My advice is that you do not use the activity staining method to identify phenolic acid decarboxylase. My experience says that you can get erratic results strewn all over the work.
I am personally against using activity staining method for identification of phenolic acid decarboxylase. I would suggest that you use the TUNEL (Terminal deoxyribonucleotidal transferase mediated deoxyuridine triphosphate (dUTP) nick end labeling assay) method. I am giving you the reference where you can find this method:
Gavrieli Y, Sherman Y, Bensasson SA: Identification of programmed cell-death in situ via specific labeling of nuclear DNA fragmentation. J Cell Biol 1992, 119(3):493-501