Relationship between the concentration of pectin-like substances and the severity of internal browning in radish (Raphanus sativus L.) roots induced by high soil temperature

Abstract

SummaryIn white radish (Raphanus sativus L.) roots, internal browning (IB), a physiological disorder characterised by a brown discolouration in the central stele, is frequently associated with summer-sown crops. In this study, nine cultivars of white radish were studied to clarify the association between pectin-like substances and the severity of IB. Radish cultivars could be classified into three types according to the severity of the IB disorder: resistant, moderately susceptible, or susceptible. An increase in the severity of the disorder was accompanied by a parallel increase in polyphenol oxidase activity.The mean cell length in each IB-resistant cultivar was < 135 µm, while the mean cell length in each moderately susceptible or susceptible cultivar, with one exception, was 1.2- to 1.8-times larger. Although there was no association between the severity of IB and the concentration of water-soluble pectin or pectic acid, the IB-resistant cultivars contained a higher concentration of protopectin than the moderately susceptible or susceptible cultivars. These results suggest that IB in radish roots is closely associated with the concentration of protopectin, which varies according to cell size.