Legumes vary strongly in their quality as hosts for parasitic plants: interactions between the root hemiparasite Rhinanthus alectorolophus and 30 legume species.

Abstract

Background: Host identity can strongly affect the performance of root hemiparasitic plants. Legumes have frequently been observed to be particularly beneficial hosts and it has been suggested that they are in general good hosts due to their association with rhizobia, but exceptions have been observed.

Methods: We studied the interactions of the root hemiparasite Rhinanthus alectorolophus with 30 legume species. We related parasite performance and morphology to traits of uninfected hosts and the number and size of haustoria. We also analysed sections of haustoria quantitatively using morphometrics.

Key results: There was enormous variation in the biomass and other traits of R. alectorolophus with the different legume species. The suitability of a legume species as a host had a significant phylogenetic component (Pagel's λ = 0.51). Parasite performance increased with the biomass of uninfected hosts. In contrast, host root traits explained little of the variation in parasite performance and there was no anatomical evidence of defence against infection. Parasite biomass was very strongly and positively related to the number of haustoria formed, but part of the variation due to host identity could not be explained by the host traits studied and the number of haustoria, suggesting a role for variation in the quality of compounds provided by the hosts. Damage to the hosts increased with mean parasite mass, but some legumes were tolerant of parasite attack. Sensitivity of the hosts to parasitism showed only a very weak phylogenetic signal (λ < 0.01).

Conclusions: Legumes are not universally good hosts for root hemiparasites. Instead their quality as hosts ranges from very beneficial to very poor. Moreover, clades within the legumes differ in their suitability as hosts for hemiparasites. Parasites grow best with fast-growing legumes that quickly produce large root systems with which many and large haustoria are formed.