全文获取类型
收费全文 | 15265篇 |
免费 | 1166篇 |
国内免费 | 1267篇 |
专业分类
17698篇 |
出版年
2024年 | 47篇 |
2023年 | 259篇 |
2022年 | 481篇 |
2021年 | 608篇 |
2020年 | 604篇 |
2019年 | 684篇 |
2018年 | 594篇 |
2017年 | 504篇 |
2016年 | 500篇 |
2015年 | 552篇 |
2014年 | 790篇 |
2013年 | 999篇 |
2012年 | 609篇 |
2011年 | 682篇 |
2010年 | 486篇 |
2009年 | 690篇 |
2008年 | 654篇 |
2007年 | 783篇 |
2006年 | 688篇 |
2005年 | 613篇 |
2004年 | 565篇 |
2003年 | 551篇 |
2002年 | 463篇 |
2001年 | 410篇 |
2000年 | 376篇 |
1999年 | 344篇 |
1998年 | 332篇 |
1997年 | 288篇 |
1996年 | 278篇 |
1995年 | 232篇 |
1994年 | 224篇 |
1993年 | 214篇 |
1992年 | 201篇 |
1991年 | 163篇 |
1990年 | 158篇 |
1989年 | 135篇 |
1988年 | 124篇 |
1987年 | 117篇 |
1986年 | 111篇 |
1985年 | 132篇 |
1984年 | 88篇 |
1983年 | 50篇 |
1982年 | 77篇 |
1981年 | 70篇 |
1980年 | 35篇 |
1979年 | 47篇 |
1978年 | 26篇 |
1977年 | 18篇 |
1975年 | 15篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
零下低温对杂交杨树皮层膜脂组成的影响 总被引:3,自引:0,他引:3
以不耐寒的美洲黑杨(Populusdeltoidescv.“Lux”I-69/55,父本)和耐寒性较强的欧美杨(P.euramericanaclcv.I-45/51,母本)的4个杂交F_1代无性系(95杨、559杨、600杨和1381杨)为材料,分析了零下低温寒潮前后枝条皮层的脂质组成。结果表明,寒潮影响下,皮层中磷脂含量增加而组成基本不变,膜脂脂肪酸组成的变化规律是:寒潮前脂肪酸不饱和指数(IUFA)值大的无性系,寒潮前后的IUFA值变化量小;寒潮前IUFA值较小的无性系,寒潮前后IUFA值变化量较大。本文借用力学概念,提出相对抗性概念,给出杨树无性系的相对抗性序列。序列表明F_1代无性系的耐寒性已较不耐寒的父本提高,这与田间观察基本一致。 相似文献
82.
W. A. Parrott J. N. All M. J. Adang M. A. Bailey H. R. Boerma C. N. Stewart Jr. 《In vitro cellular & developmental biology. Plant》1994,30(3):144-149
Summary Lepidopteran insects are major defoliating pests of soybean in the southeastern United States. Soybean plants transgenic for
a nativecryIA(b) gene fromBacillus thuringiensis var.kurstaki HD-1 were obtained. Embryogenic cultures were induced by plating cotyledons on a Murashige and Skoog-based medium supplemented
with 40 mg/liter of 2,4-dichlorophenoxyacetic acid (2,4-D). The embryogenic cultures were maintained in liquid medium containing
5 mg/liter 2,4-D. These cultures were subjected to microprojectile bombardment, followed by selection on 50 mg/liter hygromycin.
Resistant embryogenic cell lines were transferred to growth regulator-free medium to permit recovery of mature somatic embryos.
After a desiccation period, the somatic embryos were returned to growth regulator-free medium for conversion into plants.
Southern hybridization analysis verified transformation. Feeding assays of T1 plants from one cell line deterred feeding, development, and survival of velvetbean caterpillar at a level comparable to
that of GatIR81-296, a soybean breeding line with a high level of insect resistance. Reduced feeding on T1 plants correlated with the presence of the transgene. 相似文献
83.
An in vitro root explant tissue culture technique is described for determining susceptibility of tomato (Lycopersicon esculentum Mill.) breeding lines and cultivars to the root-knot nematode Meloidogyne incognita. Root explants were taken from 2-day-old seedlings cultured for 30 days at 28 C on Gamborg''s B-5 medium with or without nematode inoculum. The remaining portion of the root and stem from the excised root explants was transferred to soil in pots and grown to maturity in the greenhouse. In vitro root explants were evaluated for growth and occurrence of juveniles, adults, and egg masses. The regenerated plants were used to produce more seed, The proposed technique is simple, reliable, and adapted to routine screening of large numbers of F₁ and F₂ samples, and it utilizes less space than tests performed on intact plants in the greenhouse or growth chamber. Evidence is presented also on the breakdown of resistance to M. incognita under high temperature stress using this in vitro root explant technique. 相似文献
84.
Lines of wheat with the 6Mv chromosome from Aegilops ventricosa display partial resistance to both pathotypes Hal2 and Ha41 of Heterodera avenae. With either pathotype, the effect of this alien chromosome on cyst production, size, and fecundity was expressed in resistance tests. Partial resistance of five 6Mv(6D) substitution lines varied according to the intrinsic cyst-forming capacity of the nematode pathotypes and the recipient germplasms. Such partial resistance can be utilized in wheat breeding lines for integrated management of the cereal cyst nematode. 相似文献
85.
Study of multiplication of cucumber mosaic virus in susceptible and resistant Capsicum annuuum lines
R. NONO-WOMDIM K. GEBRE-SELASSIE A. PALLOIX E. POCHARD G. MARCHOUX 《The Annals of applied biology》1993,122(1):49-56
A previous survey on pepper lines (Capsicum annuum L.) indicated that a susceptible cultivar, Yolo Wonder, reacted to cucumber mosaic virus (CMV) by producing a systemic yellow mosaic. By contrast, CMV caused no symptoms on lines Perennial and Vania. The virus is recoverable from the uninoculated leaves of Perennial, while in Vania CMV is restricted to the inoculated leaves. To interpret these phenomena, a comparative study on CMV multiplication rates, yield, specific infectivity and relative proportion of RNAs was made in the inoculated leaves of the three pepper varieties. The rate of CMV multiplication, as estimated by the double antibody sandwich form of enzyme-linked immu-nosorbent assay, was lower in Perennial than in Vania or Yolo Wonder. The yield of virus purified from Perennial was very low when compared with Vania or Yolo Wonder. The specific infectivity of the virus extracted from Perennial was less than that from Vania or Yolo Wonder. These results suggest that Perennial is resistant to CMV multiplication, while restriction of the virus in inoculated leaves of Vania is not due to the inhibition of the virus replication. However, polyacrylamide gel electrophoresis revealed that the RNA profiles of CMV purified from the three pepper lines were similar. 相似文献
86.
J. C. Daly 《Genetica》1993,90(2-3):217-226
Pyrethroid resistance inHelicoverpa armigera provides a model system in which to study evolution in natural populations. Resistance is seen to evolve as a consequence of selection pressure that varies within and between life-stages and gene flow. Although three different mechanisms are involved, present day fluctuations in phenotype frequency can be explained by variation in only one of these, metabolic resistance, that is inherited as a single, incompletely dominant gene,mfo. Selective mortality of phenotypes occurs in both larvae and adults in the presence of the pyrethroid insecticides. Although most individuals of all three genotypes are killed in young larvae, selection in this age-class contributes significantly to evolution of resistance. While there is some evidence of reduced fitness of resistant pupae during winter diapause, most of the decline in resistance frequencies each spring occurs as a result of immigration of susceptible individuals into insecticide-treated populations. 相似文献
87.
O. P. Yadav V. K. Manga G. K. Gupta 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1993,87(5):558-560
Large-scale cultivation of pearl millet [Pennisetum glaucum (L.) R. Br. F1 hybrids in India has led to increased incidence of downy-mildew (Sclerospora graminicola). There is concern that the A1 male-sterile cytoplasm used in all the hybrids released so far is responsible for this increase. The influence of A1 malesterile cytoplasm on downy-mildew incidence in pearl millet was studied by comparing the disease reaction of 40 pairs of F1 hybrids, each pair carrying respectively a1 male-sterile and normal B cytoplasm. Mean downy-mildew incidence was similar in the hybrids carrying either A1 male-sterile or B cytoplasm. The general combining ability of lines with and without A1 cytoplasm was found to be similar for downy-mildew incidence. These results indicated that in pearl millet A1 cytoplasm is not associated with increased downymildew incidence. The possible danger of using only one source of cytoplasm has been briefly discussed. 相似文献
88.
《Saudi Journal of Biological Sciences》2023,30(8):103730
Mycobacterium tuberculosis (MTB) is becoming more and more resistant to drugs and it is a common problem, making current antimicrobials ineffective and highlighting the need for new TB drugs. One of the promising targets for treating MTB is MurB enzymes. This study aimed to identify potential inhibitors of MurB enzymes in M. tuberculosis, as drug resistance among MTB is a significant problem. Attempts are being made to conduct a virtual screening of 30,417 compounds, and thirty-two compounds were chosen for further analysis based on their binding conformations. The selected compounds were assessed for their drug-likeness, pharmacokinetics, and physiochemical characteristics, and seven compounds with binding energy lower than flavin (FAD) were identified. Further, molecular dynamics simulation analysis of these seven compounds found that four of them, namely DB12983, DB15688, ZINC084726167, and ZINC254071113 formed stable complexes with the MurB binding site, exhibiting promising inhibitory activity. These compounds have not been mentioned in any other study, indicating their novelty. The study suggests that these four compounds could be promising candidates for treating MTB, but their effectiveness needs to be validated through in vitro and in vivo experiments. Overall, the findings of this study provide new insight into potential drug targets and candidates for combating drug-resistant MTB. 相似文献
89.
90.
Wim Van de Vrie Sylke A. M. Van der Heyden Eric E. O. Gheuens Amelie M. Bijma Ernst A. De Bruijn Richard L. Marquet Allan T. Van Oosterom Alexander M. M. Eggermont 《Cancer immunology, immunotherapy : CII》1993,37(5):337-342
The development of resistance to anticancer drugs urges the search for different treatment modalities. Several investigators have reported the concomitant development of drug resistance and resistance to natural killer (NK), lymphokine-activated killer (LAK) or monocyte/macrophage cell lysis, while others described unchanged or even increased susceptibility. We investigated this subject in the rat colon carcinoma cell line, CC531-PAR, which is intrinsically multidrug-resistant (MDR), and in three sublines derived from this parental cell line: a cell line with an increased MDR phenotype (CC531-COL), a revertant line from CC531-COL (CC531-REV), which demonstrates enhanced sensitivity to anticancer drugs of the MDR phenotype, and an independently developed cisplatin-resistant line (CC531-CIS). In a 4-h51Cr-release assay we found no difference in susceptibility to NK cell lysis. No significant differences in lysability by adherent LAK (aLAK) cells were observed in a 4-h assay. In a prolonged 20-h51Cr-release assay an enhanced sensitivity to aLAK-cell-mediated lysis was observed in the revertant, P-glycoprotein-negative cell line and in the cisplatin-resistant cell line (CC531-CIS). None of the cell lines was completely resistant to lysis by aLAK cells. Therefore, a role for immunotherapy in the treatment of drug-resistant tumors remains a realistic option. 相似文献