The effect of host defoliation on hemiparasitic-host interactions between Rhinanthus serotinus and two Poa species

We determined in a greenhouse experiment whether experimental defoliation o f the host affects the performance of the parasitic plant and whether the e ffects on the success of the parasite depend on the host species. We also a sked whether two species of grass hosts differ in response to simultaneous defoliation and hemiparasitic infection. The experiment had a complete 2 x 3 x 2 factorial design, with host species (Poa annua L. or Poa pratensis L. ) combined with defoliation (undefoliated, 50% defoliated, or 100% defoliat ed) and hemiparasitic infection (parasitized or unparasitized). Defoliation reduced the final biomass of both host species. However, total biomass and the number of flowers produced by the hemiparasitic Rhinanthus serotinus ( Schonh.) Oborny was reduced only when the host was P. annua and when the ho st was completely defoliated. Rhinanthus infection significantly decreased the final biomass of both host species. However, the two hosts differed in their responses: the biomass of undefoliated P. annua plants was reduced fo ur times more than that of undefoliated P. pratensis plants. The results in dicate that the relative value of different host species depends on the int ensity of defoliation.