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   47篇
  免费   4篇
  51篇
  2022年   1篇
  2018年   2篇
  2016年   4篇
  2014年   2篇
  2013年   2篇
  2012年   8篇
  2011年   2篇
  2010年   1篇
  2009年   3篇
  2008年   5篇
  2007年   2篇
  2006年   1篇
  2005年   1篇
  2003年   2篇
  2002年   1篇
  2000年   1篇
  1995年   2篇
  1994年   1篇
  1990年   2篇
  1989年   1篇
  1987年   2篇
  1986年   1篇
  1984年   1篇
  1982年   2篇
  1981年   1篇
排序方式: 共有51条查询结果,搜索用时 15 毫秒
31.
This study presents an analysis of temperature and precipitation trends and their impact on grape harvests in the Penedès region (NE Spain). It includes analyses of maximum, minimum and mean daily temperatures (for both the growing and ripening seasons) and daily rainfall (for the hydrological year, the growing season and each phenological stage) for three observatories in the immediate area. We analysed a series of factors: beginning and end harvest dates; the day on which a given potential alcoholic degree was reached; and yield for several varieties of grape grown in the area in relation to climatic variables. Maximum temperatures increased at all the observatories, with greater values being recorded in recent years (1996–2009) than in previous decades (1960s–2000s): we observed increases in average growing season temperatures of 0.11°C per year for the period 1996–2009 vs 0.04°C per year for the period 1960–2009 at Vilafranca del Penedès. These temperature changes were due mainly to increases in maximum temperatures and an increase in the incidence of extreme heat (number of days with T > 30°C). Crop evapotranspiration also increased significantly during the same period. The Winkler index also increased, so the study area would correspond to region IV according to that climatic classification. There were no significant trends in annual rainfall but rainfall recorded between bloom and veraison decreased significantly at the three observatories, with the greatest decrease corresponding to the period 1996–2009. The dates on which harvests started and ended showed a continuous advance (of between −0.7 and −1.1 days per year, depending on the variety), which was significantly correlated with the average mean and maximum daily growing season temperatures (up to −7.68 days for 1°C increase). Winegrape yield was influenced by the estimated water deficit (crop evapotranspiration minus precipitation) in the bloom-veraison period; this value increased due to a reduction in precipitation and an increase in evapotranspiration. Yield may have been reduced by up to 30 kg/ha for each millimetre increase in the estimated water deficit. Under these conditions, new strategies need to be followed in this area in order to maintain grape quality and yield.  相似文献   
32.
Over recent years increased MS incidence, primarily in women, has been reported. We recently reported an unexpectedly high MS prevalence of 189/100,000 in Sweden. In the present study we estimated the nationwide age- and gender-specific MS incidence and the sex ratio in Sweden between 2001 and 2008. MS patients were identified by linking two nationwide health data registers, and the Swedish population register. The earliest registered date of MS diagnosis was determined. By logistic regression, the probability of the date of MS diagnosis being within the incidence period, depending on age and time was estimated for a subset of patients and applied to other patients. By Poisson regression, the hazard functions for the incidence of MS diagnosis were estimated. The expected number of MS patients was 7,361.4. The incidence in the average population of 9,054,658 was 10.2 per 100,000 person-years, and 6.2 and 14.0 per 100,000 person-years for men and women, respectively. The crude female to male ratio was 2.26. No increase of incidence or change of sex ratio was observed from 2001 to 2008. In conclusion, the average MS incidence in Sweden from 2001 to 2008 was 10.2 per 100.000, which was considerably higher than previous regional Swedish estimates of 4.3–6.4. No increase of female to male ratio of MS during the study period was observed. We provide supplementary data that can be used as tools for examining excess MS risk in different study materials.  相似文献   
33.
Gibberellins A1 (GA1), A3 and A9 were identified from extracts of shoots of 6-month old Norway spruce (Picea abies) seedlings by the use of sequential reverse and normal phase high performance liquid chromatography (HPLC), bioassay, radioimmunoassay (RIA) and combined gas chromatography-mass spectrometry (GC-MS). The bioassay and RIA were used after fractionation by HPLC to detect the GA-containing fractions, which were then examined by GC-MS. The GAs identified are considered to be endogenous.  相似文献   
34.
Assessing the recovery of species diversity and composition after major disturbance is key to understanding the resilience of tropical forests through successional processes, and its importance for biodiversity conservation. Despite the specific abiotic environment and ecological processes of tropical dry forests, secondary succession has received less attention in this biome than others and changes in species diversity and composition have never been synthesised in a systematic and quantitative review. This study aims to assess in tropical dry forests 1) the directionality of change in species richness and evenness during secondary succession, 2) the convergence of species composition towards that of old‐growth forest and 3) the importance of the previous land use, precipitation regime and water availability in influencing the direction and rate of change. We conducted meta‐analyses of the rate of change in species richness, evenness and composition indices with succession in 13 tropical dry forest chronosequences. Species richness increased with succession, showing a gradual accumulation of species, as did Shannon evenness index. The similarity in species composition of successional forests with old‐growth forests increased with succession, yet at a low rate. Tropical dry forests therefore do show resilience of species composition but it may never reach that of old‐growth forests. We found no significant differences in rates of change between different previous land uses, precipitation regimes or water availability. Our results show high resilience of tropical dry forests in term of species richness but a slow recovery of species composition. They highlight the need for further research on secondary succession in this biome and better understanding of impacts of previous land‐use and landscape‐scale patterns. Synthesis Secondary forests account for an increasing proportion of remaining tropical forest. Assessing their resilience is key to conservation of their biodiversity. Our study is the first meta‐analysis of species changes during succession focussing on tropical dry forests, a highly threatened yet understudied biome. We show a gradual species accumulation and convergence of composition towards that of old‐growth forests. While secondary tropical dry forests offer good potential for biodiversity conservation, their capacity for recovery at a sufficient rate to match threats is uncertain. Further research on this biome is needed to understand the effect of land use history and landscape processes.  相似文献   
35.
Two different series of N-substituted imidazolium oximes and their monoquaternary salts were synthesized and biologically tested with respect to their ability to inhibit growth a diverse panel of antibiotic susceptible Gram-positive and antibiotic resistant Gram-negative bacteria as well fungal strains. The newly synthesized compounds were analyzed by spectral studies to confirm their structure. The preliminary results showed that all compounds tested possess promising antimicrobial potential against both susceptible Gram-positive and antibiotic resistant Gram-negative isolates, exhibiting a wide range of MIC values from 0.14 to 100.0 μg/mL. The structure–activity relationship demonstrates that the p-methylphenyl and p-fluorophenyl groups in monoquaternary salts 6 and 7 attached directly to the imidazolium ring could be essential for observed remarkable inhibitory profiles against clinically important pathogens Pseudomonas aeruginosa (MIC = 0.14 μg/mL) and Klebsiella pneumoniae (MIC = 1.56 μg/mL). Furthermore, the broth microdilution assay was then used to investigate the antiresistance efficacy of compound 7 against fourteen extended-spectrum β-lactamase (ESBL)-producing strains in comparison to eight clinically relevant antibiotics. Compound 7 exhibited a remarkable antiresistance profiles ranging between 0.39 and 12.50 μg/mL against all of ESBL-producing strains, which leads to the suggestion that may be interesting candidate for development of new antimicrobials to combat multidrug resistant Gram-negative bacteria.  相似文献   
36.
A thermotolerant and halotolerant strain of Pycnoporus sanguineus was isolated from an oil-polluted site in a tropical area located in Veracruz, Mexico. This strain was able to grow at 47 degrees C and in culture medium containing 500 mM NaCl. The strain was also tolerant to the presence of 30,000 ppm of crude Maya oil. A 68-kDa protein purified from submerged cultures exhibited laccase activity towards 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), guaiacol, syringaldazine, and o-dianisidine, for which it presented the highest affinity (Km = 43 microM). Two-dimensional gel electrophoresis analysis showed that, unusual for laccases, the enzyme has two active isoforms, with isoelectric points of 7.00 and 7.08. The purified enzyme showed high thermostability, retaining 40% of its original activity after 3 h at 60 degrees C. This property seems to correlate with a long "shelf-life," given that at 40 degrees C enzyme activity was only gradually lost over a 5-day period incubation. Both the fungus and its laccase are likely to have high potential for biotechnological applications.  相似文献   
37.
Sero-epidemiological surveys of serum samples taken in 1982, 1987, 1994 and 1999 have been performed with hepatitis A virus-specific (HAV-specific) serological tests. Results obtained during these surveys show that the proportion of seropositive blood donors decreased from 69% to 18% within 17 years. The authors have recognised a (mainly subclinical) epidemic, affecting about 115000 teenagers in 1992-1994 in Hungary, is a threatening phenomenon. It was calculated that only about 3600 clinical diseases were associated with the epidemic, recognised retrospectively from the findings of the four sero-epidemiological surveys. Epidemiological data indicated that the excess clinical diseases caused by HAV concentrated in the southern counties of Hungary, which have been affected by the social and military activities between 1992 and 1994. Due to the decrease of subjects seropositive for HAV, sera from preselected or actively immunised donors will be required in the future and vaccination against HAV with killed virus is likely to be recommended for risk groups. Furthermore, health authorities might promote active immunisation of young children against HAV infection; for that, promotion of manufacturing combination vaccines of HAV/HBV/DPT or, for certain countries, HAV/DPT would be desirable.  相似文献   
38.
The chromatographic behaviour of abscisic acid (ABA), indole-3-acetic acid (IAA), phenylacetic acid (PAA), and gibberellins A1, A4, A8, A9, A13 and A20 on columns of Sephadex LH-20 and insoluble poly- N -vinylpyrrolidone (PVP) eluted with buffers of different pH values is described. PVP shows considerable batch differences that must be carefully checked. Chromatography of acidic ethyl acetate-soluble fractions of Scots pine ( Pinus sylvestris L.) extracts at pH 4.5 resulted in great losses of phytohormones, due to poor solubility of the extracts. If the extracts were applied to the column dissolved in buffer of pH 7.5, subsequent elution at pH 4.5 was possible with only small losses. The performance of the chromatographic column was strongly affected by the application volume. A combined PVP/Sephadex LH-20 column eluted at pH 4.5 allows remarkable purification of pine and spruce ( Picea abies (L.) Karst.) extracts, collection of IAA in a fraction that can be directly analyzed by e.g. the indolo-α-pyrone method, and collection of another fraction containing ABA, PAA and probably most of the known C19-gibberellins; whereas the C20-gibberellin A13 is eluted later (with IAA).  相似文献   
39.
The levels of endogenous gibberellin A1 (GA1), GA3, GA4, GA9 and a cellulase-hydrolysable GA9-conjugate in needles and shoot stems of Sitka spruce [Picea sitchensis (Bong.) Carr.] grafts with different coning or flowering histories were estimated by combined gas chromatography-mass spectrometry selected ion monitoring using deuterated GA3, GA4 and GA9 as internal standards. The samples were taken at the approximate time of the start of flower-bud differentiation, i.e. when the shoots had elongated approx. 95% of the final length. The needles of the good-flowering clones contained 11–12 ng per g fresh weight (FW) and 15–28 ng· (g FW) –1 of GA9-conjugate and GA9, respectively. The shoot stems of the same material contained no detectable amounts of GA9-conjugate and 11–15 ng-(g FW)–1 of GA9. The amounts of GA9-conjugate and GA9 were apparently lower in the poor-flowering clones, the needles containing 4–9 ng-(g FW)–1 and 7–17 ng·(g FW)–1, respectively. Also in this material the shoot stems contained no detectable amounts of GA9-conjugate. The amounts of GA4 were very small in both materials, ranging from 1–1.6 ng-(g FW)–1. The good-flowering clones contained no detectable amounts of the more polar gibberellins, GA1 and GA3. The poor-flowering clones, on the other hand, contained high levels of GA15 17–19ng·(gFW)–1 in the needles and 10–13 ng·(g FW) –1 in the shoot stems, and also smaller amounts of GA3, 2–3 ng·(g FW)–1 in the needles and approx. 1 ng·(g FW)–1 in the shoot stems. The results demonstrate differences in GA-metabolism between the poor- and the good-flowering clones. The higher amounts of GA9-conjugate and GA9 might indicate a higher capacity for synthesizing GA4 in the good-flowering material. This synthesis does not, however, result in a build-up of the GA4-pool, maybe because of a high rate of turnover. Gibberellin A4 was apparently neither hydroxylated to GA1 nor converted to GA3 in the goodflowering material, as was the case in the poor-flowering material. This might indicate that gibberellin metabolism in the poor-flowering material is directed towards GA1 and GA3, GAs preferentially used in vegetative growth.Abbreviations FW fresh weight - GAn gibberellin An - HPLC high-performance liquid chromatography  相似文献   
40.
Some authors have found indications of subgroup formation when domestic fowl are forced to live together in large flocks, while others have not. In this study experiments were carried out to test the hypothesis that hens in large flocks have home ranges in parts of the pen and that they form subgroups. We also studied if this is influenced by males. In a tiered aviary system (density averaged 16 hens/m(2) of floor area) eight flocks of 568+/-59 ISA Brown laying hybrids were kept in pens. Half of the pens contained 1 male per on average 24 females (mixed flocks). At peak production (36-53 weeks of age) four females roosting closely together for about 14 days and four females roosting far apart from each other were taken out from each flock and put together in separate groups in small pens. Their agonistic behaviour was studied for 2 days before they were put back. This was repeated with new birds, resulting in 16 small sample groups being studied. At 70 weeks, three groups of 10 females per flock roosting closely together in different parts of the pen were dyed with different colours and their locations were observed for 2 nights and 2 days.The incidence of aggressive pecks during day 1 among birds that had been roosting close to each other tended to be lower (P=0.05) than among birds that had been roosting far apart. This effect was not significant among birds from all-female flocks, but among birds from mixed flocks (P<0.05). However, this indicates a recognition of roosting partners and possibly also a rebound effect of the males' reduction of female aggressiveness towards strangers. Irrespective of sex composition in the flocks, birds marked while roosting at the ends of the pens were significantly more often observed within these areas than in other areas of the pen during daytime and came back to the same roosting sites at night (P<0.05-P<0.001). This was not the case for birds from the middle of the pens, where the distribution in the pen in most cases did not differ from random. These results show that laying hens in large groups are rather constant in their use of space, which indicate the presence of home ranges. However, environmental features that facilitate localisation may be important. In summary, we think that these findings indicate the existence of subgroup formation.  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