Nature of Photooxidative Events in Leaves Treated with Chlorosis-Inducing Herbicides

Abstract

Leaves of rye seedlings (Secale cereale L. cv. Petkus Kustro) grown in the presence of 4 chlorosis-inducing herbicides under a low light intensity of 10 lx formed chlorophyll [Chl]. When segments of such dim-light-grown leaves were exposed to 30,000 lx at either 0.degree. C or 30.degree. C, treatments with aminotriazole or haloxidine (group 1) showed no or only minor changes of their Chl contents. In treatments with San 6706 or difunon (group 2), rapid photodestruction of Chl occurred at 0.degree. C and at 30.degree. C and was accompanied by an increase of malondialydehyde that was not seen in the presence of group 1 herbicides. Unlike the in vivo behavior, virtually equal rates of Chl breakdown occurred for aminotriazole and San 6706 treatments in suspensions of isolated chloroplasts from 10 lx-grown leaves after exposure to strong light. The free radical scavengers p-benzoquinone and hydroquinone and the D-penicillamine Cu complex exerting superoxide dismutating activity effectively prevented photooxidation of Chl in 10 lx-grown herbicide-treated leaf segments or even restored an accumulation of Chl at 30,000 lx. Ascorbate and several singlet O2 or hydroxyl radical scavengers had no protective effects. Deuterium oxide and H2O2 did not enhance the degradation of Chl. Superoxide dismutase activity was decreased in leaves bleached in the presence of group 2 herbicides.