Involvement of ABA in induction of secondary dormancy in barley (Hordeum vulgare L.) seeds

At harvest, barley seeds are dormant because their germination is difficult above 20degC. Incubation of primary dormant seeds at 30degC, a temperature at which they do not germinate, results in a loss of their ability to germinate at 20degC. This phenomenon which corresponds to an induction of a secondary dormancy is already observed after a pretreatment at 30degC as short as 4-6 h, and is optimal after 24-48 h. It is associated with maintenance of a high level of embryo ABA content during seed incubation at 30degC, and after seed transfer at 20degC, while ABA content decreases rapidly in embryos of primary dormant seeds placed directly at 20degC. Induction of secondary dormancy also results in an increase in embryo responsiveness to ABA at 20degC. Application of ABA during seed treatment at 30degC has no significant additive effect on the further germination at 20degC. In contrast, incubation of primary dormant seeds at 20degC for 48 and 72 h in the presence of ABA inhibits further germination on water similarly to 24-48 h incubation at 30degC. However fluridone, an inhibitor of ABA synthesis, applied during incubation of the grains at 30degC has only a slight effect on ABA content and secondary dormancy. Expression of genes involved in ABA metabolism (HvABA8'OH-1, HvNCED1 and HvNCED2) was studied in relation to the expression of primary and secondary dormancies. The results presented suggest a specific role for HvNCED1 and HvNCED2 in regulation of ABA synthesis in secondary seed dormancy.