蓮華池桑寄生種子之初期存活率: 傳播媒介、寄主樹種、與寄生枝條之影響

Abstract

桑寄生(mistletoe)植物是近年來研究寄生植物演化非常重要的一類植物。其特徵為半寄生(hemiparasite)，花粉及種子多半由鳥類散播，果實量多，分布於專一種或多種類的寄主植物，且常呈群集性分布。近年許多研究認為這些特徵和桑寄生、散播者、寄主植物三者之間長期的共同演化所造成。桑寄生的群集性分布主要受生長初期的二個階段所影響，即(1)鳥類消化種子後的散播分布，及(2)種子散播於不同寄主樹種不同部位的相容性差異。但目前這二個因素影響的程度或機制還未被徹底了解，而這個過程是否導因於長期的演化也仍處於理論階段，加上台灣的研究少有對桑寄生寄生機制的探討，故本研究著重於桑寄生初期生長的存活率變動，並探討其各生理階段存活率變動的主因為何。 研究物種為臺灣特有的蓮華池桑寄生(Taxillus tsaii)，研究地點為南投縣魚池鄉的林業試驗所蓮華池研究中心，此地的蓮華池桑寄生呈現高密度且群集性的分布。研究方法是採用接種三種類型種子：(1)未去皮種子、(2)去皮種子、(3)鳥類排遺種子，於三種試驗寄主物種：(1)牛樟(Cinnamomum micranthum)、(2)土肉桂(Cinnamomum osmophloeum)、(3)油茶(Camellia oleifera)，並將寄主枝條分成二種徑級：(1)5-23 mm、(2)26-50 mm。自2007年六月至2008年一月，記錄不同種子類型、寄主物種、枝條粗細對種子黏附率、發芽率、固著性、總有效率、及存活率的影響。 結果發現，黏附率顯著受到種子類型與寄主物種的影響，發芽率顯著受種子類型的影響(p<0.01)，固著性、總有效率、及存活率顯著受種子類型與寄主物種的影響(p<0.01)。果皮若未去除，種子黏附率會下降，且若寄主物種樹皮光滑，種子黏附率也會下降。未去皮的種子無法發芽，鳥類消化種子的發芽率低於人工去皮種子。所有試驗種子的發芽率，受寄主樹種的影響不顯著。但寄主樹種對固著器建立率有關鍵性的影響，主因是桑寄生與不同種寄主相容性有所差異，不相容物種無法建立吸器。另外，鳥類的消化也會降低種子成功固著的機率。 因此蓮華池桑寄生的初期存活率確實主要由散播者和寄主物種所共同影響，但影響的機制並不相同。鳥類消化對桑寄生種子有生理上的傷害，但對種子發芽及散播還是必需的；散播者的微棲地選擇及寄主物種與桑寄生間的相容性差異，應是影響桑寄生分布的關鍵性因素。

Mistletoes have played an important role in studying the evolution of parasitical plants in recent years. Mistletoes are almost hemiparasite and parasitize in clusters on one or many species of hosts. Mistletoes produce abundant fruits and both of their pollination and seed dispersal are mostly relied on birds. Many recent studies suggest these traits of mistletoes are resulted from the long-term co-evolution among the mistletoe, dispersers, and hosts. The clumped distribution of mistletoes is affected by two phases during the early stage of life cycle. One phase is that the seeds are affected by birds’ digestion and spreading range. The other phase is that the seeds have different host compatibility with different species or branches of hosts where they are dispersed. The mechanisms of these two parasitic phases are under-studied and the parasitic mechanisms of the mistletoes of Taiwan have been rarely reported. As a result, this research focuses on the early survival rates of seeds of Taxillus tsaii, an endemic species of Taiwan, and the effects of seed dispersers, host trees, and branch diameter. This study was conducted in the Lianhuachih Research Center of Taiwan Forestry Research Institute (Yuchih Township, Nantou County, Taiwan), where has high density of Taxillus tsaii. I inoculated three types of seeds (coated seeds, uncoated seeds, and defecated seeds) on two classes of branch size (5-23 mm and 26-50 mm in diameter) of three host species, including Cinnamomum micranthum, Cinnamomum osmophloeum, and Camellia oleifera. I monitored the effects of different types of seeds, host species, and branch size on seeds adhesion, seeds germination, holdfast establishment, total active seed ratio, and seed survival ratio from June, 2007 to January, 2008. Results revealed that seed germination was significantly affected by seed types. In addition, seed adhesion, holdfast establishment, total active seed ratio, and survival ratio were significantly affected by both seed types and host species. The adhered ratio greatly decreased in the coated seeds and on the branches with smooth bark. Host species did not have significant influence on germinated ratio. Defecated seeds had significantly lower germinated ratio than uncoated seeds, while coated seeds could not germinate at all. Host species was critical for holdfast established ratio, as the compatibility of Taxillus tsaii seeds differed with tree species. Haustoria of mistletoe could not establish on antagonistic species. The holdfast established ratio of digested seeds was significantly lower than uncoated seeds.