Phytotoxic Activities of Crude Extracts of Eucalyptus camaldulensis in Taiwan

Abstract

In order to estimate the phytotoxic activity of eucalyptus (＂Eucalyptus camaldulensis＂ L.) in Taiwan for developing bioherbicide, phytotoxic activities of boiling-water extract and volatile essential oil, two main parts produced during essential oil extraction, from eucalyptus leaves were confirmed with a sensitive edible amaranth (＂Amaranthus gangeticus＂ L.). The dose-responses of this amaranth, based on its seed germination and radicle elongation, to the boiling-water extract and gallic acid (GA), a major component, suggest that some heat-stable compounds in this extract were involved in the inhibition of seed germination. Meanwhile, with the same bioassay, the dose response tendency caused by essential oil from this eucalyptus appeared to resemble that of 1,8-cineole, but the essential oil had higher activity than that of 1,8-cineole, with radicle elongation being more sensitive to the treatments than seed germination. Furthermore, taking into consideration of its estimated content of 1,8-cineole, the eucalyptus essential oil must have contained additional components with higher potency. At equivalent dosages causing 50% inhibition of radicle elongation, boiling-water extract and GA showed similar mitosis inhibition tendency, revealing a potential role of this phenolic compound in the extract; and 1,8-cineole and eucalyptus essential oil (containing only 2% of 1,8-cineole), respectively, inhibited 77% and 31% radicle elongation indicates that an arrest of root cell mitosis represented only a minor part of the mechanisms involved in the overall retardation of amaranth radicle elongation by 1,8-cineole. Microscopic observation showed neither GA nor 1,8-cineole affected the number of mitotic cells at all four division stages in the edible amaranth.