Root‐differentiated tissue cultures (PS‐R) fromPisum sativum(cv. Greenfeast) were exposed to a 5 mMsolution of the free radical‐generating compound 2,2′‐azobis (2‐amidinopropane) dihydrochloride (AAPH). The levels of mRNA transcripts for two genes were examined:chs2, encoding a chalcone synthase isozyme, andcab, encoding the chlorophylla/b‐binding protein of the light‐harvesting antenna complex. In light‐grown PS‐R, cab mRNA transcript levels decreased to 14% of controls after 6 h of exposure, whereaschs2mRNA levels increased 50‐fold. In dark‐grown PS‐R,chs2mRNA transcripts increased by 40‐fold compared with the controls. Glutathione determination in light‐grown PS‐R showed no substantial difference in total glutathione (GSHtot), whereas oxidized glutathione (GSSG) increased by 66% after 12 h of exposure. However, in dark‐grown PS‐R a decrease in both GSHtotand GSSG after 6 h was followed by an increase of about 70%, as compared with the controls, after 12 h of exposure. In conclusion, AAPH generated oxidative stress, reflected in changed glutathione levels and induced expression of thechs2gene of the flavonoid biosynthetic pathway and also caused a decreased level of