Caffeic acid inhibits the enzymatic degradation of IAA, probably by formation of semiquinones. These radicals then occupy the sites on the non-IAA degrading peroxidases which are normally used for the transfer of the IAA free radicals, thereby closing pathway 1 (see fig.l). H202 cancels this inhibiting effect by directing the flow of the IAA free radicals via pathway II to the cofactor.