On the phytoscreening potential of insect-induced plant galls

PURPOSE: Detecting belowground chemical contamination is a challenging environmental problem due to subsurface heterogeneity and limited monitoring capabilities associated with labor-intensive, intrusive, and costly sampling techniques. With their extensive root systems, vascular plants are an easily accessible aboveground link to the subsurface. Therefore, plant tissues, particularly tree cores, are used in phytoscreening applications for detecting belowground chemical contaminants. While phytoscreening has been evolving as an alternative to traditional sampling, this method has caveats: (a) easier to harvest tissues such as leaves and twigs tend to exhibit lower concentrations than tree cores; (b) coring can negatively influence tree health; and (c) trees may be unavailable for sampling at all sites. Therefore, less invasive techniques that incorporate plants other than trees could be more effective phytoscreeners. METHODS: Here, we explore the use of insect-induced plant galls–that can be found on many vascular plant species–for their phytoscreening capabilities. Driven by observations that they can act as sinks for extraordinary high concentrations of nutrients during development, we tested the following hypothesis: galls will accumulate belowground chemical contaminants in higher concentrations compared to other aboveground plant tissues. Specifically, we measured and compared the concentrations of two contaminant types, inorganic heavy metals (HMs) and a volatile organic compound (VOC) and known human carcinogen, 1,4-dioxane, in four aboveground plant tissue types (leaf, twig, core, fruit) and insect-induced plant gall, across three plant types (shrub, tree, vine) sampled from contaminated areas. RESULTS: Twelve different HMs were present in shrubs, and 1,4–dioxane was present in both trees and vines; however, the concentrations of each contaminants varied quantitatively across plant tissue types. While galls accumulated the lowest concentrations of HMs, they accumulated the highest concentration of 1,4-dioxane, at five-times that of tree cores. CONCLUSION: Given they can be found on many vascular plant species and are easy to collect when they are locally abundant, with further development, galls may be a powerful alternative for detecting belowground chemical contamination, particularly for VOCs in urban areas. We discuss the dichotomy in the ability of galls to detect inorganic HMs versus VOCs, and outline future questions that should be considered to further develop galls for phytoscreening application.