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Biological invasions adversely affect and disrupt natural ecosystems at great economic costs. The vast body of theory and research focus on which factors advance these invasions and is geared toward understanding, prevention, and management of non-native species. Roots of grasses in the genus Bromus constitutively possess high levels of the enzyme polyphenol oxidase (PPO), a catalyst for the oxidation of phenolics into visible melanin-like compounds. Phenolic substrates for PPO are plant-produced secondary metabolites with phytotoxic allelopathic properties. Through the conversion of these harmful phenolics by PPO, we hypothesized PPO may be used as a defense mechanism against phenolic-allelopathic plants and thereby contribute to the competitive success of Bromus species, many of which are non-native invaders. To test these hypotheses, we first assayed a wide range of Poaceae (grass) species for root PPO activity with a focus on bromes. Results showed significantly higher PPO levels in invasives than non-invasives, suggesting the ability to produce high root PPO concentrations is a trait contributing to invasion potential of non-native species, an important corollary that may be a useful tool for identifying future invasives. Second, through phylogenetic reconstructions, phenetic PPO was phylogenetically tractable and was only present in two taxonomically distinct genera, hinting at a high-PPO ancestral condition, later lost by some genera. Third, we examined effects of allelopathic competitor species on PPO and non-PPO-producing grasses in direct competition and exposed to leachate and litter; experiments supported our hypothesis as (a) PPO-producer Bromus tolerated allelopathic phenolic Centaurea, (b) but non-PPO Festuca was suppressed, and (c) non-phenolic allelopathic Artemisia suppressed both PPO-Bromus and non-PPO-Festuca. Fourth, field surveys showed allelopathic plants further distances from Bromus than non-allelopathic plants. Finally, we exposed a range of grass species of variable PPO activity to the phenolic-allelochemical caffeic acid (CA). PPO was constitutively expressed, but the utility was weakly observed, possibly due to sub-toxic doses. Overall, we illustrate PPO as a novel defense against phenolic-allelochemicals and as a trait correlated to invasiveness, and highlight ongoing taxonomic classifications that may shed light on evolutionary understanding of selection benefits of PPO and grass evolution, which are agriculturally, economically, and environmentally important.

ETD_6504

Ph.D.

Includes bibliographical references

Includes vita

by Kimberly L. Plank

doi:10.7282/T32809GG

ETD doctoral

The author owns the copyright to this work.