Inhibition of germination of dormant barley (Hordeum vulgare L.) grains by blue light as related to oxygen and hormonal regulation

Germination of primary dormant barley grains is promoted by darkness and temperatures below 20 degrees C, but is strongly inhibited by blue light. Exposure under blue light at 10 degrees C for periods longer than five days, results in a progressive inability to germinate in the dark, considered as secondary dormancy. We demonstrate that the inhibitory effect of blue light is reinforced in hypoxia. The inhibitory effect of blue light is associated with an increase in embryo abscisic acid (ABA) content (by 3.5- to 3.8-fold) and embryo sensitivity to both ABA and hypoxia. Analysis of expression of ABA metabolism genes shows that increase in ABA mainly results in a strong increase in HvNCED1 and HvNCED2 expression, and a slight decrease in HvABA8OH-1. Among the gibberellins (GA) metabolism genes examined, blue light decreases the expression of HvGA3ox2, involved in GA synthesis, increases that of GA2ox3 and GA2ox5, involved in GA catabolism, and reduces the GA signalling evaluated by the HvExpA11 expression. Expression of secondary dormancy is associated with maintenance of high embryo ABA content and a low HvExpA11 expression. The partial reversion of the inhibitory effect of blue light by green light also suggests that cryptochrome might be involved in this hormonal regulation. This work demonstrates that blue light inhibits the germination of primary dormant grains placed at low temperature (10 degrees C), and that exposure longer than 5 days under blue light induces of secondary dormancy.The inability of primary dormant grains to germinate under blue light and induction of secondary dormancy are correlated with an increase in embryo ABA content resulting from an induction of the HvNCED genesand a decrease in HvABA8OH-1 gene expression. Exposure to blue light also results in changes in GA metabolism gene expression and a strong decrease in HvExpA11 expression suggesting a reduced GA signalling. The reversion of blue light effect by green lightsuggests that this hormonal regulation probably involves cryptochrome.