Programmed cell death of daylily petals: Activities of wall-based enzymes and effects of heat shock

Flowers of the daylily (Hememcallis hybrid cv Stella dʹOro) provide an excellent system for the study of events related to cell death. The petals exhibit a highly predictable programme of senescence that occurs over a 24 h period after the flowers open. As petal cells age, they undergo large alterations in size, shape and cell-to-cell attachment. Specific activities of cellulase (EC 3.2.1.4) and pectin methylesterase (EC3.1.1.11) are highest before flower opening, and specific activities of polygalacturonase (EC 3.2.1.15) and β-galactosidase (EC3.2.1.23) increase after flower opening. Addition of an enzyme preparation that hydrolyzes wall materials accelerates senescence, as indicated by increases in peroxidized lipid and the onset of ion leakage. A 3 h heat shock administered 9 h before the flowers open induces two different classes of heat shock proteins and delays events that occur during senescence, such as increases in fresh weight, total protein, reducing sugars, protease activity, lipid peroxidation and ion leakage. Heat shock accelerates the decrease in cellulase activity and retards the increase in polygalacturonase activity. Possible roles for wall-based enzyme activity in ageing of daylily petals are presented.