Recalcitrant seeds are not only desiccation-sensitive, but also metabolically active. In contrast, orthodox seeds, owing to their dry state, are metabolically quiescent. Lowering the water content to a level that would preclude germination but facilitate vital metabolism has been suggested as a way to extend the life-span of recalcitrant seeds in hydrated storage. However, Daniel Côme and Françoise Corbineau of the Université Pierre et Marie Curie in Paris, and we, have independently shown that this practice of partial dehydration curtails the seeds' storage life-span.Current work by graduate students Déon Erdey and Sharon Eggers in our laboratory suggests that slight dehydration stimulates the onset of germinative metabolism, ther ...view middle of the document...
Seed-associated fungi are ubiquitous and pose a prodigious problem: They use seed tissues as their source of nutrition. As a result, the seeds rapidly weaken and die. With graduate student Claudia Calistru and others, we showed that if the seed's fungal load can be reduced or eliminated, then seed storage life-span can be doubled or even quadrupled, depending on the species. Although promising, even this advance is not enough for useful long-term storage of highly recalcitrant seeds.We also have been pursuing another strategy for halting germinative metabolism in order to increase storage times: deep-freezing. There has long been a consensus that achieving and maintaining the deep-frozen state by cryostorage--usually in liquid nitrogen at -196ºC--is the only solution for long-term storage of recalcitrant seeds.But how can this be achieved practically? Recalcitrant seeds--whether coconuts or the "pips" of a litchi, mango, or avocado pear--are generally large. These seeds also are "wet." Such large, hydrated living structures will not be able to withstand the effects of freezing; the ice crystals wreak lethal havoc on cell structures. The challenge for storing recalcitrant seeds, therefore, depends on both the hydration state and on seed size. The answer lies in reducing both.We had already begun to make progress in overcoming the obstacles posed by hydration. We previously had observed that the faster the seeds could be dried, the greater the degree of water loss that recalcitrant seeds would tolerate. But large recalcitrant seeds lose water only slowly even under conditions that hasten dehydration. Generally, therefore, the seed dies when its tissues are still too wet to be frozen.
