No abstract

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.

No abstract

Mature seeds of Helianthus annuus L. exhibit dormancy that is eliminated during storage in dry conditions. In vitro culture of immature embryos isolated at different times after anthesis showed that the youngest embryos are able to germinate, but within the third week after pollination, dormancy progressively affected most of the embryos. A radioimmunoassay showed that the endogenous abscisic acid (ABA) level, which increased sharply in the first half of the development period, fell at precisely the moment when embryo dormancy became established. An application of fluridone, before the increase of ABA level, prevented both ABA synthesis and development of embryo dormancy. Applied later, after the rise of the ABA level, fluridone could not prevent embryo dormancy development. Dormancy thus appears to be dependent on ABA synthesis but not concomitant with its accumulation; it must therefore be induced by ABA during maturation. Furthermore, a preincubation in water allowed dormant embryos to germinate. This acquisition of germinability could not be directly related to a leaching of free ABA. Possible effects of this treatment are discussed.The classic concept of the hormone balance theory supposed that induction, maintenance, and release of seed dormancy were regulated by the simultaneous action of promotive and inhibitory hormones. Recent work with hormone deficient mutants of Arabidopsis thaliana and Lycopersicon esculentum led Karssen and Groot (8) to a revision of this theory. In particular, it appears that ABA is responsible for the onset of dormancy in developing seeds but that it does not control the maintenance of dormancy in mature seeds. However, the authors noted that this conclusion might be valid for all species with so-called 'coat-imposed dormancy' but that direct evidence is missing to invoke the same hormonal control for embryo dormancy.Our first objective, then, was to look for the involvement of ABA in the onset of the dormancy of an embryo. Our plant material, Helianthus annuus seeds, exhibited at harvest a dormancy which could be eliminated by storage in dry conditions, and in vitro culture showed that embryos themselves were dormant. Furthermore, in this species, the lack of dormancy shown by a nondormant white mutant strain was correlated by Wallace and Haberman (16) (17), it may be suggested that the lack of dormancy must be correlated to the lack of ABA synthesis. This species appeared to be a good material to try to modify the physiological behavior of the embryo through an alteration of ABA synthesis. So from this perspective, we applied to the seeds during their development a solution of fluridone, a pyridinone inhibitor of carotenoid biosynthesis, in order to decrease their endogenous ABA levels. This approach allowed us to demonstrate the responsibility of ABA in the induction of the dormancy of the embryo.On the other hand, these dormant embryos could be germinated after a preincubation in water. This treatment, which evokes the leaching of an inhibitor, was imposed to discuss...

Excised Helianthus annuus L. embryos became dormant during the third week after anthesis. At this stage a short drying treatment (3 d) led to a slight improvement in germination but a 6-week dry storage caused a complete release from dormancy. The short drying treatment, however, elicited the embryos' response to an exogenous concentration of GAs which was unable to promote germination of fresh embryos. It therefore appeared that a short drying treatment changed the sensitivity to GAs but was not capable of directing embryo metabolism completely towards a germinative mode. Moreover, this drying treatment reduced considerably the ABA content in both the axis and the embryo. Nevertheless, no correlation could be established between germinability and the ABA content since the amount of ABA was not modified by the 6-week dry storage. The key step for predisposing the seeds to germinate is the suppression of the capacity for ABA synthesis in the axis, a suppression which takes place during 6-week dry storage.