Ferns living on the edge: Ecophysiology of ferns under severe water deficit

Ferns are the second most diverse lineage of vascular plants, yet we know little about how ferns have adapted to survive in habitats with limited fresh water. In this dissertation, I examined two groups of ferns that thrive in freshwater-limited ecosystems: the salt-tolerant mangrove ferns and the desiccation-tolerant “resurrection” ferns. In Chapter 1, I characterized adaptations to high salinity and high evaporative demand in the mangrove fern Acrostichum speciosum along salinity and rainfall gradients in Australia. I found that mangrove ferns exhibit the same key adaptations as salt-tolerant angiosperms: excluding the majority of salt at the roots and osmotically adjusting leaves to maintain turgor. Furthermore, the mangrove ferns showed differing responses to high salinity and high evaporative demand; only high salinity decreased water potential and rates of gas exchange. In Chapters 2 and 3, I characterized mechanisms of recovery in resurrection ferns, which completely desiccate (< -100 MPa) and revive when rehydrated. In Chapter 2, I experimentally determined that root pressure plays a critical role in restoring hydraulic flow through the vascular system during resurrection. High concentrations of sucrose in desiccated rhizomes may contribute to developing this root pressure. In Chapter 3, I discovered that vascular tissues experience reversible desiccation patterns using high resolution micro-computed tomography (microCT). Auto-fluorescence revealed that the chemical composition of these tissues likely facilitates reversible desiccation in the vascular system. Although mangrove and resurrection ferns are distinct in many ways, both rely on osmotic adjustment to survive periods of limited freshwater availability