Lignin content was determined using the method of Morrison (1972) with some modifications. Three grams of exocarp tissue were homogenized with 20 mL of 95% ethanol using a FSH-2A homogenizer and centrifuged at 12,000 × g for 15 min at 4 ◦C. The pellets were washed sequentially with distilled water (15 mL), ethanol (15 mL), acetone (15 mL), and diethyl ether (15 mL). The resulting residue was dried at 70 ◦C for 1 h, and 20 mg was digested with a solution of 25% (w/w) acetyl bromide in acetic acid (2.5 mL) and HClO4 (70%, 0.1 mL)in a water bath at 70 ◦C for 30 min.After the resulting solution was cooled in an ice bath, 10 mL of 2 M NaOH and 12 mL of acetic acid were added, and the absorbance at 280 nm was measured. Lignin content was expressed on a fresh weight basis as A280 kg−1.