Instantaneous Flooding and α-Tocopherol Content in Tuberous Roots of Sweetpotato ( Ipomoea batatas (L.) Lam.)

In a previous study (Eguchi et al., 2012), tuberous root growth and antioxidant contents of two sweetpotato Ipomoea batatas (L.) Lam.) cultivars were examined using two different irrigation schemes: periodic surface-irrigation and continuous sub-irrigation. Although no apparent differences in tuberous root development were observed between the two irrigation methods, the content of tocopherol in the surface-irrigated tuberous root was significantly higher for both the cultivars. We speculated that the periodical wetting of the tuberous root surface might increase the content of the antioxidant, -tocopherol. The inner portion of bulky plant tissues such as tuberous roots can become hypoxic because they are located at sites remote from the sites of oxygen entry (Geigenberger, 2003). Furthermore, hypoxia causes oxidative stress in plant tissue (Blokhina et al., 2003). A thin water film that covers the root surface may inhibit oxygen movement into the roots, and cause an increase in the content of the antioxidant -tocopherol, for coping with the slight oxidative stress occurring within the roots. However, there was doubt as to whether the surface-irrigated water completely coated the tuberous roots. Small container cultivation as we previously used for sweetpotatoes (Eguchi et al., 2012) can readily apply instantaneous flooding, which perfectly covers the whole surface of the tuberous root. In the previous study, the O2 concentration around the tuberous root was maintained at approximately 21% during the cultivation period and was unaffected by irrigation because of the good gas permeability and water drainage of the root media. In that case, instantaneous flooding may not greatly disturb O2 concentrations when we use the same root media. Therefore, in this study, we performed instantaneous flooding of sweetpotato plants grown in a small container. The effects of the flooding treatments with different times and different intervals were investigated with regards to the -tocopherol contents in the tuberous roots. Electrolyte leakage from the root flesh was also measured for examination of the occurrence of physiological stress within the root.