Chilling Injury of Sweet Potato Shoots Reduced by Prior Incubation of H2O2 and NaCl

Excised shoot tips of sweet potato (Ipomoea babatas L.) were incubated in H2O2 or NaCl aqueous solution for 24h or 48h prior to a 3-day chilling at 2.5°C. Severity of chilling injury was visually observed during a post-chilling 7-day recovery at 21°C, and scored at 0 to 5 (none to most severe injury). In the first experiment, when cv. Purple (PUR) sweet potato shoots were subjected to 3-day chilling at 2.5°C, a 48h pre-treatment of 150 mM H2O2 under 16h photoperiod reduced chilling injury, but H2O2 showed no effect under 8h photoperiod. An increase of Oxygen Radical Absorbance Capacity (ORAC) occurred two days after recovery at room temperature, and such increase in ORAC was negatively correlated with the severity of chilling injury symptoms observed after seven days at room temperature, indicating the possible protective nature of antioxidants. Because H2O2-reduced chilling injury occurred only on those pre-treated with 16h photoperiod, 16h was employed in subsequent NaCl experiments. In the second experiment, 4 cvs were used: Ace of Spades (ACE), B18, Purple (PUR), and Toka Toka Gold (TTG). Each cv formed a 3 x 2 factorial experiment: NaCl (0 mM, 200 mM or 400 mM) and incubation duration (24h or 48h). The effects of NaCl depended on cultivar (cv). NaCl at 200mM reduced chilling injury more for ACE than B18 and PUR, but NaCl increased the injury of TTG. The NaCl effects also depended on incubation duration (24h or 48h). Across 4 cvs the most beneficial NaCl treatment was 200 mM NaCl for 24h. In view of these results, both pre-treatments of H2O2 and NaCl reduced chilling injury of sweet potato shoots, suggesting that moderate stress imposed as a pre-treatment increased plant tolerance to subsequent chilling under specific