台灣之相思樹植物相剋作用潛能研究

Abstract

相思樹（Acacia confusa Merr.）屬豆科，相思樹屬，為常綠喬木，分佈台灣各地低海拔及丘陵地，具多用途，為本省低海拔最重要造林樹種之一。 在野外觀察，相思樹林下地被層的覆蓋度，與非相思樹林下的植被情形有明顯差異，本實驗的目的，即在探討造成此現象的原因，藉由種子生物分析，瞭解相思樹相剋作用的潛能，繼而探討各季節和各地點之間，相思樹相剋作用的變化程度，最後探討相思樹對自身幼苗是否有自毒作用。 取不同地區（屏東縣枋山鄉、台中縣大甲鎮、臺北縣林口鄉、宜蘭縣礁溪鄉）、不同季節（春、夏、秋、冬）相思樹風乾葉及枝的水溶萃取液，對分析用種子進行生物分析，觀察並比較之間的相剋作用程度。 由結果得知，四季中以1994年9月和1995年3月的相思樹葉水溶萃取液抑制作用最強，四個地區整體言之，楓港地區四季的相剋作用變化程度最小，宜蘭地區的變化程度最大。枝水溶萃取液的抑制程度，較葉水溶萃取液抑制作用為低。土壤生物分析中，三個地區（楓港、大甲、林口）之土壤對萵苣均有抑制作用而相思樹林內土壤之萵苣生長情形較林外土壤之生長情形為差。 雙向式濾紙色層分析結果，相思樹葉水層萃取液至少有13種化合物，乙醚層至少有9種化合物，而枝所含之化合物較葉少。並初步判斷化和物含量，亦隨著地點、季節不同而有變化。另外以這些化合物進行生物分析，得知水層化合物抑制程度較乙醚層化合物高。 最後，在相思樹自毒作用檢定實驗，發現相思樹葉水溶萃取液，對自身幼苗有自毒作用，此與野外觀察，相思樹林內無小苗出現相符合。

Acacia confusa Merr., a member of leguminosae, is widely distributed in foothill and lowland areas of Taiwan. The plant has been extensively used in many ways, including as a feedstock for charcoal-making and as a building material. The plant is particularly important for natural conservation of soil and water in Taiwan. It exhibits a unique pattern of dominance, and there is a relative lack of understory species on the Acacia floor. Field observations and experiments were conducted to elucidate the mechanism of its dominance. Four experimental sites were selected to represent the geographic distribution of the plants. The sites were located at Taoyuan (northern Taiwan), Taichung (central), Pington (southern) and Ilan (eastern). Physical para meters, such as soil pH, organic matter, soil moisture content and light intensity, were measured and compared among sites. Seasonal variation of physical factors were also determined for soil and plant materials. The allelopathic potential of Acacia confusa plants was evaluated by experiments conducted in the field, greenhouse and laboratory. Field measurements of understory plants in the three sites showed a relatively bare understory on the Acacia floor, in comparison with that of the adjacent grassland area. The variation of phytotoxicity, tested via a bioassay of aqueous extracts of Acacia leaves against lettuce, chinese cabbage, and radish, was significantly higher at the Ilan site than at other sites, and lowest at the Pington site. The phytotoxicity of aqueous extracts of Acacia confusa leaves was significantly higher than that of the extracts of twigs. The phytotoxicity of Acacia soil was also higher than that of the adjacent grassland soils; the degree of toxicity varied with sites. Two-dimensional paper chromatography of aqueous extracts of Acacia confusa leaves showed 13 spots, of which 9 spots were found in the ether fraction of aqueous extracts. The chromatography spots were tested via lettuce-seed bioassay; the spots eluted from the aqueous extract revealed higher toxicity than those from the ether fraction of aqueous extract, indicating that the phytotoxic elements present in Acacia leaves are water soluble. The autotoxicity of Acacia confusa was examined by watering one-month-old seedlings with aqueous extract of the leaves. A significant inhibition was found, suggesting that Acacia plants are autotoxic. It appears that allelopathy is involved in the species quote exclusion of understory plants.