全文获取类型
收费全文 | 1861篇 |
免费 | 97篇 |
国内免费 | 208篇 |
专业分类
2166篇 |
出版年
2024年 | 3篇 |
2023年 | 33篇 |
2022年 | 32篇 |
2021年 | 39篇 |
2020年 | 47篇 |
2019年 | 38篇 |
2018年 | 45篇 |
2017年 | 48篇 |
2016年 | 55篇 |
2015年 | 59篇 |
2014年 | 54篇 |
2013年 | 146篇 |
2012年 | 54篇 |
2011年 | 62篇 |
2010年 | 62篇 |
2009年 | 55篇 |
2008年 | 70篇 |
2007年 | 50篇 |
2006年 | 67篇 |
2005年 | 68篇 |
2004年 | 53篇 |
2003年 | 74篇 |
2002年 | 58篇 |
2001年 | 52篇 |
2000年 | 44篇 |
1999年 | 42篇 |
1998年 | 38篇 |
1997年 | 53篇 |
1996年 | 43篇 |
1995年 | 46篇 |
1994年 | 46篇 |
1993年 | 55篇 |
1992年 | 51篇 |
1991年 | 64篇 |
1990年 | 42篇 |
1989年 | 39篇 |
1988年 | 47篇 |
1987年 | 35篇 |
1986年 | 34篇 |
1985年 | 26篇 |
1984年 | 36篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 5篇 |
排序方式: 共有2166条查询结果,搜索用时 0 毫秒
161.
Russell DA Spatola LA Dian T Paradkar VM Dufield DR Carroll JA Schlittler MR 《Biotechnology and bioengineering》2005,89(7):775-782
Human growth hormone (hGH) is not only a valuable recombinant therapeutic protein for hormone deficiency indications, but is also an extensively characterized molecule both from recombinant bacterial systems and as circulating in humans. We describe the characterization of hGH produced in three different plant systems: tobacco cell culture, soy seed, and maize seed. The data indicate highest production in the maize seed system, with continued productivity over multiple generations, and when bred to a new host genotype for improved productivity. Purification indicated significant material of the correct structure from both plant cell culture and maize seed, with maize seed also showing correct activity relative to that produced by Escherichia coli. However, all systems showed some proteolyzed hGH, with data from gel electrophoresis, mass spectrometry, and peptide mapping localizing to a region of the protein also prone to cleavage in some other systems. Together, the data indicate the dependence of recombinant protein accumulation on posttranslational processes in different host systems. 相似文献
162.
Barrios-Ceballos MP Martínez-Gallardo NA Anaya-Velázquez F Mirelman D Padilla-Vaca F 《Experimental parasitology》2005,110(3):270-275
Serine proteases are one of the biologically most important and widely distributed enzyme families. A protease capable of degrading the substrate Suc-AAF-AMC was isolated from axenically grown trophozoites of Entamoeba histolytica. The enzyme was purified by ion-exchange chromatography and electroelution, and appeared on 2D-PAGE as a spot of 60 kDa and pI of 4.65. Data obtained from zymogram suggest the active protease is present either as homodimer (130 kDa) or homotetramer (250 kDa). The optimal temperature of the enzyme was 37 degrees C, and it exhibited activity over a broad pH range. The protease was strongly inhibited by TPCK and chelating agents. The enzymatic activity was restored upon addition of calcium. BLAST analysis with the sequence of internal peptides of the protein revealed two open reading frames within the genome of E. histolytica, homologous to members of the family S28, clan SC of serine proteases. 相似文献
163.
Competitive inhibition of soybean urease by 15 triketone oximes has been studied at 36 degrees C in aqueous solution (pH 4.95). The studied oximes are supposed chelators for the nickel atom in the urease metallocenter. The inhibition constants of urea hydrolysis (K(i)) varied in the range 2.7-248 microM depending on the oxime structure. Analysis of this dependency demonstrates that the optimal inhibitor is the one containing carbonyl group in position 1 of the cycle, the ethoxyimino group and alkyl residue in the substituent in position 2, as well as the methoxycarbonyl group in position 4 of the cycle. 相似文献
164.
Satoshi Shimamura Ryo Yamamoto Takuji Nakamura Shinji Shimada Setsuko Komatsu 《Annals of botany》2010,106(2):277-284
Background and Aims
Aerenchyma provides a low-resistance O2 transport pathway that enhances plant survival during soil flooding. When in flooded soil, soybean produces aerenchyma and hypertrophic stem lenticels. The aims of this study were to investigate O2 dynamics in stem aerenchyma and evaluate O2 supply via stem lenticels to the roots of soybean during soil flooding.Methods
Oxygen dynamics in aerenchymatous stems were investigated using Clark-type O2 microelectrodes, and O2 transport to roots was evaluated using stable-isotope 18O2 as a tracer, for plants with shoots in air and roots in flooded sand or soil. Short-term experiments also assessed venting of CO2 via the stem lenticels.Key Results
The radial distribution of the O2 partial pressure (pO2) was stable at 17 kPa in the stem aerenchyma 15 mm below the water level, but rapidly declined to 8 kPa at 200–300 µm inside the stele. Complete submergence of the hypertrophic lenticels at the stem base, with the remainder of the shoot still in air, resulted in gradual declines in pO2 in stem aerenchyma from 17·5 to 7·6 kPa at 13 mm below the water level, and from 14·7 to 6·1 kPa at 51 mm below the water level. Subsequently, re-exposure of the lenticels to air caused pO2 to increase again to 14–17 kPa at both positions within 10 min. After introducing 18O2 gas via the stem lenticels, significant 18O2 enrichment in water extracted from roots after 3 h was confirmed, suggesting that transported O2 sustained root respiration. In contrast, slight 18O2 enrichment was detected 3 h after treatment of stems that lacked aerenchyma and lenticels. Moreover, aerenchyma accelerated venting of CO2 from submerged tissues to the atmosphere.Conclusions
Hypertrophic lenticels on the stem of soybean, just above the water surface, are entry points for O2, and these connect to aerenchyma and enable O2 transport into roots in flooded soil. Stems that develop aerenchyma thus serve as a ‘snorkel’ that enables O2 movement from air to the submerged roots. 相似文献165.
For this report, we examined the toxic effects of three plant-derived isothiocyanate compounds on second-stage juveniles (J2) of Heterodera glycines. We found significant differences among compounds in the concentration required to affect nematodes, according to mortality and behavioral measurements. The concentrations required to affect behavior were significantly lower than those required for mortality. Both mortality and behavioral measurements were used to investigate whether nematodes in a quiescent state display decreased sensitivity to isothiocyanates compared with actively moving nematodes. Mortality measurements revealed that quiescent nematodes were significantly less sensitive to isothiocyanates than active nematodes. All behavioral measurements following exposure to benzyl- and phenyl isothiocyanate showed significant differences in sensitivity between quiescent and active nematodes. However, significant differences between quiescent and active nematodes were observed in only one of the five behavioral measurements following exposure to allyl isothiocyanate. These results expand the list of plant-derived compounds toxic to H. glycines and illustrate the impact of behavioral quiescence on nematode sensitivity to exogenous toxins. 相似文献
166.
This study assesses the long‐term effects of an experimental diet vs. a commercially available manufactured diet, intended to reduce clinical disease related to cystinuria, on the taurine status of captive maned wolves. For 13 weeks, two pairs of maned wolves were maintained on the commercially available maintenance diet, whereas two individually housed wolves were maintained on the experimental diet. All six wolves, at the beginning and at the end of the diet trial, had severely decreased plasma concentrations of taurine (as compared to the normal canine reference range of 60–120 nmol/ml) (National Research Council [2003] National Academies Press) with average taurine concentrations of 16 nmol/ml at the beginning of the study and 3 nmol/ml at the end of the study. There was no statistically significant difference in the taurine concentrations between animals on the maintenance vs. experimental diets. Both diets were supplemented subsequently with taurine at a concentration of 0.3%. All study animals were eventually switched to the taurine‐supplemented version of the commercially manufactured maintenance diet and subsequent samplings were carried out to monitor plasma taurine concentrations. A final sampling, carried out approximately 5 months after the initiation of taurine supplementation, showed an average taurine concentration within the target canine reference range (90.25 nmol/ml). There are numerous physiologic (e.g., possible unique metabolism and requirements for taurine in this species as compared to other canids) and dietary factors (e.g., effects of the types and concentrations of fiber and protein on nutrient availability, taurine metabolism, and enterohepatic circulation of taurine‐conjugated bile salts; impaired taurine synthesis secondary to low cysteine availability) that could be potential contributors to the development of taurine deficiency in the maned wolves in this study. Taurine supplementation should be considered in maned wolves maintained on diets intended for reduction of cystinuria‐related complications. Zoo Biol 0:1–14, 2005. © 2005 Wiley‐Liss, Inc. 相似文献
167.
Cyclosporine A from a nonpathogenic Fusarium oxysporum suppressing Sclerotinia sclerotiorum 总被引:2,自引:0,他引:2
AIMS: To evaluate the antagonistic activity of Fusarium oxysporum nonpathogenic fungal strain S6 against the phytopathogenic fungus Sclerotinia sclerotiorum and to identify the antifungal compounds involved. METHODS AND RESULTS: The antagonistic activity of Fusarium oxysporum strain S6 was determined in vitro by dual cultures. The metabolite responsible for the activity was isolated by chromatographic techniques, purified and identified by spectroscopic methods as cyclosporine A. The antifungal activity against the pathogen was correlated with the presence of this metabolite by a dilution assay and then quantified. Cyclosporine A caused both growth inhibition and suppression of sclerotia formation. In a greenhouse assay, a significant increase in the number of surviving soybean (Glycine max) plants was observed when S. sclerotiorum and F. oxysporum (S6) were inoculated together when compared with plants inoculated with S. sclerotiorum alone. CONCLUSION: Fusarium oxysporum (S6) may be a good fungal biological control agent for S. sclerotiorum and cyclosporine A is the responsible metabolite involved in its antagonistic activity in vitro. SIGNIFICANCE AND IMPACT OF THE STUDY: Cyclosporine A has not been previously described as an inhibitor of S. sclerotiorum. Its minimum inhibitory concentration (MIC) of 0.1 microg disc(-1) makes it suitable to use as a biofungicide. In vivo experiments showed that F. oxysporum (S6) is a good candidate for the biocontrol of S. sclerotiorum in soybean. 相似文献
168.
AIMS: The inhibitory effect of cowdung fumes, Captan, leaf powder of Withania somnifera, Hyptis suaveolens, Eucalyptus citriodora, peel powder of Citrus sinensis, Citrus medica and Punica granatum, neem cake and pongamia cake and spore suspension of Trichoderma harzianum and Aspergillus niger on aflatoxin B(1) production by toxigenic strain of Aspergillus flavus isolated from soybean seeds was investigated. METHODS AND RESULTS: Soybean seed was treated with different natural products and fungicide captan and was inoculated with toxigenic strain of A. flavus and incubated for different periods. The results showed that all the treatments were effective in controlling aflatoxin B(1) production. Captan, neem cake, spore suspension of T. harzianum, A. niger and combination of both reduced the level of aflatoxin B(1) to a great extent. Leaf powder of W. somnifera, H. suaveolens, peel powder of C. sinensis, C. medica and pongamia cake also controlled the aflatoxin B(1) production. CONCLUSIONS: All the natural product treatments applied were significantly effective in inhibiting aflatoxin B(1) production on soybean seeds by A. flavus. SIGNIFICANCE AND IMPACT OF THE STUDY: These natural plant products may successfully replace chemical fungicides and provide an alternative method to protect soybean and other agricultural commodities from aflatoxin B(1) production by A. flavus. 相似文献
169.
Guixiang Tang Wenjian Song Ling Xu Zonglai Jin Kasirajan Subrahmaniyan Weijun Zhou 《Acta Physiologiae Plantarum》2006,28(3):273-280
Experiments were conducted to study the influence of sowing seasons and drying methods on the seed vigour of two spring soybean
(Glycine max (L.) Merr.) cultivars. Two cultivars, ‘Huachun18’ and ‘Huachun 14’, were sown in three seasons viz., spring, summer and autumn and the harvested seeds were dried using three different methods. The results showed that soybean
sown in spring had a higher number of branches per plant, pods per branch and seed weight, and consequently resulted in higher
seed yields than that of soybean sown in autumn or summer seasons. Seeds sown in the autumn season had the lowest values of
electrical conductivity during seed imbibitions, higher peroxidase (POD) activity in germinated seedlings and lower contamination
by the seed-borne fungi on the MS medium, which indirectly improved the seed vigour, which was followed by summer sown seeds.
Seeds sown during the spring season resulted in poor seed vigour. In addition, the effect of drying methods on the seed vigour
was also clarified. Seeds that hung for four days before threshing and then air-dried had the poorest seed vigour which was
determined by germination, electrical conductivity, POD activity and seed borne fungal growth. There was no difference in
seed vigour between other methods, i.e. seeds threshed directly at harvest and then air-dried on a bamboo sifter or concrete floor. These results indicated that
autumn sowing soybean and the drying method in which seeds were threshed directly at harvest and then air-dried on a bamboo
sifter resulted in higher seed vigour. 相似文献
170.
Małgorzata Kopyra Magdalena Stachoń-Wilk Edward A. Gwóźdź 《Acta Physiologiae Plantarum》2006,28(6):525-536
Nitric oxide (NO) is a small, ubiquitous molecule, whose physiological function in plants has recently been widely investigated.
It seems that one of its pivotal properties is the antioxidant capacity, enabling plants to alleviate the effects of the oxidized
stress. In this work we investigated the role of NO in soybean (Glycine max L. Cv. Navico) cell suspension treated with cadmium. Sodium nitroprusside (SNP), nitric oxide donor, markedly decreased the
negative influence of Cd2+ on cell growth. It was also found to stimulate superoxide dismutase (SOD, EC 1.15.1.1). Using specific fluorochromes — dihydroethidine
(DHE) and 2′,7′- dichlorofluorescein (DCFH-DA) it was shown that NO was very effective in reducing the level of superoxide
anion (O
2
·−
) and hydrogen peroxide, respectively. Furthermore, as evaluated by means of NO specific fluorochrome 4,5-diaminofluorescein
diacetate (DAF-2DA), increased production of NO was found in Cd-treated cells. In cadmium-stressed cells SNP lowered the level
of oxidized proteins.
Our results suggest that the antioxidant properties of nitric oxide in Cd-treated soybean cells rely mainly on its ability
to direct scavenging of ROS and stimulation of the antioxidant system. 相似文献