Effects of ionizing radiation on nucleic acids during embryonic development. I. Quantitative and qualitative analyses at embryonic “maturity”

Abstract In many previous studies developing barley embryos have been shown to be highly radiosensitive. With acute irradiations, many of the responses exhibited are dependent in great measure upon the specific embryonic stage irradiated. The present study was undertaken to determine to what extent nucleic acid metabolism in barley embryos might be altered following acute irradiation, and whether here, too, embryonic stage would play a decisive role. Embryos developing within the ovaries of the parent plants were irradiated with 450 r acute X-radiation at one of four specific stages: early proembryo, late proembryo, mid-differentiation, or late differentiation. Fractional analyses of the individual bases of DNA and RNA were carried out at embryonic “maturity”. Because of the presence of interfering substances within the embryos, modification of previously published methods was necessary, the procedures developed being described herein. The DNA of control embryos is of the A-T type with the ratio between guanine and cytosine being unity and that between adenine and thymine being only 0·68. The total nucleic acid content of irradiated embryos showed residual depressions of 30–36 per cent, most of this being due to an effect upon RNA which makes up about 85 per cent of the total nucleic acid at embryonic maturity. Overall depression of RNA amounted to as much as 39 per cent but with a maximum difference among irradiated groups of only 5 per cent. This low variability in RNA content indicates that neither difference in embryonic stage nor post-irradiation time are important factors. In contrast, DNA depression ranged from 7 to 33 per cent among irradiated groups, the degree of depression being inversely correlated with the embryonic stage irradiated. With the exception of the oldest embryo group, residual DNA depression was independent of post-irradiation time. Of the RNA bases, guanine and cytosine showed proportionately less alteration than adenine (which was the least radiosensitive) or uracil (which was the most labile). Changes in DNA base components were generally more complex with wide fluctuations in the amount of cytosine and guanine depression. Effects on adenine were less pronounced. Of the DNA bases, thymine was characteristically the most labile, being the only DNA base to be significantly depressed in all four irradiated groups and the only one to be so affected in two of the groups. Embryos irradiated early in proembryo development or late in differentiation showed greater degrees of residual DNA depression at embryonic maturity but with little alteration in DNA base ratios. Irradiation during late proembryo development, in contrast, produced little lasting effect on DNA content, but marked changes in DNA base ratios primarily because of a decrease in thymine. When compared with results from other studies the present data suggest that a relationship exists between alteration of DNA base ratios and the occurrence of organ abnormalities during embryonic development. Metabolic disturbance as an expression of intra- or inter-tissue incompatibility mediated through radiation-induced genetic heterogeneity is considered as a possible cause.