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Preserving valuable seed germplasm in liquid nitrogen (LN2) has the potential of extending the storage life of the sample almost indefinitely. However, during the cooling and rewarming process, temperature related stresses occur that can be lethal to many biological materials, including seeds. The capacity of seeds to withstand LN2 cooling is critical to long‐term cryopreservation of these germplasm samples. Seeds from six sesame (Sesamum indicum L.) selections were used to develop procedures to overcome cryogenic damage during the LN2 cooling/rewarming process. Seed moisture content and cooling rate were identified as controlling factors in the survival of sesame seeds exposed to LN2. As seed moisture content increased from 60 to 120 g kg−1 and cooling rate increased from 10 to 200°C min−1, mortality increased. Regardless of cooling rate, all seeds of all selections were killed at moisture content levels above 120 g kg−1. At seed moisture contents below 60 g kg−1 and cooling rates of 1, 10, and 30°C min−1, all seeds of all sesame selections tested survived. At a 200°C min−2 cooling rate and moisture contents below 60 g kg−1, four of the six selections were damaged. Results from this study suggest that sesame seeds can tolerate LN2 exposure if the seed moisture content is below 60 g kg−1 and the cooling rate is between 1 and 30°C min−1.

Changes in electrolyte leakage and viablity in response to dehydration stress were examined in two species of seeds that do not survive desiccation. Leakage from silver maple (Acer saccharium L.) seeds increased markedly as seed moisture contents decreased from 45 to 35% (fresh weight basis) and germtion decreased from 97 to 5%, coincidentally. rume course curves of _mbibional leakae from areca palm (Chasldocarpus bdeswens IBoryl WendL) embryos showed an increase in both intial leakage and steady-state leakage rates in response to dehydration from an original moisture content of 84 to as low as 53%. Absorbance at 530 nanometers ofextracts from triphenyl tetrazolium chloride stained embryos of areca palm was used as a measure of vability. Absorbance decreased significantly in response to dehydration as embryo moisture content decreased from 80 to 30%. Collectively, the data suggest that membranes in the desiccation-sensitive seed tissues studied are damaged by dehydration below a critical moisture content, 40% in silver maple seed and 55% in areca palm embryos, and that the m ane damage contributes to loss of viability.Seeds of many tropical and some temperate plant species do not survive dehydration (2, 6). The seeds of silver maple are an example (8). It is known that germination of desiccation-sensitive seeds declines rapidly as seed moisture content is decreased, but little is known about the mechanism or events leading to this dehydration-induced loss of viability. Studies on the mechanism of aging and deterioration in desiccation-tolerant seeds show that an increase in electrolyte leakage is associated with a loss of germination capacity (3, 15). Because an increase in electrolyte leakage reflects an increase in membrane permeability to solutes, deterioration of cellular membranes has been implicated as a causal mechanism of deterioration and senescence in seeds (15,20).Time course curves of leakage from dehydrated seeds show initial rapid leakage rates associated with the imbibition of water (14,17). It has been proposed that this phase of rapid leakage represents a period of membrane reorganization associated with rehydration, and that subsequent linear leakage rates represent steady-state diffusion of solutes through organized membranes (16). It has also been suggested that water uptake by desiccationtolerant seeds reinstates the original structure of the cellular membranes, whereas the membranes ofdesiccation-sensitive seeds that have been dehydrated are unable to reform completely (12). If dehydration stress disrupts membrane integrity in desiccationsensitive seeds, then changes in leakage rates and increases in the amount of solutes leaked may be detectable in response to dehydration, and these changes should be associated with loss of viability. Our results show that increased imbibitional leakage is associated with dehydration stress in two species of desiccationsensitive seeds and that there is a close correlation between increased leakage and loss of viability. MATERIALS AND METHODSSeed mo...

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