Studies on the utility of artificial mutations in plant breeding : I. γ-ray sensitivity of rice and induced aberrant palnts in the X₂ generation

Abstract

With the aim of examining the utility of artificial mutations in plant breebing, several experiments have been conducted since 1956. This paper deals with the radiosensitivity of rice in the X₁ generation and the phenotypic variants appearing in ghe X₂ generation. The varietics used in this experiment were "Aikoku" and "Gimbozu" and two strains (F₇ ) derived from Aikoku X Gembozu, "Wase-ichigata" and "Goku-bansei". The genes responsible for heading period in thcse four starains have been identifled by Syakudo et al. (1954). In the spring of 1956 dry and water-soaked seeds of these strains were irradiated with Co⁶⁰ γ-rays of O.5∼5Okr and the plants from these seeds were grown in the paddy field. In the fall of 1956 10∼20 X₁ plants were selected randomly from each of the varying dosage groups in all the strains, and about three spikes were taken from each of these plants. In 1957 the progenies of these spikes were grown as separate lines, totaling 6888 lines and about 20, OO0 plants. The examination of this X₂ generation was made mainly with Gimbozu. As for the ger, omatomg ability the irradiated seed showed no decline as compared with that of the control even in the highest dose. The seedlings, however, showed a slower growth than the control and in the higher doses some proportion of them were killed in the early stage of growth. The number of plants killed incrcased as the dosage increased, in the highes dose all the plants being perished. Seed fertility of the matured plants decreased lineary as the dpsage incresed until 20 kr, from which the rate of the decrease fell down. The fertility of the plants from water-soaked seed was lower than that of the plants from dry seed at the same dosage. 50% of fertility was obtained in the dosage of 10∼15 kr with dry seed. In the X₂ generation various kinds of mutation were observed. The aberrant plants presumed as mutants are those of chlorophyll defrciency, early and late maturity, tall and dwarf type, dense and lax spike, large-and small-sized or deformed grain, sterility of various degrees and other aberrations (See Table 3). About 30% of these aberrant plants except chloropyll mutants showed simultanious change in two or more characters.