For vitamin C analysis, pulp tissue (2 g) was well homogenizedwith 20 mL of 6% metaphosphoric acid in 2 M acetic acid and centrifuged for 10 min at 13,000 × g and 4◦C. The vitamin C content in the supernatant was determined by the dinitrophenylhydrazine method of Terada et al. (1978)