Tracing nitrogen flow in a root-hemiparasitic association by foliar stable-isotope labelling

Abstract

Background and aims – The resource flows in the host-hemiparasite association have been frequently studied by applying stable isotope techniques. However, these methods of artificial labelling required sophisticated equipment preventing their application to field experiments. Here, we aimed to test the applicability of the 15N13C-urea foliar brushing method in tracing the resource flows between a root hemiparasite, Rhinanthus major, and a host, Triticum aestivum. In addition, the dynamics of the label movement was examined in order to provide an estimate of the most appropriate harvesting time. Methods – Double-labelled urea (98 atom % 15N, 99 atom % 13C) solution (2 g dm-3) was applied on host plants grown with hemiparasites by a single foliar brushing. Above- and belowground biomass of both species was harvested 3, 7, and 14 d after host labelling and its isotopic composition was analyzed. Final isotopic enrichment of biomass was expressed as the atom percent difference between labelled samples and the mean of corresponding controls. Key results – Our results showed that a single leaf-brushing with 15N13C-urea provided sufficiently 15N-labelled plant material, but it was insufficient to shift the natural abundance of 13C in both species. Similar 15N values were found for the host and hemiparasite biomass already 3 d after labelling, but the 15N enrichment of attached hemiparasite significantly increased in time. Within a week, 15N-label gradually dispersed into the host tissues and was simultaneously transferred into the hemiparasite via the root connections. Conclusions – We present foliar brushing by 15N-urea as a simple and precise labelling method, which can be widely applied in both greenhouse and field experiments to examine the nitrogen flows between root hemiparasites and their host species. The transfer of nitrogen to the hemiparasite is fast and thus an experimental period of 7 d seems largely sufficient for field studies where the equilibrium state of labeling is of interest.