PARASITE - HOST INTERACTIONS IN CASTILLEJA AND ORTHOCARPUS

Growth, reproduction, and biomass allocation were studied in three per ennial root hemiparasites, Castilleja integra Gray, Castilleja miniata Dougl., Castilleja chromosa A. Nels., and the annual hemiparasite Ort hocarpus purpurascens Benth. grown either without or with a host plant . In addition, the effects of the hemiparasites on the host plants wer e investigated. All four hemiparasites could grow without a host, indi cating that they are facultative parasites; O. purpurascens and C. chr omosa produced flowers without a host. However, shoot mass of parasite s with a host was 3-41 times that of parasites without a host. For C. miniata, C. chromosa, and O. purpurascens the growth of parasites grow n with the legume Medicago sativa L. was compared with that of parasit es grown with the grass Lolium perenne L. The legume was consistently a more beneficial host than the grass. In C. miniata and C. chromosa, patterns of biomass allocation were also influenced by the host type. The proportion of biomass allocated to roots was lower in parasites gr own with the legume than in plants without a host, whereas it was high er in parasites grown with the grass. The parasites had strong negativ e effects on host growth, but the extent of host damage depended on th e particular parasite-host combination. Castilleja chromosa and O. pur purascens affected the growth of the legume more strongly than that of the grass, whereas C. miniata affected the grass more strongly than t he legume. Grasses parasitized by C. miniata allocated more biomass to roots than unparasitized grasses. In the other parasite-host associat ions the pattern of biomass allocation of the host was not influenced by parasitization. Because the biomass produced by the four hemiparasi tes was lower than the reduction in host biomass caused by parasitism, the parasites reduced total productivity. Therefore, the studied hemi parasites may potentially affect the structure and diversity of their communities.