In photoautotrophic cultures of pak-bung hairy roots, strong light irradiation (22 W/m(2)) increased the content of reactive oxygen species (ROS) in the cells, resulting in chlorophyll (Chl) degradation and DNA injury. The Chl degradation rate, R-D, can be used as a parameter to measure the cellular damage caused by photo-induced stress. The presence of the antioxidants ascorbic acid, chlorogenic acid or catechin reduced the R-D while lowering the content of ROS and moderating the DNA injury. (C) 2009, The Society for Biotechnology, Japan. All rights reserved.