台灣欒樹雄花及兩性花花部發育與小孢子形成之研究

Abstract

台灣欒樹 (Koelreuteria henryi Dummer) 屬無患子科，欒樹屬，為台灣原生種。落葉性喬木，廣泛栽培為觀賞行道樹。台灣欒樹為雜性花同株，圓錐花序上混生有扇形繖花序，所產生之雄花及兩性花，分別產生可孕性及不孕性花粉粒。以光學顯微鏡、穿透式及掃描式電子顯微鏡，並配合組織染色觀察，研究台灣欒樹的 (1) 雄花及兩性花早期花部發育與花序上之性別表現；(2) 雄花及兩性花成熟花粉粒形態、活性及貯存物之異同；(3) 小孢子發育過程中細胞與胞器變化及花粉壁於雄花花粉粒及兩性花花粉粒之異同；(4) 比較雄花及兩性花之花藥壁構造。藉由上述研究方法，以找出台灣欒樹雄花及兩性花結構及功能上之差異性。 本研究顯示台灣欒樹圓錐花序始原於八月中旬由枝梢頂端分生組織分化形成，並為苞片所包覆，於花序上著生有柄之花朵。花部發育，最先五個萼片始原以逆時針方向螺旋狀的順序出現，5個花瓣始原則同時冒出，8個雄蕊始原接著產生，雌蕊始原則最後形成。雄花具有發育不完全之雌蕊；兩性花之雄蕊雖然正常發育，但花絲不伸長且花藥不開裂。 台灣欒樹花藥壁組織除了表皮層外，由外而內分別為內壁層、中間層 (2~3層) 及營養層，花藥組織分化屬於基本型。台灣欒樹雄花之花藥為緃向開裂並釋出雙細胞花粉粒，經由膜翅目昆蟲傳粉。以FCR進行花粉活性測試，兩性花之花粉粒雖具15.3 % 的活性，但無法萌芽；然而雄花花粉不但有75%的活性，且萌芽率可達76.8 %。不孕性花粉粒之貯存物質為脂質顆粒及少數澱粉粒而可孕性花粉粒則僅貯存豐富的脂質顆粒。 小孢子母細胞的分裂屬於同時型，產生四面體排列的小孢子。營養層為分泌型，細胞質內有二種製造脂肪球的構造，一為內質網，一為造油體。於液胞小孢子早期，營養層中於內質網首先形成脂肪顆粒。接著，飽和及不飽和脂肪球於造油體內產生。花粉粒雙細胞晚期，營養層細胞成熟，造油體中之脂肪球與內質網產生之脂肪顆粒瓦解共同形成花粉鞘附著於花粉外壁上。 花藥開裂前，表皮及內壁層仍保持完整。在雄花，緊鄰二個花藥腔之隔膜細胞瓦解、唇細胞裂開，藥隔組織分開，花粉粒即可釋出；兩性花開花時，花藥之隔膜細胞與唇細胞保持完整，無花粉粒釋出。此二種花粉粒皆有發育良好的花粉壁，花粉外壁和內壁並存。花粉外壁由頂蓋層、柱狀層、底層及外壁內層所組成。雄花花粉於孔區之花粉外壁破裂且花粉內壁具明顯三層，但兩性花所產生的不孕性花粉之孔區的內壁較厚且外壁通常不破裂。 根據本研究：台彎欒樹花部性別分化早期同時具有兩性始原，到了發育後期 (胚珠形成後)，雄花及兩性花才可以清楚辨視。雄花及兩性花除了在構造、功能上有差異之外，其產生的花粉粒亦有所不同。

Koelreuteria henryi Dummer (Sapindaceae) is a deciduous tree and endemic to Taiwan. This species has been widely cultivated as side tree. The inflorescence is thyrse mixed rhipidium. The flower is polygamous-andromonoecious with staminate and hermaphrodite flowers on the same plant, fertile and steril pollen grains are produced respectively. In this study, with the aids of LM, TEM, SEM and histochemical observations, it is attempted to reveal (1) the floral initiation, flowers' sex differentiation and sex expression in the inflorescence; (2) the similarity and dissimilarity between pollen morphology, pollen viability and cytoplasmic content in staminate and hermaphrodite flowers; (3) the cellular and organellar transformations during the microsporogenesis; and (4) the anther wall configuration at anthesis. The results show that thyrse primordium of K. henryi was emerged from the shoot apical meristem in middle August and subtended conspicuously by bracts, which possesses numerous pedicellate flowers. Five sepal primodia intiated from the floral primodium in a spiral order. Then five petal primordia arised simultaneously in a whorl arrangement, followed by eight stamen primodia arised and last by pistil primodium. The anther wall development of K. henryi conforms to the basic type, consisting of four different tissues: epidermis, endothecium, 2~3 middle layer and tapetum The anthers in staminate flower of K. henryi are opened by longitudinal splits to release 2-celled pollen with the help of insects-pollinators. Although the pollen grains from hermaphrodite flowers have ca. 15.3 % viability when tested with FCR reaction, no pollen grain germinated in vitro. In constrast, those pollen grains from the staminate flowers have higher germination rate (76.8 %) tested in vitro. The results of histochemical staining show that there is a remarkable difference in storage compounds between sterile and fertile pollen grains. In the former, lipids globules with fewer starch grains are detected in the cytoplasm, while in the latter, only lipid globules are detectable. Cytokinesis in microspore mother cells is simultaneous, forming tetrahedral tetrads. The secretory tapetum has two organelles involved in production of lipid bodies were endoplasmic reticulum (ER) and elaioplast. During early-vacuolated stage, the tapetum has lipids in the endoplasmic reticulum. The elaioplasts of the tapetum produce numerous saturated and unsaturated lipids. At maturation stage, the plastoglobules released from elaioplasts coalesced with unsaturated lipids from endoplasmic reticulum to form the pollenkitt mass deposited on the exine. At the time of pre-dehiscence, the epidermal and endothecium are still intact. Fibrous thickenings are formed in the cells of the endothecium. In staminate flowers, the septum separating anther locules was cleaved and the stomium was degenerated to release the pollen grains. In hermaphrodite flowers, all of these structures remained intact without any release of pollen grains. All two pollen grains possess a well-developed pollen wall (exine and intine). The exine is composed of tectum, columellae, foot layer and endexine. The intine (I) of aperture in fertile pollen grains was opened with conspicuous three strata. Sterile pollen grains display a thickness intine and exine in the aperture which was not opened. According to this study, the flowers' sex differentiation in K. henryi are potentially developed as bisexual and only at the final stage (ovules formed) of development sex organs could be defined. Not only the staminate and hermaphrodite flowers have functionally and structurally difference, but also the fertile and steril pollen grains produced respectively.