Phylogenetic ecology of leaf surface traits in the milkweeds (Asclepias spp.): chemistry, ecophysiology, and insect behavior

Combining transcriptome and metabolome analysis to understand the response of sorghum to Melanaphis sacchari

BACKGROUND

The sorghum aphid Melanaphis sacchari (Zehntner) (Homoptera: Aphididae) is an important insect in the late growth phase of sorghum (Sorghum bicolor L.). However, the mechanisms of sorghum response to aphid infestation are unclear.

RESULTS

In this paper, the mechanisms of aphid resistance in different types of sorghum varieties were revealed by studying the epidermal cell structure and performing a transcriptome and metabolome association analysis of aphid-resistant and aphid-susceptible varieties. The epidermal cell results showed that the resistance of sorghum to aphids was positively correlated with epidermal cell regularity and negatively correlated with the intercellular space and leaf thickness. Transcriptome and metabolomic analyses showed that differentially expressed genes in the resistant variety HN16 and susceptible variety BTX623 were mainly enriched in the flavonoid biosynthesis pathway and differentially expressed metabolites were mainly related to isoflavonoid biosynthesis and flavonoid biosynthesis. The q-PCR results of key genes were consistent with the transcriptome expression results. Meanwhile, the metabolome test results showed that after aphidinfestation, naringenin and genistein were significantly upregulated in the aphid-resistant variety HN16 and aphid-susceptible variety BTX623 while luteolin was only significantly upregulated in BTX623. These results show that naringenin, genistein, and luteolin play important roles in plant resistance to aphid infestation. The results of exogenous spraying tests showed that a 1‰ concentration of naringenin and genistein is optimal for improving sorghum resistance to aphid feeding.

CONCLUSIONS

In summary, the physical properties of the sorghum leaf structure related to aphid resistance were studied to provide a reference for the breeding of aphid-resistant varieties. The flavonoid biosynthesis pathway plays an important role in the response of sorghum aphids and represents an important basis for the biological control of these pests. The results of the spraying experiment provide insights for developing anti-aphid substances in the future.

How and why do species break a developmental trade-off? Elucidating the association of trichomes and stomata across species

PREMISE

Previous studies have suggested a trade-off between trichome density (Dt) and stomatal density (Ds) due to shared cell precursors. We clarified how, when, and why this developmental trade-off may be overcome across species.

METHODS

We derived equations to determine the developmental basis for Dt and Ds in trichome and stomatal indices (it and is) and the sizes of epidermal pavement cells (e), trichome bases (t), and stomata (s) and quantified the importance of these determinants of Dt and Ds for 78 California species. We compiled 17 previous studies of Dt-Ds relationships to determine the commonness of Dt-Ds associations. We modeled the consequences of different Dt-Ds associations for plant carbon balance.

RESULTS

Our analyses showed that higher Dt was determined by higher it and lower e, and higher Ds by higher is and lower e. Across California species, positive Dt-Ds coordination arose due to it-is coordination and impacts of the variation in e. A Dt-Ds trade-off was found in only 30% of studies. Heuristic modeling showed that species sets would have the highest carbon balance with a positive or negative relationship or decoupling of Dt and Ds, depending on environmental conditions.

CONCLUSIONS

Shared precursor cells of trichomes and stomata do not limit higher numbers of both cell types or drive a general Dt-Ds trade-off across species. This developmental flexibility across diverse species enables different Dt-Ds associations according to environmental pressures. Developmental trait analysis can clarify how contrasting trait associations would arise within and across species.

Rice physical defenses and their role against insect herbivores

MAIN CONCLUSION

Understanding surface defenses, a relatively unexplored area in rice can provide valuable insight into constitutive and induced defenses against herbivores. Plants have evolved a multi-layered defense system against the wide range of pests that constantly attack them. Physical defenses comprised of trichomes, wax, silica, callose, and lignin, and are considered as the first line of defense against herbivory that can directly affect herbivores by restricting or deterring them. Most studies on physical defenses against insect herbivores have been focused on dicots compared to monocots, although monocots include one of the most important crops, rice, which half of the global population is dependent on as their staple food. In rice, Silica is an important element stimulating plant growth, although Silica has also been found to impart resistance against herbivores. However, other physical defenses in rice including wax, trichomes, callose, and lignin are less explored. A detailed exploration of the morphological structures and functional consequences of physical defense structures in rice can assist in incorporating these resistance traits in plant breeding and genetic improvement programs, and thereby potentially reduce the use of chemicals in the field. This mini review addresses these points with a closer look at current literature and prospects on rice physical defenses.

A review of changes at the phenotypic, physiological, biochemical, and molecular levels of plants due to high temperatures

MAIN CONCLUSION

This review summarizes the physiological, biochemical, and molecular regulatory network changes in plants in response to high temperature. With the continuous rise in temperature, high temperature has become an important issue limiting global plant growth and development, affecting the phenotype and physiological and biochemical processes of plants and seriously restricting crop yield and tree growth speed. As sessile organisms, plants inevitably encounter high temperatures and improve their heat tolerance by activating molecular networks related to heat stress, such as signal transduction, synthesis of metabolites, and gene expression. Heat tolerance is a polygenic trait regulated by a variety of genes, transcription factors, proteins, and metabolites. Therefore, this review summarizes the changes in physiological, biochemical and molecular regulatory networks in plants under high-temperature conditions to lay a foundation for an in-depth understanding of the mechanisms involved in plant heat tolerance responses.

Examining the associations between a generalist feeder and a highly toxic host

Understanding the often antagonistic plant-herbivore interactions and how host defenses can influence herbivore dietary breadth is an area of ongoing study in ecology and evolutionary biology. Typically, host plants/fungi that produce highly noxious chemical defenses are only fed on by specialists. We know very little about generalist species that can feed and develop on a noxious host. One such example of generalists feeding on toxic host occurs in the mushroom-feeding Drosophila found in the immigrans-tripunctata radiation. Although these species are classified as generalists, their acceptable hosts include deadly Amanita species. In this study, we used behavioral assays to assess associations between one mushroom-feeding species, Drosophila guttifera, and the deadly Amanita phalloides. We conducted feeding assays to confirm the presence of cyclopeptide toxin tolerance. We then completed host preference assays in female flies and larvae and did not find a preference for toxic mushrooms in either. Finally, we assessed the effect of competition on oviposition preference. We found that the presence of a competitor's eggs on the preferred host was associated with the flies increasing the number of eggs laid on the toxic mushrooms. Our results highlight how access to a low competition host resource may help to maintain associations between a generalist species and a highly toxic host.

Orchids reduce attachment of herbivorous snails with leaf trichomes

Protective structures in the epidermis are essential for land plants to defend themselves against herbivores. In this study, we investigated the effect of different types of trichomes of three orchids, Calanthe triplicata, Dendrochilum pallidiflavens and Trichotosia ferox, on attachment of herbivorous land snails, using histochemistry and centrifuge experiments. Size, ornamentation and histochemistry of epicuticular trichomes on the orchid leaves were assessed with light microscopy, scanning electron microscopy and transmission electron microscopy. Total forces needed to detach two differently shaped snail species, Subulina octona and Pleurodonte isabella, were measured using a turntable equipped with a synchronized strobe. Snails were placed in two positions, either perpendicular or parallel to the main veins on the orchid leaves, both on the adaxial (= upper) or abaxial (= lower) side. The results obtained provided three new insights. First, a perpendicular or parallel position of the snails to the main veins did not significantly affect the attachment performance of either species tested. Secondly, snails detached significantly easier on leaf sides covered with a high density of lignin filled epicuticular trichomes. Thirdly, the removal of glandular trichomes did not affect the attachment forces; however, the absence of lignified trichomes increased the attachment of the snails. Our study highlights the importance of studying micro-ornamentation in combination with performance for obtaining a better understanding of the defense mechanisms employed by different species of orchids to deter herbivorous snails.

Roles of lower-side leaf trichomes in diffusion resistance and gas-exchange characteristics across environmental gradients in Metrosideros polymorpha

Leaf trichomes on the lower leaf surface are common in many plant species, especially those grown under dry and/or low-temperature conditions; however, their adaptive significance remains unclear. Lower-side leaf trichomes can directly decrease gas fluxes through increased gas-diffusion resistance but can indirectly increase gas fluxes through increased leaf temperature owing to increased heat-diffusion resistance. We examined whether the combined direct and indirect effects of trichome resistance increase photosynthetic rates and water-use efficiency (WUE) using Metrosideros polymorpha Gaud., which varies widely in the masses of lower-side non-glandular leaf trichomes across various environments on the Hawaiian Islands. We employed both field surveys, including ecophysiological measurements at five elevation sites, and simulation analyses to predict the gas-exchange rates of leaves with various trichome-layer thicknesses across a wide range of environmental conditions. Field surveys showed that the trichome-layer thickness was the largest at the coldest and driest site and the thinnest at the wettest site. Field surveys, experimental manipulations and simulation analyses demonstrated that leaf trichomes significantly increased leaf temperature owing to the increased heat resistance. Simulation analyses showed that the effect of leaf trichomes on heat resistance was much larger than that on gas-flux resistance. Leaf trichomes can increase daily photosynthesis only in cold dry areas by increasing the leaf temperature. However, the increased leaf temperature with leaf trichomes resulted in a consistent decrease in the daily WUE at all elevation sites. The magnitudes of trichome effects on gas-exchange rates were associated with the temperature difference across the elevational gradient, the strong light intensity in Hawaii, the leaf-size variation and the conservative stomatal behavior of M. polymorpha as well as the trichome-layer thickness. In summary, the lower-side leaf trichomes in M. polymorpha can be beneficial for carbon assimilation in low-temperature environments but not for water conservation in most environments in terms of diffusion resistance.

Adaptive mechanism in Quercus brantii Lindl. leaves under climatic differentiation: morphological and anatomical traits

Leaf traits, which vary across different climatic conditions, can reveal evolutionary changes within a species made to adapt to the environment. Leaf traits play major roles in a plant functions under varying climatic conditions. To examine adaptive modes and mechanisms applied by plants in different climates, we analyzed leaf morphology and anatomical structures in Quercus brantii in the Zagros forests, Western Iran. The plants adapted to the environmental differences with increased dry matter content in a Mediterranean climate, and increasing leaf length, specific leaf area, stomata length (SL), stomata width, stomatal density (SD), stomatal pore index (SPI), trichome length, and width in a sub-humid climate; trichome density was increased in a semi-arid climate. There were strong, positive correlations between SPI with SL and SD. Correlations for other leaf traits were weakly significant. Such morphological and anatomical plasticity probably leads to lower transpiration rates, control of internal temperature and water status, and improved photosynthetic capability under stressing conditions. These findings provide new insights into the adaptive strategies of plants to environmental changes at the morphological and anatomical levels.

Coleoptera claws and trichome interlocking

The present study tests the hypothesis that the specialized claws with a basal tooth found in some coccinellid beetles represent an adaptation to interlock with flexible unbranched trichomes of different plants. We compared the attachment ability of three Coleoptera species, Chnootriba elaterii, Harmonia axyridis (both Coleoptera: Coccinellidae), and Chrysolina herbacea (Coleoptera: Chrysomelidae) with claws of different shape. The attachment ability of insect individuals with or without claws to a plant with leaves bearing straight non-branched trichomes (Cucurbita moschata) and to a plant with smooth leaves (Prunus laurocerasus) was measured in traction force experiments. Insect attachment ability was also tested on a resin replica of C. moschata leaf, to variate trichome stiffness, and on glass as a reference surface. Centrifugal force tester experiments were performed to compare the attachment ability of the two ladybird species to glass and to the leaf of C. moschata. Natural and artificial substrates were characterized in cryo-SEM. The collected data reveal that plant trichomes can enhance insect attachment to plant surface compared with smooth glass by increasing insect friction force, but this is directly related to the trichome stiffness. To effectively grasp soft trichomes, insects evolved special claws-associated structures, such as the dentate claws observed in Coccinellidae.

Plant defense synergies and antagonisms affect performance of specialist herbivores of common milkweed

As a general rule, plants defend against herbivores with multiple traits. The defense synergy hypothesis posits that some traits are more effective when co-expressed with others compared to their independent efficacy. However, this hypothesis has rarely been tested outside of phytochemical mixtures, and seldom under field conditions. We tested for synergies between multiple defense traits of common milkweed (Asclepias syriaca) by assaying the performance of two specialist chewing herbivores on plants in natural populations. We employed regression and a novel application of random forests to identify synergies and antagonisms between defense traits. We found the first direct empirical evidence for two previously hypothesized defense synergies in milkweed (latex by secondary metabolites, latex by trichomes) and identified numerous other potential synergies and antagonisms. Our strongest evidence for a defense synergy was between leaf mass per area and low nitrogen content; given that these "leaf economic" traits typically covary in milkweed, a defense synergy could reinforce their co-expression. We report that each of the plant defense traits showed context-dependent effects on herbivores, and increased trait expression could well be beneficial to herbivores for some ranges of observed expression. The novel methods and findings presented here complement more mechanistic approaches to the study of plant defense diversity and provide some of the best evidence to date that multiple classes of plant defense synergize in their impact on insects. Plant defense synergies against highly specialized herbivores, as shown here, are consistent with ongoing reciprocal evolution between these antagonists.

Volatile organic compounds as mediators of plant communication and adaptation to climate change

Plant volatile organic compounds are the most abundant and structurally diverse plant secondary metabolites. They play a key role in plant lifespan via direct and indirect plant defenses, attracting pollinators, and mediating various interactions between plants and their environment. The ecological diversity and context-dependence of plant-plant communication driven by volatiles are crucial elements that influence plant performance in different habitats. Plant volatiles are also valued for their multiple applications in food, flavor, pharmaceutical, and cosmetics industries. In the current review, we summarize recent advances that have elucidated the functions of plant volatile organic compounds as mediators of plant interaction at community and individual levels, highlighting the complexities of plant receiver feedback to various signals and cues. This review emphasizes volatile terpenoids, the most abundant class of plant volatile organic compounds, highlighting their role in plant adaptability to global climate change and stress-response pathways that are integral to plant growth and survival. Finally, we identify research gaps and suggest future research directions.

Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity. Absence of a correlation between the components of host specificity results in low-performance hosts supporting limited larval development. Most studies of specificity focus on oviposition behavior leaving the importance and basis of palatability and survival under-explored. We conducted a comprehensive review of 127 plant species that have been claimed or tested to be hosts for the monarch butterfly Danaus plexippus to classify them as non-hosts, low performance, or high performance. We performed a meta-analysis to test if performance status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice larval feeding experiment to identify causes of low performance. We identified 34 high performance, 42 low performance, 33 non-hosts, and 18 species with unsubstantiated claims. Mean cardenolide concentration was greater in high- than low-performance hosts and a significant predictor of host status, suggesting possible evolutionary trade-offs in monarch specialization. Other cardenolide properties and trichome density were not significant predictors of host status. In the experiment, we found, of the 62% of larvae that attempted to eat low-performance hosts, only 3.5% survived to adult compared to 85% of those on the high-performance host, demonstrating that multiple factors affect larval host plant specificity. Our study is the first to classify all known host plants for monarchs and has conservation implications for this threatened species.

Biochemical and Morphological Mechanisms Underlying the Performance and Preference of Fall Armyworm (Spodoptera frugiperda) on Wheat and Faba Bean Plants

Simple Summary

Spodoptera frugiperda (J. E. Smith) has been recently identified as a notorious global crop pest that affects wheat production in China. Therefore, exploring preventive solutions based on agricultural strategies is a cost effective and eco-friendly approach in realizing sustainable pest management. The wheat–faba bean intercropping model mitigates the damage caused by wheat aphids, while the possible benefit of this pattern in the control of S. frugiperda remains unevaluated. To provide a fundamental basis for implementing this strategy in a wheat field for S. frugiperda management, this novel study attempted to extensively evaluate the effects of feeding wheat and faba bean plants on S. frugiperda performance and preference and to reveal the underlying mechanisms associated with the biochemical and morphological traits of the two host species. Our results suggested that the faba bean plants exhibited significant antibiosis on larvae and repellency to the females of S. frugiperda compared with wheat plants. Therefore, we concluded the potential usefulness of the faba bean plants as a push crop. These findings will facilitate the adoption of wheat and faba bean intercropping systems against S. frugiperda in the wheat-growing regions of China.

Abstract

Spodoptera frugiperda (J. E. Smith), which attacked China in 2019, remains a significant threat to wheat production. Wheat–faba bean intercropping systems prevent damage caused by wheat aphids; however, the potential role in S. frugiperda control remains unclear. Here, the adaptability and preferences of S. frugiperda to wheat and its common intercropped plant, faba bean, were evaluated to implement an eco-friendly approach for S. frugiperda management. Their adaptability showed that both hosts could support S. frugiperda to complete their life cycle; however, the larvae performed worse on faba bean compared with on wheat. The biochemical analysis revealed that faba bean plants had lower contents of soluble sugars and total proteins but higher levels of phenolics and tannins than in wheat leaves. The gravid S. frugiperda preferred (during the preference assays) to oviposit on wheat rather than on faba bean plants in cage tests. The wheat odor was preferred over the faba bean odor in the Y-tube olfactometer bioassays. The morphological scanning electron microscopy (SEM) showed increased trichome density on wheat leaves. Therefore, the faba bean plants displayed antibiosis on larvae and were repellent to female moths, thus, suggesting that faba bean plants could serve as a push crop to be intercropped with wheat for S. frugiperda control for wheat fields.

Introduced populations of the garden lupine are adapted to local generalist snails but have lost alkaloid diversity

Intraspecific variation in growth and defence among plant populations can be driven by differences in (a)biotic conditions, such as herbivory and resources. Introduction of species to novel environments affects simultaneously herbivory encountered by a plant and resource availability both directly and via altered competitive environment. Here, we address the question of how growth (leaf mass per area (LMA), plant size) and resistance traits (leaf alkaloids, leaf trichomes, resistance to a generalist snail) vary and covary between native and introduced populations of the garden lupine, Lupinus polyphyllus. We focused specifically on evolved differences among populations by measuring traits from plants grown from seed in a common environment. Plants from the introduced populations were more resistant against the generalist snail, Arianta arbustorum, and they had more leaf trichomes and higher LMA than plants from the native populations. The composition of alkaloids differed between native and introduced populations, with the native populations having more diversity in alkaloids among them. Resistance was positively associated with plant size and LMA across all populations. Other trait associations differed between native and introduced areas, implying that certain trade-offs may be fundamentally different between native and introduced populations. Our results suggest that, for the introduced populations, the loss of native herbivores and the alterations in resource availability have led to a lower diversity in leaf alkaloids among populations and may facilitate the evolution of novel trait optima without compensatory trade-offs. Such phytochemical similarity among introduced populations provides novel insights into mechanisms promoting successful plant invasions.

Biochemical response of Sonneratia alba Sm. branches infested by a wood boring moth (Gazi Bay, Kenya)

Infestation by a moth woodborer species is causing mortality of Sonneratia alba Sm. mangrove by tunneling through the inner bark, cambium and conductive tissue. Infestation leads to death of some infested branches, whereas in other cases infested branches have been observed to recover from infestation. We have used Fourier transform infrared spectroscopy (FTIR) to investigate the differences in macromolecule (polysaccharide and lignin) content present in branches that died (D) of the infestation, those that recovered (R) from the infestation and control branches (C) that were not subject to any infestation. Wood samples were taken from four sampling plots (A, B, C and D) in Gazi Bay (Kenya). From each of the four plots, 15 S. alba branches were taken from five trees, from which 1 cm thick discs were cut from each of these branches to be used as samples. To identify the most characteristic FTIR bands for the three groups of samples, Principal Component Analysis (PCA) was applied on the transposed data matrix. Furthermore, canonical discriminant analysis was applied on the data considering the main FTIR band that would be identified through the PCA factors. Finally, One-way ANOVA and post hoc test were used to verify the significance of the observed trends. Branches that recovered from infestation had higher relative abundance of lignified cells. We conclude that insect-infested S. alba undergo changes related to the lignocellulosic contents. The infestation induces a decrease of the proportion of the polysaccharide content and an increase of the proportion of the lignin contents.

Investigation of secondary metabolites in bean cultivars and their impact on the nutritional performance of Spodoptera littoralis (Lep.: Noctuidae)

Spodoptera littoralis (Boisd) is globally recognized as a destructive polyphagous insect pest of various crops in the world. It is commonly managed by chemical pesticides, which can cause deleterious effects such as environmental pollution, toxicity to non-target organisms and the emergence of secondary pests. Hence, investigations into alternative pest control strategies such as the use of resistant host plant cultivar against S. littoralis is important. This study aimed to explore the nutritional performance of S. littoralis larvae in dependence on total anthocyanin, flavonoid, and phenol levels across 11 bean cultivars (Phaseolus and Vigna spp.) under laboratory conditions. The results revealed that the Mashhad cultivar accumulated the highest amount of total phenols (13.59 mg ml-1), whereas Yaghout and Arabi cultivars posed the lowest total phenols contents (1.80 and 1.90 mg ml-1, respectively). Across larval instars (third to sixth), the highest consumption index and relative consumption rate were recorded on the Mashhad cultivar. The lowest values of efficiency of conversion of ingested food and the efficiency of conversion of digested food of total larval instars were detected in the larvae which were reared on the Mashhad cultivar. Likewise, the lowest value of the index of plant quality (IPQ) was obtained in the Mashhad cultivar; however, IPQ was figured out at the highest level in the Arabi cultivar. Our findings show that the differential accumulation of secondary metabolites would change the nutritional quality of plants for S. littoralis. Based on the findings, the Mashhad cultivar may serve as a candidate for either integrated pest management or breeding programs aiming at controlling this pest.

Evolution of shade tolerance is associated with attenuation of shade avoidance and reduced phenotypic plasticity in North American milkweeds

PREMISE

Mismatches between light conditions and light-capture strategy can reduce plant performance and prevent colonization of novel habitats. Although light-capture strategies tend to be highly conserved among closely related species, evolutionary transitions from shaded to unshaded habitats (and vice versa) occur in numerous plant lineages.

METHODS

We combined phylogenetic approaches with field and greenhouse experiments to investigate evolutionary constraints on light-capture strategy in North American milkweeds (genus Asclepias) and to determine whether colonization of shaded habitats in this heliophilic clade is associated with reduced plasticity and attenuation of the shade avoidance response.

RESULTS

Colonization of shaded habitats has occurred at least 10 times in this genus, including at least once in each major North American clade. Evolutionary transitions between habitats exhibit strong directional bias, with shifts from full-sun to shaded habitats occurring at least three times as often as the opposite transition. In field and greenhouse experiments, sun species responded to shade by increasing internode length, height, and specific leaf area, consistent with the shade avoidance response; paired shade species exhibited reduced plasticity overall, and only one trait (specific leaf area) responded to experimental shade.

CONCLUSIONS

Our results suggest that milkweeds colonized shaded environments multiple times using a light-capture strategy distinct from the ancestral (putatively shade avoidant) strategy, including a general attenuation of plasticity in response to variable light conditions. This pattern bolsters the notion that shade avoidance and tolerance represent divergent evolutionary strategies for maximizing performance under qualitatively different types of shade.

Variations in morphological and epidermal features of shade and sun leaves of two species: Quercus bambusifolia and Q. myrsinifolia

PREMISE

Microclimatic differences between the periphery and the interior of tree crowns result in a variety of adaptive leaf macromorphological and anatomical features. Our research was designed to reveal criteria for sun/shade leaf identification in two species of evergreen oaks, applicable to both modern and fossil leaves. We compared our results with those in other species similarly studied.

METHODS

For both Quercus bambusifolia and Q. myrsinifolia (section Cyclobalanopsis), leaves from single mature trees with well-developed crowns were collected in the South China Botanical Garden, Guangzhou, China. We focus on leaf characters often preserved in fossil material. SVGm software was used for macromorphological measurement. Quantitative analyses were performed and box plots generated using R software with IDE Rstudio. Leaf cuticles were prepared using traditional botanical techniques.

RESULTS

Principal characters for distinguishing shade and sun leaves in the studied oaks were identified as leaf lamina length to width ratio (L/W), and the degree of development of venation networks. For Q. myrsinifolia, shade and sun leaves differ in tooth morphology and the ratio of toothed lamina length to overall lamina length. The main epidermal characters are ordinary cell size and anticlinal wall outlines. For both species, plasticity within shade leaves exceeds that of sun leaves.

CONCLUSIONS

Morphological responses to sun and shade in the examined oaks are similar to those in other plant genera, pointing to useful generalizations for recognizing common foliar polymorphisms that must be taken into account when determining the taxonomic position of both modern and fossil plants.

Relationship of contact angle of spray solution on leaf surfaces with weed control

The weeds are important in agricultural and livestock areas because these plants can cause several damages, especially in the yield. The herbicide pulverization for weed control is the most used, but the efficiency of the control can be dependent the several factors, for example, the correct chose the herbicide and the mixture or not with adjuvant. This study aimed to evaluate the contact angle of herbicide solution droplets associated with adjuvant when deposited on the leaf surface of different weed species and their relationship with chemical control. For the contact angle experiment, the design was completely randomized, with four repetitions, while for the control experiment, a randomized block design was used, both experiments were arranged in a factorial (4 × 2 + 1) design. Factor A corresponded to four spray solutions containing the herbicide no addition of adjuvants and herbicide associated with adjuvants (vegetable oil, mineral oil, and lecithin), factor B to two herbicide dosages, and additional treatment corresponded to water. The contact angle was determined in six weed species: Crotalaria incana, Lantana camara, Ipomoea grandifolia, Asclepias curassavica, Sida obtusifolia, and Ricinus communis, on the adaxial and abaxial surface of each species, and an artificial surface. For the weed control experiment was used two weed species: C. incana and L. camara. The multivariate analysis allowed the understanding of the behavior of the contact angle of the different groups on the natural and artificial surfaces, due to the formation of factors. For all plants, except for the abaxial surface of I. grandifolia and the adaxial surface of A. curassavica, the association of herbicide and adjuvants reduced contact angle on the surfaces. The chemical control resulted in an indirect relation with contact angle, where smaller contact angles of the herbicide solution resulted in a higher percentage of plant intoxication. Therefore, for this situation, it is recommended to use the herbicide aminopyralid + fluroxypir associated with lecithin.

Variation Between Three Eragrostis tef Accessions in Defense Responses to Rhopalosiphum padi Aphid Infestation

Tef (Eragrostis tef), a staple crop that originated in the Horn of Africa, has been introduced to multiple countries over the last several decades. Crop cultivation in new geographic regions raises questions regarding the molecular basis for biotic stress responses. In this study, we aimed to classify the insect abundance on tef crop in Israel, and to elucidate its chemical and physical defense mechanisms in response to insect feeding. To discover the main pests of tef in the Mediterranean climate, we conducted an insect field survey on three selected accessions named RTC-144, RTC-405, and RTC-406, and discovered that the most abundant insect order is Hemiptera. We compared the differences in Rhopalosiphum padi (Hemiptera; Aphididae) aphid performance, preference, and feeding behavior between the three accessions. While the number of aphid progeny was lower on RTC-406 than on the other two, the aphid olfactory assay indicated that the aphids tended to be repelled from the RTC-144 accession. To highlight the variation in defense responses, we investigated the physical and chemical mechanisms. As a physical barrier, the density of non-granular trichomes was evaluated, in which a higher number of trichomes on the RTC-406 than on the other accessions was observed. This was negatively correlated with aphid performance. To determine chemical responses, the volatile and central metabolite profiles were measured upon aphid attack for 4 days. The volatile analysis exposed a rich and dynamic metabolic profile, and the central metabolism profile indicated that tef plants adjust their sugars and organic and amino acid levels. Overall, we found that the tef plants possess similar defense responses as other Poaceae family species, while the non-volatile deterrent compounds are yet to be characterized. A transcriptomic time-series analysis of a selected accession RTC-144 infested with aphids revealed a massive alteration of genes related to specialized metabolism that potentially synthesize non-volatile toxic compounds. This is the first report to reveal the variation in the defense mechanisms of tef plants. These findings can facilitate the discovery of insect-resistance genes leading to enhanced yield in tef and other cereal crops.

Trait Multi-Functionality in Plant Stress Response

Plants often experience multiple stresses in a given day or season, and it is self-evident that given functional traits can provide tolerances of multiple stresses. Yet, the multiple functions of individual traits are rarely explicitly considered in ecology and evolution due to a lack of a quantitative framework. We present a theory for considering the combined importance of the several functions that a single trait can contribute to alleviating multiple stresses. We derive five inter-related general predictions: (1) that trait multifunctionality is overall highly beneficial to fitness; (2) that species possessing multifunctional traits should increase in abundance and in niche breadth; (3) that traits are typically optimized for multiple functions and thus can be far from optimal for individual functions; (4) that the relative importance of each function of a multifunctional trait depends on the environment; and (5) that traits will be often "co-opted" for additional functions during evolution and community assembly. We demonstrate how the theory can be applied quantitatively by examining the multiple functions of leaf trichomes (hairs) using heuristic model simulations, substantiating the general principles. We identify avenues for further development and applications of the theory of trait multifunctionality in ecology and evolution.

Living in Drylands: Functional Adaptations of Trees and Shrubs to Cope with High Temperatures and Water Scarcity

Plant functioning and survival in drylands are affected by the combination of high solar radiation, high temperatures, low relative humidity, and the scarcity of available water. Many ecophysiological studies have dealt with the adaptation of plants to cope with these stresses in hot deserts, which are the territories that have better evoked the idea of a dryland. Nevertheless, drylands can also be found in some other areas of the Earth that are under the Mediterranean-type climates, which imposes a strong aridity during summer. In this review, plant species from hot deserts and Mediterranean-type climates serve as examples for describing and analyzing the different responses of trees and shrubs to aridity in drylands, with special emphasis on the structural and functional adaptations of plants to avoid the negative effects of high temperatures under drought conditions. First, we analyze the adaptations of plants to reduce the input of energy by diminishing the absorbed solar radiation through (i) modifications of leaf angle and (ii) changes in leaf optical properties. Afterwards, we analyze several strategies that enhance the ability for heat dissipation through (i) leaf size reduction and changes in leaf shape (e.g., through lobed leaves), and (ii) increased transpiration rates (i.e., water-spender strategy), with negative consequences in terms of photosynthetic capacity and water consumption, respectively. Finally, we also discuss the alternative strategy showed by water-saver plants, a common drought resistance strategy in hot and dry environments that reduces water consumption at the expense of diminishing the ability for leaf cooling. In conclusion, trees and shrubs living in drylands have developed effective functional adaptations to cope with the combination of high temperature and water scarcity, all of them with clear benefits for plant functioning and survival, but also with different costs concerning water use, carbon gain, and/or leaf cooling.

Correlated evolution of leaf and root anatomic traits in Dendrobium (Orchidaceae)

The whole-plant economic spectrum concept predicts that leaf and root traits evolve in coordination to cope with environmental stresses. However, this hypothesis is difficult to test in many species because their leaves and roots are exposed to different environments, above- and below-ground. In epiphytes, both leaves and roots are exposed to the atmosphere. Thus, we suspect there are consistent water conservation strategies in leaf and root traits of epiphytes due to similar selection pressures. Here, we measured the functional traits of 21 species in the genus Dendrobium, which is one of the largest epiphytic taxa in the family Orchidaceae, and used phylogenetically independent contrasts to test the relationships among traits, and between traits and the environment. Our results demonstrate that species with a thicker velamen tended to have thicker roots, a thicker root cortex and vascular cylinder, and a larger number of vessels in the root. Correspondingly, these species also had higher leaf mass per area, and thicker leaf lower cuticles. Leaf and root traits associated with water conservation showed significantly positive relationships. The number of velamen layers, leaf density and the ratio of vascular cylinder radius to root radius were significantly affected by the species' differing environments. Thus, traits related to water conservation and transport may play an important role in helping Dendrobium cope with the cool and dry conditions found at high elevations. These findings confirmed the hypothesis that leaf and root traits have evolved in coordination, and also provide insights into trait evolution and ecological adaptation in epiphytic orchids.

Her Majesty's Desert Throne: The Ecology of Queen Butterfly Oviposition on Mojave Milkweed Host Plants

Butterfly-host plant relationships can inform our understanding of ecological and trophic interactions that contribute to ecosystem function, resiliency, and services. The ecology of danaid-milkweed (Apocynaceae) host plant interactions has been studied in several biomes but is neglected in deserts. Our objective was to determine effects of plant traits, seasonality, and landscape-level host plant availability on selection of Mojave milkweed (Asclepias nyctaginifolia A. Gray) by ovipositing monarch butterflies (Danaus plexippus plexippus) and queen butterflies (Danaus gilippus thersippus) in the Californian Mojave Desert. We surveyed all known Mojave milkweed locations in the Ivanpah Valley, California (n = 419) during early, mid-, and late spring in 2017. For each survey, we counted monarch and queen butterfly eggs on each Mojave milkweed plant. We also measured canopy cover, height, volume, and reproductive stage of each Mojave milkweed plant. We counted a total of 276 queen butterfly eggs and zero monarch butterfly eggs on Mojave milkweed host plants. We determined that count of queen butterfly eggs significantly increased with increasing Mojave milkweed canopy cover. Additionally, count of queen butterfly eggs was: (1) greater on adult Mojave milkweed plants than on juvenile and seedling plants and greater on juvenile Mojave milkweed plants than on seedling plants; and (2) greater during early spring than mid-spring-we recorded no eggs during late spring. Based on aggregation indices, queen butterfly eggs occurred on Mojave milkweed plants in a nonrandom, clustered pattern throughout the Ivanpah Valley. We provide the first evidence of trophic interactions between queen butterflies and Mojave milkweed at multiple spatial scales in the Mojave Desert, suggesting that conservation and management practices for both species should be implemented concurrently. Given its role as an herbivore, pollinator and prey, the queen butterfly may serve as a model organism for understanding effects of anthropogenic disturbance (e.g., solar energy development) on "bottom-up" and trophic interactions among soils, plants and animals in desert ecosystems.

Leaf trichomes in Metrosideros polymorpha can contribute to avoiding extra water stress by impeding gall formation

BACKGROUND AND AIMS

Plants inhabiting arid environments tend to have leaf trichomes, but their adaptive significance remains unclear. Leaf trichomes are known to play a role in plant defence against herbivores, including gall makers. Because gall formation can increase water loss partly through increased surface area, we tested the novel hypothesis that leaf trichomes could contribute to avoiding extra water stress by impeding gall formation, which would have adaptive advantages in arid environments.

METHODS

We focused on Metrosideros polymorpha, an endemic tree species in the Hawaiian Islands, whose leaves often suffer from galls formed by specialist insects, Hawaiian psyllids (Pariaconus spp.). There is large variation in the amount of leaf trichomes (0-40 % of leaf mass) in M. polymorpha. Three gall types are found on the island of Hawaii: the largest is the 'cone' type, followed by 'flat' and 'pit' types. We conducted laboratory experiments to quantify the extent to which gall formation is associated with leaf water relations. We also conducted a field census of 1779 individuals from 48 populations across the entire range of habitats of M. polymorpha on the island of Hawaii to evaluate associations between gall formation (presence and abundance) and the amount of leaf trichomes.

KEY RESULTS

Our laboratory experiment showed that leaf minimum conductance was significantly higher in leaves with a greater number of cone- or flat-type galls but not pit-type galls. Our field census suggested that the amount of trichomes was negatively associated with probabilities of the presence of cone- or flat-type galls but not pit-type galls, irrespective of environmental factors.

CONCLUSION

Our results suggest that leaf trichomes in M. polymorpha can contribute to the avoidance of extra water stress through interactions with some gall-making species, and potentially increase the fitness of plants under arid conditions.

Plant Defense by Latex: Ecological Genetics of Inducibility in the Milkweeds and a General Review of Mechanisms, Evolution, and Implications for Agriculture

Latex occurs in 10% of plant families, has evolved independently many times, and is the most effective defense of milkweeds against its chewing herbivores. Here we report on new experiments on the heritability and inducibility of latex in several milkweed species. In addition, we review what is known about the genetic and environmental determinants of latex exudation, hormonal regulation, evolution within and among species, and the role and frequency of latex in agricultural crops. We first evaluated genotype-by-environment interactions using ~20 full-sibling genetic families in each of seven Asclepias species treated as controls or attacked by monarch butterfly caterpillars. All species showed substantial genetic variation for latex exudation and six of seven species responded to monarch herbivory (two species increased latex, two species decreased, and two showed variation among genetic families). Exogenous application of jasmonic acid (JA) to three species induced a consistent increase in latex (including species which showed a decline following caterpillar herbivory). We next evaluated three hypotheses for what determines genetic variation for induced latex in A. syriaca: 1) a trade-off with constitutive investment, 2) differential endogenous JA induction, or 3) variation in responsiveness to JA. We only found support for the second hypothesis: genetic families with a stronger JA-burst showed the greatest latex exudation following herbivory. We conclude that most species exhibit a genetic and inducible basis for latex, although genetic variation in inducibility is not pervasive. Finally, we summarized studies across 22 species of Asclepias and found that neither a species' latitude nor its phylogenetic position predicted latex inducibility. Nonetheless, a negative association between constitutive and induced latex across species indicates a macroevolutionary trade-off in allocation to this defense. Our review indicates that jasmonic acid is a key regulator of latex exudation, laticifer morphology, and defensive metabolites within latex. Biotic and abiotic factors strongly modulate latex expression. A survey of latex in food crops revealed that latex and analogous exudates (gums, resins, mucilage) are more common than expected based on their distribution across all plants. In conclusion, despite its widespread occurrence, the literature on latex is currently dominated by rubber trees and milkweeds, and we look forward to the broadening of ecological, agricultural, and mechanistic research into other systems.

Foliar plasticity related to gradients of heat and drought stress across crown orientations in three Mediterranean Quercus species

Studies on plasticity at the level of a single individual plant provide indispensable information to predict leaf responses to climate change, because they allow better identification of the environmental factors that determine differences in leaf traits in the absence of genetic differences. Most of these studies have focused on the responses of leaf traits to variations in the light environment along vertical gradients, thus paying less attention to possible differences in the intensity of water stress among canopy orientations. In this paper, we analyzed the differences in leaf traits traditionally associated with changes in the intensity of water stress between east and west crown orientations in three Quercus species. The leaves facing west experienced similar solar radiation levels but higher maximum temperatures and lower daily minimum water potentials than those of the east orientation. In response to these differences, the leaves of the west orientation showed smaller size and less chlorophyll concentration, higher percentage of palisade tissue and higher density of stomata and trichomes. These responses would confirm the role of such traits in the tolerance to water stress and control of water losses by transpiration. For all traits, the species with the longest leaf life span exhibited the greatest plasticity between orientations. By contrast, no differences between canopy positions were observed for leaf thickness, leaf mass per unit area and venation patterns.

Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies

Background: North American monarchs (Danaus plexippus) are well-known for their long-distance migrations; however, some monarchs within the migratory range have adopted a resident lifestyle and breed year-round at sites where tropical milkweed (Asclepias curassavica) is planted in the southern coastal United States. An important question is whether exposure to exotic milkweed alters monarch migratory physiology, particularly the ability to enter and remain in the hormonally-induced state of reproductive diapause, whereby adults delay reproductive maturity. Cued by cooler temperatures and shorter photoperiods, diapause is a component of the monarch’s migratory syndrome that includes directional flight behavior, lipid accumulation, and the exceptional longevity of the migratory generation. Methods: Here, we experimentally test how exposure to tropical milkweed during the larval and adult stages influences monarch reproductive status during fall migration. Caterpillars reared under fall-like conditions were fed tropical versus native milkweed diets, and wild adult migrants were placed in outdoor flight cages with tropical milkweed, native milkweed, or no milkweed. Results: We found that monarchs exposed to tropical milkweed as larvae were more likely to be reproductively active (exhibit mating behavior in males and develop mature eggs in females) compared to monarchs exposed to native milkweed. Among wild-caught fall migrants, females exposed to tropical milkweed showed greater egg development than females exposed to native or no milkweed, although a similar response was not observed for males. Conclusions: Our study provides evidence that exposure to tropical milkweed can increase monarch reproductive activity, which could promote continued residency at year-round breeding sites and decrease monarch migratory propensity.

Defence gene expression and phloem quality contribute to mesophyll and phloem resistance to aphids in wild barley

Aphids, including the bird cherry-oat aphid (Rhopalosiphum padi), are significant agricultural pests. The wild relative of barley, Hordeum spontaneum 5 (Hsp5), has been described to be partially resistant to R. padi, with this resistance proposed to involve higher thionin and lipoxygenase gene expression. However, the specificity of this resistance to aphids and its underlying mechanistic processes are unknown. In this study, we assessed the specificity of Hsp5 resistance to aphids and analysed differences in aphid probing and feeding behaviour on Hsp5 and a susceptible barley cultivar (Concerto). We found that partial resistance in Hsp5 to R. padi extends to two other aphid pests of grasses. Using the electrical penetration graph technique, we show that partial resistance is mediated by phloem- and mesophyll-based resistance factors that limit aphid phloem ingestion. To gain insight into plant traits responsible for partial resistance, we compared non-glandular trichome density, defence gene expression, and phloem composition of Hsp5 with those of the susceptible barley cultivar Concerto. We show that Hsp5 partial resistance involves elevated basal expression of thionin and phytohormone signalling genes, and a reduction in phloem quality. This study highlights plant traits that may contribute to broad-spectrum partial resistance to aphids in barley.

Trade-offs constrain the evolution of an inducible defense within but not between plant species

Inducible defense is a common form of phenotypic plasticity, and inducibility (change in defense after herbivore attack) has long been predicted to trade off with constitutive (or baseline) defense to manage resource allocation. Although such trade-offs likely constrain evolution within species, the extent to which they influence divergence among species is unresolved. We studied cardenolide toxins among genetic families in eight North American Asclepias species, spanning the full range of defense in the genus. Using common environment experiments and chemical assays, we report a consistent trade-off (negative genetic correlation) between concentrations of constitutive cardenolides and their inducibility within each species. However, no trade-off was found in a phylogenetic analysis across species. To investigate factors driving differences in defense allocation among species we used latitude as a proxy for growing season and herbivore pressure and found that divergence into lower latitudes resulted in evolution of higher cardenolides overall. Next we used an enzymatic assay of the cellular target of cardenolides (sodium-potassium ATPase) and confirm that higher cardenolides resulted in stronger toxicity to a sensitive species, but not to specialized monarch butterflies. Thus, plant speciation into biogeographic regions with alternative resources or pest pressure resulted in the macroevolution of cardenolide defense, especially against unspecialized herbivores. Nonetheless, trade-offs persist in the extent to which this defense is allocated constitutively or is inducible among genotypes within each species.

NGS-Indel Coder: A pipeline to code indel characters in phylogenomic data with an example of its application in milkweeds (Asclepias)

Targeted genome sequencing approaches allow characterization of evolutionary relationships using a considerable number of nuclear genes and informative characters. However, most phylogenomic analyses only utilize single nucleotide polymorphisms (SNPs). Studies at the species level, especially in groups that have recently radiated, often recover low amounts of phylogenetically informative variation in coding regions, and require non-coding sequences, which are richer in indels, to resolve gene trees. Here, NGS-Indel Coder, a pipeline to detect and omit false positive indels inferred from assemblies of short read sequence data, was developed to resolve the relationships among and within major clades of the American milkweeds (Asclepias), which are the result of a rapid and recent evolutionary radiation, and whose phylogeny has been difficult to resolve. This pipeline was applied to a Hyb-Seq data set of 768 loci including targeted exons and flanking intron regions from 33 milkweed species. Robust species tree inference was improved by excluding small alignment partitions (<100 bp) that increased gene tree ambiguity and incongruence. To further investigate the robustness of indel coding, data sets that included small and large indels were explored, and species trees derived from concatenated loci versus coalescent methods based on gene trees were compared. The phylogeny of Asclepias obtained using nuclear data was well resolved, and phylogenetic information from indels improved resolution of specific nodes. The Temperate North American, Mexican Highland, and Incarnatae clades were well supported as monophyletic. Asclepias coulteri, which has been considered part of the Sonoran Desert clade based on plastome analyses, was placed as sister to all the other milkweed species studied here, rather than as a member of that clade. Two groups within the Temperate North American and Mexican clades were not resolved, and the inferred relationships strongly conflicted when comparing results based on data sets that did or did not include indel characters. This new pipeline represents a step forward in making maximal use of the information content in phylogenomic data sets.

Asclepiasspeciosa (Apocynaceae, Asclepiadoideae): a rare or unrecognized alien species in Europe?

Studies on populations of Asclepiassyriaca L. in Lithuania revealed the occurrence of a new alien plant species, the North American native Asclepiasspeciosa Torr. (Apocynaceae, Asclepiadoideae), in southern parts of Lithuania - the first report of the latter species in Europe. Interestingly, a thorough analysis of herbarium specimens revealed that A.speciosa had first been collected in Lithuania in 1962, but the specimen was misidentified at the time as A.syriaca. The newly discovered population of A.speciosa occupies mesic grasslands, tall-herb fringe communities and arable field habitats. Sexual reproduction of this species was not recorded; it spreads locally by means of vegetative reproduction. We present here an exhaustive analysis of morphological characteristics and differences between A.speciosa and A.syriaca and other species of the genus, as well as a key for identification of alien Asclepias species in Europe. We predict that the effect of A.speciosa on native habitats and communities, and its economic impact, are comparable to those of the highly invasive A.syriaca. Although A.speciosa currently occurs very rarely as an alien species in Europe, its existence in other regions of Europe is highly probable.

Cardenolide Intake, Sequestration, and Excretion by the Monarch Butterfly along Gradients of Plant Toxicity and Larval Ontogeny

Monarch butterflies, Danaus plexippus, migrate long distances over which they encounter host plants that vary broadly in toxic cardenolides. Remarkably little is understood about the mechanisms of sequestration in Lepidoptera that lay eggs on host plants ranging in such toxins. Using closely-related milkweed host plants that differ more than ten-fold in cardenolide concentrations, we mechanistically address the intake, sequestration, and excretion of cardenolides by monarchs. We show that on high cardenolide plant species, adult butterflies saturate in cardenolides, resulting in lower concentrations than in leaves, while on low cardenolide plants, butterflies concentrate toxins. Butterflies appear to focus their sequestration on particular compounds, as the diversity of cardenolides is highest in plant leaves, lower in frass, and least in adult butterflies. Among the variety of cardenolides produced by the plant, sequestered compounds may be less toxic to the butterflies themselves, as they are more polar on average than those in leaves. In accordance with this, results from an in vitro assay based on inhibition of Na⁺/K⁺ ATPase (the physiological target of cardenolides) showed that on two milkweed species, including the high cardenolide A. perennis, extracts from butterflies have lower inhibitory effects than leaves when standardized by cardenolide concentration, indicating selective sequestration of less toxic compounds from these host plants. To understand how ontogeny shapes sequestration, we examined cardenolide concentrations in caterpillar body tissues and hemolymph over the course of development. Caterpillars sequestered higher concentrations of cardenolides as early instars than as late instars, but within the fifth instar, concentration increased with body mass. Although it appears that large amounts of sequestration occurs in early instars, a host switching experiment revealed that caterpillars can compensate for feeding on low cardenolide host plants with substantial sequestration in the fifth instar. We highlight commonalities and striking differences in the mechanisms of sequestration depending on host plant chemistry and developmental stage, which have important implications for monarch defense.

Hexaacetyl-chitohexaose, a chitin-derived oligosaccharide, transiently activates citrus defenses and alters the feeding behavior of Asian citrus psyllid

Plants have a perception system triggered by pathogen and pest signals to initiate defense. These signals include evolutionarily conserved molecules from microbes and insects termed pathogen/herbivore-associated molecular patterns (PAMPs/HAMPs). Here we showed that hexaacetyl-chitohexaose (HC), an oligosaccharide from chitin, a structural component in insect exoskeletons and fungi cell walls, upregulated defense-associated genes WRKY22, GST1, RAR1, EDS1, PAL1 and NPR2, and downregulated ICS1 at 1 h after HC treatment in Sun Chu Sha mandarin leaves. The effect was transient as defense gene transcriptional changes were not observed at 18 h after the treatment. Electrical penetration graph (EPG) recordings were used to study the feeding behavior of Asian citrus psyllid (ACP) following the HC treatment. ACP is the hemipteran vector of Candidatus Liberibacter asiaticus (CLas), the pathogen associated with huanglongbing (HLB). Adult ACP displayed reduced intercellular probing, reduced xylem feeding count and duration, and increased non-probing activity on HC-treated citrus compared to controls. During an 18-h recording, percentage for total duration of xylem ingestion, phloem ingestion, intercellular probing were lower, and the percentage of non-probing behavior was higher in HC-treated leaves than in controls. In host-selection behavior studies, HC treatment did not alter the attractiveness of citrus leaves under light or dark conditions. In addition, ACP feeding on HC-treated leaves did not show differences in mortality for up to 10 day of exposure. In summary, we report that HC induced a transient defense in citrus and an inhibitory effect on ACP feeding but did not affect host selection or the insect fitness under the tested conditions.

Population Variation, Environmental Gradients, and the Evolutionary Ecology of Plant Defense against Herbivory

A central tenet of plant defense theory is that adaptation to the abiotic environment sets the template for defense strategies, imposing a trade-off between plant growth and defense. Yet this trade-off, commonly found among species occupying divergent resource environments, may not occur across populations of single species. We hypothesized that more favorable climates and higher levels of herbivory would lead to increases in growth and defense across plant populations. We evaluated whether plant growth and defense traits covaried across 18 populations of showy milkweed (Asclepias speciosa) inhabiting an east-west climate gradient spanning 25° of longitude. A suite of traits impacting defense (e.g., latex, cardenolides), growth (e.g., size), or both (e.g., specific leaf area [SLA], trichomes) were measured in natural populations and in a common garden, allowing us to evaluate plastic and genetically based variation in these traits. In natural populations, herbivore pressure increased toward warmer sites with longer growing seasons. Growth and defense traits showed strong clinal patterns and were positively correlated. In a common garden, clines with climatic origin were recapitulated only for defense traits. Correlations between growth and defense traits were also weaker and more negative in the common garden than in the natural populations. Thus, our data suggest that climatically favorable sites likely facilitate the evolution of greater defense at minimal costs to growth, likely because of increased resource acquisition.

Effects of Short- and Long-Term Variation in Resource Conditions on Soil Fungal Communities and Plant Responses to Soil Biota

Soil biota can strongly influence plant performance with effects ranging from negative to positive. However, shifts in resource availability can influence plant responses, with soil pathogens having stronger negative effects in high-resource environments and soil mutualists, such as arbuscular mycorrhizal fungi (AMF), having stronger positive effects in low-resource environments. Yet the relative importance of long-term vs. short-term variation in resources on soil biota and plant responses is not well-known. To assess this, we grew the perennial herb Asclepias speciosa in a greenhouse experiment that crossed a watering treatment (wet vs. dry treatment) with a manipulation of soil biota (live vs. sterilized soil) collected from two geographic regions (Washington and Minnesota) that vary greatly in annual precipitation. Because soil biota can influence many plant functional traits, we measured biomass as well as resource acquisition (e.g., root:shoot, specific leaf area) and defense (e.g., trichome and latex production) traits. Due to their important role as mutualists and pathogens, we also characterized soil fungal communities in the field and greenhouse and used curated databases to assess fungal composition and potential function. We found that the experimental watering treatment had a greater effect than soil biota origin on plant responses; most plant traits were negatively affected by live soils under wet conditions, whereas responses were neutral or positive in live dry soil. These consistent differences in plant responses occurred despite clear differences in soil fungal community composition between inoculate origin and watering treatments, which indicates high functional redundancy among soil fungi. All plants grown in live soil were highly colonized by AMF and root colonization was higher in wet than dry soil; root colonization by other fungi was low in all treatments. The most parsimonious explanation for negative plant responses in wet soil is that AMF became parasitic under conditions that alleviated resource limitation. Thus, plant responses appeared driven by shifts within rather than between fungal guilds, which highlights the importance of coupling growth responses with characterizations of soil biota to fully understand underlying mechanisms. Collectively these results highlight how short-term changes in environmental conditions can mediate complex interactions between plants and soil biota.

Effects of Short- and Long-Term Variation in Resource Conditions on Soil Fungal Communities and Plant Responses to Soil Biota

Soil biota can strongly influence plant performance with effects ranging from negative to positive. However, shifts in resource availability can influence plant responses, with soil pathogens having stronger negative effects in high-resource environments and soil mutualists, such as arbuscular mycorrhizal fungi (AMF), having stronger positive effects in low-resource environments. Yet the relative importance of long-term vs. short-term variation in resources on soil biota and plant responses is not well-known. To assess this, we grew the perennial herb Asclepias speciosa in a greenhouse experiment that crossed a watering treatment (wet vs. dry treatment) with a manipulation of soil biota (live vs. sterilized soil) collected from two geographic regions (Washington and Minnesota) that vary greatly in annual precipitation. Because soil biota can influence many plant functional traits, we measured biomass as well as resource acquisition (e.g., root:shoot, specific leaf area) and defense (e.g., trichome and latex production) traits. Due to their important role as mutualists and pathogens, we also characterized soil fungal communities in the field and greenhouse and used curated databases to assess fungal composition and potential function. We found that the experimental watering treatment had a greater effect than soil biota origin on plant responses; most plant traits were negatively affected by live soils under wet conditions, whereas responses were neutral or positive in live dry soil. These consistent differences in plant responses occurred despite clear differences in soil fungal community composition between inoculate origin and watering treatments, which indicates high functional redundancy among soil fungi. All plants grown in live soil were highly colonized by AMF and root colonization was higher in wet than dry soil; root colonization by other fungi was low in all treatments. The most parsimonious explanation for negative plant responses in wet soil is that AMF became parasitic under conditions that alleviated resource limitation. Thus, plant responses appeared driven by shifts within rather than between fungal guilds, which highlights the importance of coupling growth responses with characterizations of soil biota to fully understand underlying mechanisms. Collectively these results highlight how short-term changes in environmental conditions can mediate complex interactions between plants and soil biota.

Screening Germplasm and Quantification of Components Contributing to Thrips Resistance in Cotton

Three hundred and ninety-one Gossypium hirsutum and 34 Gossypium barbadense accessions were screened for thrips resistance under field conditions at the Upper Coastal Plain Research Station in Rocky Mount, North Carolina in years 2014 and 2015. Visual damage ratings, thrips counts, and seedling dry weights were recorded at 2.5, 3.5, and 4.5 wk after planting, respectively. Population density and thrips arrival times varied between years. Data from the three separate damage scoring dates provided a better estimate of resistance or susceptibility to thrips than ratings from the individual dates over the season. Tobacco thrips [Frankliniella fusca (Hinds) (Thysanoptera: Thripidae)], followed by western flower thrips [Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae)], were the dominant thrips species observed in the study. Five resistant G. barbadense accessions and five moderately resistant upland cotton accessions were identified from field evaluations. Greenhouse experiments were conducted in Fall 2015 and Spring 2016 to determine if plant height, growth rate, leaf pubescence, and leaf area were significantly different in resistant and susceptible groups of G. hirsutum and G. barbadense accessions identified from the field screenings. Leaf pubescence and relative growth rate were significantly higher in resistant accessions compared with susceptible accessions in absence of thrips. There was no difference for plant height and leaf area between resistant and susceptible groups. Results suggest thrips-resistant plants have a possible competitive advantage through faster growth and higher trichome density, which limits thrips movement.

Assessing the influence of biogeographical region and phylogenetic history on chemical defences and herbivory in Quercus species

Biogeographical factors and phylogenetic history are key determinants of inter-specific variation in plant defences. However, few studies have conducted broad-scale geographical comparisons of plant defences while controlling for phylogenetic relationships, and, in doing so, none have separated constitutive from induced defences. This gap has limited our understanding of how historical or large-scale processes mediate biogeographical patterns in plant defences since these may be contingent upon shared evolutionary history and phylogenetic constraints. We conducted a phylogenetically-controlled experiment testing for differences in constitutive leaf chemical defences and their inducibility between Palearctic and Nearctic oak species (Quercus, total 18 species). We induced defences in one-year old plants by inflicting damage by gypsy moth larvae (Lymantria dispar), estimated the amount of leaf area consumed, and quantified various groups of phenolic compounds. There was no detectable phylogenetic signal for constitutive or induced levels of most defensive traits except for constitutive condensed tannins, as well as no phylogenetic signal in leaf herbivory. We did, however, find marked differences in defence levels between oak species from each region: Palearctic species had higher levels of constitutive condensed tannins, but less constitutive lignins and less constitutive and induced hydrolysable tannins compared with Nearctic species. Additionally, Palearctic species had lower levels of leaf damage compared with Nearctic species. These differences in leaf damage, lignins and hydrolysable (but not condensed) tannins were lost after accounting for phylogeny, suggesting that geographical structuring of phylogenetic relationships mediated biogeographical differences in defences and herbivore resistance. Together, these findings suggest that historical processes and large-scale drivers have shaped differences in allocation to constitutive defences (and in turn resistance) between Palearctic and Nearctic oaks. Moreover, although evidence of phylogenetic conservatism in the studied traits is rather weak, shared evolutionary history appears to mediate some of these biogeographical patterns in allocation to chemical defences.

Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies

While it is well established that climate change affects species distributions and abundances, the impacts of climate change on species interactions has not been extensively studied. This is particularly important for specialists whose interactions are tightly linked, such as between the monarch butterfly (Danaus plexippus) and the plant genus Asclepias, on which it depends. We used open-top chambers (OTCs) to increase temperatures in experimental plots and placed either nonnative Asclepias curassavica or native A. incarnata in each plot along with monarch larvae. We found, under current climatic conditions, adult monarchs had higher survival and mass when feeding on A. curassavica. However, under future conditions, monarchs fared much worse on A. curassavica. The decrease in adult survival and mass was associated with increasing cardenolide concentrations under warmer temperatures. Increased temperatures alone reduced monarch forewing length. Cardenolide concentrations in A. curassavica may have transitioned from beneficial to detrimental as temperature increased. Thus, the increasing cardenolide concentrations may have pushed the larvae over a tipping point into an ecological trap; whereby past environmental cues associated with increased fitness give misleading information. Given the ubiquity of specialist plant-herbivore interactions, the potential for such ecological traps to emerge as temperatures increase may have far-reaching consequences.

Evolution at the tips: Asclepias phylogenomics and new perspectives on leaf surfaces

PREMISE OF THE STUDY

Leaf surface traits, such as trichome density and wax production, mediate important ecological processes such as anti-herbivory defense and water-use efficiency. We present a phylogenetic analysis of Asclepias plastomes as a framework for analyzing the evolution of trichome density and presence of epicuticular waxes.

METHODS

We produced a maximum-likelihood phylogeny using plastomes of 103 species of Asclepias. We reconstructed ancestral states and used model comparisons in a likelihood framework to analyze character evolution across Asclepias.

KEY RESULTS

We resolved the backbone of Asclepias, placing the Sonoran Desert clade and Incarnatae clade as successive sisters to the remaining species. We present novel findings about leaf surface evolution of Asclepias-the ancestor is reconstructed as waxless and sparsely hairy, a macroevolutionary optimal trichome density is supported, and the rate of evolution of trichome density has accelerated.

CONCLUSIONS

Increased sampling and selection of best-fitting models of evolution provide more resolved and robust estimates of phylogeny and character evolution than obtained in previous studies. Evolutionary inferences are more sensitive to character coding than model selection.

Photoprotective Strategies of Mediterranean Plants in Relation to Morphological Traits and Natural Environmental Pressure: A Meta-Analytical Approach

Despite being a small geographic extension, Mediterranean Basin is characterized by an exceptional plant biodiversity. Adaptive responses of this biocoenosis are delineated by an unusual temporal dissociation along the year between optimal temperature for growth and water availability. This fact generates the combination of two environmental stress factors: a period of summer drought, variable in length and intensity, and the occurrence of mild to cold winters. Both abiotic factors, trigger the generation of (photo)oxidative stress and plants orchestrate an arsenal of structural, physiological, biochemical, and molecular mechanisms to withstand such environmental injuries. In the last two decades an important effort has been made to characterize the adaptive morphological and ecophysiological traits behind plant survival strategies with an eye to predict how they will respond to future climatic changes. In the present work, we have compiled data from 89 studies following a meta-analytical approach with the aim of assessing the composition and plasticity of photosynthetic pigments and low-molecular-weight antioxidants (tocopherols, glutathione, and ascorbic acid) of wild Mediterranean plant species. The influence of internal plant and leaf factors on such composition together with the stress responsiveness, were also analyzed. This approach enabled to obtain data from 73 species of the Mediterranean flora, with the genus Quercus being the most frequently studied. Main highlights of present analysis are: (i) sort of photoprotective mechanisms do not differ between Mediterranean plants and other floras but they show higher plasticity indexes; (ii) α-tocopherol among the antioxidants and violaxanthin-cycle pigments show the highest responsiveness to environmental factors; (iii) both winter and drought stresses induce overnight retention of de-epoxidised violaxanthin-cycle pigments; (iv) this retention correlates with depressions of Fv/Fm; and (v) contrary to what could be expected, mature leaves showed higher accumulation of hydrophilic antioxidants than young leaves, and sclerophyllous leaves higher biochemical photoprotective demand than membranous leaves. In a global climatic change scenario, the plasticity of their photoprotective mechanisms will likely benefit Mediterranean species against oceanic ones. Nevertheless, deep research of ecoregions other than the Mediterranean Basin will be needed to fully understand photoprotection strategies of this extremely biodiverse floristic biome: the Mediterranean ecosystem.

The Evolutionary Ecology of Plant Disease: A Phylogenetic Perspective

An explicit phylogenetic perspective provides useful tools for phytopathology and plant disease ecology because the traits of both plants and microbes are shaped by their evolutionary histories. We present brief primers on phylogenetic signal and the analytical tools of phylogenetic ecology. We review the literature and find abundant evidence of phylogenetic signal in pathogens and plants for most traits involved in disease interactions. Plant nonhost resistance mechanisms and pathogen housekeeping functions are conserved at deeper phylogenetic levels, whereas molecular traits associated with rapid coevolutionary dynamics are more labile at branch tips. Horizontal gene transfer disrupts the phylogenetic signal for some microbial traits. Emergent traits, such as host range and disease severity, show clear phylogenetic signals. Therefore pathogen spread and disease impact are influenced by the phylogenetic structure of host assemblages. Phylogenetically rare species escape disease pressure. Phylogenetic tools could be used to develop predictive tools for phytosanitary risk analysis and reduce disease pressure in multispecies cropping systems.

Ecological distribution of leaf stomata and trichomes among tree species in a Malaysian lowland tropical rain forest

The vertical structure of a tropical rain forest is complex and multilayered, with strong variation of micro-environment with height up to the canopy. We investigated the relation between morphological traits of leaf surfaces and tree ecological characteristics in a Malaysian tropical rain forest. The shapes and densities of stomata and trichomes on the abaxial leaf surfaces and their relation with leaf characteristics such as leaf area and leaf mass per area (LMA) were studied in 136 tree species in 35 families with different growth forms in the tropical moist forest. Leaf physiological properties were also measured in 50 canopy and emergent species. Most tree species had flat type (40.4 %) or mound type (39.7 %) stomata. In addition, 84 species (61.76 %) in 22 families had trichomes, including those with glandular (17.65 %) and non-glandular trichomes (44.11 %). Most leaf characteristics significantly varied among the growth form types: species in canopy and emergent layers and canopy gap conditions had higher stomatal density, stomatal pore index (SPI), trichome density and LMA than species in understory and subcanopy layers, though the relation of phylogenetically independent contrasts to each characteristic was not statistically significant, except for leaf stomatal density, SPI and LMA. Intrinsic water use efficiency in canopy and emergent tree species with higher trichome densities was greater than in species with lower trichome densities. These results suggest that tree species in tropical rain forests adapt to a spatial difference in their growth forms, which are considerably affected by phylogenetic context, by having different stomatal and trichome shapes and/or densities.

Macroevolution of leaf defenses and secondary metabolites across the genus Helianthus

Leaf defenses are widely recognized as key adaptations and drivers of plant evolution. Across environmentally diverse habitats, the macroevolution of leaf defenses can be predicted by the univariate trade-off model, which predicts that defenses are functionally redundant and thus trade off, and the resource availability hypothesis, which predicts that defense investment is determined by inherent growth rate and that higher defense will evolve in lower resource environments. Here, we examined the evolution of leaf physical and chemical defenses and secondary metabolites in relation to environmental characteristics and leaf economic strategy across 28 species of Helianthus (the sunflowers). Using a phylogenetic comparative approach, we found few evolutionary trade-offs among defenses and no evidence for defense syndromes. We also found that leaf defenses are strongly related to leaf economic strategy, with higher defense in more resource-conservative species, although there is little support for the evolution of higher defense in low-resource habitats. A wide variety of physical and chemical defenses predict resistance to different insect herbivores, fungal pathogens, and a parasitic plant, suggesting that most sunflower defenses are not redundant in function and that wild Helianthus represents a rich source of variation for the improvement of crop sunflower.