INTERACTIONS BETWEEN THE ROOT HEMIPARASITE MELAMPYRUM ARVENSE AND MIXTURES OF HOST PLANTS - HETEROTROPHIC BENEFIT AND PARASITE-MEDIATED COMPETITION

Root hemiparasitic plants frequently establish haustorial connections simultaneously to several host species, which may differ in the qualit y of their contribution to the nutrition of the parasite and in their sensitivity to parasitic attack. The obligate hemiparasite Melampyrum arvense was grown with three host species (Lolium perenne, Medicago sa tiva and Linum usitatissimum) and with all possible binary host mixtur es to rest whether (1) a mixed host supply (e.g. a legume and a grass) results in an increase in heterotrophic benefit to the parasite and ( 2) whether the hemiparasite may influence the competitive balance betw een hosts. Growth of M. arvense was much stronger with the legume (M. sativa) as host than with the two non-leguminous species. The biomass of parasites grown with binary mixtures of the host species was not hi gher than that of parasites grown with the three host species separate ly. Thus, the results do not support the hypothesis that hemiparasites benefit from a mixed host supply. The rank order of species in terms of host quality (M. sativa > L. usitatissimum > L. perenne) was the sa me as that in terms of damage suffered from the hemiparasite. Host and parasite biomass in individual pots were also negatively correlated. In two of the binary mixtures of host species M. arvense influenced th e competitive balance between hosts, because the hemiparasite reduced the growth of the legume M. sativa much more strongly than that of the other species. Overall productivity per pot (hosts + hemiparasite) wa s reduced by M. arvense, because parasite biomass did not fully compen sate for the reduction in host biomass caused by parasitism.