Ducharme, L.A., and J.R. Ehleringer. 1996. Gas exchange, d13C, and heterotrophy for Castilleja linariifolia and Orthocarpus tolmiei, facultative root parasites on Artemisia tridentata. Gr. Basin Nat. 56:333-340.

Gas exchange and carbon isotope ratios were measured on 2 facultative hemiparasites, Castilleja linariifolia Benth. (Indian paintbrush; Scrophulariaceae) and Orthocarpus tolmiei H. & A. (Tolmie owl clover; Scrophulariaceae) and their Artemisia tridentata L. (big sagebrush; Asteraceae) hosts. Photosynthetic rates differed greatly between years; rates in 1995 were more than double those in 1994, likely due to more precipitation and less water stress during 1995. Despite this difference in precipitaion, photosynthetic rates for C. linariifolia were not different from those of their hosts for either year. However, carbon isotope ratios of C. linariifolia and O. tolmiei were up to 3% more negative than those of their A. tridentata hosts. Using measured d13C ratios in conjunction with d13C ratios predicted from gas-exchange measurements, we calculated thatC. linariifolia derived, on average, 40% of its leaf carbon heterotrophically. Contrary to current suggestions that high photosynthetic rates if hemiparasites are an indication of reduced heterotrophy, C. linariifolia exhibited photosynthetic rates similar to autotrophic plants and used a substantial amount of host-derived carbon. Moreover, this evidence shows that manipulation of a heterotrophic carbon supply transcends obligate hemiparasites to include those plants whose parasitism is facultative.