Allelochemicals Biologica... - Name

IMPACT OF PATHOGENS ON PLANT INTERFERENCE AND
ALLELOPATHY
further trials, high rust severity and high soil moisture contents increased the
suppression of clover by rusted ryegrass.
The ability of rust to directly inhibit or infect clover could not explain the results
from these studies, since clover is a non-host of P. coronata and inoculation with this
rust did not reduce the yield of clover monocultures. Under some conditions, infection
by rusts can increase the scavenging ability of some hosts for water and nutrient
resources (Ahmad et al., 1982; Paul and Ayres, 1988), potentially increasing their
competitive ability. For this to explain the results from Mattner’s studies, however,
the suppression of clover by rusted ryegrass should have been greatest at high plant
densities where competition for resources was most intense. Instead, the suppression
of clover by rusted ryegrass was greatest at low densities where resources were plentiful
and competition was low. Furthermore, the ability of rusted ryegrass to suppress
clover continued even beyond the death of the plant, when it had no capacity to scavenge
resources. For these reasons, Mattner (1998) posed the hypothesis that rust reduces
the competitiveness of ryegrass, while simultaneously increasing its allelopathic ability.
In this way, it was expected that the expression of allelopathy by rusted ryegrass was
greatest at low densities, where there was little competition and the effects of rust in
reducing ryegrass competitiveness did not obscure its effects on increasing allelopathy.
Furthermore, high plant densities may detoxify or dilute the action of allelochemicals
(Thijs et al., 1994).
To test the validity of this hypothesis and to separate the effects of competition
and allelopathy, four bioassays for allelopathy were conducted (Mattner, 1998; Mattner
and Parbery, 2001). Each bioassay highlighted the potential for extracts, leachate, or
residues from ryegrass to inhibit the yield of clover through allelopathy, and for rust
to enhance this potential. For example, soil previously growing rusted ryegrass
suppressed clover biomass by 36% compared with soil previously growing healthy
ryegrass. Similarly, leachate from soil supporting rusted ryegrass suppressed clover
biomass by 27% compared with that from healthy ryegrass (Mattner and Parbery,
2001). Although some bioassays have confounding and interpretational problems
(Stowe, 1979; Inderjit and Weston, 2000), the conformity of results between these
different bioassays provides strong evidence for the hypothesis that rust infection
increases the allelopathic ability of ryegrass. Furthermore, in the field, the proportion
of prickly lettuce (Lactuca serriola) reduced in areas of a depleted pasture dominated
by rusted ryegrass, compared with areas dominated by non-rusted ryegrass (Mattner,
1998). Further bioassays provided evidence that this association potentially related
to an enhanced allelopathic ability of ryegrass rather than to differences in soil
chemistry. Thus, this study highlighted the potential for rust to increase ryegrass
allelopathy in the field.
Mattner (1998) suggested two mechanisms by which rust may increase ryegrass
allelopathy. Firstly, rust may directly stimulate the production of allelochemicals by
ryegrass in a defensive response to infection. Alternatively, or additionally, the increased
rate of tissue senescence in ryegrass induced by rust may result in a higher concentration
of plant residues reaching the soil. These residues may then form an allelochemical
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