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Previous results suggested that the amount of nuclear DNA varies in one and the same progeny of Helianthus annuus, depending on the head portion in which seeds have developed. Accordingly, cytophotometric determinations were carried out in a selfed line, after Feulgen-staining, to obtain information on the developmental stages at which DNA changes are produced and on the mechanism controlling the variation. It was found that the Feulgen absorption values of mitotic prophases in immature anthers and pistils and of meiotic prophases I are the same in any flower of the head. In contrast, the Feulgen/DNA contents of early prophases in heart-shaped embryos differ significantly, increasing from those developing at the centre of the head to those developing at its periphery, and remaining unchanged in each embryo during further development and seed germination. Variations in the number of chromosomes do not account for the differences observed in nuclear DNA contents in which sequences comprised in heterochromatic nuclear regions are involved. The Feulgen absorption values of seedlings obtained from seeds developed in different portions of single heads increase or diminish starting from those found in the mother plant. This depends on whether these latter are relatively low or high and on the gradient of seed location in the head. The variation occurring within each single progeny covers the whole range existing within the line.

Cytophotometric measurement of the root meristems of seedlings after Feulgen-staining reveals that large differences (up to 58.16%) in nuclear DNA content may occur in the thirty-one cultivated varieties or lines of Helianthus annuus tested. Significant variations (not exceeding 25%) in the amount of DNA, which does not differ between the root and the shoot meristems of a single seedling, are also found to exist within cultivars or lines; even seedlings obtained from seeds collected from different portions of single heads of plants belonging to a selfed line may vary one from the other in this respect. Variations in the number of chromosomes or alterations in the chromosome structure do not account for the differences observed in nuclear DNA content. Karyometric analyses demonstrate that the surface area of squashed interphase nuclei and metaphase chromosomes and the total length of the latter increase with the increase in Feulgen/DNA absorption. DNA thermal denaturation and reassociation kinetics indicate that a frequency variation in repeated DNA sequences goes hand in hand with changes in the size of the genome. These results, supporting the concept that a plant genome is highly flexible, are discussed in relation to other data to be found in the literature on the intraspecific variation in the nuclear DNA content and in relation to the way in which it is produced in H. annuus.