260
]oop ].A. van Loon, Marcel Dicke
chain aliphatic acids, produced through the lipoxygenase pathway (together often
designated as green leaf volatiles; GL V) and a variable range of mono- and sesquiterpenoids. When damaged by arthropods, not only may the absolute quantities
of the volatiles as emitted from undamaged plants increase, but also two different
clear-cut changes in the composition of the volatile bouquet released may occur:
(1) a quantitative change due to shifts in the relative quantities and (2) a qualitative change seen as the damage-induced release of new compounds that are not
emitted from undamaged or from mechanically damaged plants (Fig. 2).
For some herbivorous species it has been shown that they secrete elicitors into
the damaged plant tissue that induce a volatile emission that differs from that
induced by mechanical damage (Mattiacci et al. 1994; Turlings and Fritzsche
1999). In a few cases the chemical nature of these elicitor molecules has been
identified (Mattiacci et al. 1995; Alborn et al. 1997).
2.2.2 Factors Causing Variability in Plant Volatile Emissions
For any plant species, the composition of the bouquet of volatiles emitted as well
as the amount released per gram tissue is prone to a considerable degree of
variation (Dicke and Vet 1999). The most important sources of variation are
genotypic differences, developmental stage, and biotic factors such as competing
plants, infection by pathogens, and damage by arthropods. Abiotic factors that
affect volatile production are, e.g., shading and soil nutrient level.
2.3 Responses to Plant Volatiles: Herbivores
2.3.1 Responses of Herbivores to Volatiles from Undamaged Plants
For a range ofherbivorous species, plant volatiles have been shown to elicit odorconditioned anemotaxis (reviewed by Visser 1986; Metcalf and Metcalf 1992;
Bemays and Chapman 1994; Schoonhoven et al. 1998). Regarding the chemical
identity of the kairomones utilized during host-plant searching, knowledge has
increased rapidly relative to the limited information on nonvolatile chemicals that
trigger acceptance. The reasons are of a practical rather than a scientific nature:
olfactory activity of volatiles can be tested on-line, using so-called coupled gas
chromatography-electroantennography or single-cell recording techniques (Am et
al. 1975; Wadhams 1984) and analytical chemistry of volatiles is more feasible
compared to chemistry of nonvolatiles in view of the fact that the number of
potentially active compounds in the latter category is much greater. Cases have
been documented in which the detector of the gas chromatograph was much less
sensitive than the biological detector, being the olfactory receptors producing
electrophysiological responses (Pickett 1990; Weissbecker et al. 1997).
]oop ].A. van Loon, Marcel Dicke
chain aliphatic acids, produced through the lipoxygenase pathway (together often
designated as green leaf volatiles; GL V) and a variable range of mono- and sesquiterpenoids. When damaged by arthropods, not only may the absolute quantities
of the volatiles as emitted from undamaged plants increase, but also two different
clear-cut changes in the composition of the volatile bouquet released may occur:
(1) a quantitative change due to shifts in the relative quantities and (2) a qualitative change seen as the damage-induced release of new compounds that are not
emitted from undamaged or from mechanically damaged plants (Fig. 2).
For some herbivorous species it has been shown that they secrete elicitors into
the damaged plant tissue that induce a volatile emission that differs from that
induced by mechanical damage (Mattiacci et al. 1994; Turlings and Fritzsche
1999). In a few cases the chemical nature of these elicitor molecules has been
identified (Mattiacci et al. 1995; Alborn et al. 1997).
2.2.2 Factors Causing Variability in Plant Volatile Emissions
For any plant species, the composition of the bouquet of volatiles emitted as well
as the amount released per gram tissue is prone to a considerable degree of
variation (Dicke and Vet 1999). The most important sources of variation are
genotypic differences, developmental stage, and biotic factors such as competing
plants, infection by pathogens, and damage by arthropods. Abiotic factors that
affect volatile production are, e.g., shading and soil nutrient level.
2.3 Responses to Plant Volatiles: Herbivores
2.3.1 Responses of Herbivores to Volatiles from Undamaged Plants
For a range ofherbivorous species, plant volatiles have been shown to elicit odorconditioned anemotaxis (reviewed by Visser 1986; Metcalf and Metcalf 1992;
Bemays and Chapman 1994; Schoonhoven et al. 1998). Regarding the chemical
identity of the kairomones utilized during host-plant searching, knowledge has
increased rapidly relative to the limited information on nonvolatile chemicals that
trigger acceptance. The reasons are of a practical rather than a scientific nature:
olfactory activity of volatiles can be tested on-line, using so-called coupled gas
chromatography-electroantennography or single-cell recording techniques (Am et
al. 1975; Wadhams 1984) and analytical chemistry of volatiles is more feasible
compared to chemistry of nonvolatiles in view of the fact that the number of
potentially active compounds in the latter category is much greater. Cases have
been documented in which the detector of the gas chromatograph was much less
sensitive than the biological detector, being the olfactory receptors producing
electrophysiological responses (Pickett 1990; Weissbecker et al. 1997).