White light-grown seedlings of smallseed dodder were (a) provided with unilateral far-red (700 to 800 nm) at photon irradiances ranging from 20 to 110 μmol m−2 s−1 against a background of cool white light (400 to 700 nm) from above at 77 μmol m−2 s−1, or (b) transferred to darkness and provided with unilateral white light at 20 μmol m−2 s−1, unilateral blue light (400 to 500 nm) at 10 μmol m−2 s−1, unilateral red light (600 to 700 nm) at 10 μmol m−2 s−1, unilateral far-red at 50 μmol m−2 s−1, or (c) in experiments utilizing bilateral irradiations, provided with unilateral far-red perpendicular to unilateral white light. Positive phototropic curvature was induced by unilateral white light and by unilateral blue light in otherwise darkness and by unilateral far-red in a background of cool white light. Seedling vines were also phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Phototropism to unilateral white light was inhibited in seedlings treated with 200 μM norflurazon and 50 mM potassium iodide. Norflurazon- and potassium iodide-treated seedlings remained phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Seedling vines were not phototropic to unilateral red or to unilateral far-red in otherwise darkness, and seedlings in cool white light were neither skototropic (i.e., tropic toward unilateral darkness) nor tropic to or from infra-red (radiation with wavelengths greater than 900 nm). Phototropism toward regions of lowered red:far-red may aid smallseed dodder in chlorophyllous host location and attachment.

White light-grown seedlings of smallseed dodder were cyclically irradiated from above with red light for 4 h followed by red plus far-red for 4 h. Tracings of initially nearly vertical, white light-grown seedlings were made from time-lapsed video images after four cycles of red light followed by red plus far-red. Stems of seedlings cycled into red light were negatively gravitropic and stems of seedlings then cycled into red plus far-red became photoreversibly, positively gravitropic. Observations suggest gravitropic sign reversal in smallseed dodder is regulated by photochrome.