Responses to seed dormancy-breaking treatments in rice species (Oryza L.)

Seed germination tests of 18 germplasm accessions representing 16 rice species (Oryza L.) were conducted under a series of dormancy-breaking treatments. Hulled and intact seeds were exposed to (1) constant temperatures of 26 °C, 31 °C, and 36 °C or left at room temperature (28±1 °C ); and (2) alternating temperatures of 40/25 °C, 40/30 °C, and 45/30 °C (12 h/12 h), or left at room temperature. In a third treatment, intact seeds were exposed to dry heat at 50 °C for 7 or 14 d before germination of hulled and intact seeds under the optimum temperature regimes for each species determined from Treatments 1 or 2. Species that showed less than 85% germination in Treatment 3 were exposed to various chemical treatments, namely germination of intact seeds in 0.001 M potassium nitrate (KNO 3 ), or presoaking the seeds for 24 or 48 h in 0.1 or 0.2 M nitric acid (HNO 3 ), 1 M hydrogen peroxide (H 2 O 2 ), 1000 ppm gibberellic acid (GA 3 ), and a combination of 0.1 M HNO 3 and 1 M H 2 O 2 or 0.2 M HNO 3 and 1 M H 2 O 2 . Four conclusions emerge from this work: (1) removal of the seed hull is extremely effective for breaking seed dormancy; (2) species respond differently to various constant or alternating temperature regimes, and no single temperature regime is consistently effective to break seed dormancy across all Oryza species; (3) heat treatment generally promotes germination of the species that respond to certain constant or alternating temperature treatments, but no consistent differences were observed in seed germination of most species between heat treatments at 50 °C for 7 or 14 d; and (4) some species respond to certain chemical treatments effectively under the optimum temperature regimes. An appropriate combination of seed hull removal, dry heat or chemical treatments, and germination under the optimum temperature regimes for individual species provides the best results for breaking seed dormancy of rice species.