How is glyphosate resistance modified by exogenous salicylic acid application on Conyza bonariensis biotypes

Hairy fleabane (Conyza bonariensis (L.) Cronq.) is an annual weed which occurs typically on waste land and crops such as orchards and vineyards. It is considered one of the worst weeds worldwide, including Greece, because it has developed resistance to glyphosate. Glyphosate is a systemic, non-selective herbicide and it is the world’s biggest selling for weed control. Salicylic acid is a phenolic growth regulator, providing protection against biotic and abiotic stress of plants. In the present study, it is investigated the effect of a synthetic compound which is a chemical analog to salicylic acid (benzo (1,2,3) - thiadiazole −7-carbothioic acid S - methyl ester -BTH), in fleabane resistant and susceptible biotype, on glyphosate resistance. Some plants from resistant and susceptible biotype were sprayed with glyphosate, while others were sprayed with both BTH and glyphosate. The proline content, hydrogen peroxide (H2O2) content and malondialdehyde (MDA) content in fleabane leaves were then determined on the 1st, 6th and 10th days following treatment. The concentration of shikimic acid and photosynthetic activity was determined on the 3rd day after treatment. The results showed that the BTH caused protection against glyphosate damage in both fleabane biotypes, decreasing the hydrogen peroxide and MDA content and increasing the proline content. However, BTH did not affect the concentration of shikimic acid. Moreover, resistant and susceptible biotype were affected differently by BTH application. Considering all the above, we can conclude that BTH induced the defense in plants, in both biotypes, after the glyphosate application. Furthermore, susceptible biotype was affected more by BTH application than resistant biotype. Although, the defense mechanisms in susceptible biotype were induced more profoundly after BTH application compared to resistant biotype, susceptible biotype showed slow activation of defense mechanisms, thus it could not survive possibly due to the fact that glyphosate action had already caused detrimental effects on plants.