Ecophysiological responses of lichens to environmental stressors: assessing the role of stress-tolerant mechanisms in sun and shade adapted species of Afromontane lichens.

Abstract

Lichens are a relatively understudied yet significant component of South African ecosystems. They are capable of inhabiting a wide range of habitats and in these habitats are susceptible to abiotic stressors. Amongst these, drought, temperature fluctuations, and intense light exposure pose significant threats, primarily due to accumulation of reactive oxygen species (ROS), which can be lethal to lichens. Remarkably, lichens have evolved various mechanisms to counteract such stresses and mitigate the impact of ROS. These mechanisms involve either avoidance through the accumulation of secondary metabolites and non-photochemical quenching (NPQ), scavenging by antioxidants, or repairing the damage inflicted by ROS through the PSII repair cycle. However, our understanding of how these mechanisms contribute to stress tolerance in lichens remains limited. Hence, this research aims to investigate and understand stress tolerance mechanisms in lichens in response to common abiotic stresses. Furthermore, a secondary aim is to identify mechanisms in lichens that could improve stress tolerance in crops. In particular, phenotypic plasticity was assessed by comparing the roles of NPQ, melanin and glutathione in stress tolerance among lichens from shaded and exposed habitats. The findings reveal that pre-treating lichens such as Crocodia aurata under moderate light conditions increases tolerance to photoinhibition by enhancing NPQ. Comparison between sun and shade lichens revealed that rapid NPQ relaxation in shade forms optimizes light use efficiency; the higher NPQ levels in shade lichens more likely enables them to cope with rapid light changes. Interestingly, melanised thalli from slightly exposed habitats display NPQ patterns resembling shade forms. Additionally, comparison of glutathione (GSH) accumulation in melanized and pale thalli of Lobaria pulmoria during drying and wetting cycles demonstrates that melanized thalli are less affected by oxidative stress, necessitating lower levels of GSH to scavenge ROS. Overall, this research underlines the adaptability of lichen photobionts to modulate NPQ and electron transport rates (rETR) under varying light conditions, ensuring efficient photosynthesis. Moreover, melanins play a crucial role in lichen tolerance to various stresses beyond protecting from harmful UV-B and PAR radiation. Therefore, this thesis contributes to our understanding of the impacts of climate change induced abiotic stressors and may potentially aid in enhancing stress tolerance in crop species facing the challenges of global warming.