Two mechanisms have been suggested as being responsible for dormancy in barley grain: (i) ABA in the embryo, and (ii) limitation of oxygen supply to the embryo by oxygen fixation as a result of the oxidation of phenolic compounds in the glumellae. The aim of the present work was to investigate whether hypoxia imposed by the glumellae interferes with ABA metabolism in the embryo, thus resulting in dormancy. In dormant and non-dormant grains incubated at 20 degrees C and in non-dormant grains incubated at 30 degrees C (i.e. when dormancy is not expressed), ABA content in the embryo decreased dramatically during the first 5 h of incubation before germination was detected. By contrast, germination of dormant grains was less than 2% within 48 h at 30 degrees C and embryo ABA content increased during the first hours of incubation and then remained 2-4 times higher than in embryos from grains in which dormancy was not expressed. Removal of the glumellae allowed germination of dormant grains at 30 degrees C and the embryos did not display the initial increase in ABA content. Incubation of de-hulled grains under 5% oxygen to mimic the effect of glumellae, restored the initial increase ABA in content and completely inhibited germination. Incubation of embryos isolated from dormant grains, in the presence of a wide range of ABA concentrations and under various oxygen tensions, revealed that hypoxia increased embryo sensitivity to ABA by 2-fold. This effect was more pronounced at 30 degrees C than at 20 degrees C. Furthermore, when embryos from dormant grains were incubated at 30 degrees C in the presence of 10 microM ABA, their endogenous ABA content remained constant after 48 h of incubation under air, while it increased dramatically in embryos incubated under hypoxia, indicating that the apparent increase in embryo ABA responsiveness induced by hypoxia was, in part, mediated by an inability of the embryo to inactivate ABA. Taken together these results suggest that hypoxia, either imposed artificially or by the glumellae, increases embryo sensitivity to ABA and interferes with ABA metabolism.

No abstract

Pre-harvest sprouting (PHS) causes immediate loss of seed viability, making barley (Hordeum vulgare L.) grains worthless for malting purposes. Grain dormancy release rate in barley crops is genetically and environmentally controlled. A 2 year experiment was conducted to evaluate the effect of soil nitrogen and water availability during grain filling on the dormancy release pattern (and then on the PHS susceptibility) for five malting barley commercial cultivars. Drought and well-irrigated control treatments were imposed from anthesis onwards, and contrast nitrogen fertilization treatments were applied at tillering. Nitrogen availability showed no effects on dormancy release. Drought during grain filling accelerated dormancy release with respect to well-irrigated control in 2004, but not in 2005 year. Mean temperatures during the last stages of grain filling were much higher (ca. 6°C) in 2005 than in 2004, indicating that high-dormancy loss promoting temperatures had masked drought effects on dormancy release.