CHANGES IN GLASS-TRANSITION TEMPERATURES IN GERMINATING PEA-SEEDS

An element of storage stability of many orthodox seeds is that the embryo cytoplasm is vitreous at normal storage temperatures. That is, as drying proceeds during seed maturation, intracellular solutions become so concentrated and viscous that diffusional movement is all but eliminated (Williams and Leopold, 1989). In this report the relationship between desiccation tolerance and the ability to form the vitreous state as germination proceeds is examined. As pea seeds (Pisum sativum cv. Alaska) imbibed water up to 18 h, no change in the glass transition temperature was seen, and the embryos remained desiccation-tolerant. Between 18 and 44 h of imbibition, the axes lost the ability to exhibit a distinguishable vitreous transition, and the embryos had lost the ability to survive desiccation. After about 50 h, embryos again showed vitrification but only at markedly lower temperatures, as would be expected to accompany the loss of oligosaccharides (sucrose, raffinose, stachyose and verbascose) and their replacement by monosaccharides during early germination (Koster and Leopold, 1988). Thus, the loss of desiccation tolerance during germination in pea seeds appears to be associated with a loss of the high temperature oligosaccharide:water glass and a subsequent appearance of a new glass transition at a lower temperature resulting from the accumulation of monosacchrides.