but they are also used for host-plant recognition and selection by specialist insect
herbivores such as the diamond back moth (DBM, Plutella xylostella) and the small
white butterfly (Pieris rapae) [19]. Plants therefore face a dilemma with respect
to the signaling of the concentrations of SMs on the leaf surface: signaling a high
concentration may deter the generalist herbivores while attracting the specialist ones.
For both generalist and specialist herbivores, it is important whether or not
the signaling is honest, i.e., reflects the true concentration and composition of
chemicals inside the leaves. Glucosinolates have been detected on the leaf surface
of the cress species Barbarea rupicola, B. verna, and B. vulgaris, and the concentrations found on the surface were sufficient to be used by DBM as oviposition cues.
However, glucosinolates were not detected on the leaf surface of other crucifers such
as Brassica napus and Nasturtium officinale, although glucosinolates are present in
the leaf tissue of these species at levels comparable to the three Barbarea spp. [20].
Very recently, Shroff et al. [21] reported that the glucosinolate profile on the leaf
surface revealed differences from that in leaf tissue.
Hence, the relationship between chemical profiles of the leaf surface and
leaf tissue may be species-specific, and this could offer a new angle to the study of
insect-plant interactions mediated by plant SMs. The groups of SMs such as
alkaloids, terpenes, flavonoids, and phenolics present on surface involve in plant
chemical defense [17]. However, little is known about the relationship between these
SMs on plant surface and the corresponding SMs inside plants.
2
Pyrrolizidine Alkaloids (PAs)
Pyrrolizidine alkaloids (PAs) represent a class of typical SMs, which are constitutively formed in the plants containing them and mediating plant-herbivore interactions [22]. More than 400 PAs have been identified from ca. 6000 angiosperm
species [23], of which more than 95% belong to four families: Asteraceae,
Boraginaceae, Fabaceae, and Orchidaceae [24].
PAs can occur in plants in two forms: tertiary amine (free base) and N-oxide
[25–27]. Hartmann and coworkers showed that PAs are produced as N-oxides and
are dominantly present as N-oxides in Senecio plants. The reduction from N-oxides
to corresponding tertiary amines can happen spontaneously during alkaloid
extraction, and then the high amount of tertiary PAs in the samples is an artifact
[28, 29].However, recent research shows that not all PAs are exclusively present
as N-oxides in the plants of Jacobaea vulgaris and hybrids between J. vulgaris
and Jacobaea aquatica. In J. vulgaris and in hybrids between J. vulgaris and
J. aquatica, some jacobine-like PAs occur in up to 50% as tertiary amines. Moreover
the variation in ratio between the tertiary amines and N-oxides is genotype-dependent [25]. Pelser et al. [30] reported that 26 PAs (as tertiary amines) were present in
24 species of sect. Jacobaea. With more sensitive analytical methods for the
detection, more structural PA variants can been found in such species as J. vulgaris
and J. aquatica [25].
11 Variation in Leaf-Surface and Leaf-Tissue Secondary Metabolites:. . .
251
herbivores such as the diamond back moth (DBM, Plutella xylostella) and the small
white butterfly (Pieris rapae) [19]. Plants therefore face a dilemma with respect
to the signaling of the concentrations of SMs on the leaf surface: signaling a high
concentration may deter the generalist herbivores while attracting the specialist ones.
For both generalist and specialist herbivores, it is important whether or not
the signaling is honest, i.e., reflects the true concentration and composition of
chemicals inside the leaves. Glucosinolates have been detected on the leaf surface
of the cress species Barbarea rupicola, B. verna, and B. vulgaris, and the concentrations found on the surface were sufficient to be used by DBM as oviposition cues.
However, glucosinolates were not detected on the leaf surface of other crucifers such
as Brassica napus and Nasturtium officinale, although glucosinolates are present in
the leaf tissue of these species at levels comparable to the three Barbarea spp. [20].
Very recently, Shroff et al. [21] reported that the glucosinolate profile on the leaf
surface revealed differences from that in leaf tissue.
Hence, the relationship between chemical profiles of the leaf surface and
leaf tissue may be species-specific, and this could offer a new angle to the study of
insect-plant interactions mediated by plant SMs. The groups of SMs such as
alkaloids, terpenes, flavonoids, and phenolics present on surface involve in plant
chemical defense [17]. However, little is known about the relationship between these
SMs on plant surface and the corresponding SMs inside plants.
2
Pyrrolizidine Alkaloids (PAs)
Pyrrolizidine alkaloids (PAs) represent a class of typical SMs, which are constitutively formed in the plants containing them and mediating plant-herbivore interactions [22]. More than 400 PAs have been identified from ca. 6000 angiosperm
species [23], of which more than 95% belong to four families: Asteraceae,
Boraginaceae, Fabaceae, and Orchidaceae [24].
PAs can occur in plants in two forms: tertiary amine (free base) and N-oxide
[25–27]. Hartmann and coworkers showed that PAs are produced as N-oxides and
are dominantly present as N-oxides in Senecio plants. The reduction from N-oxides
to corresponding tertiary amines can happen spontaneously during alkaloid
extraction, and then the high amount of tertiary PAs in the samples is an artifact
[28, 29].However, recent research shows that not all PAs are exclusively present
as N-oxides in the plants of Jacobaea vulgaris and hybrids between J. vulgaris
and Jacobaea aquatica. In J. vulgaris and in hybrids between J. vulgaris and
J. aquatica, some jacobine-like PAs occur in up to 50% as tertiary amines. Moreover
the variation in ratio between the tertiary amines and N-oxides is genotype-dependent [25]. Pelser et al. [30] reported that 26 PAs (as tertiary amines) were present in
24 species of sect. Jacobaea. With more sensitive analytical methods for the
detection, more structural PA variants can been found in such species as J. vulgaris
and J. aquatica [25].
11 Variation in Leaf-Surface and Leaf-Tissue Secondary Metabolites:. . .
251