PAL activity was measured as described (Degl'Innocenti et al.,
2005) with some modification. About two g frozen lettuce stem discs from the mixture of 25 discs were homogenized at 4 C with
16 mL of 50 mmol L 1 borate buffer (pH 8.5) containing 5 mmol L 1
2-mercaptoethanol and 0.2 g of polyvinylpyrrolidone. The ho- mogenate was filtrated through 4 layers of cheesecloth and centrifuged at 20,000 g at 4 C for 10 min. The supernatant was assayed for PAL activity after the addition of 0.55 mL of
50 m mol L 1 L-phenylalanine and incubation at 40 C for 1 h. The
samples were measured at 290 nm using the Shimadzu PharmaS- pec UV-160A UVeVis spectrophotometer (Shimadzu Scientific In- strument, MD, USA) before and after incubation. One unit of PAL
activity equals the amount of PAL that produced 10 3 mol of
transcinnamic acid per kg fresh discs in 1 h. PAL activity was measured daily from day 0 to day 6 of storage.