Allelochemicals Biologica... - Name

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SCOTT W. MATTNER
from three ryegrass cultivars infected with three different strains of endophyte, all
inhibited the growth of clover, by up to 27% compared with extracts from endophytefree
ryegrass. Both ryegrass cultivar and endophyte strain influenced the degree that
ryegrass extracts inhibited clover, but this did not relate to the type of alkaloids produced
by the different endophyte strains. These studies suggest that both environmental
and genetic influences may moderate the triggers for enhanced allelopathy by endophyte
infected grasses, and this may explain the discrepancy in results between individual
studies. Nonetheless, the majority of evidence suggests that endophyte infection has
the capacity to increase ryegrass allelopathy, which is a further added benefit conferred
by this mutualist to its host. Although the endophyte is a non-pathogenic organism,
its ability to stimulate plant allelopathy adds further weight to the hypothesis that
infection increases the allelopahtic ability of host plants.
3.2.4. Other Systems
The ability of rusts to stimulate allelopathy in their hosts may not be limited to ryegrass,
as the rusts Puccinia hordei and Uromyces troflii-repentis increased the suppression
of white clover by barley grass (Hordeum leporinum) and subterranean clover (Trifolium
subterraneum), respectively (Mattner, 1998). Yet, the effect was not universal since
Puccinia coronata in wild oat (Avena fatua), Puccinia graminis in cocksfoot (Dactylis
glomerata) and Puccinia recondita in soft brome (Bromus mollis) all failed to increase
their host’s allelopathic ability.
Kong et al. (2002) studied the allelopathic potential of goatweed (Ageratum
conyzoides) under different environmental stresses, including infection by powdery
mildew (Erysiphe cichoracearum). Infection stimulated the production of 17 of the
24 volatile chemicals produced by goatweed that they investigated, with total volatile
production increasing by 50%. Exposure to the volatiles released by infected goatweed
stimulated the growth of peanut (Arachis hypogaea), redroot amaranth (Amaranthus
retroflexus), Italian ryegrass (Lolium multiflorum) and cucumber (Cucumis sativus)
compared with volatiles from healthy goatweed. In contrast, volatiles from infected
plants inhibited the growth of three fungal pathogens (Rhizoctonia solani, Botrytis
cinerea, and Sclerotinia sclerotiorum). For this reason, they postulated that the
allelochemicals stimulated by fungal infection in goatweed are more important in the
defence of the plant against infection, rather than against competition from
neighbouring plants. Nonetheless, this study currently provides the clearest
demonstration of the ability of pathogens to influence the allelopathic ability of plants.
4. IMPLICATIONS FOR PATHOGEN
AGGRESSIVENESS AND EVOLUTION
Plant/pathogen interactions in natural populations are often explained in terms of coevolution,
which is ‘the joint evolution of two (or more) taxa that have close ecological
relationships but do not exchange genes, and in which reciprocal selective pressures
operate to make the evolution of either taxon partially dependent upon the evolution