Comparative sensitivity and relative biological effectiveness of gamma-rays, X-rays and electron beams in aromatic Joha rice derived from different locations in Assam state.

Determining the optimum dose of a mutagen that causes a high frequency of favourable mutations with minimum damage to the plant is the vital criterion for the success of a mutation breeding programme. We evaluated the comparative effectiveness, radiosensitivity and relative biological effectiveness (RBE) of three physical mutagens, viz., electron beams, X-rays and gamma-rays, on twenty indigenous aromatic Joha rice cultivars of Assam. We exposed dry uniform seeds of 12.4% moisture content to 16 different mutagens doses ranging from 50 to 800 Gy at an interval of 50 Gy. Key results: Germination percentage and the seedling traits registered a decreasing trend towards increasing the mutagen doses in all the cultivars, showing differential radiosensitivities. The LD₅₀ values ranged from 515 to 615 Gy for electron beams and 421 to 537 Gy for X-rays and 414 to 481 Gy for gamma-rays. The GR50 values based on seedling vigour index I ranged from 125 to 304 Gy for electron beams, 191 to 313 Gy for X-rays and 178 to 267 Gy for gamma-rays. The optimum dose ranges for electron beams, X-rays and gamma-rays are 241 to 337 Gy, 228 to 324 Gy and 250 to 346 Gy, respectively. The cultivar Soru Joha-Tinsukia was the most radio tolerant, followed by Keteki Joha and Local Joha, while the cultivar Kon Joha-Moran showed the least tolerance. Joha rice cultivars showed more radiosensitivity for X-rays, followed by electron beams and gamma-rays. Similarly, a high RBE was evident for X-rays, followed by electron beams and gamma-rays, indicating their decreasing penetration capacity and lethality trend. These results would help plant breeders select the appropriate mutagen dose for mutation breeding in Assam's least researched aromatic Joha rice. [ABSTRACT FROM AUTHOR]