Field experiments with manually defoliated black alders (Alnus glutinosa) s
howed that defoliation affected herbivory by the major alder antagonist, th
e leaf beetle Agelastica alni. Herbivore damage increased with increasing d
istance to the defoliated tree, suggesting induced resistance not only on t
he damaged tree, but also on the neighbouring trees. The beetles also avoid
ed leaves from the nearest neighbours for both feeding and oviposition in a
laboratory assay, so the alders showed interplant resistance transfer. Nat
ural enemies did not appear to shape this pattern., because the number of e
ntomophagous arthropods and predator-prey ratios even increased with increa
sing distance to the defoliated tree. The numbers of all specialist, but no
t the generalist, herbivore species paralleled the increase in the attack o
f the specialist A. alni, supporting the view that specialists are more aff
ected by plant resistance than generalists.
Mechanisms causing this pattern, found in the field, were studied in more d
etail using biochemical analyses and further bioassays. Responses of alder
leaves to herbivory of A. alni were shown to include ethylene emission and
the release of a blend of volatiles with mono-, sesqui- and homoterpenes. C
hanges in leaf chemistry after herbivory included increases in the activity
of oxidative enzymes (polyphenoloxidase, PPO, lipoxygenase, LOX, and perox
idase, POD) and proteinase inhibitors (Pis). and an increase in the phenoli
c contents of the leaves. Quantification of the endogenous jasmonic acid (J
A) showed the activation of the octadecanoid pathway following herbivory.
The active components in mediating a possible interplant signal transfer vi
a airborne volatiles may have included ethylene, beta -ocimene. 4,8-dimethy
lnona-1,3,7-triene (DMNT), and 4,8,12-trimethyltrideca-1.3,7,11-tetraene (T
MTT). The incubation with volatiles resulted in an increase in the activity
of catalase (CAT) and Pis (after MeJA application) and in an increase in t
he content of phenolics and PI activity (after ethylene application). Furth
er evidence that airborne interplant communication may be important in the
response of alder trees to beetle attack came from container experiments, I
n airtight chambers, unattacked leaves significantly increased the activity
of proteinase inhibitors when they were associated with leaves previously
attacked by beetle larvae.
In conclusion, field experiments, bioassays in the laboratory as well as bi
ochemical analyses suggest the existence of interplant resistance transfer
in A. glutinosa. with airborne volatiles as a possible mechanism. However.
the relative importance of airborne and possible soil-borne signals as well
as unknown effects of intensified nutrient absorption of defoliated trees.
possibly reducing foliage quality of undamaged neighbours, remains to be s
hown. (C) 2001 Elsevier Science Ltd. All rights reserved.