References

28 Communication Between Undamaged Plants by Volatiles 423
28.1.1
Plant–Plant Communication via Volatiles – a Complex Language
Well-known examples of interactions mediated by volatiles from undamaged
plants are the dominance achieved by shrub species (e.g. Salvia leucophylla,
S. apiana, S. mellifera and Artemisia californica) through emission
of chemicals such as α-pinene, β-pinene, camphene, champhor, cienole
and dipentene that inhibit germination and root growth in seeds of herbaceous
species (Muller et al. 1964). Methyl jasmonate produced by A. tridentata
plants induced resistance to herbivores in leaves of neighbouring
tomato plants by initiating the accumulation of proteinase inhibitors
(Farmer 2001). Karban et al. (2000) showed that mechanically damaged
A. tridentata plants increase production of methyl jasmonate and induce
defensive responses to herbivores in wild tobacco, Nicotiana attenuata,
although a recent study suggests that methyl jasmonate is not the active
signal in this interaction (Preston et al. 2004).
There has been an explosion of interest in plant signalling in recent
years,withastrongemphasisonvolatilesemittedbyplantsinresponseto
attack by herbivores or infection with pathogens. Such signals can induce
a defence response in neighbouring, non-attacked plants, making them less
attractive to herbivores. Studies have also shown that volatiles produced
by plants induced in this way can promote searching behaviour of natural
enemies of the herbivores. It is not our intention to review the entire
spectrum of volatile plant–plant communication; however, a number of
excellent reviews are available, including those by Bruin and Dicke (2001)
and Farmer (2001).
28.1.2
Experimental Considerations in Plant–Plant Communication
Distinguishing the effects of plant interaction via chemicals from the
effects of competition for resources has been the major hindrance to
studies of allelopathy/allelobiosis in both laboratory and natural conditions.
The first reports of communication between infested and uninfested
plants (Rhoades 1983; Baldwin and Schultz 1983) met with some criticism
focusing on the experimental design, particularly problems with
pseudoreplication (Fowler and Lawton 1985). Pettersson et al. (1999) and
Ninkovic et al. (2002) introduced a new method for plant exposure that
separates allelobiosis from other interference mechanisms. The method
is based on a large number of two-chamber cages, each of which constitutes
an experimental replicate (Fig. 28.2). Pots containing the plants are
placed in separate chambers, preventing competition for nutrients, light