The mechanism underlying overwintering death in poplar:the cumulative effect of effective freeze-thaw damage

We analyzed the relationships linking overwintering death and frost cracking to temperature and sunlight as well as the effects of low temperatures and freeze–thaw cycles on bud-burst rates,relative electrical conductivity,and phloem and cambial ultrastructures of poplar.Overwintering death rates of poplar were not correlated with negative accumulated temperature or winter minimum temperature.Freeze–thaw cycles caused more bud damage than constant exposure to low temperatures.Resistance to freeze–thaw cycles differed among clones,and the budburst rate decreased with increasing exposure to freeze–thaw cycles.Cold-resistant clones had the lowest relative electrical conductivity.Chloroplasts exhibited the fastest and the most obvious reaction to freeze–thaw damage,whereas a single freeze–thaw cycle caused little damage to cambium ultrastructure.Several such cycles resulted in damage to plasma membranes,severe damage to organelles,dehydration of cells and cell death.We conclude that overwintering death of poplar is mainly attributed to the accumulation of effective freeze–thaw damage beyond the limits of freeze–thaw resistance.

Effect of Methylene Blue (MB)-value of Manufactured Sand on the Durability of Concretes

The relation between the methylene blue （MB） value of MS and its limestone powder content and clay content was investigated. The effects of MB values ranging from 0.35 to 2.5 on the workability of fresh concrete, the mechanical properties, the resistance to freezing as well as the resistance to chlorine ion permeation of the hardened concrete were all investigated. The experimental results showed that the MB value had no correlation with the limestone powder content of MS, while it was directly related to the clay content. With an increase of MB value, concrete workability decreased, as did the flexural and 7-day compressive strengths, however, the 28-day compressive strength was not affected. Furthermore, influence of MB value on concretes of different strength levels was different. For low-strength concretes, an increase of MB value could improve its impermeability, but this was not the case for high-strength concretes. Instead, their resistance to chloride ion permeability decreased slightly. However, even a slight increase in MB value remarkably accelerated freeze-thaw damage of MS concrete. It was thus concluded that the critical MB value of 1.4 would not cause significant deterioration in the performance of MS concretes.

Comparison of G6PDH Activity and LT50 Between P. tomentosa and P. suaveolens During Freezing Acclimation

The differences of glucose-6P dehydrogenase (G6PDH) activity and freezing resistance induced by freezing acclimation between cuttings of freezing-sensitive P. tomentosa and freezing-resistant P. suaveolens were compared for exploring the role of G6PDH on the enhancement of freezing resistance induced by freezing acclimation. After 5 d of freezing acclimation at -3 ℃, the LT50 of P. tomentosa has decreased from -6.2 ℃ in control cuttings to -14.3 ℃ in freezing acclimated ones, and the increase of G6PDH activity was observed in freezing acclimated cuttings as compared with control ones. Whereas, when P. suaveolens was freezing acclimated at -20℃ for 5 d, the LT50 has decreased from -27.1℃ in control cuttings to -43.5 ℃ in freezing acclimated ones, and the activity of G6PDH increased considerably. In addition, the increase of LT50 and the decrease of G6PDH activity resulting from 2 d of deacclimation at 25 ℃ were found in two kinds of freezing acclimated cuttings. It is concluded that the increase in the activity of G6PDH may associate with the inherited freezing resistance of species and the enhancement of freezing resistance of cuttings, and may play an important role in the antifreeze process under freezing temperature, which would provide the basis for the study on the molecular mechanism of freezing resistance in P. suaveolens and the cloning of gene associated with freezing resistance.

Salt freezing resistance improvement of cement-based materials incorporated with calcined layered double hydroxide

Salt freezing damage has severe impacts on durability of cement-based materials(CBMs).Calcined layered double hydroxide(CLDH),as an efficient environmental-friendly adsorption material,can impart excellent salt freezing resistance to CBMs.In this work,salt freezing resistance improvement of CBMs incorporated with CLDH was experimentally evaluated by chloride binding capacity,mass loss rate,relative dynamic elastic modulus,setting time,compressive strength,and micro structure tests.Beside these,the salt freezing damage model was established to effectively express the quantitative relationship between influencing factors and evaluation indexes of the salt freezing resistance of CBMs.Results show that CLDH can reconstruct its original layered structure to form reconstructed layered double hydroxide(RLDH).RLDH combines with chloride ions to form RLDH-Cl recrystallization,which can improve chloride binding capacity and pore structures of CBMs to relieve the salt freezing damage.The salt freezing damage model indicates that the suitable CLDH content can evidently alleviate the salt freezing damage,which facilitates the quantitative analysis of the effect of CLDH on the salt freezing resistance of CBMs.