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Two populations of Hordeum leporinum have evolved resistance to paraquat within a small area in central Tasmania, Australia. One population (THL1) was more than 80-fold resistant to paraquat when treated in winter, compared with a susceptible population (THL4) collected nearby, whereas the other population (THL2) was only 19-fold resistant. Translocation of paraquat was examined in all three populations at warm and cool temperature regimes. Herbicide was applied to a basal section of the second leaf of plants kept in the dark and translocation measured after 16 h of dark and during a subsequent light period. Paraquat absorption into the treated leaf was uniformly high in susceptible and resistant populations, with >93% of the applied herbicide absorbed within 16 h in the dark at both tempera-tures. Translocation of paraquat out of the treated leaf was low in the dark, with <4% of the herbicide translocated to the remainder of the plant. More herbicide was translocated out of the treated leaves in susceptible plants in the dark, compared with resistant plants at both temperature regimes and more paraquat was translocated at warmer temperatures. Extensive basipetal translocation of paraquat to the rest of the plant occurred in susceptible plants following exposure of the treated plants to light. However, basipetal translocation was much reduced in resistant plants in the light and corresponded to the degree of resistance. Resistance to paraquat in H. leporinum is the result of reduced translocation of paraquat out of the treated leaves.

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Pollen movement between individuals can increase the rate of herbicide resistance evolution by spreading resistance alleles within or between populations and by facilitating the rapid accumulation of resistance alleles within individuals. This study investigated the level of pollen-mediated gene flow between paraquat-resistant and paraquat-susceptible populations of the self-pollinated weed species hare barley. The experiment was conducted in both directions, from resistant to susceptible and susceptible to resistant, across 2 yr. To maximize the potential for pollen flow, individual plants were grown in a single pot. The level of gene flow was similar across years and between genotypes. The level of pollen-mediated gene flow ranged from 0.06 to 0.15%. Gene flow from resistant to susceptible plants was confirmed by demonstrating segregation for resistance in the progeny of suspected crosses. This study suggests that pollen-mediated gene flow will occur in this species at frequencies less than 0.16% and could assist the accumulation of resistance alleles within a population. These low levels of gene flow through pollen movement suggest that cross-pollination over larger distances would be unlikely and pollen movement probably does not contribute to gene flow between populations.