Global Atmospheric Change and Trophic Interactions: Are There Any General Responses?

Interspecific variation and elevated CO2 influence the relationship between plant chemical resistance and regrowth tolerance

To understand how comprehensive plant defense phenotypes will respond to global change, we investigated the legacy effects of elevated CO2 on the relationships between chemical resistance (constitutive and induced via mechanical damage) and regrowth tolerance in four milkweed species (Asclepias). We quantified potential resistance and tolerance trade-offs at the physiological level following simulated mowing, which are relevant to milkweed ecology and conservation. We examined the legacy effects of elevated CO2 on four hypothesized trade-offs between the following: (a) plant growth rate and constitutive chemical resistance (foliar cardenolide concentrations), (b) plant growth rate and mechanically induced chemical resistance, (c) constitutive resistance and regrowth tolerance, and (d) regrowth tolerance and mechanically induced resistance. We observed support for one trade-off between plant regrowth tolerance and mechanically induced resistance traits that was, surprisingly, independent of CO2 exposure. Across milkweed species, mechanically induced resistance increased by 28% in those plants previously exposed to elevated CO2. In contrast, constitutive resistance and the diversity of mechanically induced chemical resistance traits declined in response to elevated CO2 in two out of four milkweed species. Finally, previous exposure to elevated CO2 uncoupled the positive relationship between plant growth rate and regrowth tolerance following damage. Our data highlight the complex and dynamic nature of plant defense phenotypes under environmental change and question the generality of physiologically based defense trade-offs.

The Role of Plant Abiotic Factors on the Interactions Between Cnaphalocrocis medinalis (Lepidoptera: Crambidae) and its Host Plant

Atmospheric temperature increases along with increasing atmospheric CO2 concentration. This is a major concern for agroecosystems. Although the impact of an elevated temperature or increased CO2 has been widely reported, there are few studies investigating the combined effect of these two environmental factors on plant-insect interactions. In this study, plant responses (phenological traits, defensive enzyme activity, secondary compounds, defense-related gene expression and phytohormone) of Cnaphalocrocis medinalis (Guenée) (Lepidoptera: Pyralidae) -susceptible and resistant rice under various conditions (environment, soil type, variety, C. medinalis infestation) were used to examine the rice-C. medinalis interaction. The results showed that leaf chlorophyll content and trichome density in rice were variety-dependent. Plant defensive enzyme activities were affected environment, variety, or C. medinalis infestation. In addition, total phenolic content of rice leaves was decreased by elevated CO2 and temperature and C. medinalis infestation. Defense-related gene expression patterns were affected by environment, soil type, or C. medinalis infestation. Abscisic acid and salicylic acid content were decreased by C. medinalis infestation. However, jasmonic acid content was increased by C. medinalis infestation. Furthermore, under elevated CO2 and temperature, rice plants had higher abscisic acid content than plants under ambient conditions. The adult morphological traits of C. medinalis also were affected by environment. Under elevated CO2 and temperature, C. medinalis adults had greater body length in the second and third generations. Taken together these results indicated that elevated CO2 and temperature not only affects plants but also the specialized insects that feed on them.

Elevated CO2 differentially affects tobacco and rice defense against lepidopteran larvae via the jasmonic acid signaling pathway

Atmospheric CO2 levels are rapidly increasing due to human activities. However, the effects of elevated CO2 (ECO2 ) on plant defense against insects and the underlying mechanisms remain poorly understood. Here we show that ECO2 increased the photosynthetic rates and the biomass of tobacco and rice plants, and the chewing lepidopteran insects Spodoptera litura and Mythimna separata gained less and more mass on tobacco and rice plants, respectively. Consistently, under ECO2 , the levels of jasmonic acid (JA), the main phytohormone controlling plant defense against these lepidopteran insects, as well as the main defense-related metabolites, were increased and decreased in insect-damaged tobacco and rice plants. Importantly, bioassays and quantification of defense-related metabolites in tobacco and rice silenced in JA biosynthesis and perception indicate that ECO2 changes plant resistance mainly by affecting the JA pathway. We further demonstrate that the defensive metabolites, but not total N or protein, are the main factors contributing to the altered defense levels under ECO2 . This study illustrates that ECO2 changes the interplay between plants and insects, and we propose that crops should be studied for their resistance to the major pests under ECO2 to predict the impact of ECO2 on future agroecosystems.

Insect herbivory in a mature Eucalyptus woodland canopy depends on leaf phenology but not CO2 enrichment

Background

Climate change factors such as elevated atmospheric carbon dioxide concentrations (e[CO₂]) and altered rainfall patterns can alter leaf composition and phenology. This may subsequently impact insect herbivory. In sclerophyllous forests insects have developed strategies, such as preferentially feeding on new leaf growth, to overcome physical or foliar nitrogen constraints, and this may shift under climate change. Few studies of insect herbivory at elevated [CO₂] have occurred under field conditions and none on mature evergreen trees in a naturally established forest, yet estimates for leaf area loss due to herbivory are required in order to allow accurate predictions of plant productivity in future climates. Here, we assessed herbivory in the upper canopy of mature Eucalyptus tereticornis trees at the nutrient-limited Eucalyptus free-air CO₂ enrichment (EucFACE) experiment during the first 19 months of CO₂ enrichment. The assessment of herbivory extended over two consecutive spring—summer periods, with a first survey during four months of the [CO₂] ramp-up phase after which full [CO₂] operation was maintained, followed by a second survey period from months 13 to 19.

Results

Throughout the first 2 years of EucFACE, young, expanding leaves sustained significantly greater damage from insect herbivory (between 25 and 32 % leaf area loss) compared to old or fully expanded leaves (less than 2 % leaf area loss). This preference of insect herbivores for young expanding leaves combined with discontinuous production of new foliage, which occurred in response to rainfall, resulted in monthly variations in leaf herbivory. In contrast to the significant effects of rainfall-driven leaf phenology, elevated [CO₂] had no effect on leaf consumption or preference of insect herbivores for different leaf age classes.

Conclusions

In the studied nutrient-limited natural Eucalyptus woodland, herbivory contributes to a significant loss of young foliage. Leaf phenology is a significant factor that determines the level of herbivory experienced in this evergreen sclerophyllous woodland system, and may therefore also influence the population dynamics of insect herbivores. Furthermore, leaf phenology appears more strongly impacted by rainfall patterns than by e[CO₂]. e[CO₂] responses of herbivores on mature trees may only become apparent after extensive CO₂ fumigation periods.

Electronic supplementary material

The online version of this article (doi:10.1186/s12898-016-0102-z) contains supplementary material, which is available to authorized users.

Metabolite analysis of the effects of elevated CO2 and nitrogen fertilization on the association between tall fescue (Schedonorus arundinaceus) and its fungal symbiont Neotyphodium coenophialum

Atmospheric CO2 is expected to increase to between 550 ppm and 1000 ppm in the next century. CO2-induced changes in plant physiology can have ecosystem-wide implications and may alter plant-plant, plant-herbivore and plant-symbiont interactions. We examined the effects of three concentrations of CO2 (390, 800 and 1000 ppm) and two concentrations of nitrogen fertilizer (0.004 g N/week versus 0.2 g N/week) on the physiological response of Neotyphodium fungal endophyte-infected and uninfected tall fescue plants. We used quantitative PCR to estimate the concentration of endophyte under altered CO2 and N conditions. We found that elevated CO2 increased the concentration of water-soluble carbohydrates and decreased the concentration of plant total amino acids in plants. Fungal-derived alkaloids decreased in response to elevated CO2 and increased in response to nitrogen fertilization. Endophyte concentration (expressed as the number of copies of an endophyte-specific gene per total genomic DNA) increased under elevated CO2 and nitrogen fertilization. The correlation between endophyte concentration and alkaloid production observed at ambient conditions was not observed under elevated CO2. These results suggest that nutrient exchange dynamics important for maintaining the symbiotic relationship between fungal endophytes and their grass hosts may be altered by changes in environmental variables such as CO2 and nitrogen fertilization.

A meta-analytical review of the effects of elevated CO2 on plant-arthropod interactions highlights the importance of interacting environmental and biological variables

We conducted the most extensive meta-analysis of plant and animal responses to elevated CO(2) to date. We analysed > 5000 data points extracted from 270 papers published between 1979 and 2009. We examined the changes in 19 animal response variables to the main effect of elevated CO(2). We found strong evidence for significant variation among arthropod orders and feeding guilds, including interactions in the direction of response. We also examined the main effects of elevated CO(2) on: six plant growth and allocation responses, seven primary metabolite responses, eight secondary metabolite responses, and four physical defence responses. We examined these response variable changes under two-way and three-way interactions between CO(2) and: soil nitrogen, ambient temperature, drought, light availability, photosynthetic pathway, reproductive system, plant growth rate, plant growth form, tissue type, and nitrogen fixation. In general we found smaller effect sizes for many response variables than have been previously reported. We also found that many of the oft-reported main effects of CO(2) obscure the presence of significant two- and three-way interactions, which may help better explain the relationships between the response variables and elevated CO(2).