Mechanism Underlying the Onset of Internal Blue Discoloration in Japanese Radish (Raphanus sativus) Roots.

The internal blue discoloration observed in Japanese radish (Raphanus sativus L.) roots is a physiological phenomenon caused by storage following harvest at approximately 20 °C and poses a serious problem for farmers. Here, we describe the mechanism underlying the onset of internal blue discoloration of three cultivars: Hukuhomare, SC8-260, and Yuto. Each cultivar was maintained under the same conditions. Additionally, Hukuhomare radish roots were maintained at three different cultivation conditions in a related experiment. The blue discoloration in radish roots was caused by the oxidation of 4-hydroxyglucobrassicin as a result of an increase in oxidative stress involving peroxidase. Thus, the extent of blue discoloration was influenced by the chemical balance involving 4-hydroxyglucobrassicin content, antioxidant capacity, and oxidation activity.

Structure of a Precursor to the Blue Components Produced in the Blue Discoloration in Japanese Radish (Raphanus sativus) Roots.

The internal blue discoloration in Japanese radish (Raphanus sativus L.) roots has been reported to be a physiological phenomenon after harvest and poses a significant problem for farmers. To avoid this discoloration, the fundamental development of new radish cultivars that do not undergo discoloration and/or improved cultivation methods is required. Elucidating the chemical mechanism leading to this discoloration could help overcome these difficulties. To determine the mechanism underlying this discoloration, this study was designed to probe the structure of a precursor to the blue components generated during the discoloration process. Soaking fresh roots in aqueous H2O2 resulted in rapid blue discoloration, similar to the natural discoloration. Using a H2O2-based blue discoloration assay, the precursor was extracted and isolated from the fresh roots and identified as the glucosinolate, 4-hydroxyglucobrassicin, via spectroscopy and chemical synthesis.