Three sweet potato varieties, Taoyuan 2, Simon 1 and Sushu 18, were pretreated with four levels of CaCl2 (0, 60, 120 and 180 kg ha -1) weekly for 50 days from planting before being subjected to non-flooding (control) and flooding conditions. The experiment used a randomised complete block design with a split-split plot arrangement of treatments. Young, fully expanded leaves from each plant were clipped for measuring enzyme activities and antioxidant contents. The three genotypes exhibited unique abilities and specificities through the antioxidative systems in response to flooding stress. The level of activity of the antioxidative system in sweet potato leaves was related to CaCl2 pretreatment during flooding. The ascorbate peroxidase, superoxide dismutase, glutathione reductase, reduced ascorbate, total ascorbate, reduced glutathione and malondialdehyde contents of the three sweet potato varieties under flooding stress significantly increased because of pretreatment with 60 and 120 kg ha-1 of CaCl2. Moreover, pretreatment with 60 and 120 kg ha-1 CaCl2 enhanced the flooding tolerance of all three sweet potato varieties and mitigated the effects of flooding stress. However, pretreatment with 180 kg ha-1 CaCl2 markedly decreased some enzyme activities and antioxidant contents under a flooded condition. Calcium most likely played a role in the antioxidative system in the leaves of these three sweet potato varieties under flooding stress, as an optimum amount strengthened their oxidative systems.
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