Effects of glucosinolates and their hydrolytic products on culturable soil biota in and around garlic mustard patches

This research focuses on the ecological aspect of garlic mustard (Scientific name: Alliaria petiolata [Bieb.] Cavara & Grande), a cool season exotic biennial herb belonging to the family Brassicaceae. Since its introduction into the United States from its native Europe in mid 1800's this plant has aggressively invaded many hardwood forests of US and Canada. This plant is of particular concern because it can not only invade mature second growth forests but also has the potential to displace native vegetation in these invaded areas. Invasive plants like garlic mustard can have major direct or indirect effects on the belowground soil microbial processes which in turn can alter ecosystem properties like nutrient cycling, soil fertility, demography, and community composition and function. Glucosinolates, a major group of plant secondary metabolites found in all the members of Brassicaceae have been shown to have a major role in defining the invasion potential of garlic mustard. In the present study, I examined the effects of glucosinolates and their hydrolytic products on the density and distribution patterns of culturable soil bacteria and fungi in and around garlic mustard patches. Soils were sampled in fall-2010 and summer-2011 at set distances from garlic mustard patches that infested three replicate pine plantation stands at the Sand Ridge State Forest, IL. The densities of culturable aerobic bacteria and filamentous fungi in both the seasons were estimated by dilution plate count methods on three different media. Frequently occurring colonies of bacteria and fungi on the media plates were identified by various biochemical tests to their genus levels. A select group of these genera were then tested for their sensitivities towards crude extracts and synthetic forms of glucosinolates and their hydrolytic products. The results suggested that although the densities of culturable microbes varied significantly with change in seasons, they were not affected by their distance from the garlic mustard patches. A comparative analysis of the occurrence of the isolated microbial species with respect to their distance from garlic mustard patches showed that members of certain genera including Corynebacterium, Aureobacterium, Paenibacillus, Acinetobacter and Paecilomyces occurred at specific distances from the garlic mustard patches in both the seasons while generalist groups like Bacillus, Pseudomonas, Penicillium and Aspergillus were present at all distances from garlic mustard suggesting a pattern in distribution. However, data from the sensitivity assays showed the presence of both sensitive and resistant bacteria in the immediate vicinity of the garlic mustard plants suggesting that the pattern in distribution was probably not influenced by glucosinolates and/or their hydrolytic products and more complex interactions may be occurring which warrants future research.