Basyal, B. 2020. Aboveground-belowground interactions: roles of soil biotic and abiotic factors on switchgrass's (Panicum virgatum) growth, defense against herbivory and cell wall chemistry. Dissertation, University of Louisville, Louisville, KY.

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Plants constantly interact with their biotic and abiotic soil environments. Most terrestrial plants form beneficial associations with soil microbes such as arbuscular mycorrhizal (AM) fungi, which are widely known for their ability to transfer soil phosphorus and nitrogen to the host plants. They help plants tolerate drought stress and improve plant defense against herbivores such as plant-parasitic nematodes (PPNs).
This dissertation investigates the role of AM fungi on switchgrass’s (Panicum virgatum) growth, cell wall chemistry, and defense against PPNs using a combination of growth chamber and field studies. Switchgrass is a native warm-season species which is gaining traction as candidate lignocellulosic biofuel crop. It forms a tight association with AM fungi, and PPNs like Pratylenchus penetrans can become a potential threat when the plant is grown in monocultures. My first study was a growth chamber experiment where I manipulated the absence and presence of an AM fungal species to examine its effects on switchgrass’s growth and defense against Pratylenchus penetrans under drought conditions. I found that AM fungi increased root biomass under drought conditions and reduced the abundance of the PPN in plant roots by about 66%. I followed this study with a field experiment where I manipulated soil fungi and nematodes by applying biocides. I found that the application of biocides resulted in an altered monomeric composition of lignin in switchgrass. My final study was a growth chamber study where I looked at the effects of AM fungi on switchgrass along a drought intensity gradient. Overall, I found that AM fungi provided maximum benefit to the plants in extreme drought and the benefit declined with increasing moisture indicating that the functioning of AM fungi can span a mutualism-parasitism continuum. Mycorrhizal plants also had increased root biomass, cellulose, and hemicellulose regardless of drought treatment.
These studies provide evidence that AM fungi can have wide ranging effects on switchgrass growth and physiology and these effects can sometimes be altered by the soil abiotic environments. Overall, my results suggest that plant-AM fungal symbiosis can become ever more important in the face of climate change.

Associated Treatment Areas:

Cellulosic Biofuel Experiment

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