Lack of Cross-Resistance of Paraquat-Resistant Hairy Fleabane (Conyza bonariensis) to Other Toxic Oxygen Generators Indicates Enzymatic Protection is Not the Resistance Mechanism

Cross-resistance of the paraquat-resistant (R) hairy fleabane to other compounds that accept electrons from photosystem I (PSI) or produce toxic oxygen species was determined by chlorophyll loss, electron microscopy, and chlorophyll fluorescence suppression. Although the R bioype is approximately 100 x more resistant to paraquat than the susceptible (S) biotype based upon the assays for tissue damage, little or no cross-resistance was observed to a number of other PSI electron acceptors, including the bipyridilium herbicide morfamquat. A low level of resistance (approximately 10-fold) was noted to diquat and the singlet oxygen generator rose bengal. As measured by chlorophyll fluorescence suppression, the R biotype was about 100-fold resistant to paraquat, but only 10-fold resistant to diquat, and exhibited no resistance to morfamquat. Because differences observed with this protocol are direct measures of the ability of the herbicide to reach the active site and the results correlate with the level of resistance observed by chlorophyll bleaching or electron microscopy, these data suggest that compartmentalization is the major factor in paraquat resistance in hairy fleabane.