Enhanced Inducibility of Antioxidant Systems in a Nicotiana tabacum L. Biotype Results in Acifluorfen Resistance*

Levels of non-protein thiols (mostly glutathione, GSH), ascorbic acid (AA), and activities of the enzymes ascorbate peroxidase (AP), glutathione reductase (GR) and GSH S-transferase (GST) were determined in cell-free leaf extracts of acifluorfen-resistant and -sensitive tobacco plants. These parameters were examined also in detached leaves of the above plants exposed to acifluorfen stress. In leaves of untreated plants the AA content was by 40% higher in the resist­ ant biotype as compared to the sensitive ones, but the levels of GSH, AP, GR, and GST did not differ significantly in the two biotypes. However, in the resistant leaves stressed by aci­ fluorfen the activity of AP readily increased while in the sensitive leaves it did not change. The levels of GSH and the activities of GR and GST markedly increased in both biotypes after acifluorfen stress, but the induction in the resistant leaves was consistently stronger in each case. The AA contents were increased equally in both biotypes. These parameters were much less affected by paraquat stress. The only significant changes were observed at low concentra­ tions of this herbicide (8 x 10 '9 m): when the thiol content and the activity of GST increased in the resistant leaves. Enhanced inducibility of antioxidant systems seems to be involved in resistance of tobacco to acifluorfen stress.