DNA degradation and nuclear degeneration during programmed cell death in petals of Antirrhinum, Argyranthemum, and Petunia.

Programmed cell death (PCD) was studied in the petals of Antirrhinum majus, Argyranthemum frutescens, and Petunia hybrida, using DNA degradation and changes in nuclear morphology as parameters. The petals exhibit loss of turgor (wilting) as a visible symptom of PCD. DNA degradation, as shown on agarose gels, occurred in all species studied, prior to visible wilting. The number of DNA masses in all the petals of a flower, determined by flow cytometry, markedly increased in Argyranthemum and Petunia, but decreased in Antirrhinum. Many small DNA masses were observed in Argyranthemum and Petunia. The surface of each small DNA mass stained with the lipophilic fluorochrome 3,3'-dihexyloxacarbocyanine iodide (DiOC6), indicating that these masses were surrounded by a membrane. In Antirrhinum, in contrast, the chromatin fragmented into several small spherical clumps that remained inside a large membranous structure. Nuclear fragmentation, therefore, did not occur in Antirrhinum, whereas nuclear fragmentation possibly was a cause of the small DNA masses in Argyranthemum and Petunia. It is concluded that at least two contrasting nuclear morphologies exist during PCD. In the first, the chromatin fragments inside the nucleus, not accompanied--or followed--by nuclear fragmentation. In the second, a large number of DNA masses were observed each enveloped by a membrane. The second type was probably due, at least partially, to nuclear fragmentation.