Glyphosate is the most widely used herbicide for postemergence weed control in Florida citrus (Citrus spp.). Variation in susceptibility of certain weed species to glyphosate has been observed in last few years. Therefore, understanding the mechanism underlying such phenomenon is required. Experiments were conducted to evaluate differences in tolerance of four weed species to glyphosate by quantifying glyphosate efficacy, the amount of epicuticular wax, absorption, and translocation of carbon-14-labeled glyphosate (C-14 glyphosate). The results of glyphosate efficacy study suggested that application of glyphosate at 3 oz/acre resulted in 99%, 90%, and 84% control of florida beggarweed (Desmodium tortuosum), spanishneedles (Bidens bipinnata), and johnsongrass (Sorghum hale-pense), respectively. Increasing application rate and addition of nonionic surfactant (NIS) usually did not improve glyphosate efficacy. Ivyleaf morningglory (Ipomoea hederacea) was the most tolerant and resulted in 0% and 25% control when glyphosate applied at 3 and 24 oz/acre, respectively. Biomass reduction in all weed species reflected a similar trend to percent control in response to all glyphosate treatments. Glyphosate absorption and translocation in the weed species were differed with the quantity of wax extracted. Ivyleaf morningglory had the lowest leaf wax content (10.8 mu g.cm(-2)) and showed less absorption (62% to 79%) and translocation (15% to 39%) of C-14-glyphosate compared with other weed species. The absorption of C-14-glyphosate was in the range of 87%, 71% to 83%, and 72% to 83%; and translocation was 34% to 50%, 32% to 52%, and 53% to 58% in florida beggarweed, spanishneedles, and johnsongrass, respectively. Increasing glyphosate application rate from 6 to 12 oz/acre and addition of NIS usually increased C-14-glyphosate translocation.
