流蘇與呂宋莢迷之種子休眠與其果實與種子發育過程中形態形成之研究

Abstract

流蘇(Chionanthus retusus Lindl. & Paxt)與呂宋莢迷(Viburnum luzonicum Rolfe)皆為具園藝觀賞價值的台灣原生樹種。前人研究顯示兩者之種子可能具上胚軸休眠性質，造成以種子育苗繁殖上的困難。本論文首先檢測兩者之發芽模式，結果顯示流蘇種子之胚芽與胚根呈現不同步萌發現象，然而當胚根突出種殼後，胚芽並非處於休眠靜止狀態，而已呈緩慢生長；呂宋莢迷則不具不同步萌發現象，而是存在胚芽與胚根同步萌發的深度形態性休眠。為進一步瞭解此兩種具不同休眠模式之種子，其發育過程與種子休眠的相關性，本論文乃比較其胚胎發育過程的異同，並探討種殼（包括內果皮、種皮與胚乳）的發育對其種子發芽的影響。 流蘇與呂宋莢迷果實的各部位生長曲線類似，在長度方面均呈單S形生長曲線，果徑則呈雙S形曲線。流蘇由開花至果熟脫落約需時19週，而呂宋莢迷則約36週方落果，屬於宿存型果實。兩者果實的發育過程依胚的發育程度均可區分為形態分化期、胚體增大期及生理成熟期三階段，流蘇各階段需時約12、5及2週，呂宋莢迷則為16、12及8週。 就內果皮與種皮的發育而言，兩者在胚珠發育早期均具珠被絨氈層。流蘇內果皮自授粉後一週開始漸特化為厚壁組織，至增大期的中期方特化完成；種皮的中層薄壁組織在增大期中期逐漸被壓擠但未完全瓦解。而呂宋莢迷內果皮特化較晚，自授粉後約七週方開始，但至形態分化期末期即已特化完成；而其種皮則自形態分化期中期，最外層表皮細胞開始特化為巨大且富含單寧的細胞，中層薄壁組織則逐漸瓦解，而至形態分化期末期種皮特化完成後即沒有進一步的變化。流蘇與呂宋莢迷內果皮與種皮在成熟後均具透水性，並未造成物理性的種殼休眠機制。 流蘇與呂宋莢迷胚乳的發育皆為細胞型胚乳，發育初期胚乳細胞的細胞壁薄且液胞大，而後其細胞質內逐漸累積儲存性油質體與蛋白質體，醣類則以半纖維素累積於細胞壁，可能造成發芽時的機械性阻力。流蘇自增大期才漸由胚乳外層向內層細胞累積油質體與蛋白質體，而加厚的細胞壁則出現於增大期末期；然而，呂宋莢迷則自形態分化期末期即開始增厚細胞壁，增大期中期油質體與蛋白質體大量累積，且蛋白質體內尚有晶簇狀結晶發育，此為胚乳細胞中較特有的現象。 就胚胎的發育而言，流蘇與呂宋莢迷均經歷原胚、球形胚、心形胚與魚雷形胚的形態分化過程，再經增大期至生理成熟期。流蘇成熟胚為匙形胚型，具明顯分化完整的胚根構造，但並不具圓頂狀胚芽頂端分生組織與上胚軸構造，因此推論胚芽頂端分生組織形態分化未全，可能是造成其胚根與胚芽不同步休眠之原因。呂宋莢迷成熟胚為線形胚型，胚體微小，且懸柄仍明顯可見，而胚根內維管束組織分化並不明顯，胚芽頂端分生組織亦未分化，由此推論其胚體未發育完全，而造成種子具深度的形態性休眠。 比較流蘇胚根與胚芽頂端分生組織之發育過程中微細構造的變化，兩者在形態分化期及胚體增大期並無明顯差異，然而至成熟期，胚芽頂端分生組織細胞內具較多的油質體，且自增大期中期即不見分裂中的細胞，然而胚根分生組織則於成熟期尚可見分裂細胞，顯示胚根較晚呈現休眠狀態；然而呂宋莢迷的胚根與胚芽就儲存物的累積、細胞分裂的停止而言，幾乎是呈同步的。與流蘇相較，呂宋莢迷在增大期中期已有儲存物累積，成熟期細胞質幾乎完全為油質體與蛋白質體佔滿，而呈現休眠狀態，因而由此推論，相較於非或淺休眠性的種子，在較深度形態性休眠種子中，其發育未全的胚胎頂端分生組織的細胞內，會較早進行儲存物的累積，且會累積較多。

In Taiwan, both Chionanthus retusus Lindl. & Paxt and Viburnum luzonicum Rolfe are local trees with ornamental value in horticulture. The previous studies reported that the mature seeds of some species of these two genera maybe with epicotyl dormancy and making the seed propagations with difficulty. In this thesis, the germination mode of these two kinds of seeds were previously studied. The results showed that the seeds of C. retusus are with epicotyl dormancy, i.e. unsynchronized dormancy of plumule and radicle. However, the shoot apical meristem began to differentiate when the radicle extruded from the seed coat. The seeds of V.luzonicum showed synchronized deep morphological dormancy of the plumule and radicle. In order to understand the structural differences between these two kinds of seed dormancy, the development of their fruits and seeds were investigated. Besides, the effects of seed coat on the seed dormancy were also studied. The growth curves of fruits and seeds of C.retusus and V.luzonicum were similar and showed a single-sigmoidal curve in length and double-sigmoidal curve in width. The process of fruit development were divided into three stages: histodifferentiation, cell expansion, and maturation drying stage. The duration from the flowering to fruit ripening was 19 weeks for C. retusus and 36 weeks for V. luzonicum, i.e. for each stage 12,5, and 2 weeks in C. retusus and 16, 12, and 8 weeks in V. luzonicum. In C. retusus, stone cells of the endocarp to formed in the first week after pollination (WAP) and ended the tissue sclerification in the middle cell expansion stage. After then the mesophyll of testa were compressed but never completely disintegrated. While in V. luzonicum, the endocarp sclerification began in the 7th WAP and ended at the later histodifferentiation stage. The outer epidermis of the testa of V. luzonicum was composed of big tannin cells and the mesophyll was compressed and disintegrated at the middle histodifferentiation stage. The endocarp and testa of both species were water permeable and without physical dormancy mechanism. The pattern of endosperm development in C. retusus and V. luzonicum are cellular type. At early stage, the cell wall was very thin with a big vacuole. During the development of endosperm, oil and protein bodies accumulated in the cytoplasm and storage carbohydrates formed in the cell wall. The accumulation of food reserves started earlier in V. luzonicum than in C. retusus. It is interesting to note that in V. luzonicum lots of druse crystals were found in the protein bodies. The processes of embryo development in both species were composed of proembryo, globular, heart, torpedo, elongation and mature stages. The type of embryo in C. retusus was spatulate and with well-differentiated radicle, while the structure of dome-shaped shoot apical meristem and the epicotyl were not found, and thus the unsynchronized dormancy of seed caused. The mature embryo of V. luzonicum was linear type and small in size without well-developed radicle and epicotyl. Till the maturation of seed, the suspensor was still exist. The under-developed embryo might cause the deep morphological dormancy of seed. Comparing the ultrastructure of shoot and root apical meristem of embryo of C. retusus at the histodifferentiation and cell expansion stage, there were no obvious differences. While at maturation, more oil bodies were observed in the shoot apical meristem than in root apical meristem. Besides, dividing cell were still observed at maturation stage of the root apical meristem but not in shoot apical meristem. These phenomenon could related to unsynchronized dormancy of shoot apical meristem and root apical meristem in C. retusus. Nevertheless, the food reserves started to accumulate in V. luzonicum at the middle cell expansion stage. At the seed maturation, the oil and protein bodies almost occupied the cytoplasm and showing deep embryo dormant state. It was hypothesized that the accumulation of food reserves during the seed development started earlier and accumulated more in the under-development apical meristem cell in the deep morphological dormancy seed than in the non or non-deep dormant seeds.