全文获取类型
收费全文 | 5078篇 |
免费 | 521篇 |
国内免费 | 1022篇 |
出版年
2024年 | 8篇 |
2023年 | 141篇 |
2022年 | 95篇 |
2021年 | 209篇 |
2020年 | 235篇 |
2019年 | 259篇 |
2018年 | 199篇 |
2017年 | 201篇 |
2016年 | 228篇 |
2015年 | 240篇 |
2014年 | 227篇 |
2013年 | 341篇 |
2012年 | 250篇 |
2011年 | 279篇 |
2010年 | 242篇 |
2009年 | 256篇 |
2008年 | 289篇 |
2007年 | 293篇 |
2006年 | 265篇 |
2005年 | 249篇 |
2004年 | 180篇 |
2003年 | 197篇 |
2002年 | 211篇 |
2001年 | 188篇 |
2000年 | 126篇 |
1999年 | 127篇 |
1998年 | 122篇 |
1997年 | 121篇 |
1996年 | 109篇 |
1995年 | 86篇 |
1994年 | 84篇 |
1993年 | 63篇 |
1992年 | 70篇 |
1991年 | 57篇 |
1990年 | 19篇 |
1989年 | 35篇 |
1988年 | 28篇 |
1987年 | 27篇 |
1986年 | 18篇 |
1985年 | 39篇 |
1984年 | 35篇 |
1983年 | 32篇 |
1982年 | 36篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1979年 | 14篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1973年 | 10篇 |
排序方式: 共有6621条查询结果,搜索用时 31 毫秒
91.
A two-stage bioreactor system for the production of recombinant proteins using a genetically engineered baculovirus/insect cell system 总被引:1,自引:0,他引:1
A two-stage bioreactor scheme was developed for the large-scale production of recombinant proteins using a genetically engineered baculovirus/insect cell system. The first bioreactor was employed for cell growth and the second for cell infection. Silkworm Bm5 cells were infected with a recombinant baculovirus, BmNPV/P5.cat, containing a bacterial chloramphenicol acetyltransferase (CAT) gene under the control of the polyhedrin gene promoter of Bombyx mori nuclear polyhedrosis virus (BmNPV). This recombinant baculovirus has been used as an expression vector for the production of recombinant CAT enzyme. A specific productivity of 82 to 90 mug CAT/(10(6) cells) was obtained using the BmNPV/Bm5 expression system, a yield similar to that achieved using the AcNPV/Sf expression system. Repeated infection of high-density cell cultures did not reduce the specific productivity of the CAT enzyme. Most importantly, the problems associated with the infection of high-density cell cultures were resolved by means of controlled infection conditions and appropriate replenishment of spent culture medium following infection. The glucose uptake rate by the cells following infection was 50% higher than that by the cells before infection. Not only did the infection of high-density cell cultures result in consistent yields of 250 mg/L of CAT enzyme, but also the two-stage bioreactor system was proven to be reliable for a long-term operation beyond 600 h. (c) 1993 John Wiley & Sons, Inc. 相似文献
92.
T. Jermy 《Entomologia Experimentalis et Applicata》1993,66(1):3-12
Most hypotheses concerning the evolution of insect-plant relationships are based on the assumptions that, (1) phytophagous
insects reduce plant fitness, and that (2) insect-plant relationships are the result of unconstrained selection. It can be
shown, however, that there is little evidence to support these assumptions. As an alternative, it is proposed that the evolution
of insect-plant relationships results primarily from autonomous evolutionary events; namely from heritable functional changes
within the insects' nervous system that determine plant recognition and ultimately host plant specificity. These changes cannot
be evoked by selective ecological agents. They originate from intrinsic changes (mutationssensu lato) within the insect genome. Ecological factors play a secondary role: by either supporting or preventing the establishment
of the new genotype with the novel food preference.
This paper has been dedicated in warm friendship to Professor Louis M. Schoonhoven, the leading scientist in sensory physiology
of phytophagous insects, on the occasion of his 60th birthday. 相似文献
93.
Antisera raised to the cardioactive peptide corazonin were used to localize immunoreactive cells in the nervous system of the American cockroach. Sera obtained after the seventh booster injection were sufficiently specific to be used for immunocytology. They recognized a subset of 10 lateral neurosecretory cells in the protocerebrum that project to, and arborize and terminate in the ipsilateral corpus cardiacum. They also reacted with bilateral neurons in each of the thoracic and abdominal neuromeres, a single dorsal unpaired median neuron in the suboesophageal ganglion, an interneuron in each optic lobe, and other neurons at the base of the optic lobe, in the tritocerebrum and deutocerebrum. The presence of corazonin in the abdominal neurons and the lateral neurosecretory cells was confirmed by HPLC fractionation of extracts of the abdominal ganglia, brains and retrocerebral complexes, followed by determination of corazonin by ELISA, which revealed in each tissue a single immunoreactive peak co-eluting with corazonin in two different HPLC systems. Antisera obtained after the first three booster injections recognized a large number of neuroendocrine cells and neurons in the brain and the abdominal nerve cord. However, the sera from the two rabbits reacted largely with different cells, indicating that the majority of this immunoreactivity was due to cross-reactivity. These results indicate that the production of highly specific antisera to some neuropeptides may require a considerable number of booster injections. 相似文献
94.
Jennifer D. Watts Mary Farina John S. Kimball Luke D. Schiferl Zhihua Liu Kyle A. Arndt Donatella Zona Ashley Ballantyne Eugénie S. Euskirchen Frans-Jan W. Parmentier Manuel Helbig Oliver Sonnentag Torbern Tagesson Janne Rinne Hiroki Ikawa Masahito Ueyama Hideki Kobayashi Torsten Sachs Daniel F. Nadeau John Kochendorfer Marcin Jackowicz-Korczynski Anna Virkkala Mika Aurela Roisin Commane Brendan Byrne Leah Birch Matthew S. Johnson Nima Madani Brendan Rogers Jinyang Du Arthur Endsley Kathleen Savage Ben Poulter Zhen Zhang Lori M. Bruhwiler Charles E. Miller Scott Goetz Walter C. Oechel 《Global Change Biology》2023,29(7):1870-1889
Arctic-boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic-boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003–2015) vegetation gross primary productivity (GPP), ecosystem respiration (Reco), net ecosystem CO2 exchange (NEE; Reco − GPP), and terrestrial methane (CH4) emissions for the Arctic-boreal zone using a satellite data-driven process-model for northern ecosystems (TCFM-Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM-Arctic to obtain daily 1-km2 flux estimates and annual carbon budgets for the pan-Arctic-boreal region. Across the domain, the model indicated an overall average NEE sink of −850 Tg CO2-C year−1. Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH4 emissions from tundra and boreal wetlands (not accounting for aquatic CH4) were estimated at 35 Tg CH4-C year−1. Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink by 21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high-latitude carbon status and also indicates a continued need for integrated site-to-regional assessments to monitor the vulnerability of these ecosystems to climate change. 相似文献
95.
Danica Antonijević Mathias Hoffmann Annette Prochnow Karoline Krabbe Mirjam Weituschat John Couwenberg Sigrid Ehlert Dominik Zak Jürgen Augustin 《Global Change Biology》2023,29(13):3678-3691
Drainage and agricultural use transform natural peatlands from a net carbon (C) sink to a net C source. Rewetting of peatlands, despite of high methane (CH4) emissions, holds the potential to mitigate climate change by greatly reducing CO2 emissions. However, the time span for this transition is unknown because most studies are limited to a few years. Especially, nonpermanent open water areas often created after rewetting, are highly productive. Here, we present 14 consecutive years of CH4 flux measurements following rewetting of a formerly long-term drained peatland in the Peene valley. Measurements were made at two rewetted sites (non-inundated vs. inundated) using manual chambers. During the study period, significant differences in measured CH4 emissions occurred. In general, these differences overlapped with stages of ecosystem transition from a cultivated grassland to a polytrophic lake dominated by emergent helophytes, but could also be additionally explained by other variables. This transition started with a rapid vegetation shift from dying cultivated grasses to open water floating and submerged hydrophytes and significantly increased CH4 emissions. Since 2008, helophytes have gradually spread from the shoreline into the open water area, especially in drier years. This process was periodically delayed by exceptional inundation and eventually resulted in the inundated site being covered by emergent helophytes. While the period between 2009 and 2015 showed exceptionally high CH4 emissions, these decreased significantly after cattail and other emergent helophytes became dominant at the inundated site. Therefore, CH4 emissions declined only after 10 years of transition following rewetting, potentially reaching a new steady state. Overall, this study highlights the importance of an integrative approach to understand the shallow lakes CH4 biogeochemistry, encompassing the entire area with its mosaic of different vegetation forms. This should be ideally done through a study design including proper measurement site allocation as well as long-term measurements. 相似文献
96.
Plant-associated microbiomes can improve plant fitness by ameliorating environmental stress, providing a promising avenue for improving outplantings during restoration. However, the effects of water management on these microbial communities and their cascading effects on primary producers are unresolved for many imperiled ecosystems. One such habitat, Everglades tree islands, has declined by 54% in some areas, releasing excess nutrients into surrounding wetlands and exacerbating nutrient pollution. We conducted a factorial experiment, manipulating the soil microbiome and hydrological regime experienced by a tree island native, Ficus aurea, to determine how microbiomes impact growth under two hydrological management plans. All plants were watered to simulate natural precipitation, but plants in the “unconstrained” management treatment were allowed to accumulate water above the soil surface, while the “constrained” treatment had a reduced stage to avoid soil submersion. We found significant effects of the microbiomes on overall plant performance and aboveground versus belowground investment; however, these effects depended on hydrological treatment. For instance, microbiomes increased investment in roots relative to aboveground tissues, but these effects were 142% stronger in the constrained compared to unconstrained water regime. Changes in hydrology also resulted in changes in the prokaryotic community composition, including a >20 log2fold increase in the relative abundance of Rhizobiaceae, and hydrology-shifted microbial composition was linked to changes in plant performance. Our results suggest that differences in hydrological management can have important effects on microbial communities, including taxa often involved in nitrogen cycling, which can in turn impact plant performance. 相似文献
97.
98.
99.
100.