Abstract

Starch is the principal reserve of Araucaria araucana seeds, and it is hydrolysed during germination mainly by ?-amylase. There are several ?-amylase isoenzymes whose patterns change in the embryo and in the megagametophyte from the one observed in quiescent seeds (T0) to a different one observed 90 h after imbibition (T90). The objective of this research was to study the roles of two purified ?-amylase isoenzymes by in vitro digestion of starch granules extracted from the tissues at two times of imbibition: one is abundant in quiescent seeds and the other is abundant after 90 h of imbibition. The isoenzymes digested the starch granules of their own stage of germination better, since the isoenzyme T0 digested starch granules mainly from quiescent seeds, while the isoenzyme T0 digested starch mainly at 90 h of imbibition. The sizes of the starch granule and the tissue from which these granules originated make a difference to digestion by the isoenzymes. Embryonic isoenzyme T0 digested large embryonic starch granules better than small and medium-sized granules, and better than those isolated from megagametophytes. Similarly isoenzyme T0 digested small embryonic starch granules better than medium-sized and large granules, and better than those isolated from megagametophytes. However, a mixture of partially purified megagametophytic isoenzymes T0 and T90 digested the megagametophytic granules better than those isolated from embryos. Studies of in vitro sequential digestion of starch granules with these isoenzymes corroborated their specificity. The isoenzyme T90 digested starch granules previously digested by the isoenzyme T0. This suggests that in vivo these two isoenzymes may act sequentially in starch granule digestion.