EVIDENCE FOR VACUOLAR SEQUESTRATION OF PARAQUAT IN ROOTS OF A PARAQUAT-RESISTANT HORDEUM-GLAUCUM BIOTYPE

Paraquat resistance in the grass weed Hordeum glaucum Steud. has been proposed to result from herbicide sequestration away from the growing points. In the present study, we used roots as a model system to inves tigate cellular transport of paraquat in resistant (R) and susceptible (S) H. glaucum biotypes. Both time- and concentration-dependent kinet ics of paraquat influx across the root cell plasma membrane were simil ar in the S and R biotype. However, compartmentation analysis indicate d greater herbicide accumulation in root vacuoles of the R seedlings. In contrast, the amount of paraquat accumulated in the cytoplasm of S was double that found in R biotype. While paraquat efflux from the cyt oplasm back into the external solution was similar in the two biotypes , efflux across the tonoplast from the vacuole back into the cytoplasm was 5 times slower in the R than in the S biotype. At the end of a 48 -h efflux period, nearly 7-fold more herbicide was retained in the roo ts of the R compared with those of the S biotype. These results sugges t that paraquat resistance in H. glaucum may be due to the herbicide s equestration in the vacuole.