全文获取类型
收费全文 | 4884篇 |
免费 | 191篇 |
国内免费 | 656篇 |
出版年
2023年 | 40篇 |
2022年 | 35篇 |
2021年 | 73篇 |
2020年 | 97篇 |
2019年 | 89篇 |
2018年 | 79篇 |
2017年 | 106篇 |
2016年 | 119篇 |
2015年 | 96篇 |
2014年 | 133篇 |
2013年 | 425篇 |
2012年 | 143篇 |
2011年 | 215篇 |
2010年 | 148篇 |
2009年 | 299篇 |
2008年 | 312篇 |
2007年 | 303篇 |
2006年 | 289篇 |
2005年 | 224篇 |
2004年 | 213篇 |
2003年 | 196篇 |
2002年 | 171篇 |
2001年 | 166篇 |
2000年 | 149篇 |
1999年 | 126篇 |
1998年 | 119篇 |
1997年 | 103篇 |
1996年 | 93篇 |
1995年 | 96篇 |
1994年 | 99篇 |
1993年 | 90篇 |
1992年 | 103篇 |
1991年 | 62篇 |
1990年 | 92篇 |
1989年 | 60篇 |
1988年 | 59篇 |
1987年 | 72篇 |
1986年 | 66篇 |
1985年 | 46篇 |
1984年 | 56篇 |
1983年 | 15篇 |
1982年 | 22篇 |
1981年 | 25篇 |
1980年 | 29篇 |
1979年 | 33篇 |
1978年 | 23篇 |
1977年 | 25篇 |
1976年 | 19篇 |
1974年 | 21篇 |
1973年 | 20篇 |
排序方式: 共有5731条查询结果,搜索用时 15 毫秒
991.
L. M. Dosdall B. J. Ulmer G. A. P. Gibson H. A. C rcamo 《Biocontrol Science and Technology》2006,16(10):987-1006
Distribution patterns of the cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), and its larval parasitoids were investigated in commercial fields of spring canola (Brassica rapa L. and Brassica napus L.) in southern Alberta, Canada, from 2002 to 2004 in relation to developmental stages of its host plants. Adult weevils invaded fields along one or more fronts when crops were in bud to early flower. Significant clustering of adults along field edges in early stages of invasion was followed by more homogeneous distributions as canola reached the mid to late flowering and pod enlargement stages. Larval weevil distributions, as indicated by exit holes in siliques at the end of the season, were often aligned spatially with adult distributions, but they did not coincide in all regions of the fields. The primary ectoparasitoid species attacking weevil larvae comprised Necremnus tidius (Walker) (Hymenoptera: Eulophidae), and Trichomalus lucidus (Walker), Chlorocytus sp., and Pteromalus sp. (Hymenoptera: Pteromalidae). Parasitism rates increased from 0.1 to 5.0% over the three years of study. Parasitoid distributions were often, but not consistently, spatially associated with high densities of C. obstrictus larvae. Lack of close spatial alignment of parasitoids and their hosts probably reflects low parasitoid numbers in comparison with an abundant resource of weevil larvae, and a lack of co-evolutionary history between host and parasitoids. Some parasitoids invaded fields early in host plant development, at the same time that weevils invaded. Unfortunately the synchronous invasions of host and parasitoids indicate that insecticidal applications to reduce adult weevil infestations may be detrimental to these beneficial species. 相似文献
992.
Ci Su-Qin Chen Shan Dong-Bo Liu Hong-Mei Xia 《World journal of microbiology & biotechnology》2006,22(7):729-735
Summary
Penicillium sp. DS9713a-01 was obtained by ultraviolet (u.v.) light mutagenesis from the Penicillium sp. DS9713a which can degrade poly (3-hydroxybutyrate) (PHB). The enzymatic activity of DS9713a-01 was 97% higher than that
of the wild-type strain. The DS9713a-01 mutant could completely degrade PHB films in 5 days; however, the wild-type strain
achieved only 61% at the same time. The extracellular PHB depolymerase was purified from the culture medium containing PHB
as the sole carbon source by filtration, ammonium sulfate precipitation and chromatography on Sepharose CL-6B. The molecular
weight of the PHB depolymerase was about 15.1kDa determined by SDS-polyacrylamide gel electrophoresis. The optimum activity
of the PHB depolymerase was observed at pH 8.6 and 50 °C. The enzyme was stable at temperatures below 37 °C and in the pH
range from 8.0 to 9.2. The activity of PHB depolymerase could be activated or inhibited by some metal ions. The apparent K
m
value was 0.164 mg ml−1. Mass spectrometric analysis of the water-soluble products after enzymatic degradation revealed that the primary product
was the monomer, 3-hydroxybutyric acid. 相似文献
993.
Oily sludge degradation by bacteria from Ankleshwar, India 总被引:7,自引:0,他引:7
Shuchi Verma Renu Bhargava Vikas Pruthi 《International biodeterioration & biodegradation》2006,57(4):207-213
Three bacterial strains, Bacillus sp. SV9, Acinetobacter sp. SV4 and Pseudomonas sp., SV17 from contaminated soil in Ankleshwar, India were tested for their ability to degrade the complex mixture of petroleum hydrocarbons (such as alkanes, aromatics, resins and asphaltenes), sediments, heavy metals and water known as oily sludge. Gravimetric analysis showed that Bacillus sp. SV9 degraded approx. 59% of the oily sludge in 5 days at 30 °C whereas Acinetobacter sp. SV4 and Pseudomonas sp. SV17 degraded 37% and 35%. Capillary gas chromatographic analysis revealed that after 5 days the Bacillus strain was able to degrade oily sludge components of chain length C12–C30 and aromatics more effectively than the other two strains. Maximum drop in surface tension (from 70 to 28.4 mN/m) was accompanied by maximum biosurfactant production (6.7 g l−1) in Bacillus sp. SV9 after 72 h, these results collectively indicating that this bacterial strain has considerable potential for bioremediation of oily sludge. 相似文献
994.
三峡库区藻类水华调查及其毒理学研究 总被引:14,自引:1,他引:13
三峡水库一期工程蓄水近两年(2003年7月—2005年4月)期间,库区发生过多次藻类“水华”,尤其在阳光充足、气温回升的春季。经鉴定形成水华的藻类主要有拟多甲藻(Peridiniopsissp1,sp2),新星形冠盘藻(Stephanodiscusneoastraea),里海小环藻(Cyclotella caspia),湖沼红胞藻(Rhodomonas lacustris),实球藻(Pandorina morum)和空球藻(Eudorina elegans);其中以拟多甲藻水华涉及面积最大,持续时间最久,污染程度最为严重。通过对拟多甲藻水华水及其藻细胞提取液的生物毒性测试,表明其对其他水生生物和小白鼠没有明显的生物毒性。 相似文献
995.
基因枪法转化基因在小麦条锈菌中的瞬时表达 总被引:3,自引:0,他引:3
以小麦条锈菌(Puccinia striiformis f.sp.tritici)野生毒性菌株为转化受体,以含有gus报告基因的质粒(pGUS6L20)和潮霉素抗性基因的质粒(pKLHyg14)为载体,应用基因枪法研究了小麦条锈菌夏孢子遗传转化的瞬时表达特征。结果表明,在金粉直径为0.6μm、射程6cm、载体DNA5μL、可裂膜压力为900Psi或1100Psi时,gus基因和潮霉素抗性基因的瞬时表达率相对较高。 相似文献
996.
Susan L. F. Meyer Inga A. Zasada Daniel P. Roberts Bryan T. Vinyard Dilip K. Lakshman Jae-Kook Lee David J. Chitwood Lynn K. Carta 《Journal of nematology》2006,38(3):333-338
Extracts from the plants Plantago lanceolata and P. rugelii were evaluated for toxicity to the root-knot nematode Meloidogyne incognita, the beneficial microbes Enterobacter cloacae, Pseudomonas fluorescens and Trichoderma virens, and the plant-pathogenic fungi Fusarium oxysporum f. sp. gladioli, Phytophthora capsici, Pythium ultimum, and Rhizoctonia solani. Wild plants were collected, roots were excised from shoots, and the plant parts were dried and ground to a powder. One set of extracts (10% w/v) was prepared in water and another in methanol. Treatments included extract concentrations of 25%, 50%, 75% and 100%, and water controls. Meloidogyne incognita egg hatch was recorded after 7-day exposure to the treatments, and second-stage juvenile (J2) activity after 48 hours. All extracts were toxic to eggs and J2, with P. lanceolata shoot extract tending to have the most activity against M. incognita. Numbers of active J2 remained the same or decreased in a 24-hour water rinse following the 48-hour extract treatment, indicating that the extracts were lethal. When data from water- and methanol-extracted roots and shoots of both plant species were combined for analysis, J2 tended to be more sensitive than eggs to the toxic compounds at lower concentrations, while the higher concentrations (75% and 100%) were equally toxic to both life stages. The effective concentrations causing 50% reduction (EC50) in egg hatch and in J2 viability were 44.4% and 43.7%, respectively. No extract was toxic to any of the bacteria or fungi in our assays. 相似文献
997.
Robin M. Giblin-Davis Weimin Ye Natsumi Kanzaki Donna Williams Krystalynne Morris W. Kelley Thomas 《Journal of nematology》2006,38(1):137-149
Parasitodiplogaster comprises a potentially large radiation of nematode species that appear to be parasitically bound to their Agaonid fig wasp hosts, which are mutualistically associated in the syconia (figs) of the diverse plant genus Ficus. Parasitodiplogaster laevigata n. sp. is described and illustrated as an associate of the fig wasp, Pegoscapus sp. from Ficus laevigata from southern Florida. It is the first species of Parasitodiplogaster reported from North America and is closest to P. trigonema from F. trigonata from Panama. Parasitodiplogaster laevigata n. sp. can be differentiated from all described species of Parasitodiplogaster based on stomatal morphology (presence of a large dorsal and a right subventral tooth) in the adults of both sexes, molecular comparisons of two expansion segments (D2,D3) of the large subunit (LSU) rRNAgene, and fig-fig wasp host affinities. The ultrastructure of P. laevigata n. sp. was elucidated using TEM and SEM for comparisons with other species of Parasitodiplogaster. The stoma of P. laevigata n. sp. possesses a nonsegmented cheilostomal ring that connects to the longitudinal body musculature per- and interradially, a claw-like dorsal tooth, a right subventral tooth, and telostegostomatal apodemes arising from the dorsal side of each subventral sector. The unification of the pro-, meso-, and metastegostom with the gymnostom in P. laevigata n. sp. and further simplification in other described species may be due to derived adaptations associated with the internal parasitism of fig wasps. 相似文献
998.
通过对Streptosporangium sp.菌株发酵液的初步分离纯化,得到二个化合物,由波谱数据解析鉴定其结构分别为YXJE1(1),7,4′-二羟基黄酮(2)。其中化合物1是新化合物。活性实验证明化合物1,2具有一定的抑制乙酰胆碱酯酶活性。 相似文献
999.
盐胁迫是影响荒漠区土壤藻类生存的重要环境因子。集球藻是一种广泛分布于生物土壤结皮中的球状绿藻, 能够积累红色素(如虾青素)和油滴, 显示出其独特的生理特性和潜在的应用价值。目前对集球藻的生理、细胞结构以及色素积累的研究非常匮乏。以从荒漠生物结皮中分离的一种集球藻为材料, 在实验室条件下研究盐胁迫对集球藻生物量、光合活性、膜脂过氧化产物丙二醛含量、细胞可溶性蛋白质含量和可溶性糖含量以及细胞结构的影响。研究结果表明, 与对照处理相比, 盐胁迫导致集球藻生物量和光合活性的显著降低, 细胞可溶性蛋白和可溶性糖呈现一定的积累。同时盐处理导致集球藻膜脂丙二醛含量大量增加, SOD和CAT 活性升高。研究还表明, 对照处理下细胞结构完整, 细胞器形态清晰, 生长后期有大量脂肪体积累。在盐处理下藻体细胞形态结构出现阶段性破坏特征和脂肪体以及淀粉粒的积累, 此外细胞器结构模糊和消失, 细胞出现质壁分离和空泡化等。研究为更好地揭示集球藻在盐胁迫环境中的生理适应特性、微结构特征以及色素积累机制具有重要的科学意义, 并为该藻的基础和应用研究提供实验资料。
相似文献
1000.
松褐天牛肿腿蜂对寄主松褐天牛三龄幼虫的功能反应 总被引:4,自引:0,他引:4
为评估松褐天牛肿腿蜂Sclerodermus sp.(膜翅目: 肿腿蜂科)对其寄主松褐天牛Monochamus alternatus 3 龄幼虫的控制作用, 通过功能反应试验, 研究了松褐天牛肿腿蜂对松褐天牛3 龄幼虫补充营养时期的捕食功能反应和补充营养后的寄生功能反应以及肿腿蜂的密度效应。结果表明: 当松褐天牛肿腿蜂密度固定为1 头/缸时, 肿腿蜂因补充营养对松褐天牛3 龄幼虫的捕食功能反应符合Holling Ⅱ型方程, 1 头松褐天牛肿腿蜂雌蜂对松褐天牛3 龄幼虫的最大致死量为9.48 头, 平均为3.75 头; 寄生作用的功能反应符合Holling Ⅰ型方程, 可知肿腿蜂的寄主密度阈限为4 头就能满足补充营养和寄生的需要; 在捕食和寄生两个阶段其对松褐天牛幼虫的寻找效应随着寄主密度的增加呈线性减弱。当松褐天牛幼虫密度固定为30 头/缸时, 随着肿腿蜂的密度增加, 其补充营养阶段的捕食作用和之后的寄生作用均呈线性升高, 寻找效应则呈线性减弱; 由蜂虫比和致死总量的模型可得肿腿蜂与天牛幼虫比例为0.9805(近1∶1)时, 可使松褐天牛幼虫死亡总数最大。这些结果表明, 松褐天牛肿腿蜂是松褐天牛幼虫期的有效天敌。本研究为评价松褐天牛肿腿蜂对寄主的控制能力提供了基础数据和方法, 并为其在野外的释放量提供了依据。 相似文献