Koornneef, M., Reuling, G. and Karssen, C. M. 1984. The isolation and characterization of abscisic acid-insensitive mutants oi Arabidopsis thaliana. -Physiol. Plant. 61: 377-383.Abscisic acid (ABA) insensitive mutants of Arabidopsis ihaliana (L.) Hcynh. were isolated by selecting plants which grew well on a medium containing 10 [iM ABA. From the progeny of approximately 3500 mutagen-treated seeds, five mutants of at least three different loci were isolated. Three mutants were characterized, moreover, by a reduced seed dormancy and by symptoms of withering, indicating disturbed water relations and, therefore, resembled phcnotypically the ABA-deficient mutants we described earlier in this species. Two mutants showed in addition only a reduction of seed dormancy. Compared to wild type, all mutants showed similar or increased levels of endogenous ABA in developing seeds and fruits (siliquae). The role of the different genes involved is discussed in relation to the mechanism of ABA action.Additional key words -Dormancy, hormone receptors, seeds, water relations.M. Koornneef (reprint requests). G. Reuling.

Mutant lines of Arabidopsis thaliana (L.) Heynh., which are characterized by symptoms of withering and the absence of seed dormancy, showed much lower levels of endogenous abscisic acid (ABA) in developing seeds and fruits (siliquae) than the wild type. Reciprocal crosses of wild type and ABA-deficient mutants showed a dual origin of ABA in developing seeds. The genotype of the mother plant regulated a sharp rise in ABA content half-way seed development (maternal ABA). The genotype of the embryo and endosperm was responsible for a second ABA fraction (embryonic ABA), which reached much lower levels, but persisted for some time after the maximum in maternal ABA. The onset of dormancy correlated well with the presence of the embryonic ABA fraction and not with the maternal ABA. Dormancy developed in both the absence and presence of maternal ABA in the seeds. In this respect maternal ABA resembled exogenously applied ABA which did not induce dormancy in ABA-deficient seeds. However, both maternal and applied ABA stimulated the formation of a mucilage layer around the testa, which could be observed during imbibition of the mature seeds. In the mature state, ABA-deficient seeds germinated in the siliquae on the plant, but only when the atmosphere surrounding the plant was kept at high relative humidity. In younger stages germination in siliquae occurred after isolation from the plants and incubation on wet filter paper. Therefore, it seems that limited access to water is the primary trigger for the developmental arrest in these seeds.

No abstract

Summary1 The paper reviews the literature on seed dormancy, with special regard to inconsistencies in terms and definitions used. It presents a concept of seed dormancy in which physiology and ecology are integrated. Its aim is to increase the understanding of seed dormancy and germination, and to help defining ecological research questions. 2 It is claimed that seed dormancy should not be identified with the absence of germination. Seed dormancy should rather be defined as a characteristic, the degree of which determines the range of conditions in which a seed is able to germinate. Dormancy varies on a continuous scale, which is visualized by continuous changes in the range of conditions suitable for germination. If the conditions required by the seed are met by its environment, the seed will germinate. 3 The concept of dormancy that is described in the paper is partly based on a physiological model for the regulation of dormancy and the stimulation of germination. In this model dormancy is related to the amount of a hypothetical phytochrome receptor in the seed. 4 It is argued that the process of dormancy release should be clearly distinguished from the germination process itself. It is stated that as yet only temperature has been shown to alter the degree of dormancy in seeds. Factors like light and nitrate are often indispensable for germination, but only by promoting the germination process itself, not by mitigating the requirements for germination. 5 It is suggested that seed dormancy prevents germination when conditions are favourable for germination, at a time of the year when it can be expected that the plant originating from the seed will not survive and produce offspring. 6 It is concluded that dormancy should not be regarded as inactivity of seeds. At any degree of dormancy, seeds continuously react to their environment by adjusting their level of dormancy to the changing environment.

Two new abscisic acid (ABA)-insensitive mutants of