Modelling the mechanical behaviour of pit membranes in bordered pits with respect to cavitation resistance in angiosperms

Background and Aims

Various correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events.

Methods

Transmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff–Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on air-seeding resistance.

Key Results

The TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P₅₀).

Conclusions

The results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation.     

花楸属一新品种——中昌大果花楸

中昌大果花楸是由俄罗斯引进的欧洲花楸(Sorbus aueuparia)筛选出的新品种。其果大,最大直径达1.1cm,托叶宿存,叶背无毛,抗病虫害能力强,生长速度快。     

不同海拔天山云杉叶功能性状及其与土壤因子的关系

植物功能性状反映了植物对生长环境的响应和适应,将环境、植物个体和生态系统结构、过程与功能联系在一起。为阐明天山北坡1420-2300m天山云杉(Picea schrenkiana var.tianschanica)的叶片功能性状对环境的适应性变化,研究了天山云杉10种叶片功能性状的变化规律以及与土壤因子的关系。结果表明叶长宽比(LL/LW)、叶面积(LA)、气孔密度(SD)、单位干重的叶磷(LPC)和叶钾(LKC)含量沿海拔梯度呈上升趋势。单位干重的叶氮(LNC)含量、叶绿素含量(Chla+b)、叶片饱和含水量(LWC)、叶片干物质含量(LDMC)和比叶重(LMA)随海拔的升高呈现非线性变化,前3项在海拔约2100m处达到最大值,分别为(15.42±0.38)mg/g、(2.44±0.37)mg/g和(55.01±0.48)%,后两项在海拔约2100m处达到最小值,分别为(451.80±6.28)mg/g、(252.33±3.60)g/m2。逐步回归分析结果显示,海拔梯度上影响LPC的主要土壤因子是土壤pH值和SWC,影响LNC、LKC、Chla+b、LDMC、LMA和LWC的主要土壤因子是TN,影响LA、SD和LL/LW的主要土壤因子是SWC,可见SWC和TN是天山云杉叶片功能特征沿海拔梯度变化的主要驱动因子。在优越的环境中植物叶片虽然具有较高的光合能力和较高的相对生长速率,但是对资源的利用能力往往较低,在2100m附近LNC、Chla+b、LA最大,LMA、LDMC最小,因此判断此海拔附近为天山云杉最适宜的生长范围。     

外源激素与温度对花楸树种子萌发的影响

三种外源激素和萌发温度对花楸树种子萌发影响的研究结果表明,25℃（16h光／8h暗,10d）-5℃（暗110d）变温可极显著提高种子发芽率和种子发芽势,但极显著延迟发芽初始时间。200mg·L^-16-BA溶液中吸胀2d后经历25℃·5℃变温可使种子发芽初始时间最短（33d）,发芽率（62．25％）和发芽势（57．88％）最高。GA3或ABA与温度的结合处理对种子萌发的促进作用小于6-BA与温度的综合处理。     

Do aphids paint the tree red (or yellow)—can herbivore resistance or photoprotection explain colourful leaves in autumn?

We explored two mutually nonexclusive hypotheses on autumnal leaf colouration. The co-evolutionary hypothesis states that autumnal leaf colouration functions as a handicap signal to herbivorous insects, whereas the photoprotection hypothesis posits that plant pigments primarily protect the plant against cold-induced photoinhibition and enhance nutrient transfer. To contrast both hypotheses, we compared yellow and red leaf colouration in three groups of mountain ash (Sorbus aucuparia L.). Two montane groups of different age were characterised by low aphid numbers and low temperature, and a lowland group by high aphid numbers and high temperature. There were no consistent altitudinal differences in leaf colouration. Compared to young trees, adult trees developed fewer red but more yellow leaves at high altitude. In the lowland population, the development of red leaf colour was related to decreasing daytime temperature, whereas the appearance of yellow leaf colouration corresponded to the decreasing photoperiod. This is consistent with the photoprotection hypothesis. Individual differences in red and yellow leaf colouration were inversely correlated to the number of fruits, which might be interpreted as a trade-off between reproductive and protective commitment. Temperature effects explained variation in aphid numbers over time and leaf colouration explained aphid distribution on a given day. As predicted by the co-evolutionary hypothesis, strongly coloured individuals harboured fewer aphids than green or dull-coloured ones. Since decreasing temperature reduced the number of migrating aphids but induced red leaf colouration, these processes are not mutually fine-tuned, which likely restricts the potential for co-evolution between mountain ash and aphids.     

Short-Lived Tree Species and Their Role as Indicators for Plant Diversity in the Restoration of Natural Forests

Anthropogenic forests, particularly conifer monocultures, today constitute a large proportion of Central European woodland. Conversion of such forest stands into abundantly structured mixed‐species woodland is within the focus of ecosystem restoration and is considered to affect forest biodiversity. Short‐lived tree species play an important role in such conversion processes and may serve as focal species. However, not much is known about their relationship with forest biodiversity. In this study, the short‐lived tree species, European mountain ash (Sorbus aucuparia L.), European white birch (Betula pendula Roth), Downy birch (B. pubescens Ehrh.), and Glossy buckthorn (Frangula alnus P. Mill.), commonly occurring throughout Central Europe, are investigated with regard to their relationship with plant diversity. The focus is on their occurrences in Scots pine (Pinus sylvestris L.)–dominated forests in the Northeast German lowlands. A significant increase in vascular plant diversity is revealed in stands with the selected species’ presence, in comparison to stands without them. Increase in plant species numbers is highest where the respective species occurs in the tree and/or shrub layer, compared with their presence only in the herb layer. For bryophyte species, there is a less strong inverse relationship. An analysis of different species groups, such as threatened, woody, and typical forest species of higher plants, reveals no decrease in species numbers in these groups if short‐lived tree species are present. It is concluded that short‐lived tree species can be indicators for plant diversity assessment within forest restoration processes. As to causal explanations, effects of differing site conditions, assessed by use of Ellenberg indicator values, are discussed as well as possible active effects of the tree species changing their environment.     

Relationship between Maturity Stage,Triterpenoid Content and Cytotoxicity of Sorbus intermedia (EHRH.) PERS. Fruits – A Chemometric Approach

In this study, two ursane-type triterpenes, i. e., ursolic acid (UA) and 3-O-β-acetoxyursolic acid (AUA), were isolated and quantified in Sorbus intermedia (EHRH.) PERS. fruits. UA and AUA levels differed slightly throughout fruit maturation, and both triterpenes showed similar dynamics of accumulation with the highest levels found in ripe fruits (up to 6.33±0.56 and 1.17±0.18 mg/g dw. of UA and AUA, respectively). The extracts derived from unripe fruits displayed significant cytotoxic effects against WM793, DU-145, PC3, 8505C, Caco2 and A549 cells, but no correlation between UA and AUA levels and cytotoxicity was found. On the other hand, extracts from mature fruits were not active towards almost all of the tested cell lines. The chemometric approach showed that the extracts derived from fruits harvested earlier clustered to form a clearly distanced group from extracts prepared from more-mature fruits. The extracts at higher concentrations formed separate groups, which indicated the concentration-dependent effect of these extracts on the cells.