Submitted to: Cryobiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/6/2004
Publication Date: 4/30/2004
Citation: Walters C., L.J. Wheeler and P.C. Stanwood. 2004. Longevity of cryogenically-stored seeds. Cryobiology 48:229-244. Interpretive Summary: We know that germplasm will survive for a long time in cryogenic storage, but we do not know exactly how long. Estimates of the actual time scale for shelf life of cryogenically stored material are critical for efficient genebanking, but are difficult because of instrument limitations or the extended times required for measurements. Here we use germination results from seeds placed in storage about 25 years ago to estimate how temperature controls the kinetics of seed aging reactions. We show that low temperature storage may extend shelf-life for hundreds to thousands of years, but that there is wide variation among species and accessions of a species that preclude confident predictions. We also show that once the aging process begins, low temperature storage is increasingly ineffective at slowing deterioration.
Technical Abstract: Though cryogenic storage is presumed to provide nearly infinite longevity to cells, the actual shelf life achieved under ultra-cold temperatures has not been addressed theoretically or empirically. Here, we report measurable changes in germination of dried seeds stored under liquid nitrogen conditions for > 10 years. There was considerable variability in the extent of deterioration among species and accessions within a species. Aging time courses for lettuce seeds stored at temperatures between 50 and -196 degrees C were fit to a form of the Avrami Equation to determine rate coefficients and predict half-life of accessions. A reduction in the temperature dependency of aging rate, determined as a break in the Arrhenius plot, occurred at about -15 degrees C, and this resulted in faster deterioration than anticipated from extrapolation of kinetics measured at higher temperatures. The break in Arrhenius behavior occurred at temperatures in between the glass transition temperature and the Kauzmann temperature and also coincided with a major triacylglycerol phase change. In spite of the faster than anticipated deterioration, cryogenic storage clearly prolonged shelf life of lettuce seeds with half lives projected as ~450 and ~2600 years for fresh lettuce seeds stored in the vapor and liquid phases of liquid nitrogen, respectively. The benefit of low temperature storage (-18 degrees C or -135 degrees C) on seed longevity was progressively lost if seeds were first stored at 5 degrees C. Collectively, these results demonstrate that lowering storage temperature progressively increases longevity of seeds. However, cryogenic temperatures were not sufficient to stop deterioration, especially if initial stages of aging were allowed to progress at higher storage temperatures. This work contributes to reliable assessments of the potential benefit and cost of different genebanking strategies.