Whole plant disease severity is associated with reduced polyphenolic concentrations in lesioned but not green tissue in eelgrass Zostera marina.

Plants possess an impressive range of chemical traits to defend themselves against pathogens. Describing and interpreting these traits can provide insights into plant resistance to infection and disease progression. To examine one potentially responsive chemical trait, we measured polyphenolic content of green and lesioned tissue from Labyrinthula-infected eelgrass shoots growing in both the field and a long-term common garden. Infection of eelgrass Zostera marina with wasting disease (Labyrinthula zosterae) leads to the formation of necrotic lesions on host tissue and is known to alter host photobiology and secondary metabolite production. It is common to quantify total polyphenolics as a potential measure of eelgrass defense against infection, yet there is no clear evidence that polyphenolics are a reliable indicator of host response or immunity. We found that polyphenolic concentrations in lesion tissue fell predictably as lesion cover increased, while polyphenolic concentrations in green tissue were more variable and increased only across large ranges of lesion cover. We hypothesize that while polyphenolic induction may be a part of the host plant response to L. zosterae infection, total polyphenolic concentrations in both green and lesioned tissue may also be influenced by genetic variability and localized metabolic processes. Further research is necessary to assess a causative relationship between plant defenses and disease severity in this system and may benefit from looking beyond phenolics.