The impact of phosphorus on interactions of the hemiparasitic angiosperm Rhinanthus minor and its host Lolium perenne

The effects of phosphorus supply on the outcome of interactions between the hemiparasitic angiosperm Rhinanthus minor L. with its host species Lolium perenne L. were investigated in a glasshouse experiment. Host plants were g rown in 3-1 pots in the presence and absence of R. minor at limiting (0.13 mM P) and optimal (0.65 mM P) concentrations of phosphorus for the growth o f the host species. Phosphorus was supplied at 2-day intervals in the form of half-strength Long Ashton nitrate-based solution with phosphorus concent rations adjusted accordingly. Parasitism by R. minor significantly suppress ed host growth, with final biomass losses ranging between 32% and 44%. Phos phorus supply had a marked impact on the outcome of the host-parasite inter action. By the end of the growing period, parasite biomass at 0.65 mM P was 90% lower than that achieved at 0.13 mM P, In contrast, host biomass at 0. 65 mM P was 74% higher than achieved at 0.13 mM P, indicting that the negat ive impact of parasitism on the host species was reduced when phosphorus su pply was increased. The effects of phosphorus on the host-parasite associat ion appeared to be mediated by changes in both the morphological characteri stics of the host roots and the relative sink strengths of the host and par asite.