No abstract

The causes of extreme differences in radiosensitivities of different plant (or animal) species have long puzzled radiobiologists. Our investigations show that, for diploid species, the larger the nuclear volume, the more radiosensitive the organism. Correspondingly, species with large nuclei have more deoxyribonucleic acid (DNA) per nucleus than those possessing small nuclei. Our data now make it possible to predict fairly accurately the tolerance of plant species to ionizing radiation on the basis of average nuclear volume and DNA content. The same correlations are expected to hold for some microorganisms and for animals and may explain differences in sensitivities of different cell types in many living organisms.

Intraspecific variation of DNA per cell was established chemically and cytophotometrically for 17 seed sources of Picea glauca, and cytophotometrically for 11 sources of Pinus banksiana. The DNA Feulgen absorption per cell varied from the lowest to the highest amount by factors of 1.6 and 1.5 for Picea glauca and Pinus banksiana, respectively. Intraspecific DNA also varies with intraspecific nuclear volume. Intraspecific variation of histone was similar to the observed DNA variation. A regression analysis between DNA per cell and latitude provided evidence that eastern and western population series of Picea glauca existed in the seed sources studied. Two-year seedling height growth results demonstrated that eastern seed sources are different from western seed sources. Seedling height in the western provenances varied inversely with DNA content; i.e., seed sources with small DNA per cell displayed greater growth, whereas the eastern sources did not display the inverse relationship between DNA amount and 2-year growth.

DNA, RNA, protein and heterochromatin were measured cytophotometrically in developing soybean (Glycine max) seeds. The average 2C DNA content for the soybean genome was 2.64 pg. The amounts of nuclear DNA in embryo axes showed no significant change during embryo development, whereas the DNA content in cotyledon nuclei increased significantly from 3.58 pg to 5.49 pg. The number of endopolyploid nuclei increased from 26% to 48% and the DNA content from 4.45 to 5.49 pg after cessation of cell division. The changes in RNA and protein content during embryo development were in general similar to those in DNA content. This can be interpreted that increased DNA levels in soybean cotyledons generated during embryogeny increase the protein synthesizing capacity. During the first 15 days of germination, the number of endopolyploid nuclei in cotyledons declined from 46% to 4%, and this decline is interpreted as DNA degradation providing a ready source of nucleosides and phosphates during early embryo growth. A later decline, however, between 15 and 20 days after germination, was age related similar to leaf senescence, because the percentage of endopolyploid nuclei remained unchanged while the number of non-viable cells increased. In senescing cotyledons, 73% and 80% of RNA and protein but only 20% of DNA were lost, as compared to dormant cotyledons. The heterochromatin (condensed chromatin) measurements indicated that nuclei of metabolically inactive dormant and senescent cotyledon nuclei contained an average of 33% more heterochromatin than nuclei from the green cotyledons of seedlings.

Seeds were sown on mineral agar and grown under axenic constant conditions. Floral transition and flower primordia were observed at approximately 172 and 220 hr, respectively. The vegetative apex was flat in outline for 160 hr, after which time it was slightly dome shaped; cytohistological zonation was not readily apparent. The inflorescence apex (172 hr) was markedly convex and possessed 3–5 and 4–6 cell layers in the most axial and lateral portions of the apex, respectively. The nucleoli of cells of the T2 layer were larger than cells of the T1 and outer corpus during vegetative development; in contrast, they were equal in size at reproductive stages.