• histology;
  • mass flow;
  • mass spectrometry;
  • 15N tracer;
  • parasitic plant;
  • xylem


  • • 
    The root parasite Rhinanthus minor feeds on the xylem of a diverse range of species. Grasses and legumes are the best hosts, while on forbs R. minor typically shows poorer growth. It has been hypothesized that host quality is linked to the expression of defences against the parasite seen in forb roots, but never in grasses. The efficacy of these defence mechanisms in preventing resource loss has not, however, been measured directly.
  • • 
    Here we combine histological characterization of haustoria formed on Cynosurus cristatus (a grass), Leucanthemum vulgare and Plantago lanceolata (forbs) with 15N tracers supplied to the host to quantify the efficacy of these defence responses.
  • • 
    Rhinanthus minor penetrated only the xylem of C. cristatus, abstracting an average of 17% of the 15N tracer taken up, but only 2.5 and 0.2%, respectively, when attached to L. vulgare and P. lanceolata.
  • • 
    For the first time, this study has established that the resistance mechanisms of the forbs are effective in preventing the parasite from directly accessing their xylem solutes.