Gregory P. Cheplick

Effects of Insect Herbivory and Fungal Endophyte Infection on Competitive Interactions among Grasses

Interactions among plants may be influenced by pests or parasites that dif- ferentially affect one competitor. The purpose of this study was to determine the effects of fungal parasitism and insect herbivory, alone and in concert, on plant competitive inter- actions. The effects of fungal endophyte (Acremonium spp.) infection and fall armyworm (Spodoptera frugiperda) herbivory on competitive interactions in one- and two-species mixtures of the grasses tall fescue (Festuca arundinacea), red fescue (F. rubra), and perennial ryegrass (Lolium perenne) were examined in greenhouse experiments. In general, herbivory reduced plant biomass whereas endophyte infection increased plant biomass. Endophyte- infected (E+) plants were less damaged by herbivory than uninfected (E-) plants of the same species. Studies on fall armyworm larval feeding and choice tests with the five grass species were generally consistent with the outcome of the competition experiments; E+ plants were less nutritious and less preferred than E- plants of the same species. There were significant interactions among factors so that the outcome of competition depended on the species identities and the presence or absence of endophytes and herbivores. In competition with Kentucky bluegrass (Poa pratensis), E+ and E- perennial ryegrass pro- duced similar biomass in the absence of herbivory, but E+ perennial ryegrass had nearly twice the biomass of E- plants when herbivores were present. E+ and E- tall fescue were poor competitors with orchard grass (Dactylis glomerata) when herbivores were absent, but E + tall fescue was a better competitor than E - plants and orchard grass when herbivores were present. This study indicates that competitive hierarchies among grasses are altered by interactions with insect herbivores and fungal endophytes, which have typically been ignored in past studies.

Acquired chemical defences in grasses: the role of fungal endophytes

Microbial symbionts of plants can affect the interaction between their hosts and herbivores. Systemic fungal endophytes (Clavicipitaceae, Ascomycetes) infect grasses and produce alkaloids in host tissues. The objectives of this study were to determine the effects of these fungi infecting a variety of grasses on leaf and seedfeeding insects. Survival, growth, and development of fall armyworm (Spodoptera frugiperda) reared on infected or noninfected plants were compared. Larvae reared on infected plants tended to exhibit reduced survival and growth and retarded development compared with larvae reared on noninfected plants. There was considerable variation in these variables among host grasses and between different trials using the same grass grown in different environments. Population growth of flour beetles (Tribolium castaneum) on ground seeds from infected or noninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were also compared. Mean beetle population growth rates were significantly lower on infected seeds and especially on tall fescue. We conclude that insect resistance is a general property of endophyte-infected grasses and that infection may be beneficial to host plants in nature.

Effect of ergot alkaloids from fungal endophyte-infected grasses on fall armyworm (Spodoptera frugiperda)

Ergot alkaloids produced by endophytic fungi in the tribe Balansiae (Clavicipitaceae, Ascomycetes), which infect grasses, may provide plant defense against herbivores. This study examined the effects of six ergot alkaloids on survivorship, feeding, and growth of larvae of the fall armyworm (Spodoptera frugiperda, Lepidoptera: Noctuidae), a generalist herbivore of grasses. Corn leaf disks were soaked in solutions of individual ergot alkaloids at different concentrations and presented to neonate larvae. At the highest concentrations (77–100 mg/liter) of ergonovine, ergotamine, ergocryptine, agroclavine, and elymoclavine, larval weights and/or leaf area consumed after eight days were reduced relative to controls. Lysergol had no effect on larval weights and leaf consumption at any concentration. Although active concentrations were higher than those reported from two host grasses, in vivo levels of ergot alkaloids have not been quantified for most endophyte-infected grasses. The detrimental effects on fall armyworm observed in this study suggest that ergot alkaloids could be responsible, at least in part, for the greater insect resistance of endophyte-infected grasses.

Responses to severe competitive stress in a clonal plant : differences between genotypes

The rhizomatous perennial grass (Amphibromus scabrivalvis) has the ability to be phalanx-like and guerrilla-like in its growth form, producing new ramets both from the base of established ramets (phalanx) and spread out along elongated rhizomes (guerrilla). Changes in clonal growth and allocation patterns of A. scabrivalvis in response to severe competitive stress induced by a background of Lolium perenne, a phalanx species, were studied. Eleven genotypes of A. scabrivalvis were divided into ramets and planted into pots in the glasshouse alone or into a background of dense L. perenne. Plants were harvested after 27 wk. Competitive stress was severe, with relative competition intensity for the genotypes ranging from 0.89 to 0.97. Both ramet production and mean interramet distances were significantly depressed by competition. Under competition, A. scabrivalvis showed a much higher allocation to roots and rhizomes compared to the control. This may serve to increase storage under competitive conditions. Genotypes producing a large number of ramets with high interramet distances in the control were more strongly affected by competition than genotypes producing fewer ramets with low interramet distances. This suggests that a more phalanx-like growth strategy may be selectively favored under severe competitive stress from a phalanx species in A. scabrivalvis. In communities dominated by perennial plants, general growth strategy, as exemplified by clone architecture, may be a critical determinant of population development and maintenance under highly competitive conditions.

The ecology of amphicarpic plants

Amphicarpic plants, which produce both aerial and subterranean flowers and seeds, provide evolutionary ecologists with insight into the evolution of breeding systems involving self-fertilization, seed dimorphisms and multiple dispersal strategies. The subterranean flowers of amphicarpic species do not open, are self-fertile, and produce large seeds with a limited dispersal potential. In contrast, the aerial flowers usually open, can cross-pollinate, and produce smaller seeds with a greater potential for long-distance dispersal. The eco-genetic consequences of this reproductive system are a function of these common features of amphicarpic species, and must be considered in an ecological context when attempting to determine the selective basis of subterranean fruit and seed production.

Impact of the fungus Balansia henningsiana on Panicum agrostoides: frequency of infection, plant growth and reproduction, and resistance to pests

SummaryThe impact of the systemic fungus Balansia henningsiana (Clavicipitaceae) on the grass Panicum agrostoides was examined in field and greenhouse studies comparing infected and uninfected plants. Approximately one-half of all plants in three populations located in southern Indiana were infected. In field samples and greenhouse studies infected plants were significantly heavier than uninfected plants and produced significantly more tillers. Infection tended to suppress flowering but occasional asymptomatic tillers on infected plants produced healthy inflorescences. Although infected plants produced fewer inflorescences than uninfected plants as a proportion of total tillers, absolute numbers of inflorescences were similar in the two groups. Because other grasses infected by different species of Balansia and related fungi often are more resistant to insect damage, pest damage was quantified in one population. No differences between infected and uninfected plants were detected in levels of herbivory but infected plants had significantly less damage by the common leaf spot fungus Alternaria triticina. The results suggest that there is no selective disadvantage for plants infected by B. henningsiana.

A conceptual framework for the analysis of phenotypic plasticity and genetic constraints in plants

A plant genotype can potentially exhibit a variety of phenotypes, depending on environmental conditions. If the quantitative measure of a life history trait is defined as the phenotypic value, once can then define an optimum phenotypic value obtainable by a genotype, in an ideal, nonlimiting environment. Analogous to niche theory, the fundamental phenotype (F) describes an individual at its optimum phenotypic value, while the realized phenotype (R) describes an individual at some fraction of its fundamental phenotype, given the presence of limiting factors. I hypothesize that the range of fundamental phenotypes found in a population affects future evolutionary potential because genetically based differences between realized phenotypes translate into fitness differences between genotypes