首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   396篇
  免费   35篇
  2023年   8篇
  2022年   3篇
  2021年   6篇
  2020年   10篇
  2019年   7篇
  2018年   13篇
  2017年   6篇
  2016年   10篇
  2015年   19篇
  2014年   18篇
  2013年   30篇
  2012年   22篇
  2011年   27篇
  2010年   13篇
  2009年   17篇
  2008年   12篇
  2007年   22篇
  2006年   16篇
  2005年   18篇
  2004年   19篇
  2003年   11篇
  2002年   19篇
  2001年   6篇
  2000年   3篇
  1999年   8篇
  1998年   9篇
  1997年   2篇
  1996年   5篇
  1995年   4篇
  1994年   7篇
  1993年   2篇
  1992年   4篇
  1991年   4篇
  1990年   7篇
  1989年   7篇
  1988年   6篇
  1987年   4篇
  1986年   3篇
  1985年   2篇
  1984年   3篇
  1983年   3篇
  1982年   4篇
  1981年   2篇
  1979年   1篇
  1978年   3篇
  1976年   1篇
  1974年   2篇
  1972年   1篇
  1969年   1篇
  1956年   1篇
排序方式: 共有431条查询结果,搜索用时 125 毫秒
1.
2.
Monson RK  Fall R 《Plant physiology》1989,90(1):267-274
Isoprene emission rates from quaking aspen (Populus tremuloides Michx.) leaves were measured simultaneously with photosynthesis rate, stomatal conductance, and intercellular CO2 partial pressure. Isoprene emission required the presence of CO2 or O2, but not both. The light response of isoprene emission rate paralleled that of photosynthesis. Isoprene emission was inhibited by decreasing ambient O2 from 21% to 2%, only when there was oxygen insensitive photosynthesis. Mannose (10 millimolar) fed through cut stems resulted in strong inhibition of isoprene emission rate and is interpreted as evidence that isoprene biosynthesis requires either the export of triose phosphates from the chloroplast, or the continued synthesis of ATP. Light response experiments suggest that photosynthetically generated reductant or ATP is required for isoprene biosynthesis. Isoprene biosynthesis and emission are not directly linked to glycolate production through photorespiration, contrary to previous reports. Isoprene emission rate was inhibited by above-ambient CO2 partial pressures (640 microbar outside and 425 microbar inside the leaf). The inhibition was not due to stomatal closure. This was established by varying ambient humidity at normal and elevated CO2 partial pressures to measure isoprene emission rates over a range of stomatal conductances. Isoprene emission rates were inhibited at elevated CO2 despite no change in stomatal conductance. Addition of abscisic acid to the transpiration stream dramatically inhibited stomatal conductance and photosynthesis rate, with a slight increase in isoprene emission rate. Thus, isoprene emission is independent of stomatal conductance, and may occur through the cuticle. Temperature had an influence on isoprene emission rate, with the Q10 being 1.8 to 2.4 between 35 and 45°C. At these high temperatures the amount of carbon lost through isoprene emission was between 2.5 and 8% of that assimilated through photosynthesis. This represents a significant carbon cost that should be taken into account in determining midsummer carbon budgets for plants that are isoprene emitters.  相似文献   
3.
Hepatic plasma membrane lactate transport was studied in isolated hepatocytes prepared from fed, starved, and streptozotocin diabetic rats. Carrier-mediated lactate entry was determined using the lactate transport inhibitors alpha-cyano-3-hydroxycinnamate and D-3-hydroxybutyrate and was significantly greater in hepatocytes from starved compared to fed rats and in hepatocytes from diabetic fed compared to fed rats. The saturable component of lactate entry which corresponds to carrier-mediated transport was higher in the starved than in the fed state with results from diabetic fed being intermediate between the two. Insulin treatment prevented the increment in carrier-mediated lactate transport observed in hepatocytes from diabetic fed rats. The data indicate that hepatic plasma membrane lactate transport is increased under conditions of starvation and diabetes mellitus. This may partly explain the increased gluconeogenic flux under these conditions.  相似文献   
4.
Mulberry trees in Northern Utah were located by historical references to local sericulture and by examining vegetation in proximity to pioneer-era two-story houses. It was usually in the upstairs bedrooms of these larger homes where silkworms were raised, and many of these houses planted one or two mulberry trees in their yards. Although mulberry planting and sericulture were once advocated by early Mormon leaders as a means to achieve economic self-sufficiency, ultimately this social and economic experiment failed, leaving relic mulberry groves dotted throughout Utah. Most of these groves have now disappeared in the wake of urban expansion. Preservation of a few relic trees is proposed as a means of preserving cultural ties to the past.  相似文献   
5.
Isoprene is the primary biogenic hydrocarbon emitted from temperate deciduous forest ecosystems. The effects of varying photon flux density (PFD) and nitrogen growth regimes on rates of isoprene emission and net photosynthesis in potted aspen and white oak trees are reported. In both aspen and oak trees, whether rates were expressed on a leaf area or dry mass basis, (1) growth at higher PFD resulted in significantly higher rates of isoprene emission, than growth at lower PFD, (2) there is a significant positive relationship between isoprene emission rate and leaf nitrogen concentration in both sun and shade trees, and (3) there is a significant positive correlation between isoprene emission rate and photosynthetic rate in both sun and shade trees. The greater capacity for isoprene emission in sun leaves was due to both higher leaf mass per unit area and differences in the biochemical and/or physiological properties that influence isoprene emission. Positive correlations between isoprene emission rate and leaf nitrogen concentration support the existence of mechanisms that link leaf nitrogen status to isoprene synthase activity. Positive correlations between isoprene emission rate and photosynthesis rate support previous hypotheses that isoprene emission plays a role in protecting photosynthetic mechanisms during stress.  相似文献   
6.
Females of dioecious species usually have higher reproductive effort than males because they produce fruits in addition to flowers. Since females have higher reproductive effort, they are expected to be more negatively affected than males by low resource availability. We tested that assumption by growing females and males of Silene latifolia under low levels of light, water, nitrogen, phosphorus, and potassium. Gas exchange of the sexes did not respond differently to low resource availability; higher female reproductive effort relative to males did not differentially affect their ability to assimilate carbon. However, male photosynthesis rates and stomatal conductances were slightly, but consistently, higher than those of females. The intersexual difference in photosynthesis rate may be a proximate result of reproduction if females translocate nutrients, particularly nitrogen, from their leaves to developing fruits. Alternatively (or perhaps additionally), higher male photosynthesis and stomatal conductances relative to females may be the ultimate result of sexual selection. This could be the case if 1) reproductive effort as estimated by biomass allocation is misleading and males actually invest more in reproduction than females, or 2) females experience stronger selection than males to conserve water late in the growing season, when soil moisture is likely to be low but females need to complete fruit maturation. Our results indicated that females had slightly lower leaf nitrogen but higher photosynthetic water-use efficiency than males, so it is possible that both proximate and ultimate factors are operating simultaneously to cause lower female photosynthesis rates.  相似文献   
7.
David W. Kikuchi  William L. Allen  Kevin Arbuckle  Thomas G. Aubier  Emmanuelle S. Briolat  Emily R. Burdfield-Steel  Karen L. Cheney  Klára Daňková  Marianne Elias  Liisa Hämäläinen  Marie E. Herberstein  Thomas J. Hossie  Mathieu Joron  Krushnamegh Kunte  Brian C. Leavell  Carita Lindstedt  Ugo Lorioux-Chevalier  Melanie McClure  Callum F. McLellan  Iliana Medina  Viraj Nawge  Erika Páez  Arka Pal  Stano Pekár  Olivier Penacchio  Jan Raška  Tom Reader  Bibiana Rojas  Katja H. Rönkä  Daniela C. Rößler  Candy Rowe  Hannah M. Rowland  Arlety Roy  Kaitlin A. Schaal  Thomas N. Sherratt  John Skelhorn  Hannah R. Smart  Ted Stankowich  Amanda M. Stefan  Kyle Summers  Christopher H. Taylor  Rose Thorogood  Kate Umbers  Anne E. Winters  Justin Yeager  Alice Exnerová 《Journal of evolutionary biology》2023,36(7):975-991
Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such “defence portfolios” that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies.  相似文献   
8.
Deimatic displays, where sudden changes in prey appearance elicit aversive predator reactions, have been suggested to occur in many taxa. These (often only putative) displays frequently involve different components that may also serve antipredator functions via other mechanisms (e.g., mimicry, warning signalling, body inflation). The Colombian four-eyed frog, Pleurodema brachyops, has been suggested to gain protection against predation through putative deimatic displays where they inflate and elevate the posterior part of their body revealing eye-like colour markings. We exposed stationary artificial frogs to wild predators to test whether the two components (eyespot/colour markings, defensive posture) of their putative deimatic display, and their combination, provide protection from predation without the sudden change in appearance. We did not detect any obvious additive effect of defensive posture and eyespots/colour markings on predation risk, but found a marginally significant trend for model frogs in the resting posture to be less attacked when displaying eyespots/colour markings than when they were not, suggesting that the presence of colour markings/eyespots may provide some protection on its own. Additionally, we found that models in a resting posture were overall more frequently attacked on the head than models in a defensive posture, indicating that a defensive posture alone could help redirect predator attacks to non-vital parts of the body. The trends found in our study suggest that the different components of P. brachyops' coloration may serve different functions during a deimatic display, but further research is needed to elucidate the role of each component when accompanied by sudden prey movement.  相似文献   
9.
In perfused rat liver a decrease of cytosol pH, determined with pH-sensitive microelectrodes7 from 7.2 to 6.85 is associated with a 50% fall in ureogenesis from ammonium chloride. In isolated rat hepatocytes the fall in ureogenesis due to acidosis is associated with decrease in the mitochondrial and cytosolic concentration of citrulline. Limitation of carbamoyl phosphate synthesis and thus citrulline supply could be responsible for the inhibition of ureogenesis observed.  相似文献   
10.
When Chinese hamster ovary (CHO) cells were exposed to 22 degrees C for 2 hr prior to 42.4 degrees C hyperthermia, neither the shoulder region of the survival curve nor the characteristic development of thermotolerance after 3-4 hr of heating were observed. Absolute cell survival after 4 hr at 42.4 degrees C was decreased by a factor of between 10 and 100 (depending on the rate of heating of nonprecooled controls). Conditioning at 30 degrees C for 2 hr, 26 degrees C for 2 hr, or 22 degrees C for 20 min followed by heating to 42.4 degrees C over 30 min did not result in sensitization. Prolonged (16 hr) conditioning at 30 degrees C, however, increased the cytotoxicity of immediate exposure to 41.4 or 45 degrees C with maximum sensitization to 45 degrees C occurring after 6 hr at 30 degrees C. Both 3- and 18-hr pretreatments at 30 degrees C similarly increased the cytotoxicity of 45-41.5 degrees C step-down heating (D0 = 28 min in precooled versus 40 min in nonprecooled cells).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号