The protocols presently established for optimum seed storage do not account for the chemical composition of different seed species, the physiological status of the seed, and the physical status of water within the seed. The physiological status of seeds from five species with varying chemical compositions was determined by measurements of rates of oxygen uptake and seed deterioration. The physical status of water was determined by water sorption characteristics. For each species studied, there was a specific moisture content for the onset of respiration, chemical reactions, and accelerated aging rates. The moisture contents at which these physiological levels were observed varied among the species and correlated with the lipid content of the seed. However, the changes in physiological activifies and the physical status of water occurred at specific relative humidities: 91% for the onset of respiration, 27% for the increased rates of thermal-chemical reactions, and 19% for optimum longevity. Based on these observations, we propose that equilibrating seeds between 19 and 27% relative humidity provides the optimum moisture level for maintaining seed longevity during longterm storage.mendations of the IBPGR (3). Based on limited empirical information, calculations from the viability equations, and practical considerations, the IBPGR recommends that seeds be stored at moisture contents between 3 and 7%, depending on the seed (10). For seeds with poor storage characteristics, further drying may be recommended (3, 5, 7). Unfortunately, these recommendations do not aid the gene bank operator in determining the optimal moisture content for storage of a particular seed species, cultivar, or lot.A more theoretical approach to the study of the effect of water content on seed deterioration would alleviate some of the intrinsic difficulties of a strictly empirical approach. Protocols developed from physical principles will obviate the need to determine optimal conditions for each species, varying cultivars within a species, and specific tissues within an individual seed.Intuitively While scientists have known for many years that dry, cold conditions will increase shelf life of biological material, the optimum moisture level and temperature are poorly understood. Since studies of the optimal environment for storage are intrinsically difficult because they take years, scientists have relied on experiments of seed aging under less optimal conditions and then extrapolated beyond their data. Based on these empirical data, equations have been derived to predict the rate of seed deterioration under various conditions (6,21). The assumption of these equations has been that the effect of water content on the rate ofseed deterioration is a logarithmic relationship and thus, the drier the tissue is, the longer it will maintain viability (5-7).The viability equations are the basis of the storage recom-1019 where k2 is proportional to the rate of a reaction, Z is a coefficient related to the rate of molecular collisions, Ea is the...

The premise of this paper is that the chemical potential of water strongly influences aging reactions in seeds, and that there will be an optimal chemical potential of water for seed longevity. If this is true, the optimum moisture content for storage and the optimal drying protocols should vary with the storage temperature, but should be predictable from water sorption isotherms. Isotherms for pea, soybean and peanut are given for temperatures between 65° and −150°C. Relative humidity/moisture content relationships were determined directly for temperatures between 5° and 50°C using saturated salt solutions. At extreme temperatures, isotherms were calculated from heat capacity measurements using differential scanning calorimetry or extrapolations of van't Hoff analyses. The family of isotherms was used to predict optimum moisture contents at storage temperatures between 65°C and −150°C. The optimal moisture contents for these three species are consistently shown to increase as the storage temperature is lowered.

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Differential scanning calorimetry was used to study the relationships among drying rate, desiccation sensitivity, and the properties of water in homeohydrous (recalcitrant) seeds of Landolphia kirkW. Slow drying of intact seeds to axis moisture contents of approximately 0.9 to 0.7 gram/gram caused lethal damage, whereas very rapid (flash) drying of excised embryonic axes permitted removal of water to approximately 0.3 gram/gram. The amount of nonfreezable water in embryonic axes (0.28 gram H20/ gram dry mass) did not change with drying rate and was similar to that of desiccation-tolerant seeds. These results suggest that the amount of nonfreezable water per se is not an important factor in desiccation sensitivity. However, flash drying that removed all freezable water damaged embryonic axes. Differences between desiccation-sensitive and -tolerant seeds occur at two levels: (a) tolerant seeds naturally lose freezable water, and sensitive seeds can lose this water without obvious damage only if it is removed very rapidly; (b) tolerant seeds can withstand the loss of a substantial proportion of nonfreezable water, whereas sensitive seeds are damaged if nonfreezable water is removed.In this paper, we address (a) to what extent drying rate can affect the minimum moisture content that homeohydric tissues can survive, (b) whether the rate of water removal affects the thermal properties of water remaining in the tissues, and (c) whether there is a correlation between the thermal properties of water and expression of desiccation sensitivity. If desiccation tolerance is related to the ability of tissue to lose "bound" water without the denaturation of macromolecules, there may be differences in the thermal characteristics of water in axes of homeohydrous seeds flash dried and those dried more slowly. The relationships among dehydration rate, desiccation tolerance, and the state of water were studied using embryonic axes ofthe homeohydric seeds ofLandolphia kirkii Dyer. This species is a viny shrub native to Mozambique and the inland region of northeastern South Africa. Its large (approximately 1.5 g) endospermic seeds are typical of many other seeds from tropical species in that they are intolerant to chilling and desiccation, the embryonic axis is fully developed when it sheds, and there are no tendencies toward dormancy. If maintained at their original water content, the seed will lose viability in approximately 1 month.Desiccation tolerance of organisms involves, inter alia, the ability to withstand loss of water sorbed to macromolecular structures, particularly the surfaces of membranes, without irreversible denaturation (3, 4). A current theory, the "water replacement hypothesis," suggests that, in desiccation-tolerant tissues, water closely associated with macromolecular surfaces can be replaced by polyhydroxyl compounds that stabilize the macromolecules as water is withdrawn (3, 4). This hypothesis implies that such water replacement does not occur in desiccation-sensitive tissue.Recalcitrant, or "homeohydr...