Xylem Parenchyma—Role and Relevance in Wood Functioning in Trees

Woody plants are characterised by a highly complex vascular system, wherein the secondary xylem (wood) is responsible for the axial transport of water and various substances. Previous studies have focused on the dead conductive elements in this heterogeneous tissue. However, the living xylem parenchyma cells, which constitute a significant functional fraction of the wood tissue, have been strongly neglected in studies on tree biology. Although there has recently been increased research interest in xylem parenchyma cells, the mechanisms that operate in these cells are poorly understood. Therefore, the present review focuses on selected roles of xylem parenchyma and its relevance in wood functioning. In addition, to elucidate the importance of xylem parenchyma, we have compiled evidence supporting the hypothesis on the significance of parenchyma cells in tree functioning and identified the key unaddressed questions in the field.

Physical and Mechanical Characteristics of a Pine Thermowood Composition during Barothermal Treatment

The studies are aimed at forming ideas on the structure and properties of composite materials obtained from pine wood and the processes occurring in the structure of wood tissue. The article presents the data on the influence of the conditions of barothermal treatment of pine wood samples by the method of explosive autohydrolysis on the properties of a thermowood composition. The composite material is obtained by hot pressing. The influence on density, strength and hydrophobic characteristics was studied. A series of samples was made under different conditions of the explosive autohydrolysis rigidity factor; at a temperature of 200 °C and the process duration from 0.08 to 10 min. All samples of composite material were obtained without the use of additional components. It was found that the increase in the hydrolysis rigidity factor leads to a decrease in the density of hydrolyzed wood from 440 to ~350 kg/m3. There is no fragmentation of wood samples with the selected processing parameters. Hot pressing of hydrolyzed wood obtained under conditions of low or moderate rigidity is accompanied by a linear increase in the density of the thermowood composite material from ~440 to 500 kg/m3. The consequence of a further increase in the rigidity factor is a slowdown in the rate of increase in the density of the composite material. The conditional boundary that determines the achievement of the maximum number of cross-linked intermolecular structures in the composite material corresponds to the rigidity factor of 3000–4500 min. More rigid processing conditions cause intensification of thermal degradation processes. The dependence of hydrophobic characteristics on the rigidity of the barothermal treatment conditions is complex. At the rigidity factor of 1000–3000 min, an extreme point is observed, before which the hydrophobic properties of the material deteriorate. Its water absorption and swelling increase from 50 to 130 % and from 15 to 54 %, respectively. The hydrophobic performance is significantly improved after reaching the extreme point. Water absorption and swelling reduce to ~20 % and ~10 %, respectively. Mild hydrolysis conditions do not result in a material with consistently high hydrophobic properties. The cross-linked structures are not enough to form a strong and water-resistant composition, and as a consequence, the hydrophobic characteristics deteriorate. Increasing the value of the hydrolysis rigidity factor increases the number of active components. Additional intermolecular bonds formed during pressing improve hydrophobic characteristics. The obtained results can be used in the creation of models of processes occurring in the structure of lignocellulose substance during explosive autohydrolysis and in the preparation of composite materials based on it. Optimal parameters of barothermal treatment for obtaining composite materials with specified physical and mechanical characteristics can be determined. Barothermal treatment of solid pine wood by explosive autohydrolysis contributes to the occurrence of chemically active components in the structure of wood tissue. Their number depends on the rigidity of the processing conditions. The properties of the resulting thermowood composition depend on the conditions of explosive autohydrolysis.

First Report of Stem Canker of Almond Trees Caused by Fusarium solani in Greece

Almond (Prunus dulcis) is an important crop for Greece grown on 15.130 ha in 2019. In September 2019, a severe stem canker disease was observed in 6-year-old trees of cv Marta grafted on the rootstock ‘F675C14’, in a new almond grove of cvs Marta, Soleta, Antonela, Belona and Laurete, in Vlachiana, Heraklion, Crete, Greece. Only cv Marta trees were affected. Diseased trees exhibited cankers on trunks and branches with pale yellow to red-colored gum excreting from cankers, yellowing, leaf fall, twig and branch dieback, bark and wood tissue discoloration. Severely affected trees were killed. A Fusarium-like fungus was consistently isolated from symptomatic wood tissue previously surface-disinfested with 95% ethanol, on acidified potato dextrose agar (APDA). Emerging colonies were transferred to new PDA and the growth rate of the fungus was 7.86 mm/day at 24 °C in the dark. The abundant aerial mycelium was initially white, turning into pale orange in the centre after 7 days of growth on PDA. Microscopic observations revealed hyaline conidiophores measuring 26.74 ± 20.44 μm in length, developing microconidia 5.00 to 9.50 × 2.50 to 4.75 μm (average 6.64 × 3.50 μm) and macroconidia 10.00 to 23.25 × 3.75 to 5.50 μm (average 16.42 × 4.50 μm) in size. DNA from one representative single-spore isolate (code KOUB.AM.VR1) was extracted and the internal transcribed spacer region (ITS) of ribosomal DNA and translation elongation factor 1-alpha (EF 1-a) genes were amplified using the primer pairs ITS1/ITS4 (White et al. 1990) and EF1-F/EF2-R (O’Donnell et al. 1998), respectively. The PCR products were sequenced and deposited in GenBank (accession Nos. MW547397 and MW554492). Based on morphological characteristics (Leslie and Summerell 2006) and a BLAST search with 100.00% and 99.38% identity to published F. solani ITS and EF 1-a sequences in GenBank (KX034335.1, DQ247636.1) the fungus was identified as F. solani. Eight 3-year-old almond trees of cv. Marta were artificially inoculated in March 2020 by making a 6.0-mm-diameter hole into the trunk, inserting a 6-mm-diameter mycelial disc taken from a 10-day-old PDA culture, sealing the hole with cellophane membrane and covering with adhesive paper tape. Another eight trees of the same cultivar were mock-inoculated with sterilized PDA discs and served as controls. Potted trees were kept under ambient conditions. One month post inoculation, yellow gum was evident excreting around the inoculation point in F. solani-treated trees but not in the controls. Seven months post inoculation, longitudinal and transverse sections of inoculated trunks revealed internal and external symptoms similar to those observed under natural infection conditions and F. solani was steadily re-isolated from symptomatic wood tissue and identified by colony morphology. Neither symptoms nor positive isolations were observed in control trunks. Pathogenicity tests were repeated twice. Fusarium solani has been reported as the causal agent of stem canker or wood decay diseases in several woody hosts including bitternut hickory, black walnut, mulberry and pistachio trees (Crespo et al. 2019; Markakis et al. 2017; Park and Juzwik 2012; Tisserat 1987). To the best of our knowledge, this is the first worldwide report of stem canker caused by F. solani on almond tree. This disease could potentially be an increasing problem in almond growing areas and result in severe crop losses. Hence, effective management practices should be investigated and applied.

Palynological diagrams of Holocene sediments and ice wedges at the mouth of the Seyakha River (Zelenaya), Yamal Peninsula

This article examines the sections of the high floodplain of the Seyakha River (Zelenaya) and sections of Holocene peat layers with ice wedges on the third terrace. Palynospectra from sediments and ice wedges of the high floodplain are characterized by a rhythmic fluctuations typical to floodplain facies. The presence of spruce pollen is related to resedimentation, since spruce pollen is detected in the samples with the composition of verifiably resedimented palynomorphs around 10% or higher. The change in composition of the pollen of Siberian pine, scots pine, and birch tree is associated with a change in wind drift, since fluctuations in the composition of the pollen of these taxonomic units do not correlate with fluctuations in resedimented palynomorphs. Therefore, the three periods of increased wind drift and possible increase in pollen productivity can be determined based on mid-period contrast changes in the structure of palynospectra. At the same time, the local peak of cereals is replaced three times by the maximum pollen of dwarf birch and alder. The absence of larch trunks can be substantiated by fires, the traces of which are observed in the section, as well as that larch at the northern limit of its habitat has crumbly wood tissue, which is being rapidly destroyed. Tree limb, needles, and cones usually remain, while the wood tissue is absent. It is assumed that ice wedges formed here 8.5-6 thousand years ago during a single cycle of the change in wind direction and speed , when prevalence of birch tree pollen with some alder pollen at a relatively low pollen concentration was replaced by the dominance of gramineae pollen, and then, dwarf birch pollen in the spring pollen rain. Palynospectra of these ice wedges indicates an increase in the sum of positive temperatures from 8.3 to 6 thousand years.

Micro/nano-structural evolution in spruce wood during soda pulping

Alkaline delignification of wood tissue is the core of the global pulping technology and the most prominent large-scale separation of the main wood components. This work aims at improved understanding of the interplay between the topochemistry of alkaline pulping and the associated morphological changes. Morphology and chemical structure of partially soda-delignified wood chips were studied combining X-ray tomography (XRT), X-ray diffraction analysis and compositional characterization (lignin and carbohydrate content). The XRT studies of wet samples (providing 3D structural information without interfering drying effects), allowed observation of the cell wall separation as an increasing amount of lignin was removed with the increasing pulping time. Comparison between the microstructure of the surface and the central parts of the treated chips showed a more delignified microstructure at the surface, which highlights the dependence of the delignification process on the mass transport (hydroxide ions and lignin fragments) through the wood tissue. The crystallite size of cellulose increased in the <200> crystal planes during the early stage of pulping while there was little effect on the <110> plane.

Direct exposure to solar radiation causes radial growth eccentricity at the beginning of the growing season in Robinia pseudoacacia

Our study investigated the effect of stem temperature increase on xylem formation in Robinia pseudoacacia tree-trunks, caused by direct exposure to solar radiation. It is important to determine factors which may improve the concentricity of deposited wood tissue and intensify xylogenesis because a strong irregularity of wood tissue deposited in the radial direction in mature trees of R. pseudoacacia reduces the commercial value of the wood. Samples of vascular cambium along with adjacent tissues were collected from the southern (illuminated) and northern (shaded) side of tree-trunks growing in the inner and peripheral (thus exposed to direct sunlight) zones of the research plot. Sampling was performed several times during the growing season. The collected material was examined by epifluorescence microscopy and the thickness of deposited tissue comprising cambial xylem derivatives was measured. Deposition of a markedly greater amount of xylem on the southern side of tree-trunks in the peripheral zone of the plot was observed before full leaf development. Instrumental climatic data confirmed that in the early stage of the growing season, temperature on the southern side of the peripheral zone tree-trunk was higher than on the northern side. No clear response in terms of directional deposition of xylem was noticed in the inner zone trees and in peripheral zone trees after full leaf development. This study highlights the importance of temperature increase, caused by solar radiation, for R. pseudoacacia xylogenesis, which may be considered as a factor that affects the course of the radial growth before full leaf development.

Wood capacitance is related to water content, wood density, and anatomy across 30 temperate tree species

SUMMARYWater released from wood tissue during transpiration (capacitance) can meaningfully affect daily water use and drought response. To provide context for better understanding of capacitance mechanisms, we investigated links between capacitance and wood anatomy. On twig wood of 30 temperate angiosperm tree species, we measured capacitance, water content, wood density, and anatomical traits, i.e., vessel dimensions, tissue fractions, and vessel-tissue contact fractions (fraction of vessel circumference in contact with other tissues). Across all species, the strongest predictors of capacitance were wood density (WD) and predawn lumen volumetric water content (VWCL-pd, r2adj=0.44, P<0.0001). Vessel-tissue contact fractions explained an additional ∼10% of the variation in capacitance. Regression models were not improved by including predawn relative water content (RWCpd) or tissue lumen fractions. Among diffuse-porous species, VWCL-pd and vessel-ray contact fraction were the best predictors of capacitance, whereas among ring/semi-ring-porous species, VWCL-pd, WD and vessel-fibre contact fraction were the best predictors. Mean RWCpd was 0.65±0.13 and uncorrelated with WD. VWCL-pd was weakly negatively correlated with WD. Our findings imply that capacitance depends on the amount of stored water, tissue connectivity and the bulk wood properties arising from WD (e.g., elasticity), rather than the fraction of any particular tissue.

Sodium interception by xylem parenchyma and chloride recirculation in phloem may augment exclusion in the salt tolerant Pistacia genus: context for salinity studies on tree crops

Working in tandem with root exclusion, stems may provide salt-tolerant woody perennials with some additional capacity to restrict sodium (Na) and chloride (Cl) accumulation in leaves. The Pistacia genus, falling at the nexus of salt tolerance and human intervention, provided an ideal set of organisms for studying the influences of both variable root exclusion and potentially variable discontinuities at the bud union on stem processes. In three experiments covering a wide range of salt concentrations (0 to 150 mM NaCl) and tree ages (1, 2 and 10 years) as well as nine rootstock-scion combinations we show that proportional exclusion of both Na and Cl reached up to ~85% efficacy, but efficacy varied by both rootstock and budding treatment. Effective Na exclusion was augmented by significant retrieval of Na from the xylem sap, as evidenced by declines in the Na concentrations of both sap and wood tissue along the transpiration stream. However, while we observed little to no differences between the concentrations of the two ions in leaves, analogous declines in sap concentrations of Cl were not observed. We conclude that some parallel but separate mechanism must be acting on Cl to provide leaf protection from toxicity specific to this ion and suggest that this mechanism is recirculation of Cl in the phloem. The presented findings underline the importance of holistic assessments of salt tolerance in woody perennials. In particular, greater emphasis might be placed on the dynamics of salt sequestration in the significant storage volumes offered by the stems of woody perennials and on the potential for phloem discontinuity introduced with a bud/graft union.

Morphological and phytochemical characterization of susceptible and resistant sengon (Falcataria moluccana) tree to gall rust disease

Rahmawati D, Khumaida N, Siregar UJ. 2019. Morphological and phytochemical characterization of susceptible and resistant sengon (Falcataria moluccana) tree to gall rust disease. Biodiversitas 20: 907-913. Gall rust disease has severely attacked sengon tree (Falcataria moluccana Miq. Barneby & J.W. Grimes)) and it rapidly spread in tree plantations causing the decrease of wood productivity. Although sengon is widely cultivated in several areas in Indonesia, there are limited studies on secondary metabolite composition of its wood. This research was aimed to study differences on morphology, wood anatomy and secondary metabolites content of resistant and susceptible sengon tree trunk to gall rust disease. Both resistant and susceptible tree samples to gall rust were selected from the same plantation plots to minimize environmental effects. Structure of wood samples was observed using Scanning Electron Microscope (SEM) JSM-5310LV. Seven types of secondary metabolites, i.e., alkaloids, flavonoids, saponins, phenolic hydroquinones, tannins, triterpenoids, and steroids were qualitatively analyzed from powdered wood samples. Based on field observations, the gall rust fungi changed the color, texture, pores of the tree surface and decreased wood density. In microscopic observations, sengon trunk, which is resistant to gall rust disease, showed organized and regularly structured wood anatomy and did not show hyphae of the infecting fungi inside the wood. The fungi damaged susceptible sengon trunk as indicated by the dispersed and irregular structure of wood tissue, while the presence of fungal hyphae among wood cells was detected. Phytochemical analyses indicated that trunk of sengon containing flavonoids, saponins, phenolic hydroquinones, tannins, triterpenoids, and steroids, except alkaloids. Sengon trunk, which is resistant to gall rust disease, had stronger reactions of flavonoids, saponins, triterpenoids, and steroids compared to that susceptible one.