Long-term survival of cryopreserved germplasm: contributing factors and assessments from thirty year old experiments.

摘要

Cryobiologists assume that the extreme low temperatures of liquid nitrogen stop chemical and physical reactions that lead to sample aging and loss of viability. This assumption, based on extrapolations of temperature-reaction kinetic relationships, is not completely supported by accumulating evidence that dried seeds can deteriorate during cryogenic storage. After 30 years of cryogenic storage, seeds of some species exhibited quantitatively lower viability and vigor. Loss of viability during storage reflects molecular mobility within the system - in other words, relaxation of glassy matrices. Stability of biological glasses is not currently understood. We present a conceptual model to explain mobility within glasses and how it can differ depending among species and tissue types based on developmental programs during embryogenesis or acclimation and additions of exogenous cryoprotectants. Hence, the same thermodynamic models developed using a seed system may be applicable to a wide variety of germplasm and may provide a priori estimates of achievable longevity. Testing this hypothesis is difficult because of the long time frame needed to validate changes in viability during cryogenic storage; however, long term experiments are becoming increasingly available. For example, cryogenic storage of dormant buds is a highly efficient way to back up orchard collections and some buds have been cryogenically stored at NCGRP for 15-20 years allowing us to investigate effects of weather patterns and harvest dates. In this paper, we explore the thermodynamic principles that contribute to temperature dependency of glassy relaxation as the context for understanding potential changes in viability of cryogenically stored germplasm.