Effects of heat stress on the biological Maillard reaction, oxidative stress, and occurrence of internal browning in Japanese radish (Raphanus sativus L.).

Internal browning (IB) of Japanese radish, in which the entire central part of the root changes from light tan to dark brown, is a physiological disorder caused by heat stress at the root maturation stage and represents significant economic losses to farmers. In this study, we cultivated three cultivars with different tolerance to IB disorder under high-temperature conditions, and examined the physiological factors involved in IB disorder differences among cultivars. There was a close correlation between the severity of IB disorder among cultivars and the size of root parenchymal cells. Significantly smaller cells were observed in the IB-resistant cultivars compared to the IB-susceptible one. Glucose concentration and invertase gene expression level tended to be higher in the normal roots of the IB-susceptible cultivar than in those of the resistant cultivars. Results of immunohistochemical analysis revealed that hexanoyl-lysine and acrolein, which are indicators of oxidative stress, in parenchymal cells were significantly elevated in the normal roots of the IB-susceptible cultivar than in those of the resistant ones. Similarly, the normal roots of the IB-susceptible cultivar showed elevated levels of Nε-(carboxymethyl)lysine, which is an indicator of the Maillard reaction, compared with those of the resistant cultivars. In addition, the immunohistochemistry of these indicators of oxidative stress and the Maillard reaction were more strongly observed in the damaged roots of the IB-susceptible cultivar. These results indicate that the occurrence of IB in Japanese radish is closely related to the increase in cell size, enhancement of the Maillard reaction by elevation in reducing sugar concentration, and increase in intracellular oxidative stress.
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