WALTERS, C.*; HILL, L.M.; WHEELER, L.M.; USDA-ARS; USDA-ARS; USDA-ARS: Dying while Dry: Kinetics and Mechanisms of Deterioration in Desiccated Organisms

Persistence of anhydrous organisms in nature is dependent on how long they remain viable in dry environments. Longevity is determined by interactions of humidity, temperature, and unknown cellular factors that affect the propensity for damaging reactions. Here we describe our research to elucidate those cellular factors and to ultimately predict how long a population can survive under extreme conditions. Loss of viability typically follows a sigmoidal pattern, where a period of small changes precedes a cataclysmic decline. The time for viability to decrease to 50% ranged among 10 phylogenetically diverse species from about a week for spores of Serratia marcescens stored at ambient conditions to several years for fronds of Selaginella lepidophylla in the same environment. Most of the species studied survived longest at low humidity, but suffered under complete dryness. Aging kinetics at relative humidity > 75% varied among species. Temperature dependencies of aging kinetics appear similar among diverse organisms despite the disparate longevities. The effect of temperature on seed aging rates was consistent with the temperature dependency of molecular mobility of aqueous glasses, with both showing a reduction by several orders of magnitude when seeds were cooled from 60C to 0C. However, the final stages of deterioration (marked by the cataclysmic loss in viability) did not appear to be strongly dependent on temperature. Evidence for this comes from long-term seed storage studies where even cryogenic conditions do not halt the progress of aging once it has started. Longevity is an inherited trait in seeds, but its complex expression pattern among clades in Angiospermae suggests that it developed through multiple pathways.