Variation of desiccation tolerance and longevity in fern spores

This work contributes to the understanding of plant cell responses to extreme water stress when it is applied at different intensity and duration. Fern spores are used to explore survival at relative humidity (RH)<85% because their unicellular nature eliminates complexities that may arise in multicellular organisms from slower drying and variable responses of different cell types. Fern spore cytoplasm solidifies between 30 and 60% RH and spores survive this transition, but subsequently lose viability. We characterized the kinetics of viability loss in terms of the fluid to solid transition using concepts of water activity (i.e., sorption) and glass transition (Tg), two concepts that dominate studies of food and pharmaceutical stability. For all fern species studied, longest survival times were observed in spores placed at about 10-25% RH and mortality rates increased sharply above and below this moisture level. A RH of 10-25% corresponds well to sorption behavior parameters and is below the glass transition, measured using differential scanning calorimetry. Though response to RH was similar among species, the kinetics of deterioration varied considerably among species and this implies differences in the structure or mobility of molecules within the solidified cytoplasm. Our work suggests that desiccation damage occurs in desiccation tolerant cells, and that it is expressed as a time-dependent response, otherwise known as aging.