RAPID GERMINATION OF A BARLEY MUTANT IS CORRELATED WITH A RAPID TURNOVER OF ABSCISIC-ACID OUTSIDE THE EMBRYO

In our study of the role of abscisic acid (ABA) in controlling the ger mination of barley grains, we tested a barley mutant line with a gigan tum appearance (Hordeum distichum cv Quantum) for an ABA-insensitive p henotype by assaying germination in the presence of 10(-4) M ABA. Diss ected embryos of the mutant germinated at least 10 h earlier than did those of the wild type. The half-maximal concentrations of ABA inhibit ory for germination were determined to be 5 x 10(-4) M for the mutant and 10(-6) M for the wild type. Expression of an ABA-induced Rab gene was studied to determine ABA responsiveness. The ABA concentration req uired for a half-maximal induction of Rab gene expression was 4 x 10(- 6) M in isolated embryos of both the mutant and wild type. This result suggests that ABA signal transduction pathways were not affected in t he mutant. When isolated embryos were allowed to imbibe in water, ABA was released from the mutant and wild-type embryos at the same rate. H owever, the free ABA level in the incubation medium of the mutant show ed a much faster decrease than that of the wild type, as demonstrated by two independent ABA assay methods (high-performance liquid chromato graphy and enzyme-linked immunosorbent assay). Our results suggest tha t turnover of ABA outside the embryo is a determining factor in the ge rmination of barley seeds.