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 pea (Pisum sativum L., cv Baccara) seeds and after artificial drying at 25 degreesC and various relative humidities (20, 75 and 99% RH) to investigate whether the occurrence of desiccation tolerance was related to sucrose, raffinose and stachyose contents. Seeds became completely tolerant to fast drying at 25 degreesC and 20% RH a few days after the end of reserve accumulation, i.e. when their moisture content dropped to approx. 50% (fresh weight basis). This acquisition of desiccation tolerance was associated with an accumulation of raffinose and stachyose, the latter being more abundant in the embryonic axis than in the cotyledons. The (raffinose+stachyose)/sucrose ratio 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 was 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 deterioration of cell membrane properties and lipid peroxidation. In contrast, immature 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 oligosaccharide content and a (raffinose+stachyose)/sucrose ratio higher than 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, whereas 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 oligosaccharides may well be involved in the protection of membranes during dehydration.