Hydraulic properties of European elms: xylem safety-efficiency tradeoff and species distribution in the Iberian Peninsula

Key message

Ulmus minor and U. glabra show a trade-off between safety and efficiency in water transport, and U. laevis shows adaptations to waterlogged environments.

Abstract

Three native elm species grow in Europe: Ulmus minor Mill., U. glabra Huds. and U. laevis Pall., and within the Iberian Peninsula their habitats mainly differ in water availability. We evaluated firstly whether vulnerability to xylem embolism caused by water-stress has been a determinant factor affecting their distribution; secondly, if their xylem anatomy differs due to water availability dissimilarities; and thirdly, if these species present a trade-off between water transport safety and efficiency. Plants of the three species were grown in a common-garden in Madrid, Central Spain. The centrifuge method was used for constructing the vulnerability curves, and anatomical measurements were carried out with an optical microscope. We found clear differences in conductivity and cavitation vulnerability between the three species. Although all three elms were highly vulnerable to cavitation, U. minor was significantly more resistant to water stress cavitation. This species reached 50 % loss in conductivity at ?1.1 MPa, compared to U. glabra that did so at ?0.5 MPa, and U. laevis at ?0.4 MPa. Maximum xylem specific conductivity and maximum leaf specific conductivity were two to three times higher in U. glabra when compared to U. minor. A clear trade-off between safety against losses of conductivity and water transport efficiency was observed considering both U. minor and U. glabra samples. Ulmus minor’s hydraulic configuration was better adapted to overcome drought episodes. The expected aridification of the Iberian Peninsula could compromise Ulmus populations due to their high vulnerability to drought stress.     

Hybridization and introgression between the exotic Siberian elm, Ulmus pumila, and the native Field elm, U. minor, in Italy

In response to the first Dutch elm disease (DED) pandemic, Siberian elm, Ulmus pumila, was planted to replace the native elm, U. minor, in Italy. The potential for hybridization between these two species is high and repeated hybridization could result in the genetic swamping of the native species and facilitate the evolution of invasiveness in the introduced species. We used genetic markers to examine the extent of hybridization between these two species and to determine the pattern of introgression. We quantified and compared the level of genetic diversity between the hybrids and the two parental species. Hybrids between U. pumila and U. minor were common. The pattern of introgression was not as strongly biased towards U. pumila as was previously observed for hybrids between U. rubra and U. pumila in the United States. The levels of heterozygosity were similar between U. minor and the hybrids and both groups had higher levels of heterozygosity relative to U. pumila. The programs Structure and NewHybrids indicated the presence of first- (F₁) and second- generation (F₂) hybrids and of backcrosses in the hybrid population. The presence of healthy DED resistant U. minor individuals combined with the self-compatibility of U. minor could help explain the presence of F₂ individuals in Italy. The presence of F₂ individuals, where most of the variability present in the hybrids will be released, could facilitate rapid evolution and the potential evolution of invasiveness of U. pumila in Italy.     

Characterization of microsatellite loci in Ulmus minor Miller and cross‐amplification in U. glabra Hudson and U. laevis Pall.

We developed five microsatellites from Ulmus minor using an AFLP protocol followed by an enrichment step, with biotinylated primers containing tandem repeats. Loci were characterized using a total of 30 elms from six Spanish populations. Three microsatellites were also transferred to U. laevis and U. glabra.     

A last stand in the Po valley: genetic structure and gene flow patterns in Ulmus minor and U. pumila

Background and Aims Ulmus minor has been severely affected by Dutch elm disease (DED). The introduction into Europe of the exotic Ulmus pumila, highly tolerant to DED, has resulted in it widely replacing native U. minor populations. Morphological and genetic evidence of hybridization has been reported, and thus there is a need for assessment of interspecific gene flow patterns in natural populations. This work therefore aimed at studying pollen gene flow in a remnant U. minor stand surrounded by trees of both species scattered across an agricultural landscape.Methods All trees from a small natural stand (350 in number) and the surrounding agricultural area within a 5-km radius (89) were genotyped at six microsatellite loci. Trees were morphologically characterized as U. minor, U. pumila or intermediate phenotypes, and morphological identification was compared with Bayesian clustering of genotypes. For paternity analysis, seeds were collected in two consecutive years from 20 and 28 mother trees. Maximum likelihood paternity assignment was used to elucidate intra- and interspecific gene flow patterns.Key Results Genetic structure analyses indicated the presence of two genetic clusters only partially matching the morphological identification. The paternity analysis results were consistent between the two consecutive years of sampling and showed high pollen immigration rates (∼0·80) and mean pollination distances (∼3 km), and a skewed distribution of reproductive success. Few intercluster pollinations and putative hybrid individuals were found.Conclusions Pollen gene flow is not impeded in the fragmented agricultural landscape investigated. High pollen immigration and extensive pollen dispersal distances are probably counteracting the potential loss of genetic variation caused by isolation. Some evidence was also found that U. minor and U. pumila can hybridize when in sympatry. Although hybridization might have beneficial effects on both species, remnant U. minor populations represent a valuable source of genetic diversity that needs to be preserved.     

Feeding by Scolytus bark beetles to test for differently susceptible elm varieties

Dutch elm disease (DED), caused by the fungi Ophiostoma ulmi and O. novo‐ulmi, has reduced elm populations severely in Europe and North America. Breeding programmes are in action to find less susceptible elm varieties suitable for re‐establishing elm stands. Bark beetles, mainly Scolytus spp., are the only known natural vectors of DED. During twig feeding, beetles transfer Ophiostoma spores to healthy elms. Thus, less palatable elms should run a lower risk of DED infections. In feeding preference bioassays, we offered twigs from elms exhibiting different degree of susceptibility to O. novo‐ulmi, together with non‐host trees to Scolytus beetles. Scolytus multistriatus preferred wych elm, Ulmus glabra, to 100% in two‐choice tests, whereas S. laevis did not discriminate between a tolerant and a susceptible variety of field elm, U. minor. We suggest that the feeding assay is useful as a low‐tech method in breeding programmes for evaluating the suitability of promising elm genotypes to vector insects.     

Identification of RAPD markers linked to a black leaf spot resistance gene in Chinese elm

Black leaf spot (Stegophora ulmea) is a common foliage disease on Chinese (Ulmus parvifolia) and Siberian elms (U. pumila), two species which have been widely used as sources of Dutch-elm disease-resistance genes for interspecific elm hybrids. A dominant gene controlling resistance to black leaf spot was identified in a population derived from self-pollination of a single U. parvifolia tree. Using RAPD markers, in combination with bulked segregant analysis, we have identified three markers linked to this resistance gene. A survey of Chinese-elm hybrids revealed that the same gene is likely to confer a high level of resistance to black leaf spot in interspecific elm hybrids, although other genetic factors may also be involved in the determination of a disease phenotype.     

Vessel length and conductivity of Ulmus branches: ontogenetic changes and relation to resistance to Dutch elm disease

Changes in age of the hydraulic architecture of Ulmus minor and U. minor × U. pumila juvenile wood were studied and related to tolerance to Dutch elm disease (DED). The xylem vessel dimensions and the conductivity to air of 2- to 7-year-old branches were analyzed and quantified. No obvious differences in vessel length distribution and conductivity were found to explain differences in DED tolerance among the U. minor clones, or, at the taxon level, the higher DED tolerance of U. minor × U. pumila. Among the U. minor clones, the more susceptible one had wider vessels and a higher maximum vessel diameter than the more tolerant clone. Relations between vessel lengths, vessel diameters and branch sizes were highly significant, and varied between taxa. The diameter and length of vessels increased with age, and average values stabilized 1–2 years earlier for U. minor than for U. minor × pumila. Mean maximum vessel length was significantly higher in U. minor and increased more with age and maximum vessel diameter than in U. minor × pumila. With each 0.2 m increase in height up the stem, conductivities for U. minor and U. minor × U. pumila decreased by 59 and 50 %, respectively, probably due to shortening of the vessels. The implications of xylem structure for the means of pathogen movement and resistance to DED are discussed.     

Patterns of hybridization and introgression between invasive Ulmus pumila (Ulmaceae) and native U. rubra

Natural hybridization between introduced species and their native congeners occurs frequently and can create serious conservation concerns. Ulmus pumila (Siberian elm) is an introduced Asian elm species that has naturalized in the United States and is now considered invasive in 41 states. Red elm (U. rubra), a native to the eastern United States, often occurs in sympatry with Siberian elm, and the two species are thought to hybridize. Here, we genetically characterized reference populations of the two elm species to identify species-specific microsatellite alleles. These markers were used to classify individuals in putative hybrid zones as parental species or hybrids, assess the extent of hybridization, and track patterns of introgression. We identified nine U. rubra, 32 U. pumila, and 51 hybrid individuals in our hybrid zones. Of the 51 hybrids, 35 were classified as first-generation hybrids and 16 as backcrosses. The majority of the backcrosses (88%) were introgressed toward U. pumila. Our classification of genotypes was consistent whether we used manual classification, principal coordinate analyses or Bayesian clustering. We observed greater genetic diversity and new combination of alleles in the hybrids. Our study indicates widespread hybridization between U. pumila and U. rubra and an asymmetric pattern of introgression toward U. pumila.     

Conservation of genetic diversity in slippery elm (<Emphasis Type="Italic">Ulmus rubra</Emphasis>) in Wisconsin despite the devastating impact of Dutch elm disease

Forest pest epidemics are responsible for many population declines reported in forest trees. While forest tree populations tend to be genetically diverse, in principle mortality resulting from disease could diminish that genetic diversity and alter the genetic structure of the remnant populations with consequences for the ability of a species to adapt to changing environments. Slippery elm (Ulmus rubra Muhl.) is a long-lived, wind-pollinated forest tree with a native range covering essentially all of eastern North America. Dutch elm disease (DED) caused by an introduced fungal pathogen (Ophiostoma ulmi) devastated North American elm populations, including slippery elm, beginning in the 1930s. Estimates of the numbers of elms lost to DED are unknown but range into the hundreds of millions of trees given their former abundance. In this study, the genotypes of 77 herbarium specimens collected between 1890 and 2004 in Wisconsin, and of 100 slippery elm trees from five wild Wisconsin populations, were characterized using 13 microsatellite loci. Levels of genetic diversity were compared between the herbarium specimens collected pre- and post-DED spread in Wisconsin. In addition, the levels of genetic diversity and degree of genetic differentiation were quantified in the five wild populations. The allelic diversity and expected levels of heterozygosity were similar between the pre- and post-DED herbarium specimens. The five wild populations were only slightly differentiated and no genetic bottleneck was detected for any population. At least in Wisconsin, slippery elm apparently has maintained levels of genetic diversity that could facilitate adaptation to future climatic and environmental changes.     

Phenylalanine ammonia-lyase activity in suspension cultures of Ulmus pumila and U. campestris treated with spores of Ceratocystis ulmi

Summary Cell suspension cultures of a Ceratocystis ulmi-resistant (Ulmus pumila) and a -susceptible elm (U.campestris) were established from leaf callus tissue. Treatment of cultures with spores of C.ulmi induced a large increase in the activity of phenylalanine ammonialyase, only in the cells of the resistant species U.pumila with a maximum after 24 h. Inoculated U.pumila cells also excreted a red unidentified chemical into the culture medium. Neither responses were induced in inoculated U.campestris cultures. The results are discussed in relation to the development of the elm cell culture system as a model for studying the differential biochemical mechanisms of disease resistance in elms.     

Intra- and interspecific hybridization in invasive Siberian elm

Hybridization creates unique allele combinations which can facilitate the evolution of invasiveness. Frequent interspecific hybridization between the Siberian elm, Ulmus pumila, and native elm species has been detected in the Midwestern United States, Italy and Spain. However, Ulmus pumila also occurs in the western United States and Argentina, regions where no native elm species capable of hybridizing with it occurs. We examined whether inter- or intraspecific hybridization could be detected in these regions. Nuclear markers and the program STRUCTURE helped detect interspecific hybridization and determine the population genetic structure in both the native and the two non-native ranges. Chloroplast markers identified sources of introduction into these two non-native ranges. No significant interspecific hybridization was detected between U. pumila and U. rubra in the western United States or between U. pumila and U. minor in Argentina and vice versa. However, the genetic findings supported the presence of intraspecific hybridization and high levels of genetic diversity in both non-native ranges. The evidence presented for intraspecific hybridization in the current study, combined with reports of interspecific hybridization from previous studies, identifies elm as a genus where both inter- and intraspecific hybridization may occur and help maintain high levels of genetic diversity potentially associated with invasiveness.     

Exogenous phenol increase resistance of Ulmus minor to Dutch elm disease through formation of suberin-like compounds on xylem tissues

The survival of some elms to Dutch elm disease (DED) epidemics could be related with the application of disinfectant products based on simple phenols. To test this hypothesis, the protective effect of different phenolic treatments in Ulmus minor trees was evaluated through inoculations with Ophiostoma novo-ulmi, the current DED pathogen. During spring 2004 and spring 2005, 4-year-old elms were: (i) watered with a 0.02% solution of the phenolic fraction of phenolic oil, (ii) watered with a 0.02 and 0.2% solution of a phenol–cresol mixture, and (iii) trunk injected with a 0.2% solution of phenol–cresol mixture. In May, trees were artificially inoculated with O. novo-ulmi. At the end of the 2004 and 2005 vegetative periods, phenol-treated trees showed significantly lower wilting values than control trees. One week of bud break delay was observed in trees watered with the 0.2% solution of phenol–cresol mixture. Fourier transform-infrared spectroscopy and electrospray ionisation mass spectrometry evidenced enhanced levels of suberin-like compounds in phenol-treated trees with respect to non-treated trees. The deposition of suberin in xylem tissues, as a response to phenol treatments, might be considered as one of the mechanisms of resistance of elms to O. novo-ulmi.     

Smelling the tree and the forest: elm background odours affect egg parasitoid orientation to herbivore induced terpenoids

Parasitic insects use herbivore induced plant volatiles as signals for host location. However, their responses to these volatiles in the background of natural habitat odours need further evaluation for developing successful biological control strategies. Field elms (Ulmus minor Miller (Ulmaceae)) release a blend of volatiles in response to oviposition of the elm leaf beetle, Xanthogaleruca luteola Müller (Coleoptera: Chrysomelidae), a major urban and forest pest in the USA and Australia. This induced blend attracts the beneficial egg parasitoid Oomyzus gallerucae Fonscolombe (Hymenoptera: Eulophidae). Our olfactory assays showed that an odorous background of non-attractive host plant volatiles from feeding damaged elms or (Z)-3-hexenyl acetate masks the attractive effect of the host-induced (E)-β-caryophyllene to O. gallerucae. Quantitative GC–MS analyses revealed decreased concentrations of (Z)-3-hexenyl acetate accompanied by highly increased concentrations of sesquiterpenes in oviposition and feeding treated elms compared to undamaged elms. This finding hints to how the parasitoid might distinguish between different odorous backgrounds. It is corroborated by the outcome of our field study in natural elm stands, where the egg parasitoid parasitized more host egg masses due to an artificially induced blend of elm terpenoids.     

Effects of induced water stress on leaf trichome density and foliar nutrients of three elm (Ulmus) species: implications for resistance to the elm leaf beetle

Seedlings of three elm species with variable susceptibility to the elm leaf beetle (Pyrrhalta luteola Müller) (Coleoptera: Chrysomelidae) were subjected to three water stress treatments (no stress, low stress, and high stress) in a greenhouse experiment. The species tested were Ulmus pumila L. (Siberian elm = highly susceptible), U. parvifolia Jacq. (Chinese elm = resistant), and U. americana L. (American elm = intermediate). The seedlings were analyzed for changes in the levels of selected host traits (trichome density, foliar concentration of nitrogen [N], phosphorus [P], potassium [K], calcium [Ca], magnesium [Mg], iron [Fe], and manganese [Mn]), some of which had previously been implicated in resistance to the elm leaf beetle. Density of leaf abaxial surface trichomes (simple, bulbous, and total trichomes) and foliar Fe and Mg concentrations increased significantly in the highly susceptible Siberian elms under water stress. In contrast, stress reduced trichome density in the moderately susceptible American elms, but it had no effect on levels of foliar mineral nutrients. The stress treatments had no influence on host traits in the resistant Chinese elms. The results suggest that environmental stress can alter plant traits that are likely involved in determining resistance of elms to the elm leaf beetle.     

First record of fossil angiosperm wood (Ulmoxylon,Ulmaceae) from the famous locality of Bı́lina (Czech Republic,Early Miocene)

A fossil angiosperm wood is described for the first time from the famous Early Miocene locality of Bı́lina. It represents a fossil elm wood, attributed to Ulmoxylon marchesonii Biondi. The fossil wood can be compared to extant North American soft elms, also to Ulmus macrocarpa Hance and U. parvifolia Jacq. from China or to the European common elm U. carpinifolia Gled. The wood together with fossil leaves/fruits of Ulmus pyramidalis Goeppert forms a single natural fossil species that lived in the Bı́lina area during the Early Miocene. The influence of two types of preservation, permineralised and xylitic, on the same wood species is also discussed. To cite this article: J. Sakala, C. R. Palevol 1 (2002) 161–166.     

Influence of a Ceratocystis ulmi Toxin on Water Relations of Elm (Ulmus americana)

Water-soluble glycopeptides isolated from cultures of Ceratocystis ulmi have been reported to be toxins involved in Dutch elm disease. The influence of the glycopeptides on the water relations of Ulmus americana seedlings was tested by placing cut stems in glycopeptide preparations. After 4 hours in 200 micrograms per milliliter toxin the stem conductance of the seedlings was reduced by 79% and the leaf water potential was reduced by 3 bars to that at which the seedlings wilted, the stomata closed, and transpiration decreased. Decrease in stem conductance as the mode of action of the toxin was further confirmed by forcing toxin through the stem and petiole of elm and measuring the effects on stem conductance. High molecular weight dextrans were found to mimic the action of toxin on stem and petiole conductance, and their ability to do so was found to be correlated with their molecular weight. As low as 4 micrograms of toxin or dextrans were found to measurably decrease the stem and petiole conductance of elms. Disruption of the water-conducting system of elms and other plants by small quantities of high molecular weight compounds may be a factor in diseases with wilting symptoms.     

Resistance to Dutch Elm Disease Reduces Presence of Xylem Endophytic Fungi in Elms (Ulmus spp.)

Efforts to introduce pathogen resistance into landscape tree species by breeding may have unintended consequences for fungal diversity. To address this issue, we compared the frequency and diversity of endophytic fungi and defensive phenolic metabolites in elm (Ulmus spp.) trees with genotypes known to differ in resistance to Dutch elm disease. Our results indicate that resistant U. minor and U. pumila genotypes exhibit a lower frequency and diversity of fungal endophytes in the xylem than susceptible U. minor genotypes. However, resistant and susceptible genotypes showed a similar frequency and diversity of endophytes in the leaves and bark. The resistant and susceptible genotypes could be discriminated on the basis of the phenolic profile of the xylem, but not on basis of phenolics in the leaves or bark. As the Dutch elm disease pathogen develops within xylem tissues, the defensive chemistry of resistant elm genotypes thus appears to be one of the factors that may limit colonization by both the pathogen and endophytes. We discuss a potential trade-off between the benefits of breeding resistance into tree species, versus concomitant losses of fungal endophytes and the ecosystem services they provide.     

Isolation and characterization of microsatellite markers for red elm (Ulmus rubra Muhl.) and cross-species amplification with Siberian elm (Ulmus pumila L.)

Ulmus pumila is an elm species, non-native to the USA that hybridizes with Ulmus rubra. In order to study the genetic structure and hybridization patterns between these two elm species, we developed 15 primer pairs for microsatellite loci in U. rubra and tested their cross-amplification in U. pumila. All 15 primers amplified in both species, 11 of which possessed species-specific alleles. Eight loci were polymorphic in U. pumila and eight in U. rubra, each with two to eight alleles per locus. In addition, five primer pairs previously developed in U. laevis and U. carpinifolia (syn. U. minor) cross-amplified and showed polymorphic loci in U. pumila and/or U. rubra. These markers will facilitate the study of genetic structure and gene flow between U. rubra and exotic, invasive U. pumila.     

Endophyte inoculation enhances Ulmus minor resistance to Dutch elm disease

Certain fungal endophytes improve plant resistance to biotic stresses in forest trees. In this study, three stem fungal endophytes belonging to classes Cystobasidiomycetes, Eurotiomycetes and Dothideomycetes were selected from 210 isolates for their potential as enhancers of Ulmus minor resistance to Ophiostoma novo-ulmi. We evaluated phenotypic traits of these endophytes that could be beneficial for inhibiting O. novo-ulmi in the host plant. Under in vitro conditions, the Dothideomycetous isolate YCB36 strongly inhibited O. novo-ulmi growth, released antipathogenic VOCs, chitinases and siderophores, and overlapped with the pathogen in nutrient utilization patterns. These functional traits could explain the 40% reduction in leaf wilting due to O. novo-ulmi in elm trees pre-inoculated with this endophyte. Ulmus minor trees inoculated with this endophyte showed increased leaf stomatal conductance and higher concentrations of flavonoids and total phenolic compounds in xylem tissues, suggesting induction of defence metabolism.     

Specificity of chemical cues used by a specialist egg parasitoid during host location

Plant synomones and host kairomones are known to guide the egg parasitoid Oomyzus gallerucae to its specific host, the elm leaf beetle Xanthogaleruca luteola (= Pyrrhalta) (Muller) feeding upon elm leaves (Ulmus spp.). In this study, we investigated whether the activities of these plant synomones and kairomones are specific for the plant and herbivore species, respectively. Olfactometer and contact bioassays were used. In habitat location, O. gallerucae (Fonscolombe) is known to use synomones from Ulmus minor (Miller) that are induced by egg depositions of X. luteola. The attractiveness of such induced volatiles was shown to be specific both for the Ulmus species and the herbivore species depositing eggs. Neither leaves of U. glabra Hudson (= U. montana) carrying eggs of X. luteola nor leaves of U. minor (= U. campestris = U. procera) carrying eggs of the chrysomelid species Galeruca tanaceti L. emitted attractive synomones. O. gallerucae is also known to be attracted by volatile kairomones from faeces of X. luteola feeding on U. minor and to show prolonged antennal drumming when contacting substrates contaminated with these faeces. The kairomonal activity of the faeces was proved to be independent of the Ulmus species, since also faeces from elm leaf beetles feeding upon U. glabra emitted attractive volatiles. However, the faecal kairomones were specific for the herbivorous species, since faeces from a lepidopteran larva (Opisthograptis luteolata L.) feeding upon elm hardly elicited any antennal drumming in O. gallerucae. The egg parasitoid studied is known to recognize host eggs of X. luteola by contact kairomones extractable from the egg shell. O. gallerucae clearly differentiated between host eggs and eggs of another closely related chrysomelid species, Galerucella lineola L., as was shown by comparing duration of antennal drumming on host eggs and eggs of G. lineola.