Biophysical boundaries for dormancy release in sunflower achenes

Reactions leading to dormancy release (DR) in 'dry', orthodox seeds are still poorly understood, as well as their dependence on moisture content (MC) and temperature. In this study, sunflower achenes were used to explore the effects of MC combined with a wide range of storage temperatures (ST°) on DR dynamics, tested at 10 °C and 25 °C. Embryo sensitivity to abscisic acid and oxygen uptake were determined and ageing indicators were assessed and complemented with dynamics for predicted viability loss. Water status was examined using sorption isotherms and the glass transition temperature (Tg). The results showed that small variations in MC and ST° had profound effects on achene dormancy and embryo responsiveness to ABA. Using a 'temperature-by-humidity' coordinate system we mapped three regions allowing DR during the glassy, rubbery, and fluid states of the achenes. Full DR (allowing germination at 10 °C and 25 °C) occurred only in the rubbery state (above Tg), and the optimal ST° for full DR dropped from 30 °C to -18 °C with MC increasing from 0.045 gH2O g-1 DW to 0.10 gH2O g-1 DW. In contrast, under both glassy and fluid states, DR was incomplete (allowing germination at 25 °C but not at 10 °C) and required warm ST° that promoted ageing.