The Role of Mycorrhizal Associations in Modulating Poplar Growth, Phytohormone Responses, and Mortality under Elevated CO2 and Temperature Conditions

Joshua JR Frank-Webb, Western University

Abstract

Climate change poses significant challenges to forests worldwide, particularly the Canadian boreal forest. Populus spp. are ecologically and economically important tree species that have had declining growth and survival due to elevated temperatures and droughts associated with climate change. Symbiotic microbes, such as mycorrhizal fungi, may increase plant growth under climate change conditions by altering tree metabolic profiles and increasing tree access to water and nutrients. My thesis explores the relationship between mycorrhizal fungi and a Populus hybrid (Populus x canadensis) grown under a range of future climate scenarios: ambient CO₂ (400 ppm) or elevated CO₂ (750 ppm) with either ambient temperatures or a +4 ˚C or +8˚C warming treatment. My primary objective is to assess how mycorrhizae influence growth, stress phytohormone concentrations, and stress tolerance of hybrid poplar under predicted future climatic scenarios. Additionally, I identify compounds exuded by mycorrhizal fungi and evaluate their potential to enhance plant growth. My findings reveal that different boreal mycorrhizal fungi produce similar profiles of phytohormones, amino acids, and organic acids in their exudates. The exudates of some mycorrhizal fungi enhanced plant growth, while others caused mortality. In my hybrid poplar growth study, the colonization of poplar roots by mycorrhizal fungi increased with elevated temperature and CO₂. Inoculation with mycorrhizal fungi did not increase tree height or mass, with the exception of trees grown under +4 ˚C warming, where total biomass increased by ~15% compared to control trees. Unexpectedly, inoculation with mycorrhizal fungi almost always increased hybrid poplar mortality. To understand why mycorrhizal inoculation increased mortality but improved the growth of surviving hybrid poplars, I investigated the impacts of climate change and mycorrhizal inoculation on plant stress hormone concentrations. Mycorrhizal inoculation generally increased leaf concentrations of the stress hormone jasmonic acid, while the stress hormones salicylic acid and abscisic acid had reduced leaf concentrations across elevated temperature and CO₂ treatments. Overall, my research contributes valuable insights into the intricate connections between mycorrhizal fungi, trees and climate change, offering a better understanding of forest ecosystem resilience in the face of environmental challenges.