Inducible plant defense is a beneficial strategy for plants, which imply that plantsshould allocate resources from growth and reproduction to defense when herbivoresattack. Plant ecologist has often studied defense responses in wild populations by biomassclipping experiments, whereas laboratory and greenhouse experiments in additionapply chemical elicitors to induce defense responses. To investigate whether fieldecologists could benefit from methods used in laboratory and greenhouse studies, weestablished a randomized block-designin a pine-bilberryforest in Western Norway.We tested whether we could activate defense responses in bilberry (Vaccinium myrtillus)by nine different treatments using clipping (leaf tissue or branch removal) with orwithout chemical treatment by methyljasmonate (MeJA). We subsequently measuredconsequences of induced defenses through vegetative growth and insect herbivoryduring one growing season. Our results showed that only MeJA-treatedplants showedconsistent defense responses through suppressed vegetative growth and reducedherbivory by leaf-chewinginsects, suggesting an allocation of resources from growthto defense. Leaf tissue removal reduced insect herbivory equal to the effect of theMeJa treatments, but had no negative impact on growth. Branch removal did not reduceinsect herbivory or vegetative growth. MeJa treatment and clipping combineddid not give an additional defense response. In this study, we investigated how to inducedefense responses in wild plant populations under natural field conditions. Ourresults show that using the chemical elicitor MeJA, with or without biomass clipping,may be a better method to induce defense response in field experiments than clippingof leaves or branches that often has been used in ecological field studies.
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