Great Basin Naturalist


Gas exchange and carbon isotope ratios were measured on 2 facultative hemiparasites, Castilleja linariifolia Benth. (Indian paintbrush; Scrophulariaceae) and Othrocarpus tomiei 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 the difference in precipitation, 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 δ13C ratios in conjunction with δ13C predicted from gas exchange measurements, we calculated that C. linariifolia derived, on average, 40% of its leaf carbon heterotrophically. Contrary to current suggestions that high photosynthetic rates of 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.