食品中低温微生物的适冷机制研究进展

低温贮藏是延长食品货架期、维持食品鲜度和质量安全的重要方法,然而仍有部分微生物能适应低温环境,使食品发生腐败变质。主要从细胞膜、适冷酶、冷休克蛋白、冷适应蛋白、代谢水平及低温防护剂等角度阐述国内外食品中低温微生物适冷机制的研究进展,为低温微生物在食品领域的应用与防护提供参考。     

植被结构对严寒地区城市居住区冬夏微气候的影响研究

城市绿地对于改善城市微气候、缓解城市热岛效应有重要作用,且不同的植物配置、绿地形态以及植被结构所产生的效果不同。以严寒地区典型城市哈尔滨为例,采用现场实测的方法,针对不同植被结构的居住区绿地进行冬夏两季微气候现场实测,对不同植被结构的微气候调节性能进行深入研究,比较分析不同植被结构影响下的空气温度、相对湿度和风速。结果表明：随着植被结构趋于复杂,植被对于冬季冷风的遮挡作用和夏季的降温增湿作用均随之增强。该研究为严寒地区城市居住区景观设计提供参考依据。     

Effects of plasma jet,dielectric barrier discharge,photodynamic therapy and sodium hypochlorite on infected curved root canals

The aim of this investigation was to evaluate the effects of 2 different cold atmospheric plasma (CAP) sources, photodynamic therapy and sodium hypochlorite (NaOCl), on infected root canals. Therefore, 50 standardized curved human root canals were infected with Enterococcus faecalis and assigned to 5 groups—negative control (NC), plasma jet (CAP I), dielectric barrier discharge (CAP II), photodynamic therapy (PDT) and NaOCl + passive ultrasonic irrigation—for 30 s. Colony forming units (CFUs) were determined. NaOCl was significantly more effective at reducing CFUs than all test groups (P < .0001 [Mann‐Whitney U test]) in both parts of the root canal. CFUs in PDT were significantly lower than those in CAP II (P = .015), and those in CAP I were lower than those in CAP II (P = .05). Among all other groups and in the apical parts, no significant differences were found (P > .05).      

Dehydrins: A commonalty in the response of plants to dehydration and low temperature

Among proteins that accumulate in plants in response to dehydrative forces or low temperature, dehydrins (late embryogenesis abundant [ Lea ] D11 family) have been the most commonly observed. Dehydrins are composed of several typical domains joined together in a few characteristic patterns, with numerous minor permutations. These domains include one or more putative amphipathic a -helix forming consensus regions, a phosphorylatable tract of Ser residues, and an N-terminal consensus sequence. Lesser conserved domains are also present at various positions, particularly between the putative a -helix forming domains, where they may occur as tandem repeats. This medley of permutations is mirrored by a wide size range of dehydrin polypeptides from less than 100 to nearly 600 amino acid residues. As of yet, the fundamental biochemical mode of action of dehydrins has not been demonstrated, but a number of immunolocalization and cell fractionation studies have established that dehydrins can be located in the nucleus or cytoplasm. Furthermore, it appears that these proteins associate with macromolecules ranging from nucleoprotein complexes in the nucleus to an endomembrane sheath in the cytoplasm. At present, all observations are consistent with a hypothesis that dehydrins are surfactants capable of inhibiting the coagulation of a range of macromolecules, thereby preserving structural integrity.     

Antifreeze protein accumulation in freezing-tolerant cereals

Freezing-tolerant plants withstand extracellular ice formation at subzero temperatures. Previous studies have shown that winter rye ( Secale cereale L.) accumulates proteins in the leaf apoplast during cold acclimation that have antifreeze properties and are similar to pathogenesis-related proteins. To determine whether the accumulation of these antifreeze proteins is common among herbaceous plants, we assayed antifreeze activity and total protein content in leaf apoplastic extracts from a number of species grown at low temperature, including both monocotyledons (winter and spring rye, winter and spring wheat, winter barley, spring oats, maize) and dicotyledons (spinach, winter and spring oilseed rape [canola], kale, tobacco). Apoplastic polypeptides were also separated by SDS-PAGE and immunoblotted to determine whether plants generally respond to low temperature by accumulating pathogenesis-related proteins. Our results showed that significant levels of antifreeze activity were present only in the apoplast of freezing-tolerant monocotyledons after cold acclimation at 5/2⁰C. Moreover, only a closely related group of plants, rye, wheat and barley, accumulated antifreeze proteins similar to pathogenesis-related proteins during cold acclimation. The results indicate that the accumulation of antifreeze proteins is a specific response that may be important in the freezing tolerance of some plants, rather than a general response of all plants to low temperature stress.     

Freezing exposure releases bud dormancy in Betula pubescens and B. pendula

Bud dormancy in woody plants is released by long-term exposure to non-freezing chilling temperatures, whereas freezing temperatures have been considered to have little or no effect. However, the present results demonstrate that short-term exposure to freezing can release bud dormancy in Betula pubescens (Ehrh.) and B. pendula (Roth). Short-term freezing during the dormancy induction phase improved the release of bud dormancy only if an adequate level of dormancy had been reached. In fully dormant or chilled plants both the percentage and the speed of bud-burst increased, the more so the lower the temperature. Our results rule out the possibility that endogenous abscisic acid could be directly involved in the physiological control of bud dormancy release. The fast, easily applicable method presented here for bud dormancy release could further investigations into the biochemical and biophysical background to the process. The mechanisms of bud dormancy release and its relationship to cold acclimation are discussed in the light of these results, as also are the implications of the findings for modelling of bud dormancy.     

Expression of a synthetic antifreeze protein in potato reduces electrolyte release at freezing temperatures

A synthetic antifreeze protein gene was expressed in plants and reduced electrolyte leakage from the leaves at freezing temperatures. The synthetic AFP was expressed as a fusion to a signal peptide, directing it to the extracytoplasmic space where ice crystallization first occurs. The gene was introduced to Solanum tuberosum L. cv. Russet Burbank by Agrobacterium-mediated transformation. Transformants were identified by PCR screening and expression of the introduced protein was verified by immunoblot. Electrolyte-release analysis of transgenic plant leaves established a correlation between the level of transgenic protein expression and degree of tolerance to freezing. This is the first identification of a phenotype associated with antifreeze protein expression in plant tissue.     

Effects of sub-lethal cold stress on the Western Flower Thrips, Frankliniella occidentalis

In order to establish in a new geographical area, introduced insects must be able to survive any period of adverse conditions such as a temperate winter and be capable of subsequent development to adulthood and/or reproduction. However, this aspect of insect overwintering and cold tolerance has been poorly studied. At high latitudes, Frankliniella occidentalis is typically associated with artificially heated glasshouses, but has some ability to tolerate low temperatures and may survive winter field conditions for short periods, or for longer periods of time during mild winters. The effects of overwintering on the viability of survivors are, however, unknown. In this study, acute and chronic cold exposure regimes were imposed on first instar larvae and adult female Western Flower Thrips, after which the longevity, development and reproductive capacity of the survivors were monitored and compared to those of non-stressed individuals. Survival of cold exposure did not affect subsequent survivorship of immature or adult insects, though cold treated larvae took approximately two days longer to reach adulthood than untreated individuals (at 20°C, 18L:6D). Chill treatment of adult females significantly reduced their rate of reproduction (from 1.45 to 0.93 larvae day^-1), reproductive lifespan (from 13.3 to 9.2 days) and as a result, total reproductive output (from 20.4 to 10.8 larvae), compared to control females. Acute exposure resulted in non-significant decreases of the same parameters. The relevance of the above effects to overwintering of F. occidentalis is discussed.