Soil nitrogen availability and herbivore attack influence the chemical defenses of an invasive plant (Linaria dalmatica; Plantaginaceae)

Evolution of host plant use and diversification in a species complex of parasitic weevils (Coleoptera: Curculionidae)

Weevils (Coleoptera: Curculionoidea) represent one of the most diverse groups of organisms on Earth; interactions with their host plants have been recognized to play a central role in their remarkable diversity, yet the exact mechanisms and factors still remain poorly understood. Using phylogenetic comparative analyses, here we investigate the evolution of host use and its possible role in diversification processes of Rhinusa and Gymnetron, two closely related groups of weevils that feed and develop inside plant tissues of hosts within the families Scrophulariaceae and Plantaginaceae. We found strong evidence for phylogenetic conservatism of host use at the plant family level, most likely due to substantial differences in the chemical composition of hosts, reducing the probability of shifts between host families. In contrast, the use of different plant organs represents a more labile ecological trait and ecological niche expansion that allows a finer partitioning of resources. Rhinusa and Gymnetron weevils initially specialized on plants within Scrophulariaceae and then shifted to the closely related Plantaginaceae; likewise, a gall inducing behavior evolved from non-galler weevils, possibly in response to resource competition, as galls facilitate larval development by providing enhanced nutrition and a favorable microhabitat. Results from trait-dependent diversification analyses suggest that both use of hosts within Plantaginaceae and parasitism on fruits and seed capsules are associated with enhanced diversification of Rhinusa and Gymnetron via low extinction rates. Our study provides quantitative evidence and insights on the ecological factors that can promote diversification in phytophagous insects that feed and develop inside plant tissues.

Light and Nutrient Dependent Responses in Secondary Metabolites of Plantago lanceolata Offspring Are Due to Phenotypic Plasticity in Experimental Grasslands

A few studies in the past have shown that plant diversity in terms of species richness and functional composition can modify plant defense chemistry. However, it is not yet clear to what extent genetic differentiation of plant chemotypes or phenotypic plasticity in response to diversity-induced variation in growth conditions or a combination of both is responsible for this pattern. We collected seed families of ribwort plantain (Plantago lanceolata) from six-year old experimental grasslands of varying plant diversity (Jena Experiment). The offspring of these seed families was grown under standardized conditions with two levels of light and nutrients. The iridoid glycosides, catalpol and aucubin, and verbascoside, a caffeoyl phenylethanoid glycoside, were measured in roots and shoots. Although offspring of different seed families differed in the tissue concentrations of defensive metabolites, plant diversity in the mothers' environment did not explain the variation in the measured defensive metabolites of P. lanceolata offspring. However secondary metabolite levels in roots and shoots were strongly affected by light and nutrient availability. Highest concentrations of iridoid glycosides and verbascoside were found under high light conditions, and nutrient availability had positive effects on iridoid glycoside concentrations in plants grown under high light conditions. However, verbascoside concentrations decreased under high levels of nutrients irrespective of light. The data from our greenhouse study show that phenotypic plasticity in response to environmental variation rather than genetic differentiation in response to plant community diversity is responsible for variation in secondary metabolite concentrations of P. lanceolata in the six-year old communities of the grassland biodiversity experiment. Due to its large phenotypic plasticity P. lanceolata has the potential for a fast and efficient adjustment to varying environmental conditions in plant communities of different species richness and functional composition.

The influence of maturity and variety of potato plants on oviposition and probing of Bactericera cockerelli (Hemiptera: Triozidae)

The ecological theory on host plant choice by herbivores suggests that mothers should choose plants that will maximize their offspring's success. In annual host plants, physiology (and therefore host suitability) is sometimes influenced by maturity and growth stage, which may influence female choice. Potato plants were grown under greenhouse conditions and used in choice and no-choice bioassays to determine the effect of plant maturity and variety on oviposition and number of stylet sheaths (which approximate stylet insertions) by tomato/potato psyllids. No-choice bioassays suggested that maturity (time since planting) did not influence oviposition behavior, but oviposition varied significantly among potato plant varieties. There was a significant effect of both maturity and variety on the number of stylet sheaths, which peak toward the middle of the growing season. We also examined tomato/potato psyllid responses to plants grown in a commercial field and again found no effect on oviposition but differences in stylet sheaths. The results suggest that differential susceptibility to zebra chip disease may be associated with unequal feeding rates. Future studies should examine whether the maturity of plants influences larval fitness. Finally, potato variety has an influence on both oviposition and "probing," and has implications for management strategies and the development of resistant potato varieties.

Interactive effects of simulated nitrogen deposition and altered precipitation patterns on plant allelochemical concentrations

Global environmental change alters the supply of multiple limiting resources that regulate plant primary and secondary metabolism. Through modifications in resource availability, acquisition, and allocation, global change is likely to influence plant chemical defenses, and consequently species interactions that are mediated by these compounds. While many studies focus on individual global change factors, simultaneous changes in abiotic factors may interact to influence plant allelochemicals. In this study, we examined the individual and interactive effects of nitrogen enrichment and altered precipitation patterns on chemical defense compounds (iridoid glycosides) of an invasive plant, Linaria dalmatica. Plants were grown from seed in native mixed-grass prairie for 2 years. Nitrogen and water treatments were applied in each growing season over this period. Results indicate that soil water and nitrogen availability interact to shape plant chemical defense concentrations in L. dalmatica. Nitrogen addition decreased iridoid glycoside concentrations by approximately 25% under reduced water availability, increased concentrations by 37% in ambient water plots, and had no effect on these chemical defenses for plants growing under augmented water supply. Thus, results show differing patterns of allelochemical response to nitrogen enrichment, with respect to both the magnitude and direction of change, depending on water availability. Our study demonstrates the importance of examining multiple environmental factors in order to predict potential changes in plant chemical defenses with climate change.

Nitrogen enrichment differentially affects above- and belowground plant defense

PREMISE OF THE STUDY

Human nitrogen (N) inputs to terrestrial ecosystems have greatly increased in recent years and may have important consequences for plant growth, reproduction, and defense. Although numerous studies have investigated the effects of nitrogen addition on plants, few have examined both above- and belowground responses within a range of predicted increase and apart from concomitant increases in other nutrients. •

METHODS

We conducted a greenhouse experiment to study the consequences of increased nitrogen inputs, such as those from atmospheric N deposition, on plant performance, chemical defenses, and allocation tradeoffs for an invasive species, Linaria dalmatica. This plant produces iridoid glycosides, which are a group of terpenoid compounds. •

KEY RESULTS

Soil nitrogen enrichment increased growth, reproduction, and whole-plant iridoid glycosides while decreasing some costs of defense. Interestingly, nitrogen addition had varying effects on defense allocation to above- and belowground tissues. Specifically, there was no change in iridoid glycoside concentrations of shoots, whereas concentrations decreased in flowers by ~35% and increased in roots by >400%. •

CONCLUSIONS

Observed increases in plant performance and chemical defenses may have implications for the invasion potential of L. dalmatica. Moreover, our results highlight the importance of evaluating both above- and belowground plant defenses. In particular, findings presented here indicate that research focused on leaf-level defenses may not detect key allelochemical responses, including changes in plant resistance traits that could affect consumers (e.g., herbivores and pathogens) that specialize on different plant tissues as well as plant fitness and invasion success.