Effects of sulfur application on enzyme activities in relation to the ascorbate-glutathione cycle and the occurrence of internal browning in Raphanus roots

The occurrence of internal browning (IB) was more frequent in sublimed sulfur applications at the levels of 0 and/or 10 g center dot m(-2); however, an increase in the severity of IB symptoms was not nearly so marked when sublimed sulfur was applied, approximately at the level of 30 g center dot m(-2). The activities of L-phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) became low in proportion to the increasing application of sublimed sulfur from 0 to 30 g center dot m(-2). Conversely, a pronounced increase in ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GSHR) activities were detected with a larger amount of this element. These observations suggest that the application of sublimed sulfur resulted in an increase in H2O2 decomposing capacity via the ascorbate-glutathione cycle, so that IB symptoms seldom intensified because of inactivation of the peroxide detoxifying pathway derived from PPO.