Mechanism of Paraquat Resistance in Hordeum glaucum . 11.* Paraquat Uptake and Translocation

Abstract

A biotype of the important and widespread annual grass weed Hordeum glaucum Steud. is resistant to the bipyridyl herbicides. This resistance is not due to any changes at the chloroplast or protoplast level. Experiments utilising [methyl-I4c]paraquat show no difference between leaves of susceptible and resistant biotypes in the penetration of the cuticle by paraquat. Whole leaf photosynthesis in the resistant biotype is dramatically less sensitive to paraquat than in the susceptible biotype. This indicates that paraquat is sequestered in the leaf outside the cytoplasm thus preventing paraquat reaching the active site. Translocation of [I4c]paraquat from a droplet on a leaf is negligible in the resistant biotype but there is some translocation in the susceptible biotype. Paraquat remains localised in, or close to, the vascular tissue when excised leaves are supplied [14C]paraquat through the transpiration stream. It is proposed that the mechanism of resistance to paraquat is exclusion from the cytoplasm by sequestration in the apoplast.