Dormancy in red rice. IV. Response of unimbibed and imbibing seeds to nitrogen dioxide

Nitrogen dioxide at 4 to 8 ml 1⁻¹ breaks dormancy of dehulled, unimbibed seeds of red rice ( Oryza sativa L.) with minimum exposure times to the gas of 20 min or less. The response is independent of incubation medium pH following treatment. Germination is complete within 2 days but proceeds 4 to 6 h more slowly than non-dormant controls. The magnitude of response at a constant NO₂ dose is dependent upon initial seed moisture. There was less than 40% germination at 9.6% initial moisture while germination was 95% at 12% initial moisture. NO₂ treatment also breaks dormancy of imbibing, dehulled seeds, but the response is dependent upon the pH of the medium. Dormancy is broken at pHs close to the pK_a of nitrous acid. Intact, unimbibed or imbibing seeds respond to NO₂ treatment only if partially dry-afterripened prior to exposure.