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Title: Variation of desiccation tolerance and longevity in fern spores

Author
item BALLESTEROS, DANIEL - Royal Botanical Gardens
item Hill, Lisa
item Walters, Christina

Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2017
Publication Date: 4/30/2017
Citation: Ballesteros, D., Hill, L.M., Walters, C.T. 2017. Variation of desiccation tolerance and longevity in fern spores. Journal of Plant Physiology. 211:53-62 doi.org/10.1016/j.jplph.2017.01.003.

Interpretive Summary: Optimizing moisture conditions during storage is critical for maximizing shelf life. In this paper, we draw upon principles developed by the food and pharmaceutical industries to predict best practices for stored germplasm. We use fern spores as a model system because they are unicellular, thereby avoiding complications associated with structure and diffusion in multicellular tissues. Our work reveals that preserved cells are solids and follow biophysical principles of structure and motion typical of other solids, such as plastics. This is quite a departure from fluid-based systems that most biologists study. Longevity of fern spores vary widely among species, but principles determining the optimum moisture are fairly similar among species. When fern spores are dried below their optimum water content, they age rapidly.

Technical Abstract: This work contributes to the understanding of plant cell responses to water stress when 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 15% RH and mortality rates increased sharply above and below this moisture level. A RH of 15% 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.