Severity of Chilling Injury of Sweet Potato Shoots Is Affected by Pre-Treatment of H2O2 and NaCl

Excised shoot tips of sweet potato (Ipomoea babatas L.) were incubated in H2O2 or NaCl aqueous solution for 24 or 48h prior to a 3-day chilling at 2.5 degrees C. Severity of injury was visually observed during a post-chilling 7-day recovery at 21 degrees C, and scored at 0 to 5 (none to most severe injury). There were three experiments: the first two experiments were conducted in summer, and the third in winter. In experiment 1, when 'Purple' (PUR) sweet potato shoots were subjected to 3-day chilling at 2.5 degrees 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 pretreated with 16h photoperiod, 16h was employed in the second experiment involving NaCl. In experiment 2, 4 cultivars were used: 'Ace of Spades' (ACE), 'B18', 'PUR' and 'Toka Toka Gold' (TTG). A 3x2 factorial experiment was applied to each of the four cultivars: 3 NaCl rates (0 mM, 200 mM or 400 mM) and 2 incubation times (24h or 48h). The effects of NaCl depended on cultivar (CV). Sodium chloride at 200 mM reduced chilling injury more for ACE than 'B18' and PUR, but NaCl increased the injury of TTG. The NaCl effects also depended on incubation time (24h or 48h). Across 4 cultivars the most beneficial NaCl treatment was 200 mM NaCl for 24h. In experiment 3, a factorial of 2 NaCl rates (0 mM vs. 200 mM NaCl) and 2 photoperiods (8h vs. 16h), showed different effects: photoperiods immediately prior to chilling showed no effect on resulting severity of chilling injury, and the benefit of NaCl was profound in 'B18', marginal in TTG and ACE, and detrimental in PUR. In view of these results, pre-treatments of H2O2 or NaCl may reduce chilling injury of sweet potato shoots under specific conditions, depending on cultivars and growing seasons.