Parasite–host interactions in Castilleja and Orthocarpus

Abstract

Growth, reproduction, and biomass allocation were studied in three perennial root hemiparasites, Castilleja integra Gray, Castilleja miniata Dougl., Castilleja chromosa A. Nels., and the annual hemiparasite Orthocarpus purpurascens Benth. grown either without or with a host plant. In addition, the effects of the hemiparasites on the host plants were investigated. All four hemiparasites could grow without a host, indicating that they are facultative parasites; O. purpurascens and C. chromosa produced flowers without a host. However, shoot mass of parasites 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 grown with the legume Medicago sativa L. was compared with that of parasites 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 grown with the legume than in plants without a host, whereas it was higher in parasites grown with the grass. The parasites had strong negative effects on host growth, but the extent of host damage depended on the particular parasite–host combination. Castilleja chromosa and O. purpurascens affected the growth of the legume more strongly than that of the grass, whereas C. miniata affected the grass more strongly than the legume. Grasses parasitized by C. miniata allocated more biomass to roots than unparasitized grasses. In the other parasite–host associations the pattern of biomass allocation of the host was not influenced by parasitization. Because the biomass produced by the four hemiparasites was lower than the reduction in host biomass caused by parasitism, the parasites reduced total productivity. Therefore, the studied hemiparasites may potentially affect the structure and diversity of their communities. Key words: biomass allocation, host damage, parasite benefit, parasitic plant.