaction. 相似文献
127.
Effects of calcium,magnesium, potassium and boron on sperm cells isolated from pollen of Zea mays L.
G. Zhang C. M. Williams M. K. Campenot L. E. McGann A. J. Cutler D. D. Cass 《Sexual plant reproduction》1995,8(2):113-122
Our previous studies showed that Brewbaker and Kwack salts, which have been widely used in pollen germination and sperm isolation, are not appropriate for the maintenance of isolated maize (Zea mays L.) sperm cells. In the present study, we have characterized the effects of each BKS component salt on the integrity of isolated sperm cells using hemacytometry. At 0.01 and 0.1 mM, there were no differences in cell number between control and any salt-treated cells except a 22% decrease with 0.1 mM MgSO4 at 48 h. At the 1 mM level, cell number decreased with time in the presence of Ca(NO3)2 and MgSO4, with loss of integrity of most cells at 48 h, while KNO3 and H3BO3 had little or no effect. Further characterization of calcium-induced reduction in cell integrity using flow cytometry showed that depletion of possible residual free calcium by addition of EGTA to the suspension medium improved cell longevity and viability. Exposure of isolated sperm cells to 1 mM calcium had no effect on cell integrity and viability in 5 h; however, only 12% of cells remained intact at 24 h. The reduction in cell integrity was hastened when cells were pretreated with the calcium ionophore A23187 prior to exposure to 1 mM calcium, with a 54% reduction in cell number at 1 h and complete cell lysis at 24 h. However, depletion of cytosolic free calcium by pretreatment of cells with the calcium ionophore followed by resuspension in the presence of EGTA resulted in rapid reduction of cell integrity as well. These results collectively suggest that maize sperm cells are sensitive to exogenous free calcium; however, a certain level of cytosolic free calcium is necessary for maintenance of integrity. Mechanisms of calcium-induced reduction in cell integrity are discussed along with possible roles of the sensitivity of sperm cells to calcium in fertilization. 相似文献
128.
J. A. Ampofo 《Biocontrol Science and Technology》1995,5(4):417-424
Locally available raw materials were used as fermentation media for the preparation of an effective bacterial insecticide of Bacillus sphaericus in Ghana. The choice of materials for media production was based on their availability, their cost and how well they supported growth and sporulation of the bacterium. The materials used were anchovy (Engraulis encrasicolus), spent grain from breweries, bambara beans (Vigna subterranea), sprout maize (Zea mays) and B. sphaericus strain IAB 881. The larvicidal activities of each final whole culture of the insecticides prepared were evaluated against third and fourth instar larvae of the mosquito species Culex quinquefasciatus. The levels of larvicidal activity exhibited by preparations using the raw materials were similar to those where the bacterium was cultured in a synthetic medium, with median lethal concentrations ranging from 0.30 10- 5 to 0.68 10-6. Cell counts were in the range 11 108 - 36 108 colony-forming units ml-1, and spore counts were between 29 107 and 61 107 ml-1. 相似文献
129.
Breeding for resistance to downy mildews and stalk rots in maize 总被引:1,自引:0,他引:1
S. Lal I. S. Singh 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1984,69(2):111-119
Summary The present review includes information on distribution, symptoms, inoculation techniques, disease rating, sources of resistance, genetics of resistance, breeding approaches for resistance, and the present status of resistance breeding with respect to Sclerophthora and Peronosclerospora downy mildews and Erwinia, Cephalosporium and Fusarium stalk rots. Some suggestions highlighting research gaps pertinent to future breeding strategies are mentioned.Publication No. 2993, Experiment Station, GB Pant University of Agriculture and Technology, Pantnagar, India 相似文献
130.
D. Przybyl E. Fritzsche K. Edwards H. Kössel H. Falk J. A. Thompson G. Link 《Plant molecular biology》1984,3(3):147-158
Summary The genes coding for rRNAs from mustard chloroplasts were mapped within the inverted repeat regions of intact ctDNA and on ctDNA fragments cloned in pBR322. R-loop analysis and restriction endonuclease mapping show that the genes for 16S rRNA map at distances of 17 kb from the junctions of the repeat regions with the large unique region. The genes for 23S rRNA are located at distances of 2.8 kb from the junctions with the small unique region. Genes for 4.5S and 5S rRNA are located in close proximity to the 23S rRNA genes towards the small unique region. DNA sequencing of portions of the 5 terminal third from the mustard 16S rRNA gene shows 96–99% homology with the corresponding regions of the maize, tobacco and spinach chloroplast genes. Sequencing of the region proximal to the 16S rRNA gene reveals the presence of a tRNAVal gene in nearly the same position and with identical sequence as in maize, tobacco and spinach. Somewhat less but still strong homology is also observed for the tDNA Val/16S rDNA intercistronic regions and for the regions upstream of the tRNAVal gene. However, due to many small and also a few larger deletions and insertions in the leader region, common reading frames coding for homologous peptides larger than 44 amino acids can not be detected; it is therefore unlikely that this region contains a protein coding gene. 相似文献