Water content: a key factor of the induction of secondary dormancy in barley grains as related to ABA metabolism

Abstract

Primary dormant barley (Hordeum vulgare) grains germinate at 10-15°C but not at 30°C, and there exist a positive correlation between embryo ABA content after 24 h on water and the depth of dormancy. Incubation at 30°C results in a progressive loss of the ability to germinate at 15°C. This induction of a secondary dormancy is optimal after 3 days and requires an embryo water content higher than 0.50 g H₂O g⁻¹ DW, this corresponding with activation of the cell cycle. There exists no correlation between ABA content after 3 days at 30°C and the induction of secondary dormancy. However, at high water content (1.60-1.87 g H₂O g⁻¹ DW), secondary dormancy is associated with an high embryo ABA content after transfer to 15°C, resulting from an increase in HvNCED1 and HvNCED2 expression and a decrease in HvABA8'OH-1. Such changes are not observed at 0.45 g H₂O g⁻¹ DW. Incubation at 30°C also results in an increase in expression of genes involved in GA catabolism (HvGA2ox1, HvGA2ox3 and HvGA2ox5) and synthesis (HvGA3ox2, HvGA20ox1 and HvGA20ox3). The HvGA3ox2/HvGA2ox3 transcript ratio remains low (0.27-0.37) at 30°C and after transfer to 15°C in secondary dormant seeds, but it is higher than two when secondary dormancy is not induced. Changes in HvExpA11 expression indicate that GA signaling decreases when a secondary dormancy is expressed. Our results clearly indicate that expression of genes involved in ABA and GA metabolism differs in primary and secondary dormancies and furthermore, their expression is related to embryo water content.