Distinguishing Carbon Gains from Photosynthesis and Heterotrophy in C3-hemiparasite-C3-host Pairs.

Background and Aims Previous carbon stable isotope (C-13) analyses have shown for very few C-3-hemiparasites utilizing C-4- or CAM-hosts the use of two carbon sources, autotrophy and heterotrophy. This C-13 approach, however, failed for the frequently occurring C-3-C-3 parasite-host pairs. Thus, we used hydrogen stable isotope (H-2) natural abundances as a substitute for C-13 within a C-3-Orobanchaceae sequence graded by haustoria complexity and C-3-Santalaceae. Methods Parasitic plants and their real or potential host plants as references were collected in Central European lowland and alpine mountain meadows and forests. Parasitic plants included the xylem-feeding holoparasite Lathraea squamaria parasitizing on the same carbon nutrient source (xylem-transported organic carbon compounds) as potentially Pedicularis, Rhinanthus, Bartsia, Melampyrum and Euphrasia hemiparasites. Reference plants were used for an autotrophy-only isotope baseline. A multi-element stable isotope natural abundance approach was applied. Key Results Species-specific heterotrophic carbon gain ranging from 0 to 51 % was estimated by a H-2 mixing-model. The sequence in heterotrophic carbon gain mostly met the morphological grading by haustoria complexity: Melampyrum- < Rhinanthus- < Pedicularis-type. Conclusion Due to higher transpiration and lower water-use efficiency, depletion in C-13, O-18 and H-2 compared to C-3-host plants should be expected for tissues of C-3-hemiparasites. However, H-2 is counterbalanced by transpiration (H-2-depletion) and heterotrophy (H-2-enrichment). Progressive H-2-enrichment can be used as a proxy to evaluate carbon gains from hosts.