全文获取类型
收费全文 | 14641篇 |
免费 | 1284篇 |
国内免费 | 1篇 |
专业分类
15926篇 |
出版年
2024年 | 22篇 |
2023年 | 155篇 |
2022年 | 118篇 |
2021年 | 287篇 |
2020年 | 233篇 |
2019年 | 241篇 |
2018年 | 602篇 |
2017年 | 502篇 |
2016年 | 531篇 |
2015年 | 459篇 |
2014年 | 474篇 |
2013年 | 835篇 |
2012年 | 1183篇 |
2011年 | 1352篇 |
2010年 | 704篇 |
2009年 | 454篇 |
2008年 | 994篇 |
2007年 | 905篇 |
2006年 | 887篇 |
2005年 | 673篇 |
2004年 | 699篇 |
2003年 | 656篇 |
2002年 | 584篇 |
2001年 | 411篇 |
2000年 | 522篇 |
1999年 | 258篇 |
1998年 | 137篇 |
1997年 | 100篇 |
1996年 | 93篇 |
1995年 | 77篇 |
1994年 | 79篇 |
1993年 | 76篇 |
1992年 | 70篇 |
1991年 | 63篇 |
1990年 | 40篇 |
1989年 | 29篇 |
1988年 | 31篇 |
1987年 | 30篇 |
1986年 | 15篇 |
1985年 | 40篇 |
1984年 | 27篇 |
1983年 | 25篇 |
1982年 | 32篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 22篇 |
1978年 | 21篇 |
1977年 | 18篇 |
1974年 | 18篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
51.
Patterns and controls of the variability of radiation use efficiency and primary productivity across terrestrial ecosystems 总被引:2,自引:0,他引:2
Martín F. Garbulsky Josep Peñuelas Dario Papale Jonas Ardö Michael L. Goulden Gerard Kiely Andrew D. Richardson Eyal Rotenberg Elmar M. Veenendaal Iolanda Filella 《Global Ecology and Biogeography》2010,19(2):253-267
Aim The controls of gross radiation use efficiency (RUE), the ratio between gross primary productivity (GPP) and the radiation intercepted by terrestrial vegetation, and its spatial and temporal variation are not yet fully understood. Our objectives were to analyse and synthesize the spatial variability of GPP and the spatial and temporal variability of RUE and its climatic controls for a wide range of vegetation types. Location A global range of sites from tundra to rain forest. Methods We analysed a global dataset on photosynthetic uptake and climatic variables from 35 eddy covariance (EC) flux sites spanning between 100 and 2200 mm mean annual rainfall and between ?13 and 26°C mean annual temperature. RUE was calculated from the data provided by EC flux sites and remote sensing (MODIS). Results Rainfall and actual evapotranspiration (AET) positively influenced the spatial variation of annual GPP, whereas temperature only influenced the GPP of forests. Annual and maximum RUE were also positively controlled primarily by annual rainfall. The main control parameters of the growth season variation of gross RUE varied for each ecosystem type. Overall, the ratio between actual and potential evapotranspiration and a surrogate for the energy balance explained a greater proportion of the seasonal variation of RUE than the vapour pressure deficit (VPD), AET and precipitation. Temperature was important for determining the intra‐annual variability of the RUE at the coldest energy‐limited sites. Main conclusions Our analysis supports the idea that the annual functioning of vegetation that is adapted to its local environment is more constrained by water availability than by temperature. The spatial variability of annual and maximum RUE can be largely explained by annual precipitation, more than by vegetation type. The intra‐annual variation of RUE was mainly linked to the energy balance and water availability along the climatic gradient. Furthermore, we showed that intra‐annual variation of gross RUE is only weakly influenced by VPD and temperature, contrary to what is frequently assumed. Our results provide a better understanding of the spatial and temporal controls of the RUE and thus could lead to a better estimation of ecosystem carbon fixation and better modelling. 相似文献
52.
Steven D. Johnson Marco G. Balducci Alison Bijl Miguel Castañeda-Zárate Ruth J. Cozien Carmen R. Ortmann Timotheüs van der Niet 《Ecological Entomology》2020,45(3):751-755
1. Diel rhythms of foraging activity by animal flower visitors can reflect niche partitioning and are considered an important component of selection on floral traits. However, it has been notoriously difficult to obtain objective information on the patterns of flower visitation by crepuscular and nocturnal insects. 2. Motion-activated cameras were used for field-based studies of hawkmoth foraging behaviour on six African plant species. 3. The results showed that short-tongued hawkmoth species forage mainly around dusk and then sporadically throughout the night, whereas long-tongued hawkmoth species feed consistently throughout the night, with a peak shortly before midnight. 4. These results provide the first quantitative estimates of diel patterns of interactions between multiple hawkmoth and plant species and, when combined with qualitative reports from other studies, suggest that differences in diel activity between the two main hawkmoth functional groups (short- and long-tongued) are consistent across the Old and New Worlds. 相似文献
53.
54.
ATP (1 mM) increased the phospholipase D (PLD) activity of rat submandibular gland (RSMG) ductal cells in a concentration-dependent and calcium-sensitive manner. The response to ATP was reproduced by benzoylbenzoyl-ATP (Bz-ATP, 100 microM) and also partly by adenosine 5'-(gamma-thio)triphosphate (ATPgammaS, 1 mM). A similar stimulation was observed in control mice (P2X7R+/+ mice) but not in mice lacking the P2X7 receptors (P2X7R-/- mice). Oxidized ATP and Coomassie blue or the addition of magnesium or nickel to the incubation medium inhibited the response to ATP. The stimulation of PLD by purinergic agonist was inhibited by about 50% by calphostin C and chelerythrine, two protein kinase C (PKC) inhibitors. The stimulation of PLD by Bz-ATP and by o-tetradecanoylphorbol 13-acetate (TPA), a phorbol ester which activates PKC, were not additive.From these results we can conclude that the activation of P2X7 receptors in RSMG ductal cells is coupled to the activation of a PLD. This activation is partly mediated by protein kinase C. 相似文献
55.
Rosalie J. Harris Callum Bryant Melinda A. Coleman Andrea Leigh Verónica F. Briceño Pieter A. Arnold Adrienne B. Nicotra 《Journal of phycology》2023,59(1):179-192
Foundation seaweed species are experiencing widespread declines and localized extinctions due to increased instability of sea surface temperature. Characterizing temperature thresholds are useful for predicting patterns of change and identifying species most vulnerable to extremes. Existing methods for characterizing seaweed thermal tolerance produce diverse metrics and are often time-consuming, making comparisons between species and techniques difficult, hindering insight into global patterns of change. Using three kelp species, we adapted a high-throughput method – previously used in terrestrial plant thermal biology – for use on kelps. This method employs temperature-dependent fluorescence (T–F0) curves under heating or cooling regimes to determine the critical temperature (Tcrit) of photosystem II (PSII), i.e., the breakpoint between slow and fast rise fluorescence response to changing temperature, enabling rapid assays of photosynthetic thermal tolerance using a standardized metric. This method enables characterization of Tcrit for up to 48 samples per two-hour assay, demonstrating the capacity of T–F0 curves for high-throughput assays of thermal tolerance. Temperature-dependent fluorescence curves and their derived metric, Tcrit, may offer a timely and powerful new method for the field of phycology, enabling characterization and comparison of photosynthetic thermal tolerance of seaweeds across many populations, species, and biomes. 相似文献
56.
4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization. 相似文献
57.
Mayra Domínguez-Pérez Arturo Simoni-Nieves Patricia Rosales Natalia Nuño-Lámbarri Mónica Rosas-Lemus Verónica Souza Roxana U. Miranda Leticia Bucio Salvador Uribe Carvajal Jens U. Marquardt Daekwan Seo Luis E. Gomez-Quiroz María Concepción Gutiérrez-Ruiz 《Journal of cellular physiology》2019,234(5):7213-7223
58.
59.
Synaptosomes were isolated from cerebrums of rats fed standard (20% protein) or protein-free diets for 30 days. Arrhenius plots of their (Na+/K+)ATPase activities revealed a transition temperature of 25.5°C for control rats and 23.4°C for rats on protein-free diet, indicating that the latter increases synaptosomal membrane fluidity. The only change observed in the composition of the synaptosomal membranes was a 26% decrease of sialic acid. In synaptosomes from rats on protein-free diet the uptake of tyrosine was slightly reduced while that of glutamate was not affected. However, the exit of glutamate was reduced. 相似文献
60.
Grijota Francisco Javier Muñoz Diego Bartolomé Ignacio Siquier-Coll Jesús Robles María Concepción Maynar Marcos 《Biological trace element research》2020,195(1):39-45
Biological Trace Element Research - The aim of the present study was to determine changes occurring in the erythrocyte concentrations of arsenic (As), cadmium (Cd) and lead (Pb) in highly trained... 相似文献