| Radiosensitivity of aged wheat seeds Seiji MATSUMURA and Taro FUJII National Institute of Genetics, Misima, Japan Difference in radiation susceptibility between aged and fresh seeds was examined. 7.5 and 15 kr were applied to seeds of diploid wheat. No germination was observed in the control lot of two year old seeds, and we had no seedlings for irradiation. Non-irradiated one year old seeds showed very low germination rate; a few seeds germinated in the 7.5 kr lot and no germination was observed in the 15 kr lot. Unfortunately germination rate of fresh seeds was as low as 60%; 7.5 kr of gamma-rays did not much reduce it (54 per cent), and a few seeds germinated in the 15 kr lot. Thus, the decrease in germinability by irradiation of one year old seeds was in the whole series markedly lower than that of fresh seeds. A similar tendency was also observed in seedling length which was measured 14 days after sowing, and the decrease of survival rate in April was also marked in the one year old series as compared with that of fresh seeds. 10 and 20 kr of gamma-rays were given to tetra- and hexaploid seeds taking into consideration their higher resistance to radiation than that of diploid seeds. Germinability of two year old seeds was very low in both, tetra- and hexaploid lots, and no germination was obrerved at 20 kr irradiation. But germination rate of control lots was better for younger than for older seeds. This trend was marked in irradiated lots. For instance, germination rate in 20 kr lots of one year old tetraploid seeds was 11.4% amounting to index 55 when the index of 100 is given to the germination rate in the control lot. On the other hand, 68% germinated in the 20 kr lot of fresh seeds, the index being 73 against that of the control. Seedling length and survival rates also showed a similar tendency. In the hexaploid species, a similar decrease of germinability was observed both in irradiated fresh and in one year old seeds. Namely, sensitivity was almost the same in fresh and in one year old seeds of this species whereas two year old seeds had higher sensitivity. From the results, it can be said in general that older seeds have higher radiation susceptibility than younger ones. The decrease in germinability due to age was largest in diploid wheat and smallest in hexaploid wheat. We may assume from the results that at a higher ploidy level, we may expect a higher stability not only in respect to radiations but also to the physiological changes caused by aging. |