Chemical Ecology & Phytochemistry-2006

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6 SCHMIDT, et al. The finding has been corroborated by a study including small plants and larger trees of Norway spruce, where methyl jasmonate induced increased resin flow and other defense reactions when applied externally to intact bark. 32 The concentrations of methyl jasmonate found to be effective in spraying P. abies saplings in our work (maximum effect at 10 mM) were relatively high compared to those typically used on angiosperm foliage: 10 µM – 1 mM. 33-35 This may only be a consequence of the need for higher concentrations to penetrate the thick cuticle of conifer needles. More recently, we have shown that a 100 μM spray of methyl jasmonate is effective in inducing terpene accumulation in P. abies saplings when formulated as a 0.5 % solution in Tween 20 detergent (Schmidt, A., unpublished results). Methyl Jasmonate Application to Mature Trees in the Field We were curious to see what effect methyl jasmonate treatment would have on the terpene oleoresin content of established trees in the field. If we could manipulate the amount of terpenes in mature trees, this might allow us to learn something about the effects of resin on many serious pest insects, such as bark beetles. Several clones of 30 year-old P. abies were treated with methyl jasmonate by painting an aqueous solution of 100 mM methyl jasmonate in 0.1 % Tween 20) on the bark with a paint roller. Samples taken from the methyl jasmonate-treated zones after four weeks showed that wood tissue from these areas had five times as many traumatic resin ducts as samples from untreated control trees (Fig. 1.1C, 1.1D) and a content of monoterpenes that was 2 – 2.5 times greater than that of control trees. Although the increase of terpenes in wood was much less that that observed in 1-2 year-old saplings after methyl jasmonate spraying in the laboratory, the mature trees in this study showed considerable flow of resin on the outer bark surface (Fig. 1.2). Presumably, this resin originates in the traumatic ducts, flowing to the outer bark via rays, 19,32 and so should be added to that found in the wood to get a true measure of terpene production triggered by methyl jasmonate. Significant variation was observed, however, among the clones used in this study, with one of the six clones showing no significant differences from control trees after the 100 mM methyl jasmonate treatment.
INDUCED CHEMICAL DEFENSES IN CONIFERS 7 Fig. 1.2: External resin flow on a ca. 40-year-old Norway spruce clone after application of methyl jasmonate (100 mM in 0.1% Tween 20) and subsequent inoculation with the blue stain fungus Ceratocystis polonica. To determine if the increased terpene content of methyl jasmonate-treated trees might be associated with increased resistance to enemies, we inoculated treated trees with the blue-stain fungus, Ceratocystis polonica, four weeks after methyl jasmonate application. Treatment with 100 mM methyl jasmonate dramatically reduced fungal growth in sapwood (2 % of control) and cambium necrosis caused by the fungus (19 % of control) (Fig. 1.3). It was satisfying to see how significantly the defensive potential of Norway spruce could be manipulated by methyl jasmonate in the field. However, jasmonates trigger a variety of induced defense systems in angiosperms, 22,36 and so resistance to C. polonica cannot be attributed to the increased terpene level without further experiments.
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