Abstract Petals of the daylily, Hemerocallis hybrid cv Stella d'Oro, show visible signs of degradation just 24 h after flower opening. This is preceded by increases in ion leakage. Since losses of membrane differential permeability have been linked to reactive oxygen species (ROS), we are asking if daylily senescence is controlled in part by reactions associated with ROS. Ion leakage is prevented by hypoxic conditions, and peroxidized lipid, lipoxygenase activity and H 2O 2 increase even before flower opening. Treatments that stimulate cell death also prematurely increase levels of H 2O 2. The specific activity of protective enzymes such as catalase and ascorbate peroxidase decreases during flower senescence as do levels of ascorbic acid. Superoxide dismutase and peroxidase specific activities increase. We suggest that membrane changes leading to cell death may be induced in part by lipoxygenase activity and by ROS that are increasing because of reduced effectiveness of certain protective enzymes.