Chemical Studies on Plant-Plant Interactions of Eucalyptus and Mentha Species

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Chemical interferences by one plant with seed germination and seedling growth of another higher plant in nature are called as allelopathy. Since Molisch defined the term "allelopathy" in 1937, many scientists have been concerned with the exploration and exploitation of allelochemics. It is well known that some Eucalyptus species are surrounded by bare (grass-free) ground which are formed by the allelopathic effect in a plant ecosystem.The allelochemics of E. citriodora (lemon-scented gum) has been isolated from the leaves and identified as p-menthane-3,8-diols (cis and trans). Regio-isomers and ester derivatives of p-menthane-3,8-diols are prepared and bio-assayed on germination and growth inhibitions against several higher plants. As a results, it was elucidated that a distance between two oxygens of OH groups and a length of hydrophobic ester groups were very significant for the inhibitory activity. Furthermore, as the allelochemics of E. camaldulensis, E. delegatensis, E. pauciflora and E. viminalis, interesting terpenoids and aromatic compounds were identified.On the other hand, an allelochemics of a spearmint plant such as Mentha spicata was examined. (-)-Carvone which is a major component of spearmint oil exhibited very weak inhibition against germinating seeds and seeding of weeds. However, there is a possibility that the transformation products of (-)-carvone by microorganisms in soil may interfere with other plants. The conversion of (-)-carvone by Streptomyces species gave rise to a novel monoterpene named (+)-bottrospicatol which exhibited very strong inhibitory activity against several grasses. Studies on structure-activity relationships of allelochemics are in progress.