Application of salicylic acid induces antioxidant defense responses in the phloem of Picea abies and inhibits colonization by Ips typographus

Abstract

The adaptive plasticity of Norway spruce (Picea abies) against attack by Ips typographus depends on systemic acquired resistance which involves salicylic acid (SA), and an antioxidant system both recognized as valuable stress markers in ecophysiological studies. In the presented field experiment, 100mM SA was applied to the bark sections of Norway spruce prior to being attacked by bark beetles, in order to study interactions with antioxidants and its significance for mediating stress-tolerance under natural conditions. SA-treatments significantly elevated the total SA levels over the whole sampling period. Total glutathione (tGSH) and total cysteine (tCys) increased by 167% and 80%, respectively, two weeks after treatment, in comparison with controls. In contrast, SA-treatment caused an initial deterioration in total ascorbic acid (tASC) and enhanced the percentage of dehydroascorbic acid (DHA), but activated tASC levels over later sampling dates. The initial bark beetle attack was characterized by a significant decline in total SA levels, which was accompanied by a transient degradation and oxidation of their ascorbate-glutathione system. This initial reaction was significantly alleviated by SA-application and characterized by 175% higher tGSH contents, when compared to moderately-affected untreated trees. One month after pheromone dispensers were placed on trees, an intensification of ascorbate-glutathione system occurred within moderately-affected bark, but to a greater extent after SA-treatment. Total SA levels within SA-treated moderately-affected trees remained at the control level until June. In contrast, strong attack was characterized by a successive increase in total SA up to 252% following SA-treatment in June, whereas a 110% increase of SA was determined within severely affected control-bark. A strong attack was further characterized by a degradation of tGSH and total phenolics (tPH), a moderate increase in tASC and an oxidation of the ascorbate-glutathione pool within untreated bark. In the SA-treated trees the redox state was unaffected by severe colonization and the degradation of antioxidants was significantly alleviated. In addition, SA-treated bark had significantly less entrance holes and exhibited fewer and shorter maternal galleries than control-bark. From this perspective, exogenous SA was successfully implicated as an activator of systemic acquired resistance in Norway spruce, providing tolerance against the complex interactive effects of bark beetle attack and environmental factors.