References

28 Communication Between Undamaged Plants by Volatiles 427
In laboratory experiments, barley responses to allelobiotic interactions
via volatiles from a different barley cultivar were measured in terms of
changes in leaf temperature using an infrared camera (Pettersson et al.
1999). All possible pairwise combinations of four barley cultivars were
tested. When certain cultivars were exposed to volatiles from certain other
cultivars, significant reductions in leaf temperature were found.
Changes in leaf temperature result from active regulation of stomatal
aperture transpiration, which depends on the water status of the plant
modified by prevailing environmental conditions. Stomatal conductance,
in turn, is a measure of transpiration. It has been reported that infrared
measurements correlate well with data obtained using a diffusion porometer
and with gas-exchange measurements, and studies have also shown local
leaf temperature increases in regions where stomatal closure was induced
by initial virus infection, before disease symptoms were visible (Chaerle
and van der Streaten 2000).
Reduction in barley leaf temperature following allelobiosis indicates
increased transpiration rates in exposed plants. It is probable that an
increased need for water necessitates increased allocation of available
biomass to roots in these plants. Higher stomatal conductance enhances
the influx of CO2, which is required to maintain a higher photosynthetic
activity. The ULR is a physiological component of the RGR that is generally
strongly correlated with the rate of photosynthesis (Poorter and Nagel
2000). To maintain the same level of the ULR, it seems that the photosynthetic
activity of allelobiosis-exposed plants increased.
28.3
Allelobiosis and Insect Responses
Studies on the effects of plant–plant communication on insects have focused
almost exclusively on interactions in which the responding plant is
exposed to volatiles from herbivore- or pathogen-attacked plants. Volatiles
produced by plants attacked in this way can induce responses in neighbouring
undamaged plants, making them less attractive to herbivores (Bruin
and Dicke 2001) and more attractive to the herbivores’ natural enemies
(Dicke and Van Loon 2000). Recent studies at the biochemical and genetic
level have started to clarify the set of changes induced in responding
plants by exposure to volatiles from herbivore- or pathogen-attacked plants
(Arimura et al. 2000; Farmer 2001; Pickett et al. 2001). However, volatile
communication between undamaged plants, and its implications for higher
trophic levels, i.e. insect herbivores and their natural enemies, has been less
studied.