Resistance to paraquat has been studied in detail in many weed species for more than a decade, wit h the precise mechanism of resistance still unclear. Several studies have indicated that reduced movement of the herbicide to the site of action in the chloroplast is at least partly responsible for endowing resistance. Although paraquat translocation studies have been performed in the past it has been rare for these studies to have been conducted;on whole-plants in the light, despite early observations which clearly showed that paraquat translocation is minimal unless treated plants are exposed to light. This study has addressed this issue in Arctotheca calendula by tracing the movement of C-14-paraquat in resistant and susceptible plants in both the dark and light. Differences in paraquat translocation between. the resistant and susceptible biotypes of A. calendula were only observed when treated plants were exposed to light. It was observed that paraquat translocation was significantly reduced in the resistant compared to the susceptible biotype, when plants were exposed to light but not in the dark. It is postulated that paraquat translocation is dependent on light mediated damage. As paraquat-induced damage is less severe in paraquat resistant plants, overall paraquat translocation is reduced in the resistant biotype. (C) 2003 Elsevier Science (USA). All rights reserved.