The possible ecological role of the root enzyme polyphenol oxidase in the invasive plant genus Bromus
In plant community ecology, one of the principal questions is what enables species to be invasive. The novel weapons hypothesis states that some plant species produce phytotoxic allelochemicals unknown to the invaded environment, thereby allowing them to successfully establish. Phenolics are one type of notoriously allelopathic chemicals, which are plant-produced secondary metabolites with phytotoxic effects, entering the environment through leachate or litter. In the phenolic conformation, these allelochemical compounds can serve as substrates for the enzyme polyphenol oxidase (PPO), which is known to be exuded by roots of the grass genus Bromus but not by other grass genera. As seedlings of the highly invasive plant genus Bromus constitutively possess high levels of PPO, I hypothesized PPO may be used as a defense against allelopathic plants; non-PPO plants were predicted to be suppressed by allelopathic plants. Methodology includes: (1) greenhouse experiments to understand responses of PPO/non-PPO-producing grasses to phenolic/non-phenolic-allelopathic plants in (a) competition, (b) decomposing litter, and (c) leachate experiments and (2) field surveys to examine relationships between phenolic-allelopthic Centaurea stoebe and non-phenolic allelopathic Artemisia vulgarisand PPO/non-PPO-producing grasses in Northern New Jersey.
Greenhouse competition results supported the hypothesis as (a) PPO-plants responded to allelopathic Centaurea stoebe by producing greater biomass than controls and (b) both PPO/non-PPO grasses were suppressed by allelopathic Artemisia vulgaris. When grown with decomposing litter of either C. stoebe or A. vulgaris both PPO/non-PPO-producers produced greater biomass, suggestive of an overall net benefit of growing with added carbon that possibly overwhelmed potential allelopathic effects. In the leachate experiment we found that A. vulgaris leachate was as expected significantly allelopathic to both PPO-producer Bromus inermis and non-PPO Festuca rubra. Also as predicted, B. inermis tolerated leachate of C. stoebe whereas F. rubra was significantly suppressed. Results of field surveys indicate that Bromus tectorum occurs at larger distances from A. vulgaris compared to C. stoebe (phenolic-alleopathic) and other non-allelopathic species. In conclusion, our experiments and field survey lend support to the hypothesis that the unique presence of root PPO in the genus Bromus enables these grasses to cope with allelopathic competitors that use phenolics as weapons. This PPO enzyme therefore might act as a novel defense in invaded ranges.