Resistance to paraquat in Hordeum glaucum is temperature dependent and not associated with enhanced apoplasmic binding

Summary Response to paraquat was investigated in two Hordeum glaucum Steud. (wall barley) biotypes grown at warm (30^C) and cool (15^C) temperatures. Paraquat‐resistant (R) plants were nearly seven‐fold more tolerant when grown at 15^C than when grown at 3CPC. In contrast, there was a tendency for susceptible (S) plants to be more tolerant when grown at the higher temperature. The difference in response between the two biotypes grown at 30^ and 15^C increased from 3‐ to >40‐fold. Forty‐eight hours after foliar application, 62% less radiolabelled paraquat had translocated basipetally in the R compared with the S biotype. In addition, 26% more herbicide was retained within the treated zone of R compared with S plants. Thus, paraquat movement was restricted in the R biotype. It has been postulated that reduced mobility of the herbicide in the R plants is due to enhanced apoplasmic binding. In this study, paraquat binding to the cell walls and its release into the external solution was investigated in roots of R and S biotypes of H. glaucum grown under warm or cool conditions. No significant differences between the two biotypes were measured at either growth temperature. We propose that the mechanism of resistance to paraquat may involve a temperature‐dependent alteration in symplasmic transport of the herbicide.