臺灣欒樹的抗張材在傾斜苗木與枝條的解剖構造與生物力學功能

Abstract

抗張材在維持被子植物的生物力學穩定上扮演重要的角色，因此在研究傾斜主幹恢復直立以及枝條角度變化的機制時，必須同時關注抗張材的生物與物理特性。本論文以台灣欒樹為材料，研究在恢復直立的傾斜樹苗主幹中以及樹冠不同角度的枝條內，抗張材的形成、構造與分布等生物特徵以及應變分布的物理特徵，以期了解抗張材在傾斜主幹與不同角度枝條中所扮演的角色。 在人為傾倒的兩年生台灣欒樹樹苗主幹中，抗張材的生成與恢復直立的過程歷時約三個月。透過插針法，我們確認：當樹苗傾倒時，主幹上側的形成層區包含形成層與發育中的木質部纖維細胞均感受到力學變化（相對位置的改變），開始形成抗張材，其中的膠質纖維可產生強烈的收縮應變，將主幹拉回到直立的位置。在傾斜主幹之基部形成的抗張材比在半株高處者多，產生的收縮應變也較大，顯示在台灣欒樹樹苗傾斜主幹恢復直立的過程中，主幹基部扮演較關鍵的角色。此外，傾斜主幹伴隨上側抗張材形成的偏心生長有助於主幹恢復直立，而在主幹下側部分測量點所量到的壓縮應變亦可靠著推力協助主幹恢復直立。針對樹冠之不同角度枝條的研究則顯示，台灣欒樹枝條內存在著和傾斜主幹不同的應變分布、抗張材分布與偏心生長。枝條之生長應變參數隨著木材細胞次生細胞壁的成熟而有季節性的變化；角度大的傾斜枝條上下側可能具有收縮或壓縮應變，然而角度較小的近直立枝條的上下側多為收縮應變，顯示這兩種枝條可能具有不同的功能。抗張材可能分布在枝條的各個位置，和抗張材主要分布在傾斜樹苗主幹的上側不同，因此，抗張材有助於枝條角度的動態調整。枝條的偏心生長位於枝條下方，可能阻礙枝條的上揚，甚至促進枝條的下壓。枝條上多樣的應變分布與抗張材分布，顯示各枝條可能為了因應環境因子如重力和光線的差異，而有不同的生物力學需求。

Tension wood plays a role in maintaining the mechanical stability of angiosperm trees. Both biological and physical aspects of tension wood are essential in understanding the mechanism of trunk or branch reorientation. In this dissertation, we first worked on both the tension wood formation and its biomechanical function in artificially inclined 2-year-old Koelreuteria henryi seedlings. The tension wood formation and reorientation process of the trunk last for about 3 months. With pinning method, we confirmed that at the beginning of inclination the cambial zone including the vascular cambium and the developing normal wood fibers on the upper side of the inclined trunk perceives the onset of mechanical change and starts to produce G-fibers that generate a strong contractile released growth strain (RGS) for gravitropic correction. Stronger contractile RGS and more tension wood were found at the trunk base than at the half-height, suggesting that the trunk base plays a key role in trunk uprighting of K. henryi seedlings. The eccentric cambial growth in the tension wood side increases the efficiency of gravitropic correction and the compressive strains measured in the opposite wood of some inclined seedlings also help the upright movement. Then we further discriminated the biomechanical behavior of branches from leaning trunks. We thus investigated the development of growth strains, distribution of tension wood, and eccentricity on the branchwood of K. henryi. The results revealed the unusual distribution of released growth strain and tension wood as well as growth eccentricity. The growth strain parameter showed seasonal changes, possibly due to the maturation of secondary cell wall. Both sides of the plagiotropic branches exhibited either contractive or extensive growth strains, whereas the orthotropic branches exhibited mostly contractive strains on both sides, which implied different physiological function of the two branch types. The tension wood arcs may occur in any direction of the branchwood which is different from the inclined trunk with tension wood on the upper side, suggesting dynamic adjustment in branch reorientation. In contrast to trunks, the hypotrophic eccentric growth in branches functioned in obstructing upward movement and even facilitates downward movement, probably because the dissociation between tension wood and eccentric growth. Diversified growth strain and tension wood distribution on the branches may reflect the individual biomechanical requirements for each branch depending on the environmental factors, possibly gravitropic and phototropic stimuli.