Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor?

The high quality of leguminous hosts for the parasitic plant Rhinanthus minor (in terms of growth and fecundity), compared with forbs (non-leguminous dicots) has long been assumed to be a function of the legume's ability to fix atmospheric nitrogen (N) from the air and the potential for direct transfer of compatible amino compounds to the parasite. Using associations between Rhinanthus minor and Vicia faba (Fabaceae) that receive N either exclusively via symbiotic associations with rhizobia supplying organic N fixed from N(2) or exclusively through the supply of inorganic nitrate to the substrate, the underlying reasons for the quality of legumes as hosts for this parasite are unravelled. It is shown that sole dependence of the host, V. faba, on N fixation results in lower growth of the attached parasite than when the host is grown in a substrate supplied exclusively with inorganic N. In contrast, the host plants themselves achieved a similar biomass irrespective of their N source. The physiological basis for this is investigated in terms of N and abscisic acid (ABA) partitioning, haustorial penetration, and xylem sap amino acid profiles. It is concluded that legume N fixation does not underpin the quality of legumes as hosts for Rhinanthus but rather the well-developed haustorium formed by the parasite, coupled with the lack of defensive response of the host tissues to the invading haustorium and the presence of sufficient nitrogenous compounds in the xylem sap accessible to the parasite haustoria, would appear to be the primary factors influencing host quality of the legumes.