Using autoradiographic technique with 3H-TdR as a marker for DNA synthesis, the average duration of the each phase of mitotic cycle in barley roots was estimated and was found to be as follows: G1 (preDNA synthesis stage) 4.5 hrs., S (DNA synthesis stage) 4.5 hrs., G2 (post-DNA synthesis stage) 4 hrs., and M (mitosis) 2 hrs. Autoradiographic studies also showed that an asynchronous replication of DNA was occurring in a barley chromosome, terminal portions of the chromosome arms being first replicated. The examination of radiation-induced terminal deletions in anaphase cells revealed that chromosome sensitivity changed according to the stage of the cell cycle at which the cells were irradiated. The chromosome was more sensitive in late S and G2 than in G1 and the most sensitive in early S and/or late G1. It was discussed that DNA is a primary target for cell killing caused by irradiation.
Since all solirces of ionizing radiation seem to produce the same spectra of mutations, further research should be conducted in developing techniques for an induction of sizable mutation effects on various characters and for a selection of desirable mutations from a number of induced mutant lines. This approach was carried out by "recurrent" irradiation continued for many cycles (Figure 2). In such cases, X2 progenies, in which radiation induced mutations were detected, could be obtained after repeated irradiation of successive generations. According to a mathematical con.sideration, plants heterozygous for radiation-induced mutations attained equilibrium frequency by several generations of recurrent irradiation, if the selective mortality of the different genotypes was disregarded. And, after the attainment of the equilibrium in heterozygote frequency, an increase of mutant freque'ncy brought about by one generation of repeated irradiation equaled a rate of newly mutated genes per generation (Figure 1). Dormant seeds of barley and rice were irradiated regularly with a constant dosage at successive generations, to find out what was the effect of this recurrent irradiation on the fertility and the Fertilities genotype frequency in the population. of the populations did not reduce with a recurrence of irradiation (Table 1). It was shown in Table 1 that there was a striking improvement in seed fertility for X2 population as compared with that for X1. Thus it seemed that with repeated irradiation the seeds did not become sensitive to the treatment. Effects of treatment upon mutation in rice were remarkable for increasing its frequencies and for extending its spectra (Figure 6, and Table 2). An increase in the frequency of mutation was obtained noticeably in the population irradiated repeatedly with l0, 000 r, but the increase was slight when that treated recurrently with 30, 000r. A possible explanation fcr the smaller increase of mutations in the latter population could be sought in their lowered viability caused by the high dose of irradiation. By analyzing the genetic constitution of a group of individuals replicated with X-irradiation the proportion of heterozygotes involved and the frequency of mutation induced newly in X 1/2/3 Population were estimated, as shown in Tables 3 and 4. The frequency of mutation induced newly in X1/2/3 was of the same magnitude as that produced in X1' This result indicates that neither increase nor decrease occurred in radiosensitivity of the seed from repeatedly irradiated population.
A method for isolation of the nuclei from embryo of dormant barley seed is described. In them, metaphasic chromosome figures were found, and somatic association of metaphasic chromosomes was repeatedly observed.
A dwarf rice mutant was induced through the chronic exposure of 60Co gamma-rays. This mutant was crossed with a heavy-panicle type variety, and semi-dwarf plants permitting a more efficient distribution of light within the canopy were obtained. They along with current commercial varieties and the parent mutant were assessed for yield components, the yielding ability at three levels of nitrogen fertilizer by varying plant density and several other agronomic traits. The ssmidwarf rice possessed significantly higher rates of net photosynthetic CO2 uptake compared with that of current commercial varieties, and it was improved in yield return by increasing nitrogen application. Manner of display of leaves could be manipulated by induced mutations.
