The phenolic compounds synthesized by PAL can be oxidized, by PPO to quinones, which spontaneously polymerize to brown pig- ments responsible for tissue browning (Ke & Saltveit, 1986, 1989). It has been reported that ethanol retardation of browning develop- ment resulted from its inhibiting effects on the formation of reac- tive oxygen species and the activity of various oxidases (Hu et al.,
2010). Thus we compared the changes in contents of total phenolic compounds and quinones in different treatments during 6 days storage. Soluble phenolic compounds accumulated in the control group after cutting, reached a peak on day 4, and then decreased. The soluble phenolic compounds in the ethanol treated discs also increased during storage, but lagged behind the controls. However, they surpassed them in the control group on day 5 and peaking on day 6 (Fig. 4A). Choi, Toma s-Barberan, and Saltveit
(2005) reported that treatment with 0.1 mol L 1 1-butanol for 2 h
or exposure to 0.02 mol kg 1 FW 1-butanol vapors for 12 h reduced the accumulation of wound-induced phenolic compounds in let-
tuce mid-rib tissue. Quinone concentration in all samples increased after cutting, fluctuated, and reached a peak at day 4 and then decreased. However, in ethanol treated samples, quinone concen- tration remained lower and kept more stable than the control (Fig. 4B). These results suggest that ethanol treatment can reduce the accumulation of polyphenols and quinones so as to retard the browning.