Effects of intensity and duration of infection by a hemiparasitic plant, Rhinanthus serotinus, on growth and reproduction of a perennial grass, Agrostis capillaris

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Abstract
Arising from annual variation in parasitic plant population densities, substantial yearly changes may occur in the parasitic load of an individual perennial host. We conducted two two‐year greenhouse pot experiments to examine the effects of varying intensities and duration of infection by an annual root hemiparasitic plant.Rhinanthus serotinus, on the growth and reproduction of its perennial host grass.Agrostis capillaris.In the first experiment, one host plant was growing either alone or under a load of 1 or 3 root hemiparasitic plants for one growing season, and during the next season all hosts continued their life free of hemiparasites. In the second experiment, the host plants either grew alone or were parasitised by 1 or 2 root hemiparasitic plants either during the first growing season only or during two successive seasons (the parasitic load being the same in the two seasons). In both experiments, the root hemiparasites markedly reduced the growth and reproduction of their perennial hosts. In the first experiment, the negative effects of parasites on host performance increased with the increase in intensity of parasitic infection from one to three parasites. The harmful effects of hemiparasitim were carried over to the following season; hosts parasitised during the previous season with one or three parasites produced significantly less biomass than those without parasites. In addition, hosts parasitised by three parasites during the first season produced significantly less panicles in the second season than those parasitised by one parasite and those without parasites. The second experiment showed that the production of biomass ofA. capillarisduring the second season was, but the production of panicles was not affected by the duration of parasitic infection. In addition, in this experiment, the second season biomass ofA. capillarisdepended on the intensity of infection (1 vs 2 parasites), but the production of panicles was unaffected by the number of parasites.