Robust, universal, and persistent bud secretion adhesion in horse-chestnut trees.

Buds of horse-chestnut trees are covered with a viscous fluid, which remains sticky after long-term exposure to heat, frost, radiation, precipitation, deposition of aerosols and particles, attacks by microbes and arthropods. The present study demonstrates that the secretion does not dry out under arid conditions, not melt at 50 °C, and not change significantly under UV radiation or frost at a microscopic level. It is slightly swellable under wet conditions; and, it universally wets and adheres to substrates having different polarities. Measured pull-off forces do not differ between hydrophilic and lipophilic surfaces, ranging between 58 and 186 mN, and resulting in an adhesive strength up to 204 kPa. The mechanical and chemical properties of secretion resemble those of pressure-sensitive adhesives. The Raman, infrared, and nuclear magnetic resonance spectra show the clear presence of saturated aliphatic hydrocarbons, esters, free carboxylic acids, as well as minor amounts of amides and aromatic compounds. We suggest a multi-component material (aliphatic hydrocarbon resin), including alkanes, fatty acids, amides, and tackifying terpenoids embedded in a fluid matrix (fatty acids) comprising nonpolar and polar portions serving the universal and robust adhesive properties. These characteristics matter for ecological-evolutionary aspects and can inspire innovative designs of multifunctional, biomimetic pressure-sensitive adhesives and varnishes.

Spatial genetic structure and diversity of natural populations of Aesculus hippocastanum L. in Greece.

Horse-chestnut (Aesculus hippocastanum L.) is an endemic and relict species from the Mediterranean biodiversity hotspot and a popular ornamental tree. Knowledge about the evolutionary history of this species remains scarce. Here, we ask what historical and ecological factors shaped the pattern of genetic diversity and differentiation of this species. We genotyped 717 individuals from nine natural populations using microsatellite markers. The influence of distance, topography and habitat variables on spatial genetic structure was tested within the approaches of isolation-by-distance and isolation-by-ecology. Species niche modeling was used to project the species theoretical range through time and space. The species showed high genetic diversity and moderate differentiation for which topography, progressive range contraction through the species' history and long-term persistence in stable climatic refugia are likely responsible. A strong geographic component was revealed among five genetic clusters that are connected with very limited gene flow. The environmental variables were a significant factor in the spatial genetic structure. Modeling results indicated that future reduction of the species range may affect its survival. The possible impact of climate changes and high need of in situ conservation are discussed.

Short photoperiod reduces the temperature sensitivity of leaf-out in saplings of Fagus sylvatica but not in horse chestnut.

Leaf phenology is one of the most reliable bioindicators of ongoing global warming in temperate and boreal zones because it is highly sensitive to temperature variation. A large number of studies have reported advanced spring leaf-out due to global warming, yet the temperature sensitivity of leaf-out has significantly decreased in temperate deciduous tree species over the past three decades. One of the possible mechanisms is that photoperiod is limiting further advance to protect the leaves against potential damaging frosts. However, the "photoperiod limitation" hypothesis remains poorly investigated and experimentally tested. Here, we conducted a photoperiod- and temperature-manipulation experiment in climate chambers on two common deciduous species in Europe: Fagus sylvatica (European beech, a typically late flushing species) and Aesculus hippocastanum (horse chestnut, a typically early flushing species). In agreement with previous studies, we found that the warming significantly advanced the leaf-out dates by 4.3 and 3.7 days/°C for beech and horse chestnut saplings, respectively. However, shorter photoperiod significantly reduced the temperature sensitivity of beech only (3.0 days/°C) by substantially increasing the heat requirement to avoid leafing-out too early. Interestingly, the photoperiod limitation only occurs below a certain daylength (photoperiod threshold) when the warming increased above 4°C for beech trees. In contrast, for chestnut, no photoperiod threshold was found even when the ambient air temperature was warmed by 5°C. Given the species-specific photoperiod effect on leaf phenology, the sequence of the leaf-out timing among forest tree species may change under future climate warming conditions. Nonphotoperiodic species may benefit from warmer springs by starting the growing season earlier than photoperiodic sensitive species, modifying forest ecosystem structure and functions, but this photoperiod limitation needs to be further investigated experimentally in numerous species.

Influence of leaf damage by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimic) on mycorrhiza of Aesculus hippocastanum L.

In many parts of Europe, the white horse chestnut (Aesculus hippocastanum L.) has been attacked by the horse chestnut leafminer (Cameraria ohridella Deschka & Dimic), which causes premature leaf dieback. A. hippocastanum L. establishes mutualistic symbiosis with arbuscular mycorrhizal (AM) fungi. This study involved a comparison of mature A. hippocastanum individuals susceptible to C. ohridella and individuals resistant to this insect after a one-time treatment with a chemical preparation injected into the tree trunks 7 years before the investigation began. Concentration of macronutrients in soil and the activity of soil nonspecific dehydrogenase did not differ between soils under canopies of the treated and untreated trees. Concentrations of C and N were significantly higher in leaves of the treated than those of the untreated trees. The infestation by C. ohridella and defoliation of leaves of the untreated trees did not significantly influence the frequency and intensity of AM colonization compared to the chemically treated trees, although a tendency towards higher average AM colonization of roots of the untreated trees, infested by the herbivores, than roots of the non-infested trees was observed. The results also indicated a tendency for higher biomass of fine roots per soil volume under the trees treated against C. ohridella than under the trees invaded by the insect.

A signature of tree health? Shifts in the microbiome and the ecological drivers of horse chestnut bleeding canker disease.

Host susceptibility to pathogens can be shaped by genetic, ecological, and evolutionary factors. The ability to predict the spread of disease therefore requires an integrated understanding of these factors, including effects of pests on pathogen growth and competition between pathogens and commensal microbiota for host resources. We examined interactions between the leaf-mining moth Cameraria ohridella, the bacterial causal agent of bleeding canker disease Pseudomonas syringae pv aesculi, and the bark-associated microbiota of horse chestnut (Aesculus hippocastanum) trees. Through surveys of > 900 trees from 60 sites in the UK, we tested for ecological or life history predictors of leaf miner infestation, bleeding canker, or coinfection. Using culture-independent sequencing, we then compared the bark microbiomes from 46 trees to measure the association between microbiome composition and key ecological variables, including the severity of disease. Both pest and pathogen were found to respond to tree characteristics, but neither explained damage inflicted by the other. However, we found a clear loss of microbial diversity and associated shift in microbiome composition of trees as a function of disease. These results show a link between bark-associated microbiota and tree health that introduces the intriguing possibility that tree microbiota play key roles in the spread of disease.

Small genomes and large seeds: chromosome numbers, genome size and seed mass in diploid Aesculus species (Sapindaceae).

Background and Aims: Aesculus L. (horse chestnut, buckeye) is a genus of 12-19 extant woody species native to the temperate Northern Hemisphere. This genus is known for unusually large seeds among angiosperms. While chromosome counts are available for many Aesculus species, only one has had its genome size measured. The aim of this study is to provide more genome size data and analyse the relationship between genome size and seed mass in this genus. Methods: Chromosome numbers in root tip cuttings were confirmed for four species and reported for the first time for three additional species. Flow cytometric measurements of 2C nuclear DNA values were conducted on eight species, and mean seed mass values were estimated for the same taxa. Key Results: The same chromosome number, 2 n = 40, was determined in all investigated taxa. Original measurements of 2C values for seven Aesculus species (eight taxa), added to just one reliable datum for A. hippocastanum , confirmed the notion that the genome size in this genus with relatively large seeds is surprisingly low, ranging from 0·955 pg 2C -1 in A. parviflora to 1·275 pg 2C -1 in A. glabra var. glabra. Conclusions: The chromosome number of 2 n = 40 seems to be conclusively the universal 2 n number for non-hybrid species in this genus. Aesculus genome sizes are relatively small, not only within its own family, Sapindaceae, but also within woody angiosperms. The genome sizes seem to be distinct and non-overlapping among the four major Aesculus clades. These results provide an extra support for the most recent reconstruction of Aesculus phylogeny. The correlation between the 2C values and seed masses in examined Aesculus species is slightly negative and not significant. However, when the four major clades are treated separately, there is consistent positive association between larger genome size and larger seed mass within individual lineages.

Physiological aspects of seed recalcitrance: a case study on the tree Aesculus hippocastanum.

Recalcitrant seeds are typical of some tropical and subtropical trees. Their post-shedding life activity proceeds in humid air and wet litter. They are desiccation sensitive and, for this reason, have a short life span and need some special procedures for cryopreservation. This review is devoted to the post-shedding life strategy of recalcitrant seeds, which includes the maintenance of high hydration status, metabolic readiness and ability to rapidly germinate before desiccation-induced damage exerts a lethal effect. The main physiological aspects of recalcitrant seeds are considered starting from mature seeds, followed during dormancy if occurs and resulting in germination. The collected data embrace the metabolic processes in embryonic axes and whole seeds. The up-to-date results are integrated covering the main metabolic processes, namely water status and transport, protein and carbohydrate metabolism, antioxidant defense, axis-cotyledon relations, hormonal control and germination. Among the representatives of various taxa, the seeds of which exhibit recalcitrance, attention was given to horse chestnut seeds as one of most studied recalcitrants.

The content of phenolic compounds in leaf tissues of Aesculus glabra and Aesculus parviflora walt.

In plants, flavonoids play an important role in biological processes. They are involved in UV-scavenging, fertility and disease resistance. Therefore, in this study, we attempted to quantify and characterize phenolic compounds in Aesculus parviflora Walt. leaves and Aesculus glabra leaves partly suffering from attack by a leaf mining insect (C. ohridella). A total of 28 phenolic compounds belonging to the hydroxycinnamic acid, flavan-3-ols and flavonol groups were identified and quantified in Aesculus parviflora and A. glabra leaf extracts. Significantly decreased concentrations of some phenolic compounds, especially of flavan-3-ols, were observed in infected leaves compared to the non-infected ones. Additionally, a higher content of polymeric procyanidins in leaves of Aesculus parviflora than in Aesculus glabra may explain their greater resistance to C. ohridella insects.

Leafminers help us understand leaf hydraulic design.

Leaf hydraulics of Aesculus hippocastanum L. were measured over the growing season and during extensive leaf mining by the larvae of an invasive moth (Cameraria ohridella Deschka et Dimic) that specifically destroy the palisade tissue. Leaves showed seasonal changes in hydraulic resistance (R(lamina)) which were related to ontogeny. After leaf expansion was complete, the hydraulic resistance of leaves and the partitioning of resistances between vascular and extra-vascular compartments remained unchanged despite extensive disruption of the palisade by leafminers (up to 50%). This finding suggests that water flow from the petiole to the evaporation sites might not directly involve the palisade cells. The analysis of the temperature dependence of R(lamina) in terms of Q(10) revealed that at least one transmembrane step was involved in water transport outside the leaf vasculature. Anatomical analysis suggested that this symplastic step may be located at the bundle sheath where the apoplast is interrupted by hydrophobic thickening of cell walls. Our findings offer some support to the view of a compartmentalization of leaves into well-organized water pools so that the transpiration stream would involve veins, bundle sheath and spongy parenchyma, while the palisade tissue would be largely by-passed with the possible advantage of protecting cells from short-term fluctuations in water status.

Aesculus pavia foliar saponins: defensive role against the leafminer Cameraria ohridella.

BACKGROUND: Recently, the leafminer Cameraria ohridella Deschka & Dimic has caused heavy damage to the white-flowering horse chestnut in Europe. Among the Aesculus genus, A. pavia L. HBT genotype, characterised by red flowers, showed an atypical resistance towards this pest. Its leaves, shaken in water, originated a dense foam, indicating the presence of saponins, unlike the common horse chestnut tree. The aim was to isolate and identify these leaf saponins and test their possible defensive role against C. ohridella. RESULTS: Spectroscopic analyses showed that A. pavia HBT genotype leaves contained a mixture of saponins, four of which were based on the same structure as commercial escin saponins, the typical saponin mixture produced by A. hippocastanum and accumulated only within bark and fruit tissues. The mixture showed a repellent effect on C. ohridella moth. The number of mines detected on the leaves of A. hippocastanum plants treated with A. pavia HBT saponins through watering and stem brushing was significantly lower than the control, and in many cases no mines were ever observed. CONCLUSION: The results showed that the exogenous saponins were translocated from roots/stem to the leaf tissues, and their accumulation seemed to ensure an appreciable degree of protection against the leafminer.

Triazole induced drought tolerance in horse chestnut (Aesculus hippocastanum).

We determined the influence of the triazole derivatives paclobutrazol, penconazole, epixiconazole, propiconazole and myclobutanil on the drought tolerance and post drought recovery of container-grown horse chestnut (Aesculus hippocastanum L.) saplings. Myclobutanil neither conferred drought resistance, as assessed by its effects on a number of physiological and biochemical parameters, nor affected growth parameters measured after recovery from drought. Chlorophyll fluorescence (F(v)/F(m)), photosynthetic rates, total foliar chlorophyll and carotenoid concentrations, foliar proline concentration and superoxide dismutase and catalase activities were consistently higher and leaf necrosis and cellular electrolyte leakage was lower at the end of a 3-week drought in trees treated with paclobutrazol, penconazole, epixiconazole or propiconazole than in control trees. Twelve weeks after drought treatment, leaf area and shoot, root and total plant dry masses were greater in triazole-treated trees than in control trees with the exception of those treated with myclobutanil. In a separate study, trees were subjected to a 2-week drought and then sprayed with paclobutrazol, penconazole, epixiconazole, propiconazole or myclobutanil. Chlorophyll fluorescence, photosynthetic rate, foliar chlorophyll concentration and catalase activity over the following 12 weeks were 20 to 50% higher in triazole-treated trees than in control trees. At the end of the 12-week recovery period, leaf area and shoot, root and total plant dry masses were higher in triazole-treated trees than in control trees, with the exception of trees treated with myclobutanil. Application of triazole derivatives, with the exception of myclobutanil, enhanced tolerance to prolonged drought and, when applied after a 2-week drought, hastened recovery from drought. The magnitude of treatment effects was in the order epixiconazole approximately propiconazole > penconazole > paclobutrazol > myclobutanil.

[Physiological and biochemical characteristics of recalcitrant seed under the condition of true dormancy: a review].

The review considers and sums up the results of studies of physiological and biochemical characteristics of dormant and germinating recalcitrant seed (the object of the study, the seed of common horse chestnut, Aesculus hippocastanum L., is viewed as an exemplary case). The results of analysis of the proteomes of the axis and cotyledons have been studied and the effects of the stratification, assessed. Gene expression has been studied at the level of protein synthesis; the protein-synthesizing capacity of the cells of the embryonic axis and cotyledon storage parenchyma of mature seed and seed undergoing stratification. The extent to which the functionally active translation machinery of ripe seed depends on transcription has been assessed, and the ability to synthesize protein under the conditions of stratification has been established. It is concluded that the embryonic axis of dormant seed lacks innate dormancy and that the isolated axis exhibits diverse sensitivity to exogenous abscisic acid and other physiologically active compounds.

Physiology-based phenology models for forest tree species in Germany.

Models of phenology are needed for the projection of effects of a changing climate on, for example, forest production, species competition, vegetation-atmosphere feedback and public health. A new phenology model for deciduous tree bud burst is developed and parameters are determined for a wide geographical range (Germany) and several forest tree species. The new model is based on considerations of simple interactions between inhibitory and promotory agents that are assumed to control the developmental status of a plant. Several alternative model structures were formulated emphasizing different hypothetical physiological processes. The new models fitted the observations better than classical models. The bias of the classical models, i.e. overestimation of early observations and underestimation of late observations, could be reduced but not completely removed. Differences in the best-fitting model equations for each species indicated that, for the late spring phases (bud burst of Fagus sylvatica and Quercus robur), the photoperiod played a more dominant role than for early spring phases (bud burst of Betula pendula and Aesculus hippocastanum). Chilling only plays a subordinate role for spring bud burst compared to temperatures preceding this event in our data. The presented modeling approach allowed for a species-specific weighting of the dominant processes. The model results are in accordance with experimental findings that indicate an important role of day length in late spring BB. Potentials for model improvement are discussed.

Water and oxygen permeance of phellems isolated from trees: the role of waxes and lenticels.

The outermost phellems of Abies alba Mill., Acer pseudoplatanus L., Aesculus hippocastanum L., Betula potaninii L.C. Hue and Sambucus nigra L. have been isolated enzymatically, resulting in membranes with five to seven heavily suberized cork cell layers. Water and oxygen permeances were determined for the phellem areas without lenticels. A special diaphragm made it possible to quantify permeances of single lenticels for the first time. The water permeance of phellems was in the range of 3x10(-5) to 9x10(-5) ms(-1) and can be predicted from the density of the phellem membranes with 93% accuracy. Embedded waxes amounted to 3% ( Aesculus) and up to 35% ( Betula) of the dry weight but affected water permeance only to a small degree. The sorption isotherms describing the water content of the phellems in relation to relative humidities followed a hyperbolic shape and indicated varying water contents among plant species. It is argued that water transfer across the phellems occurs via the middle lamellae. Phellem membranes were impermeable to oxygen. Removal of the waxes hardly changed this situation. Single lenticels from Betula and Sambucus were significantly more permeable to water and oxygen than phellem areas without lenticels. The water permeance was elevated by factors of 39 for Betula and 12 for Sambucus, the oxygen permeance by factors of 1,202 for Betula and 53 for Sambucus. Extraction of lenticels did not affect permeance. A quantitative comparison of the gas-exchange capacity of lenticels and stomata demonstrated the superiority of stomata. However, differences may be not more than one order of magnitude.