Abstract :
Leaves of eucalyptus (Eucalyptus urophylla) were infiltrated
with a cell suspension of the incompatible pathogen
Acidoiorax avenae pv. avenae and showed a typical
hypersensitive response within 24 h. Necrotic leaf areas
were excised, vacuum infiltrated with 40% ethanol and
left under continuous agitation at room temperature for
24h. The diffusate was concentrated, partitioned with
ethyl-acetate, concentrated to dryness and resuspended
in a small volume of methanol. The biological activity of
extracts was evaluated by an agar diffusion method
against noncompatible bacteria (A. avenae pv. avenae
and Xanthomonas axonopodis pv. manihotis) and fungi
{Penicitlium sp. and Aspergillus sp.). Inhibition haloes,
when present, were always larger in extracts from leaves
infiltrated with the incompatible bacterium than the
water control. Thin layer chromatography resolution of
crude extracts from leaves infiltrated with both incompatible
pathogen cell suspension and water, followed by
bioautography with Thieteviopsis paradoxa, consistently
rendered, in both situations, a large, diffuse inhibition
halo near the origin, assumed to be due to preformed
antimicrobial substances. However, extracts from leaves
infiltrated with the living cells of the incompatible pathogen
gave rise to a smaller, second inhibition halo, near
the front, that was interpreted as being one or several
phytoalexins
