全文获取类型
收费全文 | 3460篇 |
免费 | 199篇 |
国内免费 | 9篇 |
专业分类
3668篇 |
出版年
2023年 | 51篇 |
2022年 | 128篇 |
2021年 | 159篇 |
2020年 | 97篇 |
2019年 | 127篇 |
2018年 | 127篇 |
2017年 | 88篇 |
2016年 | 138篇 |
2015年 | 176篇 |
2014年 | 198篇 |
2013年 | 285篇 |
2012年 | 235篇 |
2011年 | 273篇 |
2010年 | 139篇 |
2009年 | 132篇 |
2008年 | 135篇 |
2007年 | 155篇 |
2006年 | 125篇 |
2005年 | 113篇 |
2004年 | 102篇 |
2003年 | 86篇 |
2002年 | 91篇 |
2001年 | 48篇 |
2000年 | 30篇 |
1999年 | 38篇 |
1998年 | 25篇 |
1997年 | 23篇 |
1996年 | 13篇 |
1995年 | 11篇 |
1994年 | 22篇 |
1993年 | 13篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 24篇 |
1987年 | 13篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 9篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 13篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1974年 | 6篇 |
1973年 | 10篇 |
1971年 | 7篇 |
排序方式: 共有3668条查询结果,搜索用时 19 毫秒
941.
Stuart W. Smith James D. M. Speed John Bukombe Shombe N. Hassan Richard D. Lyamuya Philipo Jacob Mtweve Anders Sundsdal Bente J. Graae 《Oikos》2019,128(4):596-607
Decomposition is a vital ecosystem process, increasingly modified by human activity. Theoretical frameworks and empirical studies that aim to understand the interplay between human land‐use, macro‐fauna and decomposition processes have primarily focused on leaf and wood litter. For a whole‐plant understanding of how land‐use and macro‐fauna influence decomposition, investigating root litter is required. Using litterbags, we quantified rates of root decomposition across contrasting tropical savanna land‐uses, namely wildlife and fire‐dominated protected areas and livestock pastureland without fire. By scanning litterbags for termite intrusion, we differentiated termite and microbial driven decomposition. Root litter was buried underneath different tree canopies (leguminous and non‐leguminous trees) and outside canopies to account for savanna landscape effects. Additionally, we established a termite cafeteria‐style experiment and common garden to explore termite selectivity of root litter and root trait relationships, respectively. After one year, we found no significant differences in root litter mass loss between wildlife dominated areas and pastureland. Instead, we found consistent species differences in root litter mass loss across land‐uses and additive and non‐additive effects of termites on root decomposition across plant species. Termite selectivity for root litter species occurred for both root and leaf litter buried near termite mounds, but was not explained by root traits measured in the common garden. Termite foraging was greater under leguminous tree canopies than other canopies; however, this did not influence rates of root decomposition. Our study suggests that land‐use has a weak direct effect on belowground processes in savannas. Instead, changes in herbaceous species composition and termite foraging have stronger impacts on belowground decomposition. Moreover, termites were not generalist decomposers of root litter, but their impact varies depending on plant species identity and likely associated root traits. This root litter selectivity by termites is likely to be an important contributor to spatial heterogeneity in savanna nutrient cycling. 相似文献
942.
Jamshid Marzanghi Mohammad Hassan SafarAlizadeh Peyman Najafi Moghaddam 《Archives Of Phytopathology And Plant Protection》2013,46(6):704-709
Applying essential oils, fumigant toxicity in stored products pests control has sharpened recently. Since these products have low penetration power and vapour pressure, their use in crop depth is restricted. The aim of this work was using controlled release technology to solve this problem. In this research, 1,8-cineole-based pellets were prepared by dry mixing method and physical load of 1,8-cineole on strach, which followed by pressing the mixture, using pellet maker apparatus, to form pellets. Insecticidal efficacy of produced pellets was investigated against adults (1–3?days old) Callosobruchus maculatus F. (Col.: Bruchidae) compared with phostoxin under 28?±?2?°C and darkness in laboratory condition. LC50 and LC95 of pellets against C. maculatus in 24?h were 0/017 and 0/050?ml active ingredient per L air, respectively. LC50 and LC95 of phosphine pellets against C. maculatus in 24?h were 0/124 and 1/852?ml active ingredient per L air, respectively. 相似文献
943.
Treatment of 14-day-old maize cultivars (Hybrid351 and Giza2) with 250 mM NaCl significantly reduced shoot fresh and dry weights and protein content during the subsequent 12 days. The magnitude of reduction was more pronounced in Giza than Hybrid. Both cultivars contained converging levels of protein for the enzymes phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH), pyruvate phosphate dikinase (PPDK) and ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) under normal conditions; however, NaCl led to increase these levels in Hybrid and decrease them in Giza. Moreover, NaCl significantly inhibited the activities of PEPC, MDH and PPDK in both cultivars during the first 2 days, thereafter the inhibition nullified only in Hybrid; nonetheless, Rubisco was the least affected enzyme in both cultivars. In addition, NaCl slightly increased V max of PEPC, MDH and PPDK in Hybrid with no change in K m; nevertheless V max dropped in Giza with an increase in K m of only PEPC and MDH. Also K cat, K cat/K m and V max/K m of all enzymes were lower in treated Giza than in treated Hybrid. The increased V max of all enzymes in only Hybrid by NaCl confirms that they were synthesised more in Hybrid than in Giza. However, the decreased V max in Giza concomitant with the increased K m points to an interference of salinity with synthesis of enzymes and their structural integrity. This would lead to a noncompetitive inhibition for the enzymes. These findings declare that maize tolerance to NaCl was larger in Hybrid compared to Giza due to a role for C4 enzymes. 相似文献
944.
Hassan W Al-Gendy A Al-Youssef H El-Shazely A 《Zeitschrift für Naturforschung. C, Journal of biosciences》2012,67(3-4):144-150
A new eremophilane sesquiterpene, 1-beta-hydroxy-8-oxoeremophila-7,9-dien-12-oic acid (1), in addition to two known flavonol glycosides, rutin (2) and quercetin-3-O-glucoside-7-O-rutinoside (3), was isolated from the ethyl acetate fraction obtained from the aqueous alcoholic extract of the aerial parts of Senecio aegyptius var. discoideus Boiss. (family Asteraceae). The chemical structures of the isolated compounds were established by 1D and 2D NMR analysis (1H, 13C, COSY, HMQC, HMBC), MS and UV data, and through comparison with the literature. The ethyl acetate fraction and the isolated rutin showed significant cytotoxic activity against colorectal carcinoma (HCT 116) and to less extent against brain (U 251) and breast carcinoma (MCF 7). The ethyl acetate fraction showed a significant level of activity against Klebsiella pneumoniae, while the total extract showed the best antifungal activity against Candida albicans and Saccharomyces cerevisiae. DPPH radical scavenging activity of the ethyl acetate fraction was significant (96.7%) when compared to ascorbic acid. It also showed anti-inflammatory activity but no diuretic effect. 相似文献
945.
Asadullah Madni Muhammad Abdur Rahim Muhammad Ahmad Mahmood Abdul Jabar Mubashar Rehman Hassan Shah Arshad Khan Nayab Tahir Aamna Shah 《AAPS PharmSciTech》2018,19(4):1544-1553
Proniosomes (PN) are the dry water-soluble carrier systems that may enhance the oral bioavailability, stability, and topical permeability of therapeutic agents. The low solubility and low oral bioavailability due to extensive first pass metabolism make Pentazocine as an ideal candidate for oral and topical sustained release delivery. The present study was aimed to formulate the PNs by quick slurry method that are converted to niosomes (liquid dispersion) by hydration, and subsequently formulated to semisolid niosomal gel. The PNs were found in spherical shape in the SEM and stable in the physicochemical and thermal analysis (FTIR, TGA, and XRD). The quick slurry method produced high recovery (>?80% yield) and better flow properties (θ?=?28.1–37.4°). After hydration, the niosomes exhibited desirable entrapment efficiency (44.45–76.23%), size (4.98–21.3 μm), and zeta potential (??9.81 to ??21.53 mV). The in vitro drug release (T100%) was extended to more than three half-lives (2–4 h) and showed good fit to Fickian diffusion indicated by Korsmeyer-Peppas model (n?=?0.136–0.365 and R2?=?0.9747–0.9954). The permeation of niosomal gel was significantly enhanced across rabbit skin compared to the pure drug-derived gel. Therefore, the PNs are found promising candidates for oral as dissolution enhancement and sustained release for oral and topical delivery of pentazocine for the management of cancer pain. 相似文献
946.
Juan José Gallardo Rodríguez Asterio Sánchez Mirón María del Carmen Cerón García El Hassan Belarbi Francisco García Camacho Yusuf Chisti Emilio Molina Grima 《Harmful algae》2009,8(2):239-246
The basal L1 medium was found to be unsatisfactory for culturing the red tide dinoflagellate Protoceratium reticulatum at a high growth rate and biomass yield. The L1 medium enhanced with phosphate to a total concentration of 217 μM supported the highest attainable growth rate and biomass yield. Once the phosphate concentration exceeded 6× L1, phosphate inhibited the dinoflagellate growth and negatively affected cell viability. At the optimal phosphate concentration of 217 μM, an increase in nitrate concentration over the range of 882–8824 μM, did not affect cell growth and yield. Nitrate did not inhibit growth at any of the concentrations used. Clearly, the basal nitrate level in L1 is sufficient for effectively culturing P. reticulatum. At the ranges of phosphate and nitrate concentrations tested, cell volume was not sensitive to the concentration of nutrients but the concentration of phosphate affected both the specific cell number and cell volume growth rates. Elevated levels of nutrients supported their intracellular accumulation. Cell-specific production of yessotoxin was not influenced by concentration of phosphate in the culture medium, but elevated (>1764 μM) nitrate concentration did enhance the yessotoxin level. Phosphate concentration that maximized biomass yield also maximized volumetric production of yessotoxin in the culture broth. 相似文献
947.
948.
Treatment of D-arabino-hexose phenylosotriazole with conc. hydrochloric acid afforded a new type of alpha- and beta-glycosides of D-erythrose formed by reaction of the 3,6-anhydro derivative with the in situ formed 2-phenyl-4-(formylmethyl)- 1,2,3-triazole. 相似文献
949.
950.
Jason Liang Pin Ng Samira Hassan Thy T. Truong Charles H. Hocart Carole Laffont Florian Frugier Ulrike Mathesius 《The Plant cell》2015,27(8):2210-2226
Initiation of symbiotic nodules in legumes requires cytokinin signaling, but its mechanism of action is largely unknown. Here, we tested whether the failure to initiate nodules in the Medicago truncatula cytokinin perception mutant cre1 (cytokinin response1) is due to its altered ability to regulate auxin transport, auxin accumulation, and induction of flavonoids. We found that in the cre1 mutant, symbiotic rhizobia cannot locally alter acro- and basipetal auxin transport during nodule initiation and that these mutants show reduced auxin (indole-3-acetic acid) accumulation and auxin responses compared with the wild type. Quantification of flavonoids, which can act as endogenous auxin transport inhibitors, showed a deficiency in the induction of free naringenin, isoliquiritigenin, quercetin, and hesperetin in cre1 roots compared with wild-type roots 24 h after inoculation with rhizobia. Coinoculation of roots with rhizobia and the flavonoids naringenin, isoliquiritigenin, and kaempferol, or with the synthetic auxin transport inhibitor 2,3,5,-triiodobenzoic acid, rescued nodulation efficiency in cre1 mutants and allowed auxin transport control in response to rhizobia. Our results suggest that CRE1-dependent cytokinin signaling leads to nodule initiation through the regulation of flavonoid accumulation required for local alteration of polar auxin transport and subsequent auxin accumulation in cortical cells during the early stages of nodulation. 相似文献