References

28 Communication Between Undamaged Plants by Volatiles 431
28.4
Conclusions and Future Prospects
The work reviewed here shows that chemical communication takes place
between undamaged plants, supporting aspect 1 in our definition of allelobiosis
(see “Introduction”). It appears that this communication affects
not only the plant itself, but also organisms at higher trophic levels, namely
insect herbivores and their natural enemies (aspect 3). It also raises the idea
that a barley individual may benefit from the communication (aspect 2) by
detecting neighbouring plants via the airborne volatiles they produce. It is
most relevant to discuss this latter aspect in the context of the results on
plant biomass allocation.
In experiments with two barley cultivars, Ninkovic (2003) showed that
when plants of a particular barley cultivar were exposed to volatiles from
a different cultivar the exposed plant changed its pattern of biomass allocation,
allocating more biomass to roots and less to leaves (Fig. 28.3).
This pattern did not arise when plants were exposed to volatiles from the
same cultivar (self-induction). This selectivity may be due to the presence
or absence of specific substances in the volatile blends, but this is not necessarily
the case. It is important not to overlook the possibility that the
amounts of trivial plant compounds or the specific ratios between some of
these might constitute the active signal. This presents a challenge to the
chemical identification of the signal mechanism itself, but clearly this has
high priority for study since increased knowledge in this area will lead both
to greater understanding of the ecological role of allelobiosis as well as to
possible applications in crop management.
Even though plants of the exposed cultivar modified their pattern of
biomass allocation, both the total biomass and the RGR were not significantly
different from those of unexposed plants. Further, exposed plants
did not undergo any alteration of physiological activity compared with
unexposed plants. Here it is interesting to speculate whether, in the barley
model system, volatiles from a different cultivar represent a stimulus
that mobilises a morphological plasticity in the exposed cultivar that allows
it to respond to potential competition from a neighbouring plant. In
this scenario, allelobiosis would represent a source of information for the
responding plant.
Allelobiosis causes changes in exposed barley plants that make them less
suitable for aphid settling than unexposed plants. From the perspective of
the responding plant, allelobiosis can be viewed as a route for obtaining
information on plant competitors, which should give an advantage. Alternatively,
it can be considered as a chemical disturbance that is detrimental.
For the herbivore, however, it should be advantageous to detect the changes
in plant status associated with either situation, if these make the plant less