Carbohydrate composition during seed development and germination in two sub-tropical rainforest tree species (Erythrina caffra and Guilfoylia monostylis)

The low temperature limits to germination (Tb) and carbohydrate composition have been investigated in seeds of two sub-tropical rainforest tree species, Erythrina caffra (with orthodox viability characteristics) and Guilfoylia monostylis (with recalcitrant viability characteristics). Sub-populations of E. caffra seeds demonstrated a requirement for a common base temperature (8.5 °C), contrasting those of G. monostylis that required different base temperatures (6-10 °C). Chilling (at 6-7 °C) of rehydrated dry seeds of E. caffra induced chilling stress (loss of germinability) within 8 hours, unlike seeds of G. monostylis and physiologically mature (but without maturation drying) seeds of E. caffra which tolerated chilling for several weeks. At physiological maturity, seeds of G. monostylis displayed more constituent sugars than those of E. caffra. Maturation drying in seeds of E. caffra was associated with a sharp increase (about 200 % or higher) in starch, disaccharide and oligosaccharide contents. This increase was coincident with a marked decline (over 90 %) or loss of monosaccharides. There was a reduction (generally less than 50 %) in the content of all sugars at maturity in seeds of G. monostylis. In seeds of both species, the time of highest inositol concentration was at physiological maturity when moisture content was about 60-65 % (fresh weight basis). Alpha-amylolytic activity was extremely low or absent during seed development in E. caffra, unlike G. monostylis that had relatively high α-amylolytic activity and monosaccharides content. Seeds of both species exhibited similar sucrose and monosaccharide patterns during germination sensu stricto (when levels decreased) and following radicle emergence (when levels increased). The nature and content of sugars in seeds of these species at physiological maturity might be associated with protection against the effects of chilling. Also, different di-/oligosaccharide accumulation patterns shown by seeds of both species with desiccation appeared consistent with the need for specific protective sugars within seed tissues to ensure desiccation tolerance.