Compared to sub-irrigated sweetpotatoes (Ipomoea batatas (L.) Lam.), periodic surface-irrigated plants, i.e., twice a week on root media, showed increased α-tocopherol content in their tuberous roots with no apparent changes in both of tuberous root development and oxygen concentration around the roots. We speculated that surface irrigation might temporarily cover the tuberous root surface with water and inhibit oxygen movement into the roots, thereby increasing the antioxidant α-tocopherol content, for coping with the slight oxidative stress occurring within the roots. Therefore, we performed 1-3 times instantaneous flooding, with different intervals, which perfectly covered the whole root surface with water, of sweetpotato plants grown in a phytotron glass room (25°C, 70%RH). Electrolyte leakage from the tuberous root flesh cells showed a temporal increase for the flooding treatment, while it immediately recovered within 24 h. Instantaneous flooding did not affect the storage root development in any of the experiments. Apparent increases in the α-tocopherol content were observed during the 3-time flooding at 3-day intervals. Our results suggest that more frequent root surface wetting is necessary for increasing the α-tocopherol content, which is released because of the oxidative stress that occurs within the roots.
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