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.

中文翻译：

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热应激对萝卜 (Raphanus sativus L.) 生物美拉德反应、氧化应激和内部褐变发生的影响

萝卜内部褐变（IB）是指根部整个中央部分由浅棕褐色变为深褐色，是根成熟阶段热应激引起的生理紊乱，给农民带来了重大的经济损失。在本研究中，我们在高温条件下培育了 3 个对 IB 病害具有不同耐受性的品种，并考察了影响品种间 IB 病害差异的生理因素。品种间IB病的严重程度与根实质细胞的大小密切相关。与 IB 易感品种相比，在 IB 抗性品种中观察到明显更小的细胞。IB 易感品种的正常根中的葡萄糖浓度和转化酶基因表达水平往往高于抗性品种的正常根。免疫组织化学分析结果表明，与抗性品种相比，IB 易感品种正常根的实质细胞中作为氧化应激指标的己酰赖氨酸和丙烯醛显着升高。同样，与抗性品种相比，IB 易感品种的正常根显示出 Nε-（羧甲基）赖氨酸水平升高，这是美拉德反应的指标。此外，这些氧化应激指标的免疫组织化学和美拉德反应在 IB 易感品种的受损根中观察到更强烈。