Activities of Various Peptidases in the Senescing Petals of Tulip

The hydrolysis of seven peptidase substrates by petal extracts of tulip (Tulipa sp. cv. Guillaume Tell) was assayed at four stages in the life cycle of the petals. During senescence pronounced increases from three- to sixfold took place in all the activities. At the same time the amount of soluble protein decreased about 75% and there was a simultaneous increase in free amino acids. The activity levels (on a dry weight basis) in the petals at the beginning of withering were rather similar to those present in normal leaves. Despite their different pH-optima (5-9), all the peptidases probably act in the mobilization of protein amino acids in the senescing petals.