Roles of abscisic acid and gibberellins in maintaining primary and secondary dormancy of Korean pine seeds

Primary dormancy of seeds of Korean pine (Pinus koraiensis Sieb. et Zucc.) after dispersal in the autumn and the induction of secondary dormancy the first summer following seed dispersal limit the regeneration of mixed broadleaved Korean pine forests in Northeast China. This study was to determine how changes in the levels of abscisic acid (ABA) and gibberellic acid (GA) maintain primary and secondary dormancy of Korean pine seeds under germination conditions. We transferred seeds with one of five primary dormancy states or three secondary dormancy states to germination conditions and measured changes in the levels of ABA, GA1+3 (GA1 and GA3) and GA4+7 (GA4 and GA7) in the seed coat, megagametophyte and embryo during incubation. Seed coat ABA levels in primary dormant seeds (PDS) and ABA levels in various parts of secondary dormant seeds (SDS) gradually declined during incubation but were still higher than in seeds for which dormancy was progressively released. GA4+7 and GA1+3 levels in embryos greatly decreased 35% and 24%, respectively, during incubation of SDS, and thus, the ratio of ABA to GA4+7 in embryos and megagametophytes significantly increased. The ratio of ABA to GA1+3 in various parts of SDS increased slightly during incubation. In contrast, in seeds for which secondary dormancy was already released, GA4+7 and GA1+3 levels in the embryo, GA4+7/ABA ratio in the embryo and seed coat, and the GA1+3/ABA in the embryo and megagametophyte significantly increased during incubation. There was no trend in the changes in the levels of ABA, GA4+7 or GA1+3 in embryos and megagametophytes of PDS or the levels of GA4+7 or GA1+3 in megagametophytes of SDS during incubation. The results suggest that high ABA levels in the seed coat maintain primary dormancy of Korean pine seeds. Maintenance of secondary dormancy involves a reduction of GA4+7, GA1+3, GA4+7/ABA, and GA1+3/ABA and the retention of high ABA levels.