Mode of Evolved Photooxidant Resistance to Herbicides and Xenobiotics

A few species have evolved resistance to paraquat after repeated selection. As paraquat still inhibited NADP reduction, we hypothesized that resistance might be due to (a) detoxification of the paraquat-generated active oxygen species and (b) that resistant plants would have some cross resistance to other xenobiotic oxidants as well as to photoinhibition, which we subsequently demonstrated. The levels of plastid isozymes of the oxygen detoxification pathway. (CuZn) superoxide dismutase, ascorbate peroxidase and glutathione reductase were genetically higher in the resistant than in the sensitive biotype of Conyza bonariensis through the F2 generation. Resistance was suppressed by chelators of copper and/or zinc. Intact chloroplasts from resistant plants had less membrane damage with and without paraquat, than those from sensitive plants. Resistant Conyza plants recover from paraquat inhibition of photosynthesis in 3–4 h in high light, whereas sensitive plants died. Both resistant and sensitive plants recovered from paraquat in 3–4 h in low light intensities. Paraquat-resistant Conyza plants were cross-tolerant to SO2, atrazine, acifluorfen and to photoinhibition. Drought-tolerant maize inbreds were cross-tolerant to paraquat, SO2and acifluorfen (compared to sensitive lines) and they also possessed higher levels of (Cu/Zn) superoxide dismutase and glutathione reductase. The tolerance to oxidant stresses in Conyza and maize increases with plant age, suggesting that the shift to resistance is a constitutive, earlier expression of the genes normally expressed later in development.