The effects of three postemergence herbicides that inhibit carotenoid biosynthesis were tested on field dodder. Flurochloridone, sulcotrione, and mesotrione treatments led to bleaching symptoms in field dodder stems. The effect of flurochloridone was rapid; 2 d after treatment (DAT) the stem was bleached and contained only 2% β-carotene, with a massive accumulation of phytoene in comparison with the control. However, flurochloridone treatment did not inhibit stem elongation, and full recovery of pigment composition at newly elongated stems was recorded 6 DAT. The effects of sulcotrione and mesotrione were similar, but the recovery was slower than with flurochloridone. The developing stems were fully bleached 6 DAT, with no detectable β-carotene, and subsequently no recovery was observed. All three herbicides led to mass destruction of the plastids in the parenchyma cells of the cortex and pith tissue. This disruption of the plastids was associated with depletion in starch content. Sulcotrione and mesotrione treatments reduced field dodder biomass accumulation, whereas flurochloridone delayed biomass accumulation by 6 d only. The differences in phytotoxicity between flurochloridone, sulcotrione, and mesotrione might be due to their different translocation patterns. Flurochloridone is xylem-mobile and is therefore localized at the applied area because of the low transpiration rate of field dodder. Sulcotrione and mesotrione are phloem-mobile and tend to accumulate in sinks such as the primordial tissue. The massive destruction of the amyloplasts and the rapid decline in starch content in response to all the herbicide treatments may indicate the possible involvement of carotenoids in stabilizing the amyloplast outer membrane in field dodder.

Nomenclature: Flurochloridone; mesotrione, 2-(4-mesyl-2-nitrobenzoyl)-3-hydroxycyclohex-2-enone; sulcotrione, 2-[2-chloro(4-methylsulfonyl)benzoyl]cyclohexane-1,3-dione; field dodder, Cuscuta campestris Yuncker CVCCA; carrot, Daucus carota L. ‘Tip Top’.