Some eucalyptus species can be associated to the production of zones where vegetation is absent, sparse and/or less diverse and vigorous than surrounding zones away from eucalypts. These zones have been documented with eucalypts growing as native or introduced species. Yield reduction and poor plant performance is frequent when some crops, annual or perennial, are planted within or around eucalypt stands. Although competition for water, light and nutrients can explain some of these inhibition patterns, it is insufficient to explain others. Field evidence suggests that allelopathy explains, at least partially, the aforementioned inhibition areas. Inhibition zones are absent where the soil does not accumulate allelochemicals and the watering or rainy regime leaches them out, or the plants surronding eucalypts are unaffected by the chemicals. Phenolic acids, tannins, flavonoids and/or terpenoids have been isolated from eucalypt bark, litter and leaves; leaf hates or extracts from these parts have been shown to be phytotoxic in vitro and in greenhouse experiments for most target speciestested. Allelochemicals are normally released, from intact, dead or alive, eucalypt tissues and accumulated in water or soil in concentrations high enough to produce allelopathic effects. Milled or chopped eucalypt parts release more allelochemicals and faster than intact parts. Although no published work contains a li the undisputed evidence required to demonstrate eucalypts allelopathy, the body of evidence in the published works suggests that some eucalypt species do produce allelopathic effects in natural conditions.
In vitro allelopathic effects of extracts and fractions of five plants on tomato seed germination and vigor index