References

28 Communication Between Undamaged Plants by Volatiles 425
Intraspecific allelobiotic interactions between plants have rarely been
studied. In the case of interaction between barley plants, this phenomenon
has been addressed in very few studies and usually from the viewpoint of
induced resistance. Fujiwara et al. (1987) reported that volatile compounds
released after pruning of barley leaves induced systemic resistance against
powdery mildew fungus in exposed, intact barley seedlings. This resistance
was more prominent in the primary leaf than in the secondary leaf. From the
perspective of plant resistance to aphid, Pettersson et al. (1996) tested aphid
acceptance of plants at the two-leaf stage that were exposed to volatiles
from aphid-attacked plants or to powdery mildew-infested plants. In both
cases aphid acceptance of exposed plants was significantly decreased in
comparison with that of plants treated with clean air. The results of this
study support a link between induced resistance to herbivores and disease
in barley.
28.2.2
Allelobiosis and Plant Responses
In productive habitats such as fertilized arable land, similarities in resource
requirements between individual plants of the same species intensify the
struggleforcaptureofavailableresources.Intheearlystagesofplant
growth (seedling phase), shoots and leaves scarcely interfere with each
other and competitive interactions, where they occur at all, are most likely
to be limited to those operating within the soil. Plants can modify their
growth in response to environmental conditions, allocating biomass to either
aboveground or belowground organs in a way that maximizes growth.
This trade-off between allocation to shoots and roots may be one of the
plant’s primary responses to competition with other plants (Grime 2001).
Ifthisisso,thenitshouldbenefitplantstodetectcompetitionnotonly
through the depletion of resources, but by responding to the actual presence
of potentially competing plant individuals. In this way plants could
respond promptly to the presence of competitors, e.g. during seed germination
or the early seedling phase, allowing them to minimize the negative
effects of competition.
In a particular combination of barley cultivars, in which interaction
between roots was prevented, seedlings responded to volatiles from neighbouringplantswithchangesintheir
patternofbiomassallocation(Ninkovic
2003). Plants exposed to volatiles from a different cultivar allocated more
biomass to roots than did plants exposed to volatiles from the same cultivar,
or to air alone. However, the total dry weight did not differ between
treatments, and thus the principal effect was on the allocation of biomass,
not on the total biomass (Fig. 28.3).