2.6. Determination of vitamin C
Vitamin C content was determined by the method of Gahler
et al. (2003). A total of 20 mg lettuce samples (shoot FW) were
mixed with1.5 mL of 4.5%aqueous phosphoric acid.After shaking at
300 rpm for 30 min in the darkness, the mixtures were centrifuged
at 16,000 × g for 10 min. The supernatants were used to determine
the concentration of Vitamin C with the HPLC system equipped
with C18 column (Restek USA, Bellefonte, PA, USA) at 30 ◦C. Extract
was eluted with mobile phase (0.21% phosphoric acid) at a flow rate
of 0.8 mL/min, and the concentrations were determined at 254 nm
against ascorbic acid standards (Standard Substance Center, China).