Changes in oligosaccharide content and antioxidant enzyme activities in developing bean seeds as related to acquisition of drying tolerance and seed quality

Seeds of bean (Phaseolos vulgaris cv. Vernel) were collected throughout the ir development on the plant and dried at 15 degreesC and 75% relative humid ity to a final moisture content of about 16% (fresh weight basis) to determ ine whether the onset of tolerance to this drying condition was related to changes in soluble sugars or the activities of the main antioxidant enzymes , namely superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase ( APX), and glutathione reductase (GR). Measurements of soluble sugars and en zyme activities were made after drying the seeds, and drying tolerance was evaluated by the ability of dried seeds to germinate and to produce normal seedlings. Seeds became tolerant to drying at 45 d after anthesis, a time m arking physiological maturity. At physiological maturity, the moisture cont ent of seeds was about 50-55% (fresh weight basis) and seed dry matter reac hed about 190 mg per seed. Seed vigour, evaluated by controlled deteriorati on and conductivity measurements, continued to increase after seed mass mat urity, but decreased when seeds remained thereafter for more than 7 d on th e plant. Acquisition of drying tolerance was coincident with an accumulatio n of raffinose and stachyose. Dried-tolerant seeds were also characterized by a high amount of sucrose, the most abundant sugar, and by a low content of monosaccharides. The (raffinose + stachyose)/sucrose ratio increased dur ing seed filling, reaching a value close to 1 when all the seeds became tol erant to drying, and maintaining this proportion during the final stages of maturation. Acquisition of drying tolerance was also related to a reorient ation of the enzymatic antioxidant defence system. Drying-tolerant dried se eds displayed high CAT and GR activities and low SOD and APX activities, wh ile the opposite condition was observed in immature dried seeds. The shift in antioxidant enzymes corresponded to the beginning of the maturation-dryi ng phase. These results suggest that oligosaccharide metabolism and enzymat ic antioxidant defences may be involved in acquisition of drying tolerance during bean seed development, but are not related to seed vigour.