Effects of sowing time, soil temperatures, and shade on internal browning and polyphenol concentration in roots of Japanese radish [Raphanus sativus]

Effects of sowing time, soil temperatures, and shading with cheesecloth on the internal browning in roots of Japanese radish and relationship between the disorder and the polyphenol concentration and oxidase activity were investigated. 1. The occurrence of internal browning differed greatly with sowing dates; it was maximum in the late June sowing, and less in earlier and later sowings, no symptom appeared when seeds were sown after 15 August. The higher the air temperature and the more the amount of insolation during the late stage of growth, the more serious was the internal browning in the roots. The internal browning was also severe when the soil temperatures were high during the late stage of growth. The incidence of internal browning decreased when the plants were shaded with cheesecloth. 2. The polyphenol concentration and the polyphenol oxidase activity in the roots were higher in plants growing while high soil temperatures prevailed. Hence, the incidence of internal browning was positively correlated with both the polyphenol concentration and the polyphenol oxidase activity in the roots.