Effects of boron and phosphate on overcoming internal browning in roots of Japanese radish [Raphanus sativus]

Effects of boron and phosphate applied to the different kinds of soils and solution culture on the internal browning of roots in summer-sown Japanese radish were investigated. With hydroponics, the relationship between the incidence of internal browning and polyphenol concentrations and polyphenol oxidase activity in roots was also examined. 1. Borax, applied to the brown lowland soils (sand) and medium and coarse-textured yellow soils (sandy loam), was effective in reducing internal browning and increasing boron concentrations in the roots, but that applied to the andosols (loam) was ineffective. Super phosphate application which increased phosphorus concentrations in the roots was effective in decreasing internal browning even in andosols. 2. The occurrence of internal browning induced by the absence of boron in the hydroponic solution was severe at 29 °C as compared to 21 °C, although boron concentration in the root was relatively high at the higher temperature. These results suggest that the occurrence of internal browning can not be explained merely by concentrations of boron in the roots. 3. The polyphenol oxidase activity but not the polyphenol concentrations in the roots increased in the absence of boron in the hydroponic solution. Therefore, the internal browning is considered to be caused by the increased polyphenol oxidase activity in response to the boron deficiency in the root. In the presence of boron in the solution, the internal browning did not occur, irrespective of phosphorus concentrations in the solution. On the other hand in the absence of boron, the occurrence of internal browning and polyphenol oxidase activity in the roots decreased with the increase in phosphate levels.