Breakage of Pseudotsuga menziesii seed dormancy by cold treatment as related to changes in seed ABA sensitivity and ABA levels.

The main aims of the present work were to investigate whether a chilling treatment which breaks dormancy of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) seeds induces changes in the sensitivity of seeds to exogenous ABA or in ABA levels in the embryo and the megagametophyte, and whether these changes are related to the breaking of dormancy. Dormant seeds germinated very slowly within a narrow range of temperatures (20-30 degrees C), the thermal optimum being approximately 25 degrees C. The seeds were also very sensitive to oxygen deprivation. Treatment of dormant seeds at 5 degrees C improved further germination, and resulted in a widening of the temperature range within which germination occurred and in better germination in low oxygen concentrations. In dry dormant seeds the embryo contained about one-third of the ABA in the megagametophyte. ABA content of both organs increased during the first 4 weeks of chilling. It then decreased sharply in the megagametophyte to the level in the embryo after 7-15 weeks of chilling. At 15 degrees C, a temperature at which dormancy was expressed, the ABA level increased in the embryo and the megagametophyte of dormant unchilled seeds whereas it decreased in the organs of chilled seeds. The longer the chilling treatment, the faster the decrease in ABA after the transfer of seeds from 5 degrees C to higher temperatures, and the decrease was faster at 25 than at 15 degrees C. These results suggest that the breaking of dormancy by cold was associated with a lower capacity of ABA biosynthesis and/or a higher ABA catabolism in the seeds subsequently placed at 15 or 25 degrees C. Moreover, the chilling treatment resulted in a progressive decrease in the sensitivity of seeds to exogenous ABA. However, seeds remained more sensitive to ABA at 15 than at 25 degrees C. The possible involvement of ABA synthesis and of responsiveness of seeds to ABA in the breaking of dormancy by cold treatment is discussed.