Effects of dehydration conditions on desiccation tolerance of developing pea seeds as related to oligosaccharide content and cell membrane properties

Germination and carbohydrate metabolism were studied in fresh developing pe a (Pisum sativum L., cv Baccara) seeds and after artificial drying at 25 de greesC and various relative humidities (20, 75 and 99% RH) to investigate w hether the occurrence of desiccation tolerance was related to sucrose, raff inose and stachyose contents. Seeds became completely tolerant to fast dryi ng at 25 degreesC and 20% RH a few days after the end of reserve accumulati on, i.e. when their moisture content dropped to approx. 50% (fresh weight b asis). This acquisition of desiccation tolerance was associated with an acc umulation of raffinose and stachyose, the latter being more abundant in the embryonic axis than in the cotyledons. The (raffinose+stachyose)/sucrose r atio increased during seed development and reached 1.1 in the axis and 0.2 in the cotyledons just before the onset of desiccation tolerance. When the natural acquisition of desiccation tolerance occurred on the mother plant, artificial drying of isolated seeds induced an increase in oligosaccharide content in the cotyledons. Immature seeds, the moisture content of which wa s higher than about 60% (fresh weight basis), did not tolerate fast drying (25 degreesC and 20 or 75% RH). Such drying did not result in the synthesis of stachyose and induced an increase in electrolyte leakage, a decrease in the ability of seeds to convert I-aminocyclopropane l-carboxylic acid (ACC ) to ethylene and an increase in ethane synthesis, thus indicating a deteri oration of cell membrane properties and lipid peroxidation. In contrast, im mature seeds tolerated drying either in the pods or at 25 degreesC and 99% RH, and such drying induced a decrease in sucrose content, an increase in o ligosaccharide content and a (raffinose+stachyose)/sucrose ratio higher tha n around 1. Soluble sugar contents of dried immature seeds depended on the conditions of dehydration. In cotyledons, the (raffinose+stachyose)/sucrose ratio reached 0.61 when seeds were dried at 25 degreesC and 99% RH, wherea s it was as low as 0.15 when drying was performed at 25 degreesC and 20% RH . All the results obtained are consistent with the concept that oligosaccha rides may well be involved in the protection of membranes during dehydratio n.