Emerald ash borer (Agrilus plannipennis) (EAB) is an invasive wood-boring beetle introduced to North America which has caused extensive ash (Fraxinus spp.) mortality. Closely related white fringetree (Chionanthus virginica) was also found to support EAB larval development, although it is a lower quality host than highly susceptible black ash (F. nigra). Mechanisms mediating white fringetree susceptibility have yet to be assessed, and the influence of environmental stress on the species’ potential defenses is unclear. However, observations suggest that host trees grown in full sun are more resistant to EAB than those grown in shaded conditions. Previous studies have indicated that the activities of well-known defense-associated enzymes within phloem tissue mediate levels of susceptibility in ash species. We quantified the constitutive and induced activities of defensive enzymes in the phloem tissue of white fringetree and black ash under differential light conditions. Sixteen individuals of each species were subjected to either normal sunlight or 80% shade, and within each light treatment eight individuals were treated with methyl jasmonate to elicit defense responses. Enzyme activities were assayed in phloem protein extracts. After sampling, all trees were inoculated with EAB eggs, and larval development rates were assessed.
Results/Conclusions
We observed significantly lower activities of defense-associated enzymes in black ash trees when compared to white fringetrees at both the constitutive and induced levels. Our results indicate that light limitation negatively impacted the tree’s ability to mount induced defenses via the enzymes measured, when compared to trees grown under normal light conditions. This study is the first to examine the potential physiological mechanisms responsible for variations in the susceptibility of white fringetree to EAB infestation. Many studies in plant physiology have assessed the relationship between light and defense mechanisms, but usually in the context of light stress caused by high levels of irradiance. Our study directly explores the potential relationship between light limitation and defense-associated enzyme activity in woody deciduous trees. In addition, as EAB continues to spread into the native range of white fringetree, the results from my study may have silvicultural applications, informing consumers if resistance to EAB may increase by planting in full sun conditions.