Induced plant responses in willow to a gall-forming insect

Höglund, S. 2005: Induced plant responses in willow to a gall-forming insect. Licentiate’s dissertation. ISBN:91-576-7155-9 The interaction between the basket willow Salix viminalis, and the gall midge Dasineura marginemtorquens, includes responses known to enhance as well as reduce herbivore performance i.e., gall formation, and hypersensitive response (HR), respectively. There is large genotypic variation in resistance in S.viminalis against D. marginemtorquens. In the first of two studies in this thesis we report, variation in larval survival and induced plant responses within a full-sib S. viminalis family. Resistance (larval mortality) was associated with HR, but to different degrees among willow genotypes. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR but not in resistant genotypes without symptoms. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, yet unknown, mechanism of resistance may therefore be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. Resistant willows usually have a very few galled leaves, whereas on the great majority of leaves all larvae die before any galls are formed. This observation, of high but not complete resistance, formed the basis for the second study. We found support for rapid induced susceptibility i.e., factors acting on the target leaf immediately before and during gall formation, but no support for delayed induced susceptibility, i.e., responses induced in the shoot the previous year and expressed when leaves develop the next season. In a combined greenhouse and field experiment the possibilities that susceptibility was induced by abiotic factors at the time of gall initiation were tested. Although resistance remained strong, significantly more leaves with surviving larvae, and galls were found on potted plants moved to the field, compared with identical plants that remained in the greenhouse. These findings suggest that plasticity in plant resistance may be linked to ambient light conditions. This system is a semi-natural one, where no intentional breeding for insect resistance has taken place. The interaction between S. viminalis and D.marginemtorquens includes responses and phenomena important for understanding plant-herbivore interactions in general. Thus, results from this research contribute to general models of plant resistance as well as to breeding programs in connection with short-rotation forestry based on willow.