RESISTANCE TO PARAQUAT IN HORDEUM-GLAUCUM IS TEMPERATURE-DEPENDENT AND NOT ASSOCIATED WITH ENHANCED APOPLASMIC BINDING

Response to paraquat was investigated in two Hordeum glaucum Steud. (w all barley) biotypes grown at warm (30 degrees C) and cool (15 degrees C) temperatures. Paraquat-resistant (R) plants were nearly seven-fold more tolerant when grown at 15 degrees C than when grown at 30 degree s C. In contrast, there was a tendency for susceptible (S) plants to b e more tolerant when grown at the higher temperature. The difference i n response between the two biotypes grown at 30 degrees and 15 degrees C increased from 3- to >40-fold. Forty-eight hours after foliar appli cation, 62% less radiolabelled paraquat had translocated basipetally i n the R compared with the S biotype. In addition, 26% more herbicide w as retained within the treated zone of R compared with S plants. Thus, paraquat movement was restricted in the R biotype. It has been postul ated that reduced mobility of the herbicide in the R plants is due to enhanced apoplasmic binding. In this study, paraquat binding to the ce ll walls and its release into the external solution was investigated i n roots of R and S biotypes of H. glaucum grown under warm or cool con ditions. No significant differences between the two biotypes were meas ured at either growth temperature. We propose that the mechanism of re sistance to paraquat may involve a temperature-dependent alteration in symplasmic transport of the herbicide.