Abstract
Radiation injuries caused bv acute irradiation were studied in potted rice plants (varie.ty Gimbozu) exposed to X-rays (doses: 0.5-30 kr, dose rates: 12-llO r/min.) at several stages of plant development from about five weeks to a few days before heading. The experiments were conducted in 1960, 1961, and 1962, and three experimental sections different in the method of plant culture were prepared in 1961. As similar results were obtained in each year and among three sections in 1961, those from a section in 1961 are mainiy reported here, in which the heading date (usually Sept, 1) was delayed about ten days by long-day treatment. For the long-day treatment section in 1961, X-ray irradiation was administered on five different dates from Aug. Il to Sept.8 with one week interval. The irradiated groups were designated as I to V in the order of the irradiation date. Developmental stages of panicles of main tiller in each group at the irradiatior; were estimated as follows: I, panicle not yet differentiated; II, differentiating stage of first bract prillrlordium; III, differentlating stage of secondary branch primordia to beginning stage of flower primordia differentiation; IV, Iate differentiating stage of flower primordia to differentiating stage of PMC; and V, extine formation stage to ripe stage of pollen. After irradiation, various injuries lvere found on the irradiated plants according to the dose of X-rays and the developmental stage at the irradiation. (1) The occurrence and degree of injury varies with the developmental stage as well as with the character observed. In the above experimental section, the highest radiation sensitivity was observed in the irradiated groups II and III for heading behavior, in I for plant height, in 11 for cuhrl length, in I-III fcr number of florets per panicle, in IV for seed fertility, in 111 and IV for shape of panicle, and in 111 for shape of floret. (2) The degree of injury in each character increases linearly with the dose of X-rays in all the irradiated groups. The dose that brings about easily detectable injuries in the above characters is generally more than 5 kr. The experimental results suggest that the seed sterility may be caused not only by chromosomal aberrations but even more frequently by extrachromosomal or physiological effects of radiation. The malformed floret with degenerate inner glume can be easily induced only at the beginning stage of flower primordia differentiation of a panicle, indicating that the primordium of inner glume may have a marked sensitivity to radiation. These results suggest that acute irradiation may be effectively utilized for histological analysis of plants and also for tracing induced mutations.
