PPO activity was determined based on the method of Zauberman
et al. (1988). Each 3 mL reaction mix contained 0.1 M catechol, 0.1 M
sodium phosphate buffer, pH 6.8, and 100 L enzyme extract. The
increase in absorbance at 420 nm was monitored. One unit of PPO
activity was defined as the amount of enzyme that caused a change
in A420 of 0.01 U g−1 FW min−1