全文获取类型
收费全文 | 684篇 |
免费 | 49篇 |
出版年
2023年 | 10篇 |
2022年 | 6篇 |
2021年 | 19篇 |
2020年 | 17篇 |
2019年 | 15篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 31篇 |
2015年 | 31篇 |
2014年 | 43篇 |
2013年 | 43篇 |
2012年 | 56篇 |
2011年 | 52篇 |
2010年 | 37篇 |
2009年 | 35篇 |
2008年 | 44篇 |
2007年 | 35篇 |
2006年 | 27篇 |
2005年 | 33篇 |
2004年 | 26篇 |
2003年 | 38篇 |
2002年 | 23篇 |
2001年 | 14篇 |
2000年 | 6篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 4篇 |
1995年 | 5篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有733条查询结果,搜索用时 218 毫秒
131.
132.
133.
The bark beetle, Dendroctonus rhizophagus Thomas & Bright, is endemic to the Sierra Madre Occidental (SMOC) in México. This bark beetle is a major pest of the seedlings and young saplings of several pine species that are of prime importance to the nation's forest industry. Despite the significance of this bark beetle as a pest, its biology, ecology, and distribution are poorly known. Three predictive modeling approaches were used as a first approximation to identify bioclimatic variables related to the presence of D. rhizophagus in the SMOC and to obtain maps of its potential distribution within the SMOC, which is a morphotectonic province. Our results suggest that the bark beetle could have an almost continuous distribution throughout the major mountain ranges of the SMOC. This beetle has a relatively narrow ecological niche with respect to some temperature and precipitation variables and inhabits areas with climatic conditions that are unique from those usually prevalent in the SMOC. However, the bark beetle has a broad ecological niche with respect to the number of hosts that it attacks. At the macro-scale level, the D. rhizophagus distribution occurs within the wider distribution of its main hosts. The limit of the geographical distribution of this bark beetle coincides with the maximum temperature isotherms. Our results imply a preference for temperate habitats, which leads to the hypothesis that even minor changes in climate may have significant effects on its distribution and abundance. 相似文献
134.
Blanca P. Martínez-Bonfil Guadalupe Salcedo-Morales Alma R. López-Laredo Elsa Ventura-Zapata Silvia Evangelista-Lozano Gabriela Trejo-Tapia 《Plant Cell, Tissue and Organ Culture》2011,107(2):195-203
Castilleja tenuiflora is a medicinal plant that grows in pine–oak woods primarily in southern and central Mexico. It is highly valued for its medicinal
properties, which have been attributed to aucubin-like iridoids. In the present study, we developed an efficient protocol
for in vitro shoot proliferation and ex vitro rooting of C. tenuiflora. Using a colorimetric method, we determined total iridoid contents of various different tissues of propagated plants. The
shoots were induced from nodal explants cultured on Murashige and Skoog (MS) (1962) medium supplemented with indole-3-butyric
acid (IBA) (0 and 0.5 μM) and different concentrations of thidiazuron (TDZ), 6-benzyladenine (BA), or kinetin (KIN) (0–20 μM).
Of the cytokinins tested, KIN was more effective for shoot induction than TDZ or BA, and the highest shoot proliferation rate
was achieved with 5 μM KIN (4 shoots per explant). Plantlets were rooted on MS medium, nutrient solution, or potting mix,
alone or in combination with auxins. The best responses (100% rooting efficiency) were obtained by dipping shoots in half-strength
MS medium containing 7.5 μM IBA before transfer to potting mix. On average, each shoot formed 9 roots of 39.3 ± 3.8 mm in
length after 21 days. These roots appeared to be more functional than those that developed in nutrient solution, and were
associated with a high survival rate (95%) during acclimatization and cultivation in a greenhouse, where flowering occurred
after 4 months. Propagated plants accumulated iridoids, thus representing a potential source of pharmacologically useful compounds. 相似文献
135.
Flavonoids and oxidative stress in Drosophila melanogaster 总被引:1,自引:0,他引:1
Flavonoids are a family of antioxidants that are widely represented in fruits, vegetables, dry legumes, and chocolate, as well as in popular beverages, such as red wine, coffee, and tea. The flavonoids chlorogenic acid, kaempferol, quercetin and quercetin 3β-d-glycoside were investigated for genotoxicity using the wing somatic mutation and recombination test (SMART). This test makes use of two recessive wing cell markers: multiple wing hairs (mwh) and flare (flr(3)), which are mutations located on the left arm of chromosome 3 of Drosophila melanogaster and are indicative of both mitotic recombination and various types of mutational events. In order to test the antioxidant capacities of the flavonoids, experiments were conducted with various combinations of oxidants and polyphenols. Oxidative stress was induced using hydrogen peroxide, the Fenton reaction and paraquat. Third-instar transheterozygous larvae were chronically treated for all experiments. The data obtained in this study showed that, at the concentrations tested, the flavonoids did not induce somatic mutations or recombination in D. melanogaster with the exception of quercetin, which proved to be genotoxic at only one concentration. The oxidants hydrogen peroxide and the Fenton reaction did not induce mutations in the wing somatic assay of D. melanogaster, while paraquat and combinations of flavonoids produced significant numbers of small single spots. Quercetin 3β-d-glycoside mixed with paraquat was shown to be desmutagenic. Combinations of the oxidants with the other flavonoids did not show any antioxidant activity. 相似文献
136.
Belinda Pérez-Bibbins Ricardo Pinheiro de Souza Oliveira Ana Torrado María Guadalupe Aguilar-Uscanga José Manuel Domínguez 《Applied microbiology and biotechnology》2014,98(1):151-161
Cell immobilization has shown to be especially adequate for xylitol production. This work studies the suitability of the air lift bioreactor for xylitol production by Debaryomyces hansenii immobilized in Ca-alginate operating in fed-batch cultures to avoid substrate inhibition. The results showed that the air lift bioreactor is an adequate system since the minimum air flow required for fluidization was even lower than that leading to the microaerobic conditions that trigger xylitol accumulation by this yeast, also maintaining the integrity of the alginate beads and the viability of the immobilized cells until 3 months of reuses. Maximum productivities and yields of 0.43 g/l/h and 0.71 g/g were achieved with a xylose concentration of 60 g/l after each feeding. The xylose feeding rate, the air flow, and the biomass concentration at the beginning of the fed-batch operation have shown to be critical parameters for achieving high productivities and yields. Although a maximum xylitol production of 139 g/l was obtained, product inhibition was evidenced in batch experiments, which allowed estimating at 200 and 275 g/l the IC50 for xylitol productivity and yield, respectively. The remarkable production of glycerol in the absence of glucose was noticeable, which could not only be attributed to the osmoregulatory function of this polyol in conditions of high osmotic pressure caused by high xylitol concentrations but also to the role of the glycerol synthesis pathway in the regeneration of NAD+ in conditions of suboptimal microaeration caused by insufficient aeration or high oxygen demand when high biomass concentrations were achieved. 相似文献
137.
Carlos Peña Silvia López Andrés García Guadalupe Espín Angel Romo-Uribe Daniel Segura 《Annals of microbiology》2014,64(1):39-47
The aim of this study was to characterize the influence of the aeration conditions on the production of PHB and its molecular mass in a mutant strain of Azotobacter vinelandii (OPN), which carries a mutation on ptsN, the gene encoding enzyme IIANtr, previously shown to increase the accumulation of PHB. Cultures of A. vinelandii wild-type strain OP and its mutant derivative strain OPN were grown in 500-mL flasks, containing 100 and 200 mL of PY sucrose medium. PHB production and its molecular mass were analyzed at the end of the culture. The molecular mass (MM) was significantly influenced by the aeration conditions and strain used. A polymer with a higher molecular weight was produced under low aeration conditions for both strains. A maximal molecular mass of 2,026 kDa (equivalent to 3,670 kDa measured by GPC) was obtained with strain OPN cultured under low-aeration conditions, reaching a value two-fold higher than that obtained from the parental strain OP (MM?=?1,013 kDa) grown under the same conditions. Aeration conditions and the ptsN mutation influence the molecular mass of the PHB produced by A. vinelandii affecting in turn its physico-chemical properties. 相似文献
138.
María Florencia Gottardo Gabriela Jaita María Laura Magri Sandra Zárate Mariela Moreno Ayala Jimena Ferraris Guadalupe Eijo Daniel Pisera Marianela Candolfi Adriana Seilicovich 《PloS one》2014,9(10)
Humanin (HN) is a 24-amino acid peptide with cytoprotective action in several cell types such as neurons and testicular germ cells. Rattin (HNr), a homologous peptide of HN expressed in several adult rat tissues, also has antiapoptotic action. In the present work, we demonstrated by immunocytochemical analysis and flow cytometry the expression of HNr in the anterior pituitary of female and male adult rats as well as in pituitary tumor GH3 cells. HNr was localized in lactotropes and somatotropes. The expression of HNr was lower in females than in males, and was inhibited by estrogens in pituitary cells from both ovariectomized female and orquidectomized male rats. However, the expression of HNr in pituitary tumor cells was not regulated by estrogens. We also evaluated HN action on the proapoptotic effect of TNF-α in anterior pituitary cells assessed by the TUNEL method. HN (5 µM) per se did not modify basal apoptosis of anterior pituitary cells but completely blocked the proapoptotic effect of TNF-α in total anterior pituitary cells, lactotropes and somatotropes from both female and male rats. Also, HN inhibited the apoptotic effect of TNF-α on pituitary tumor cells. In summary, our results demonstrate that HNr is present in the anterior pituitary gland, its expression showing sexual dimorphism, which suggests that gonadal steroids may be involved in the regulation of HNr expression in this gland. Antiapoptotic action of HN in anterior pituitary cells suggests that this peptide could be involved in the homeostasis of this gland. HNr is present and functional in GH3 cells, but it lacks regulation by estrogens, suggesting that HN could participate in the pathogenesis of pituitary tumors. 相似文献
139.
Guillermo Castillo Laura L. Cruz Rosalinda Tapia-López Eika Olmedo-Vicente Diego Carmona Ana Luisa Anaya-Lang Juan Fornoni Guadalupe Andraca-Gómez Pedro L. Valverde Juan Nú?ez-Farfán 《PloS one》2014,9(7)
Selection exerted by herbivores is a major force driving the evolution of plant defensive characters such as leaf trichomes or secondary metabolites. However, plant defense expression is highly variable among populations and identifying the sources of this variation remains a major challenge. Plant populations are often distributed across broad geographic ranges and are exposed to different herbivore communities, ranging from generalists (that feed on diverse plant species) to specialists (that feed on a restricted group of plants). We studied eight populations of the plant Datura stramonium usually eaten by specialist or generalist herbivores, in order to examine whether the pattern of phenotypic selection on secondary compounds (atropine and scopolamine) and a physical defense (trichome density) can explain geographic variation in these traits. Following co-evolutionary theory, we evaluated whether a more derived alkaloid (scopolamine) confers higher fitness benefits than its precursor (atropine), and whether this effect differs between specialist and generalist herbivores. Our results showed consistent directional selection in almost all populations and herbivores to reduce the concentration of atropine. The most derived alkaloid (scopolamine) was favored in only one of the populations, which is dominated by a generalist herbivore. In general, the patterns of selection support the existence of a selection mosaic and accounts for the positive correlation observed between atropine concentration and plant damage by herbivores recorded in previous studies. 相似文献
140.
Martín Castellanos-Joya Guadalupe Delgado-Sánchez Leticia Ferreyra-Reyes Pablo Cruz-Hervert Elizabeth Ferreira-Guerrero Gabriela Ortiz-Solís Mirtha Irene Jiménez Leslie Lorena Salazar Rogelio Montero-Campos Norma Mongua-Rodríguez Renata Baez-Salda?a Miriam Bobadilla-del-Valle Jesús Felipe González-Roldán Alfredo Ponce-de-León José Sifuentes-Osornio Lourdes García-García 《PloS one》2014,9(9)