Genetic determinants of semi-dwarf characters induced in Oryza sativa for upland rice cultivation

Numerous semi-dwarf mutants have been obtained by applying gamma irradiation to tall upland varieties. They show better lodging resistance and retain their adaptation to upland cultivation conditions (MARIE, 1974). These mutants are also of interest as cross breeding parents. If the breeder knows the genes for semi-dwarfness, he can use different ones so as to avoid the risk of a narrow genetic base. Five induced mutants and one deviant from a sixth one, were used in this study. The semi-dwarf characters were analysed in two steps: Comparison of the stem morphology of the conventional varieties and of the induced mutants; Studying the behaviour of hybrid F1 and F2 segregations. Three types of genetic control for induced semi-dwarfness have been found on the basis of the six mutants used: A recessive single-gene determinant in IRAT 115; A recessive single-gene determinant different from the preceding one and common to IRAT 13, IRAT 177, IRAT 250 and IRAT 257; A semi-dominant single-gene determinant in IRAT 194. For his hybridization programme, the rice breeder therefore has at his disposal various upland semi-dwarf mutants, which are capable of transmitting this character in accordance with simple Mendelian rules. However, he has to test also their general cross breeding value. IRAT 13, from which most IRAT varieties released so far originated (including IRAT 104, IRAT 170 and IRAT 112), is undoubtedly a good parent. Mutants IRAT 177, IRAT 250 and IRAT 257, which have the same gene for semi-dwarfness, have been used more recently. They exhibit an excellent ability for combination with tall upland varieties, while being differentiated by a few features (better recombination for seed shedding or resistance to Pyricularia oryzae with IRAT 257 and for grain character with IRAT 177 or 250). The recombination observed in the crosses involving IRAT 115 and conventional upland varieties seems poor. Paradoxically, IRAT 115 has been found to become an excellent parent after hybridization with upland varieties of Laos and Thailand. IRAT 194, carrier of the semi-dominant gene, seems by far the most disappointing parent. This study is not complete. In particular, no account was taken of certain mutants of 63-83 such as M 312 A or M 327 A, whose semi-dwarf height is controlled by a recessive gene different from that of IRAT 13 (JACQUOT, 1975, 1986). At present, all the IRAT varieties released or in the process of being released have the semi-dwarf gene of IRAT 13. The varieties obtained from IRAT 177, IRAT 250 or IRAT 257 should be available soon. It is also hoped that soon the promising recombinations observed in crosses between IRAT 115 and the Laotian-Thai varieties will contribute to solve the problem of genetic uniformity for semi-dwarfness in Asia. IRAT 194 has also been included in the crossing programme so that its value can be verified