Nuclear DNA changes within Helianthus annum L.: changes within single progenies and their relationships with plant development.

The variations in the basic nuclear DNA content, which previous results indicated to occur within one and the same progeny of Helianthus annuus, were studied in detail and correlated with certain developmental features of the plants. The size and organization of the genome of seedlings obtained from seeds (achenes) collected at the periphery (P-seedlings) or in the middle (M-seedlings) of the flowering heads of plants belonging to a line selfed for 10 years were compared. Cytophotometric determinations indicated that the nuclear DNA content of P-seedlings is 14.7% higher than that of M-seedlings. Thermal denaturation and reassociation kinetics of extracted DNAs showed that variations in the redundancy of repetitive DNA, in particular of a family of medium repeated sequences with a Cot range of 2-100, account for the differences in genome size. These findings were confirmed by the results of molecular hybridizations (slot blots), which also indicated a higher amount of ribosomal DNA in the P-seedlings than in the M-seedlings. Cell proliferation is affected by DNA content, and mitotic cycle time is 1h30' longer in the P-seedlings. By studying mature plants, positive correlations were also found between genome size and both the surface area of leaf epidermal cells (P≤0.01) and flowering time (P≤0.001). It is suggested that the variations of nuclear DNA content and organization observed play a role in determining developmental variability in plant populations, which may be of importance in buffering the effects of changing environmental conditions.