赤桉(Eucalyptus camaldulensis)為桃金孃科(Myrtaceae)桉樹屬(Eucalyptus)植物，原產於澳洲，剋他效果十分明顯。本研究以赤桉隔離林區內所採集之新鮮桉葉之水萃液(1:3.25(w/v))進行生物檢定(bioassay)，結果發現白莧種子之發芽反應、白蘿蔔幼苗下胚軸之伸長反應以及白莧胚根之伸長反應對於水萃液十分敏感，故可作為桉樹剋他作用之生物檢定指標。根據林區內外之草相與土壤種子庫的調查結果顯示，林區內之草種與其發芽可能因剋他作用、遮蔭與競爭等現象抑制生長。進一步以桉葉粉末混合土壤處理，發現其強烈抑制莧科(Amaranthaceae)雜草的幼苗生長，應可作為後續莧科雜草管理之有效資材。本研究將收集之桉葉以蒸餾法分離水溶性及揮發性之剋他物質，並進一步以HPLC系統分析桉葉沸水萃取物之剋他物質gallic acid (GA)含量，並以生物檢定系統確認桉葉沸水萃取物中之GA及桉葉精油中之1, 8-cineole可能扮演桉樹剋他現象之重要角色。為了瞭解桉葉產生剋他作用的原因，利用生物檢定材料-白莧胚根細胞，觀察經GA及1, 8-cineole處理後之白莧胚根，結果顯示兩處理皆會降低白莧胚根細胞有絲分裂指數，但不影響有絲分裂各時期之相對細胞數量。 River red gum (Eucalyptus camaldulensis), native to Australia, has been reported that the secondary metabolites released from this plant have allelopathic effects. Based on the responses of several bioassay materials to allelochemicals released from river red gum, the water extract highly inhibited seed germination, radicle and hypocotyl elongation of vegetables including edible amaranth (Amaranthus gangeticus L.) and radish (Raphanus sativus L.), which were ideal candidates for detecting allelopathic activity. Field survey of weed flora and soil seed bank in and out of the Eucalyptus forest suggested that some of the native weed flora were suppressed or even failed to germinate due to the competition, shading or allelopathic effect of Eucalyptus. Experiment of 7-weed species cultivation with eucalyptus leaf powder and soils showed that seedling growth of Amaranthaceae family was strongly inhibited by leaf powder. For identifying the volatile and water-soluble allelopathic compounds, the gallic acid (GA) in boiling water extract was quantified by HPLC and tested by bioassay systems. In addition, the allelopathic activity of the essential oil was detected by bioassay systems. The result showed that GA and 1, 8-cineole played an important role in allelopathic effect of Eucalyptus. Microscopic observation of chromosomes in Amaranth radicles also showed that mitosis was retarded significantly by GA and 1,8-cineole, especially the later, but there was no difference in the relative percentage of cells stayed in different mitotic cell division stages.