全文获取类型
收费全文 | 3318篇 |
免费 | 293篇 |
国内免费 | 293篇 |
出版年
2024年 | 6篇 |
2023年 | 42篇 |
2022年 | 58篇 |
2021年 | 68篇 |
2020年 | 113篇 |
2019年 | 144篇 |
2018年 | 155篇 |
2017年 | 119篇 |
2016年 | 126篇 |
2015年 | 144篇 |
2014年 | 200篇 |
2013年 | 257篇 |
2012年 | 178篇 |
2011年 | 194篇 |
2010年 | 127篇 |
2009年 | 134篇 |
2008年 | 141篇 |
2007年 | 162篇 |
2006年 | 163篇 |
2005年 | 141篇 |
2004年 | 101篇 |
2003年 | 101篇 |
2002年 | 89篇 |
2001年 | 88篇 |
2000年 | 66篇 |
1999年 | 43篇 |
1998年 | 70篇 |
1997年 | 46篇 |
1996年 | 47篇 |
1995年 | 48篇 |
1994年 | 46篇 |
1993年 | 45篇 |
1992年 | 45篇 |
1991年 | 33篇 |
1990年 | 29篇 |
1989年 | 21篇 |
1988年 | 29篇 |
1987年 | 26篇 |
1986年 | 28篇 |
1985年 | 31篇 |
1984年 | 32篇 |
1983年 | 29篇 |
1982年 | 27篇 |
1981年 | 29篇 |
1980年 | 26篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有3904条查询结果,搜索用时 15 毫秒
61.
62.
Chlorophyll fluorescence analysis is one of the most convenient and widespread techniques used to monitor photosynthesis performance in plants. In this work, after a brief overview of the mechanisms of regulation of photosynthetic electron transport and protection of photosynthetic apparatus against photodamage, we describe results of our study of the effects of actinic light intensity on photosynthetic performance in Tradescantia species of different ecological groups. Using the chlorophyll fluorescence as a probe of photosynthetic activity, we have found that the shade-tolerant species Tradescantia fluminensis shows a higher sensitivity to short-term illumination (≤20 min) with low and moderate light (≤200 μE m−2 s−1) as compared with the light-resistant species Tradescantia sillamontana. In T. fluminensis, non-photochemical quenching of chlorophyll fluorescence (NPQ) and photosystem II operational efficiency (parameter ΦPSII) saturate as soon as actinic light reaches ≈200 μE m−2 s−1. Otherwise, T. sillamontana revealed a higher capacity for NPQ at strong light (≥800 μE m−2 s−1). The post-illumination adaptation of shade-tolerant plants occurs slower than in the light-resistant species. The data obtained are discussed in terms of reactivity of photosynthetic apparatus to short-term variations of the environment light. 相似文献
63.
It has been well-established that many epiphytic bromeliads of the atmospheric-type morphology, i.e., with leaf surfaces completely covered by large, overlapping, multicellular trichomes, are capable of absorbing water vapor from the atmosphere when air humidity increases. It is much less clear, however, whether this absorption of water vapor can hydrate the living cells of the leaves and, as a consequence, enhance physiological processes in such cells. The goal of this research was to determine if the absorption of atmospheric water vapor by the atmospheric epiphyte Tillandsia usneoides results in an increase in turgor pressure in leaf epidermal cells that subtend the large trichomes, and, by using chlorophyll fluorescence techniques, to determine if the absorption of atmospheric water vapor by leaves of this epiphyte results in increased photosynthetic activity. Results of measurements on living cells of attached leaves of this epiphytic bromeliad, using a pressure probe and of whole-shoot fluorescence imaging analyses clearly illustrated that the turgor pressure of leaf epidermal cells did not increase, and the photosynthetic activity of leaves did not increase, following exposure of the leaves to high humidity air. These results experimentally demonstrate, for the first time, that the absorption of water vapor following increases in atmospheric humidity in atmospheric epiphytic bromeliads is mostly likely a physical phenomenon resulting from hydration of non-living leaf structures, e.g., trichomes, and has no physiological significance for the plant's living tissues. 相似文献
64.
The group 1 pathogenesis-related (PR-1) proteins have long been considered hallmarks of hypersensitive response/defense pathways in plants, but their biochemical functions are still obscure despite resolution of the NMR/X-ray structures of several PR-1-like proteins, including P14a (the prototype PR-1). We report here the characterization of two basic PR-1 proteins (PR-1-1 and PR-1-5) recently identified from hexaploid wheat (Triticum aestivum). Both proteins were expressed in Pichia pastoris as a single major species of ∼15 kDa. Sequence identity of the expressed PR-1 proteins was verified by MALDI-TOF/TOF analysis. Accumulation of the native PR-1-5 protein in pathogen-challenged wheat was confirmed by protein gel blot analysis. Low-temperature SDS-PAGE and yeast two-hybrid assays revealed that PR-1-1 exists primarily as a monomer whereas PR-1-5 forms homodimers. Both PR-1 proteins are resistant to proteases compared to bovine serum albumin, but PR-1-1 shows resistance mainly to subtilisin and protease K (serine proteases) whereas PR-1-5 shows resistance to subtilisin, protease K and papain (a cysteine protease). Site-specific mutations at the five putative active sites in the PR-1 domain all affected dimerization, with the mutations at Glu-72 and Glu-102 (in the PR-1-5 numeration) also diminishing protease resistance. Sequence analysis revealed that the Glu-72 and Glu-102 residues are located in motif-like sequences that are conserved in both PR-1 and the human apoptosis-related caspase proteins. These findings prompt us to examine the function of PR-1 for a role in protease-mediated programmed cell death pathways in plants. 相似文献
65.
66.
M. Lotfalian M. Schaffie E. Darezereshki Z. Manafi M. Ranjbar 《Geomicrobiology journal》2013,30(8):697-703
Bacterially assisted heap leaching is an economical technology for treating low grade copper sulphides. In the present research, bioleaching of low grade chalcopyrite ore (1% chalcopyrite, and 3% pyrite) have been investigated using moderate thermophilic bacteria. The ore sample has low solubility in acid solution (about 5%). Experiments were carried out using column reactors and the effect of particle size (?12.7, ?19.07 and ?25.04 mm) and external addition of carbon dioxide to the induced air (10% v/v) have been investigated. Results have shown that the copper recovery increased with reducing particle size, and carbon dioxide addition improved bacterial activity and copper dissolution. In the optimum condition, i.e., particles finer than 12.07 mm and 10% (v/v) carbon dioxide addition, 69.68% of copper were extracted. 相似文献
67.
Ajay Kumar D.M. Pandey Thomson Abraham Jacob Mathew Padma Ramachandran 《Archives Of Phytopathology And Plant Protection》2013,46(6):710-720
Tapping panel dryness (TPD) syndrome affecting rubber tree (Hevea brasiliensis) is known to reduce natural latex production. Its aetiology remains ambiguous despite long years of research. A low molecular weight RNA similar to viroid RNA was isolated from TPD-affected samples of rubber trees. In the present study, a modified return-polyacrylamide gel electrophoresis procedure was standardised. The viroid-like low molecular weight (LMW) RNA was found associated with leaf, bark and root tissues and rubber seedlings. The technique was employed to detect LMW RNA in different clones of rubber planted in different locations and in bud-grafted plants. The LMW RNA isolated from TPD-affected trees was found infectious on seedlings of tomato cv Pusa Ruby. The LMW RNA was reisolated from symptomatic tomato leaves but not from control plants. This is for the first time that a biotic agent, a viroid RNA, is found consistently associated with the syndrome. The technology developed can be useful to demonstrate the onset of TPD in untapped trees in the absence of other methods such as nucleic acid hybridisation. 相似文献
68.
69.
Sandhya Sankaranarayanan Margarita de la Llera-Moya Denise Drazul-Schrader Michael C. Phillips Ginny Kellner-Weibel George H. Rothblat 《Journal of lipid research》2013,54(3):671-676
An important mechanism contributing to cell cholesterol efflux is aqueous transfer in which cholesterol diffuses from cells into the aqueous phase and becomes incorporated into an acceptor particle. Some compounds can enhance diffusion by acting as shuttles transferring cholesterol to cholesterol acceptors, which act as cholesterol sinks. We have examined whether particles in serum can enhance cholesterol efflux by acting as shuttles. This task was accomplished by incubating radiolabeled J774 cells with increasing concentrations of lipoprotein-depleted sera (LPDS) or components present in serum as shuttles and a constant amount of LDL, small unilamellar vesicles, or red blood cells (RBC) as sinks. Synergistic efflux was measured as the difference in fractional efflux in excess of that predicted by the addition of the individual efflux values of sink and shuttle alone. Synergistic efflux was obtained when LPDS was incubated with cells and LDL. When different components of LPDS were used as shuttles, albumin produced synergistic efflux, while apoA-I did not. A synergistic effect was also obtained when RBC was used as the sink and albumin as shuttle. The previously observed negative association of albumin with coronary artery disease might be linked to reduced cholesterol shuttling that would occur when serum albumin levels are low. 相似文献
70.
Marta R. Hentschke Carlos E. Poli-de-Figueiredo Bartira E. Pinheiro da Costa Lesia O. Kurlak Paula J. Williams Hiten D. Mistry 《Journal of lipid research》2013,54(10):2658-2664
Atherosis of spiral arteries in uteroplacental beds from preeclamptic women resemble those of atherosclerosis, characterized by increased plasma lipids and lipoproteins. We hypothesized that: 1) lipoprotein receptors/transporters in the placenta would be upregulated in preeclampsia, associated with increased maternal and fetal lipoprotein concentrations; and 2) expression of these would be reduced in preeclamptic placentae from women delivering small-for-gestational-age (SGA) infants. Placental biopsies and maternal and umbilical serum samples were taken from 27 normotensive and 24 preeclamptic women. Maternal/umbilical cord serum LDL, HDL, total cholesterol, and triglycerides were measured. Placental mRNA expression of lipoprotein receptors/transporters were quantified using quantitative RT-PCR. Protein localization/expression of LDL receptor-related protein 1 (LRP-1) in the preeclamptic placentae with/without SGA was measured by immunohistochemistry. Placental mRNA expression of all genes except paraoxonase-1 (PON-1), microsomal triglyceride transfer protein (MTTP), and protein disulfide isomerase family A member 2 (PDIA2) were observed. No differences for any lipoprotein receptors/transporters were found between groups; however, in the preeclamptic group placental LRP-1 expression was lower in SGA delivering mothers (n = 7; P = 0.036). LRP-1 protein was localized around fetal vessels and Hofbauer cells. This is the first detailed study of maternal/fetal lipoprotein concentrations and placental lipoprotein receptor mRNA expression in normotensive and preeclamptic pregnancies. These findings do not support a role of altered lipid metabolism in preeclampsia, but may be involved in fetal growth. 相似文献