Induction of secondary dormancy in sunflower seeds by high temperature. Possible involvement of ethylene biosynthesis

High temperature (45°C) inhibits seed germinition and seedling sunflower (Helianthus annuusL. cv. Mirasol). Treatment of imbibed seeds at 45°C for more than 48 h induces a secondary dormancy, which is associated with progressive decrease of germination ability at optimal temperature (25°C) as well as with abnormal seedling growth. Ethylene (55μl l−1) and 2‐chloroethylphosphonic acid (ethephon) (2.5 mM) improve germination of thermodormant seeds at 25°C. but the abnormal growth of the seedlings remains. O2‐enriched atmosphere and dry storage improve germination and normal seedling growth. The induction of thermodormancy in sunflower seeds seems associated with loss of their ability to convert 1‐aminocyclopropane‐1‐carboxylic acid (ACC) to ethylene. Possible effects of high temperature on membranes and ethylene forming enzyme (EFE) are discussed.