Dodder Seeds and Growing Conditions

Cuscuta gronovii (dodder) seeds were collected from a wild population along Spruce Creek, in Huntingdon County, Pennsylvania (40°37′17′′ N, 78°07′25′′ W). To initiate germination, C. gronovii seeds were soaked in concentrated sulfuric acid for 1 h (to scarify the hard seed coat), rinsed in running deionized water for 2 min, cleaned on paper towels, and then sprinkled onto moistened filter paper in 9-cm-diameter petri dishes at a density of about 50 seeds per dish. Dishes were covered, sealed with Parafilm, and incubated in a growth chamber (25°C) for 5 d. Seedlings that shed their seed coats were placed at the base of flowering Arabidopsis (Arabidopsis thaliana) plants (radicles were slightly submerged in the soil to anchor the seedlings). To facilitate attachment, fluorescent growth chamber light was enriched in far-red wavelengths by the addition of approximately 180-W incandescent light fixtures per flat of host plants and, in some experiments, by reducing fluorescent light to one-third of the original strength during the 3-d period of attachment. After this period, light conditions were returned to their original settings so that parasites did not make subsequent attachments.