【作者】 方长华；

【导师】 刘盛全； Joseph GRIL；

【作者基本信息】 安徽农业大学 ， 木材科学与技术， 2007， 博士

【摘要】 本文从微观水平到宏观水平对应拉木的特性进行了深入的观测和研究,并将之与正常木进行比较。本研究选择杨树（从法国采的Populus cv.I4551和从中国采的Populus deltoids cv.I-69/55）作为实验材料,因为杨树中很容易产生有胶质纤维的应拉木并伴有大的纵向生长应力;另外,杨树也是一个在全球都非常重要的速生人工林树种。本研究中纵向生长拉应力被用作应拉木严重程度的指标,而生长应力水平又由生长应变来指示。根据不同研究目的,本文运用了两种方法来测定生长应变:1.为了微观水平上的研究:应变片法,此方法可以直接给出生长应变值（GrowthStrain,简称GS）;2.为了宏观水平上的研究:单孔法,直接测出的值称作“生长应力指示值”（GrowthStress Indicator,简称GSI）,基本和生长应变值成正比。本文的研究内容和意义主要有如下几点:●树木生长应力的产生机理一直是林业科研工作者热烈争论的疑点课题,本文进一步推进了这一的讨论并且尝试了在细胞壁和组织水平上揭示生长应力现象。这部分实验使用了与传统的常规切片不同的方法。在传统的未包埋试样上制得的常规切片上发现胶质纤维中的胶质层呈可疑的脱离和膨胀状态。为了能够观测到实木中胶质层的实际状态从而使得进一步的研究结果的真实可信,本文使用了包埋过的试样,用玻璃刀片和钻石刀片进行超薄切片。●本文对不同严重程度的应拉木以的解剖特征、物理力学性质和化学组成做了全面的测定和深入的分析讨论,并与相应的正常木进行比较,建立了木材特性与生长应力水平之间的关系,为进一步了解和合理利用应拉木,特别是杨树应拉木提供了理论基础。●由于干缩对木材的加工利用有着显著的影响,因此本文特别对不同严重程度的应拉木的横向干缩进行深入的研究并与正常木的干缩进行比较。通过对细胞壁水平、组织水平以及实木水平上干缩特征的研究,提出了胶质纤维和正常纤维的干缩模型。●杨树的一个重要用途是用来旋切制成单板再进一步加工利用,而在由含有应拉木的杨木旋切的单板中,起毛和翘曲是两个主要问题。因此,本文对由不同程度的应拉木区域旋切的单板的起毛和翘曲进行了测定并和正常木区域的进行对比。通过以上几个方面的研究,得出如下主要结论:●进一步证实了Clair等（2005b;2005a）的报道,在传统的未包埋试样上制得的常规切片上发现的胶质层脱离和膨胀状态是一种在切片过程中造成的人为破坏结果,并不是它的实际状态。在立木和实木中,无论是在生材还是干燥后的木材中,胶质层是并没有脱离细胞壁的其他层,并且不是膨胀状态,它的实际厚度比在常规切片上观测到的要小。因此,要想研究胶质层的真实状态和性质,传统的常规切片方法是不合适的,建议使用本实验中所运用的方法,即使用包埋试样切片,并且要考虑要使用的切片到试样端头的距离。●胶质纤维中胶质层与S₂层的粘着程度不如细胞壁中其它层（即胞间层、初生壁、S₁层和S₂层以及正常纤维中的S₃层）之间的粘着程度,但这种粘着程度足够强而不至于因干燥而分开,尽管胶质层有很大的轴向横向干缩率。这一发现也确定了胶质层对应拉木物理和力学性质有着重要的影响。●根据在立木中测得的GS而划分的不同严重程度的应拉木中,通过对包埋试块端头切制的切片上胶质层膨胀率的测定,发现此膨胀率和GS呈正相关。由于端头处胶质层中的纵向拉应力在试块包埋前制备时已完全释放,因此胶质层纵向收缩,根据Poisson原理,胶质层横向膨胀。综合以上发现和推测,我们可以推导出在严重应拉木胶质纤维的胶质层中的纵向拉应力要比在轻微应拉木的胶质层中的纵向拉应力大。●在倾斜的立木中,倾斜树干的上部有着最大的纵向生长应力,沿树干外围到倾斜树干下部,生长应力呈下降分布。相应的大多数木材性质有着相似或相反的规律,与GSI呈正相关或负相关。●在用应变片法测定的试样中,在GS小于610με的试样中没有胶质纤维出现,在大于816με的试样中胶质纤维的面积比率达到50%或更大,这意味着胶质纤维出现的可能临界点在这两个值之间。另一个100%胶质纤维出现的可能临界点是在1540和1935με之间。●在测定的有胶质纤维的应拉木试样中,单位面积的组织中胶质纤维越多而且其胶质层越厚,则相应的纵向生长应力（和GS成比例）越大。这意味着这些因素影响着生长应力的产生,其中胶质层在产生高生长应力中起着最重要的作用,胶质层的量对生长应力的水平有着决定性的影响。这一结论可以推测为由下面的假说来解释:应拉木中的纵向生长拉应力产生是由细胞壁中微纤丝的拉应力来决定的（Bamber 1978;Okuyama et al.1986;Bamber 1987;Clair et al.2006a）,因为胶质层中的木质素含量甚微。●正常木纤维的胞壁厚度明显小于应拉木的包括胶质层在内的细胞壁厚度,而又显著大于胶质纤维中除胶质层以外的其它层细胞壁的厚度。在应拉木中,生长应力随着胶质纤维中的胶质层厚度的增加而增加,而其它层厚度则减小。纤维长度和直径分别与生长应力水平呈正相关和负相关。生长应力水平与纤维和导管的组织比量分别呈正相关和负相关。●根据厚度的相对减小率而计算得出的胶质层干缩率（也即径向干缩率）显著地大于其它层细胞壁的干缩率。胶质层以及总体细胞壁的干缩率与GS没有显著的相关关系,而除胶质层之外的其他层细胞壁的干缩率则与GS有着显著的负相关关系。●胶质层与GS之间没有明显相关关系意味着从低GS到高GS的应拉木胶质纤维中的胶质层没有质的上的区别,这也证实了前面的推论,即胶质层量的上的差异决定着生长应变的变动,也即与之成比例的生长应力的变动。●在干燥过程中,应拉木胶质纤维的胞腔增大而且其增大量与GS呈正相关,而正常木纤维的胞腔在本实验中则发现干燥后变小了。这一发现说明胶质层在干燥过程中是由里（胞腔一侧）向外（细胞外围）收缩的（也即它的内周长增大）,因此它的干缩对细胞的整体干缩率几乎没有影响,因而其整体干缩率主要受其他层细胞壁干缩的控制,因为我们发现其他层细胞壁是由细胞外围向内干缩的（也即其外围周长变小）。●无论是正常木还是应拉木,在组织和宏观水平上（分别基于切片和实木上的观测）,弦向的干缩率都明显高于径向的,弦向和轴向干缩率分别与GS/GSI呈显著的负相关和正相关,而径向干缩率则与GS/GSI之间没有发现明显相关关系。●木材的基本密度和纤维饱和点分别与GSI有着显著但微弱的正相关关系。随着GSI的增大,弹性模量和比弹性模量增大而顺纹抗压强度和比抗压强度则降低。从正常木到应拉木,木材的红色和黄色程度降低,颜色偏深的更接近b^*轴。●与倾斜树干一周其他部位相比,在GSI最大的树干上侧,纤维素含量最高,而木质素含量则相反。●由于有应拉木的存在,在位于倾斜树干上侧的旋切单板上有严重的起毛现象,并且这部分单板严重翘曲变形。而在倾斜树干下侧的单板上,这些现象基本可以忽略。整体上,单板的起毛率和翘曲程度与GSI呈正相关。更多还原

【Abstract】 In this study,tension wood behaviours from microscopic to macroscopic levels were observed and measured compared to normal wood.Poplar（Populus cv.I4551 from France and Populus deltoids cv.I-69/55 from China） was chosen as plant material,because poplar is known to have a characteristic tension wood with G-layer and to produce a high longitudinal stress.Poplar is also a very important fast growth plantation tree in the world.Longitudinal growth stress was regarded as indication of tension wood severity and growth strain（GS） was used to indicate the growth stress level.Two methods were used in this study to measure the GS according to the different purposes:- for the microscopic study,the strain gauge method giving a direct estimate of the GS;- for the macroscopic study,the single hole drilling method providing with a "growth stress indicator"（GSI） approximately proportional to the GS.This study advances the discussion on the origin of growth stresses generation and reveals the growth stresses at cell wall and tissue levels.The measurements were performed on embedded samples sectioned with glass knife and diamond knife,which allows to avoid the uncontrolled swelling and detachment of G-layer during sample preparation with conventional sectioning method.From normal wood to different severities of tension wood,defined by measured growth strain,wood properties were measured including anatomical,physical,mechanical and chemical properties,as well as the properties of rotary cutting veneer including woolly area proportion and distortion,which are the two most important problems for poplar veneer utilizations.The relationships between these properties and growth stress level are established.Due to its high importance for wood utilizations,the transverse drying shrinkage of tension wood was studied in particular compared to that of normal wood at cell wall,tissue and massive wood level.A shrinking model is proposed for tension wood and normal wood. From this study,following conclusions can be drawn:- This study further confirmed the observations of Clair et al.（2005b;2005a） that G-layer observed with the conventional method of sectioning is in a swollen state and detached from S₂ layer,due to cutting end-effect artefacts.In living trees or massive tension wood in both wet and dry conditions,G-layer is well adhered with S₂ layer and not swollen.Thus for studying the real state and properties of G-layer,conventional sectioning method is not appropriate and embedded sample method,taking into account the distance of the sectioning location to the border,is propositional.- The adherence between G-layer and S₂ layer are not as strong as that among other layers,i.e.among compound middle lamella,primary layer,S₁,S₂ and S₃ layer in normal wood.But this adherence is strong enough not to be altered by the high transverse and longitudinal shrinkage of G-layer after drying of tension wood.It confirms the contribution of G-layer to the physical and mechanical behaviour of tension wood.- For the severe tension wood compared to mild tension wood as defined by the GSI measured at macroscopic level,higher longitudinal tensile stress was deduced in G-layer.- The highest growth stress values were located in the upper sides of the inclined trunks. Other growth stress values mostly distributed between those of upper and lower sides. Most of wood characteristics exhibited similar distributions around the inclined trunks, their correlation with GSI being either positive or negative.- In the samples examined,no G-fibres were observed for a GS up to 610μεwhile their surface ratio amounted to 50%or more from 816με,suggesting a hypothetical threshold for G-fibres occurrence between these two GS values.Almost 100%of the fibres contained G-fibres above another hypothetical GS threshold between 1540 and 193.5με.- In the samples examined,more G-fibres per unit of tissue area and thicker G-layer accompany higher longitudinal growth stress（proportional to GS） in tension wood with G-fibres.It suggests that these factors contribute to the growth stress generation and hereinto G-layer plays the most important role in high growth stress generation,which supposedly can be explained by the hypothesis that the tensile stress of microfibrils governs the longitudinal tensile stress in tension wood（Bamber 1978;Okuyama et al. 1986;Bamber 1987;Clair et al.2006a）.- The thickness of normal wood cell wall was notably lower than that of the tension wood cell wall including G-layer but markedly larger than that of the other layers excluding G-layer.In tension wood the thickness of G-layer and other layers increased and decreased,respectively,with the increase of growth stress.Negative and positive correlations were found for cell diameter and fibre length,respectively,against growth stress level.Fibre and vessel proportion increased and decreased,respectively,with the increase of growth stress.- The drying shrinkage,measured as a relative thickness decrease,was significantly higher for G-layer than other layers.There were no significant correlation between GS and G-layer or all layers shrinkage,but a negative one was observed with other layers shrinkage.- The absence of relationship between G-layer drying shrinkage and GS tend to prove that G-layer would be nor qualitatively different from low GS to high GS and would confirms the ideas that just the quantity of G-layer would be the driving force of growth strain,i.e.growth stress.- In G-fibre,lumen size increased during drying and this increase was positively related with GS,but in normal wood fibre lumen size decreased during drying in our observations.These findings suggest that G-layer shrank outwards（i.e.its internal perimeter increases）,so that its shrinkage weakly affected the total cell shrinkage and the mesoscopic shrinkage was controlled by the other layers shrinkage which shrank inwards（i.e.its external perimeter decreases）.- At both tissue and macroscopic levels,based on the observations on sections and massive wood,respectively,tangential shrinkage was notably higher than radial one both in normal and tension wood.The shrinkages in tangential and longitudinal directions were negatively and positively,respectively,correlated with GS/GSI,while no clear relation was found between radial shrinkage and GS/GSI.- GSI was significantly but weakly correlated positively with basic density and negatively with FSP.With the increase of GSI,MOE and specific MOE increased, compressive strength and specific compressive strength decreased.From normal wood to tension wood the colour of wood became less reddish and less yellow,and darker specimens tend to approach the b^* axis.- The cellulose content rose to a maximum in the upper sides where highest GSI values were measured,and the opposite for the lignin content.- Severe veneer distortion and woolly phenomena happened in the veneers located in the upper sides of inclined trunks.In the lower sides the distortion and woolly phenomena were negligible.Positive correlations were found for them with growth stress level.更多还原