Freshly harvested oat (Avena sativa L.) seeds are considered to be dormant because they are unable to germinate at relatively high temperatures (above 20°–25°C). This primary dormancy results partly from the structures surrounding or adjacent to the embryo (pericarp, testa and endosperm) and partly lies in the embryo itself. Incubation of imbibed dormant seeds at 30°C induces a thermodormancy which is expressed as germination in a narrower temperature range. Induction of thermodormancy is apparent after 3–8 hours at 30°C, is optimal after about 1–2 days at this temperature, but diminishes with longer treatment times. Although whole grains become more dormant as a result of such treatment, the embryos themselves become less dormant, therefore the tissues covering or adjacent to the embryos must have become more inhibiting or the embryos have become more sensitive to their inhibitory action. The germination and sensitivity to ABA or oxygen of embryos isolated from such seeds are no longer so dependent on temperature. The effect of high temperature on primary dormancy and induction of thermodormancy is discussed with reference to sensitivity to ABA and temperature.

In the work reported in this paper, we attempted to elucidate the nature of the different abscisic acid (ABA) sensitivities presented by developing embryos from sorghum [Sorghum bicolor (L.) Moench] lines with contrasting pre-harvest sprouting (PHS) behaviour (Redland B2, susceptible; IS 9530, resistant). We explored two different hypotheses for a possible mechanism: (1) a different functionality of the ABA signalling pathway, and (2) a different rate of ABA degradation/conjugation in the apoplast of embryos from these genotypes. To assess the first possibility, we used an ABA-responsive gene (Rab17) as a reporter of changes in endogenous ABA content, which were artificially induced in embryos from both genotypes by means of fluridone application immediately after anthesis, to reduce ABA content, and embryo incubation in the presence of ABA. A defect in ABA signalling should be seen as a level of Rab17 expression that is independent of endogenous ABA content. For testing the second possibility at two stages of development, embryos from both lines were isolated and incubated in water for different periods. ABA concentrations in embryos and the incubation media were quantified through radioimmunoassay. In contrast to our findings for the resistant IS 9530 line, Rab17 expression did not respond to changes in ABA levels in sensitive Redland B2 embryos. The ABA degradation/conjugation rates in embryos and incubation media did not show clear differences between sorghum lines for any of the developmental stages analysed. These results suggest that a disruption in the ABA signal transduction pathway in Redland B2 underlies the low ABA sensitivity shown by embryos from this line.