Barley mutants with low rates of endosperm starch synthesis have low grain dormancy and high susceptibility to preharvest sprouting.

• Studies of embryo dormancy in relation to preharvest sprouting (PHS) in cereals have focused on ABA and other hormones. The relationship between these phenomena and the rate of grain filling has not been investigated. • A collection of barley mutants impaired in starch synthesis was assessed for preharvest sprouting in the field. In subsequent glasshouse experiments, developing grains were assayed for germination index, sugars, abscisic acid (ABA) and the effects of temperature and exogenous ABA on germination. • Mutant lines displayed greater preharvest sprouting in the field than parental lines. In the glasshouse, nondeep physiological dormancy was reduced in developing grains of five lines with mutations affecting proteins involved in endosperm starch synthesis. Inhibition of germination by exogenous ABA and elevated temperature was decreased in developing mutant grains. Sugar concentrations were high but embryo and endosperm ABA contents were unaltered. • We reveal a direct connection between grain filling and the extent of grain dormancy. Impaired endosperm starch synthesis directly influences the acquisition of embryo dormancy, perhaps because endosperm sugar concentrations modulate the ABA responsiveness of the embryo. Thus environmental or genetic factors that reduce grain filling are likely to reduce dormancy and enhance susceptibility to PHS.