Distinguishing carbon gains from photosynthesis and heterotrophy in C3-hemiparasite-C3-host-pairs

Abstract

Abstract Background and Aims Previous carbon stable isotope ( 13C) analyses showed for very few C3-hemiparasites utilizing C4- or CAM-hosts a usage of two carbon sources, autotrophy and heterotrophy. This 13C approach, however, failed for the frequently occurring C3-C3 parasite-host-pairs. Thus, we used hydrogen stable isotope ( 2H) natural abundances as substitute for 13C within a C3Orobanchaceae-sequence graded by haustoria complexity and C3Santalaceae. 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, 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 2H 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 13C, 18O and 2H compared to C3-host plants should be expected for tissues of C3-hemiparasites. However, 2H is counterbalanced by transpiration ( 2H-depletion) and heterotrophy ( 2H-enrichment). Progressive 2H-enrichment can be used as a proxy to evaluate carbon gains from hosts.