Veikko Salonen

RESISTANCE AND TOLERANCE IN A HOST PLANT–HOLOPARASITIC PLANT INTERACTION: GENETIC VARIATION AND COSTS

Abstract Host organisms are believed to evolve defense mechanisms (i.e., resistance and/or tolerance) under selective pressures exerted by natural enemies. A prerequisite for the evolution of resistance and tolerance is the existence of genetic variation in these traits for natural selection to act. However, selection for resistance and/or tolerance may be constrained by negative genetic correlations with other traits that affect host fitness. We studied genetic variation in resistance and tolerance against parasitic infection and the potential fitness costs associated with these traits using a novel study system, namely the interaction between a flowering plant and a parasitic plant. In this system, parasitic infection has significant negative effects on host growth and reproduction and may thus act as a selective agent. We conducted a greenhouse experiment in which we grew host plants, Urtica dioica, that originated from a single natural population and represented 20 maternal families either uninfected or infected with the holoparasitic dodder, Cuscuta europaea, that originated from the same site. We calculated correlations among resistance, tolerance, and host performance to test for costs of resistance and tolerance. We measured resistance as parasite performance (quantitative resistance) and tolerance as the slopes of regressions relating the vegetative and reproductive biomass of host plants to damage level (measured as parasite biomass). We observed significant differences among host families in parasite resistance and in parasite tolerance in terms of reproductive biomass, a result that suggests genetic variation in these traits. Furthermore, we found differences in resistance and tolerance between female and male host plants. In addition, the correlations indicate costs of resistance in terms of host growth and reproduction and costs of tolerance in terms of host reproduction. Our results thus indicate that host tolerance and resistance can evolve as a response to infection by a parasitic plant and that costs of resistance and tolerance may be one factor maintaining genetic variation in these traits.

LOCAL ADAPTATION, RESISTANCE, AND VIRULENCE IN A HEMIPARASITIC PLANT–HOST PLANT INTERACTION

Abstract.— Coevolution may lead to local adaptation of parasites to their sympatric hosts. Locally adapted parasites are, on average, more infectious to sympatric hosts than to allopatric hosts of the same species or their fitness on the sympatric hosts is superior to that on allopatric hosts. We tested local adaptation of a hemiparasitic plant, Rhinanthus serotinus (Scrophulariaceae), to its host plant, the grass Agrostis capillaris. Using a reciprocal cross‐infection experiment, we exposed host plants from four sites to hemiparasites originating from the same four sites in a common environment. The parasites were equally able to establish haustorial connections to sympatric and allopatric hosts, and their performance was similar on both host types. Therefore, these results do not indicate local adaptation of the parasites to their sympatric hosts. However, the parasite populations differed in average biomass and number of flowers per plant and in their effect on host biomass. These results indicate that the virulence of the parasite varied among populations, suggesting genetic variation. Theoretical models suggest that local adaptation is likely to be detected if the host and the parasite have different evolutionary potentials, different migration rates, and the parasite is highly virulent. In the interaction between R. serotinus and A. capillaris all the theoretical prerequisites for local adaptation may not be fulfilled.

Plant colonization of a bare peat surface: population changes and spatial patterns

. Changes in size and spatial arrangement of plant populations established on an initially bare peat surface were described over a period of 5 yr by following plant individuals on a 1-cm grid in an area of 10 m x 25 m. The spatial pattern of populations and association between species was analyzed statistically. The study site was very slowly colonized by 14 perennial plant species. The early successional stage was dominated by Carex rostrata, with a clumped spatial distribution, and the homogeneously distributed Eriophorum vaginatum and Pinus sylvestris. Both the growth in size of populations and changes in their spatial distribution were interpreted as a result of species dispersal ability, tolerance to severity of the substrate and pattern of reproduction.

Revegetation of harvested peat surfaces in relation to substrate quality

. The relationship between substrate quality and pattern of revegetation of harvested peat surfaces was studied by means of a survey and a field experiment examining influences of modest NPK-fertilization on plant colonization of an initially bare peat surface. The harvested peat surfaces varied a great deal in their chemical and physical characteristics and the sites differed in revegetation pattern. Early successional vegetation was dominated by perennial species native to nutrient-poor habitats on all sites. Soluble phosphorus and ash content, mean particle size of surface peat, and thickness of peat layer had the strongest influence in a CCA-ordination of species. The species composition depended on the amount and form of soluble nitrogen in the surface peat. Sites with a high content of phosphorus and ammonium nitrogen, and with a thick peat layer were usually densely revegetated by Eriophorum vaginatum alone, while sites characterized by thin peat layers associated with a high ash content, large particle size and a high content of nitrate nitrogen were mainly dominated by different grass and weed species. Deschampsia cespitosa clearly favoured sites with a high potassium content and small particle sizes of the peat. The importance of nutrient availability for the rate and pattern of colonization was also demonstrated by the field experiment. Application of 20 g/m2 of NPK-fertilizer resulted in a significant increase in the number of established plant individuals and marked differences in species composition compared to unfertilized plots.

Studying the effects of plant species richness on ecosystem functioning: does the choice of experimental design matter?

Abstract. We established two parallel greenhouse experiments to empirically test the effect of experimental design on the conclusions made of the effects of plant species richness on ecosystem functioning. The experiments included an identical group of six grassland plants and were performed under identical environmental conditions, but were set up according to two different designs. In the richness design (RD), which specifically aims at testing the effect of species richness, each replicate at each richness level was chosen by a separate random draw from the total pool of species, whereas in the richness and composition design (RCD), which aims at testing the effect of both richness and composition, richness levels included deliberately replicated monocultures (at the one-species level) and mixtures (at other levels) of constituent species. When regression analysis was applied, both experimental designs found a positive effect of plant species richness on primary production (estimated using shoot mass accumulation), with species richness explaining 34% and 16% of the total variation in production in RD and RCD, respectively. Based on an overyielding analysis, this positive overall effect of species richness on primary production was in both experiments caused by the sampling effect rather than complementarity. When analysis of variance was applied, RCD also found that shoot production was greatly affected by the presence of one species, the legume Trifolium hybridum, and that 98–99% of variation in primary production within richness levels, i.e. the variation not explained by species richness, was explained by species composition. It appears that while the RD experiment suggests that plant species richness was a significant controller of primary production in our experimental grassland community, the RCD experiment suggests that primary production was mainly determined by plant species composition, i.e. the identity of the species present. Our results are consistent with earlier mathematical simulations in that experimental designs differ in their ability to discriminate the effects of species richness and composition on ecosystem functioning and may therefore lead to different conclusions of the role of species richness in functioning. We propose that this may partly explain the different relative role of plant species richness in ecosystem functioning in earlier investigations using RD and RCD.

The effect of host mycorrhizal status on host plant–parasitic plant interactions

Abstract Two pot experiments were conducted to examine three-level interactions between host plants, mycorrhizal fungi and parasitic plants. In a greenhouse experiment, Poa annua plants were grown in the presence or absence of an AM fungus (either Glomus lamellosum V43a or G. mosseae BEG29) and in the presence or absence of a root hemiparasitic plant (Odontites vulgaris). In a laboratory experiment, mycorrhizal infection (Glomus claroideum BEG31) of Trifolium pratense host plants (mycorrhizal versus non-mycorrhizal) was combined with hemiparasite infection (Rhinanthus serotinus) of the host (parasitized versus non-parasitized). Infection with the two species of Glomus had no significant effect on the growth of P. annua, while hemiparasite infection caused a significant reduction in host biomass. Mycorrhizal status of P. annua hosts (i.e. presence/absence of AM fungus) affected neither the biomass nor the number of flowers produced by the attached O. vulgaris plants. Infection with G. claroideum BEG31 greatly increased the biomass of T. pratense, but hemiparasite infection had no effect. The hemiparasitic R. serotinus plants attached to mycorrhizal hosts had higher biomass and produced more flowers than plants growing with non-mycorrhizal hosts. Roots of T. pratense were colonized by the AM fungus to an extent independent of the presence or absence of the hemiparasite. Our results confirm earlier findings that the mycorrhizal status of a host plant can affect the performance of an attached root hemiparasite. However, improvement of the performance of the parasitic plant following attachment to a mycorrhizal host depends on the extent to which the AM fungi is able to enhance the growth of the host.

Effects of artificial plant cover on plant colonization of a bare peat surface

. This paper describes the effect of artificial plant cover on plant colonization of a bare peat surface, resulting from peat harvesting. Plant species colonization was compared on plots supplied with plastic models simulating Vaccinium vitis-idaea plants and plots without this artificial cover. After two growing seasons, species composition and total biomass of the established plant cover were similar in the two plot types. However, the number of established seedlings in the plots with artificial cover was significantly higher than that in the plots without cover. Out of 13 species observed four differed significantly in their performance on the two plot types. Betula spec. had both higher seedling numbers and higher biomass on the test plots; Deschampsia cespitosa had a higher biomass, whereas the seedlings were too numerous to be counted; Salix phylicifolia had higher seedling numbers. On the contrary, the number of seedlings of Epilobium angustifoliwn was lower on plots with artificial cover. It is suggested that colonization by Betula, D. cespitosa and S. phylicifolia was facilitated mainly by the improved microclimatic and soil moisture conditions under the artificial plant cover. On the other hand, germination of E. angustifolium may be negatively influenced by the increased shade on the test plots.

The interplay between Pinus sylvestris, its root hemiparasite, Melampyrum pratense, and ectomycorrhizal fungi: Influences on plant growth and reproduction

Abstract Despite the extensive literature on mutual interactions between plants and mycorrhizal fungi, and host plants and parasitic plants, little is known about the outcomes of interactions when the three organisms exist in concert with one another. We investigated, in a microcosm experiment, the interactions between Pinus sylvestris L. and the root hemiparasitic Melampyrum pratense L. in the presence/absence of ectomycorrhizal (EM) fungi. Mycorrhizal infection significantly increased the biomass of P. sylvestris, whereas the parasitic infection decreased it. Concentration of host phosphorus was greatly increased by the presence of EM fungi. M. pratense plants attached to mycorrhizal pines had higher biomass and produced more flowers than those growing with non-mycorrhizal pines. We attribute the stimulation of parasite performance in the presence of EM fungi to enhanced nutrient availability to the host individuals, and to increased photosynthetic capacity of hosts as a result of increased above-ground biomass.

Success of a Root Hemiparasitic Plant is Influenced by Soil Quality and by Defoliation of Its Host

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Interaction of a host plant and its holoparasite: effects of previous selection by the parasite.

If parasites decrease the fitness of their hosts one could expect selection for host traits (e.g. resistance and tolerance) that decrease the negative effects of parasitic infection. To study selection caused by parasitism, we used a novel study system: we grew host plants (Urtica dioica) that originated from previously parasitized and unparasitized natural populations (four of each) with or without a holoparasitic plant (Cuscuta europaea). Infectivity of the parasite (i.e. qualitative resistance of the host) did not differ between the two host types. Parasites grown with hosts from parasitized populations had lower performance than parasites grown with hosts from unparasitized populations, indicating host resistance in terms of parasite’s performance (i.e. quantitative resistance). However, our results suggest that the tolerance of parasitic infection was lower in hosts from parasitized populations compared with hosts from unparasitized populations as indicated by the lower above‐ground vegetative biomass of the infected host plants from previously parasitized populations.