PARASITIC AND COMPETITIVE INTERACTIONS BETWEEN THE HEMIPARASITES RHINANTHUS-SEROTINUS AND ODONTITES-RUBRA AND THEIR HOST MEDICAGO-SATIVA

1 Host plants are for hemiparasites both their main source of water an d nutrients and potential competitors for light. To investigate the ba lance between costs and benefits of host presence a greenhouse experim ent was performed with the two facultative root hemiparasites Rhinanth us serotinus and Odontites rubra and their host Medicago sativa. Above -ground (light competition) and below-ground (parasitic) interactions were separated and the host plants were killed 4, 8 and 12 weeks after planting by cutting off the shoots. 2 R. serotinus remained smaller t han 0. rubra when grown autotrophically and its growth was more stimul ated by the presence of the host, indicating that R. serotinus has a h igher degree of host dependence. The marked reduction in root allocati on by both hemiparasites in the presence of the host was more pronounc ed in R. serotinus. 3 Above-ground competition by the host reduced the growth of both hemiparasites by more than 30%. However, the negative effect of removing the host after 4, 8 and 12 weeks of development ind icated that the beneficial effects of the host predominated at all sta ges. 4 Both parasites had negative effects on host growth that were mu ch stronger than that of another M. sativa individual. Above-ground se paration of parasites and hosts had no effect on host biomass, indicat ing that the negative effect was due exclusively to parasitism. 5 Host root biomass was reduced more strongly than host shoot biomass by the parasites, resulting in a lower biomass allocation to roots. 6 The pr esence of the hemiparasites reduced the total productivity per pot. Th is suggests that the parasites have a lower efficiency of resource uti lization than the hosts. 7 It is suggested that the balance between th e effects of root parasitism on the host (e.g. via nutrient competitio n) and the effects of light competition by the host on the parasite re stricts hemiparasites to relatively nutrient-poor habitats.