The programmed senescence of flower petals has been shown to involve the fragmentation of nuclear DNA. Nuclear DNA fragmentation, as determined by the TUNEL assay, was detected in Petunia x hybrida corollas during both pollination-induced and age-related senescence. DNA fragmentation was detected late in the lifespan of the flower when corollas were wilting and producing ethylene. The induction of a 43 kDa nuclease (PhNUC1) correlated with increased DNA fragmentation. PhNUC1 is a glycoprotein with activity against DNA and RNA and a pH optimum of 7.5. EDTA was found to inhibit PhNUC1 activity, but the addition of Co2+ restored activity in the presence of the chelating agent. When total protein extracts from senescing petals were fractionated by differential centrifugation, PhNUC1 activity was detected in the nuclear but not the cytoplasmic fraction. Activity of PhNUC1 was induced in non-senescing corollas by treatment with ethylene. Delayed increases in PhNUC1 activity observed in ethylene-insensitive flowers (35S:etr1-1) suggest that ethylene modulates the timing of PhNUC1 induction, but that it is not an absolute requirement for its activation.