The function of intercellular spaces along the ray parenchyma in sapwood, intermediate wood, and heartwood of Cryptomeria japonica (Cupressaceae)

? Premise of the study: Intercellular spaces along ray parenchyma (ISRP) are common in many conifer xylems, but their function is uncertain because the in-situ structural network among ISRP, ray parenchyma, and tracheids has not been evaluated. Analysis of water distribution in ISRP from sapwood to heartwood is needed to elucidate the function of ISRP in sapwood, intermediate wood, and heartwood. ? Methods: We used cryo-scanning electron microscopy, x-ray photography, and water content measurement in xylem to analyze the presence of liquids in ISRP, ray parenchyma, and tracheids from sapwood to heartwood in Cryptomeria japonica (Cupressaceae). ? Key results: In sapwood, almost all ISRP were empty. "Cingulate-cavitated regions", which lose water along the tangential direction within one annual ring, formed in the earlywood tracheids, and their frequency increased toward the inner annual rings, whereas ray parenchyma cells were alive and not involved in the partial cavitation. In intermediate wood, almost all ISRP and earlywood tracheids and many of the ray cells were empty, and only some latewood tracheids retained liquid in their lumina. The ISRP were connected with tracheids via gas-filled ray parenchyma cells. ? Conclusions: The ISRP work as a pathway of gas for aspiration of ray parenchyma cells in sapwood. On the other hand, the occurrence of a gas network between ISRP, ray parenchyma, and tracheids facilitates cavitation of tracheids, resulting in the generation of low-moisture, intermediate wood.     

马尾松正常木与应压木的比较解剖

用光学显微镜和扫描电镜观察比较了马尾松(Pinus massoniana)正常木与应压木的结构差异。研究结果表明:马尾松应压木从早材到晚材呈缓变,管胞在横切面上除生长轮分界处的两侧外均为圆形,管胞之间具明显的胞间隙;管胞的次生壁仅有 S_1和 S_2两层;次生壁上螺纹间隙和螺纹裂隙都很明显,并与 S_2层微纤丝平行。此外,还初步讨论了应压木形成的可能机理。     

Size and function in conifer tracheids and angiosperm vessels

The wide size range of conifer tracheids and angiosperm vessels has important consequences for function. In both conduit types, bigger is better for conducting efficiency. The gain in efficiency with size is maximized by the control of conduit shape, which balances end-wall and lumen resistances. Although vessels are an order of magnitude longer than tracheids of the same diameter, they are not necessarily more efficient because they lack the low end-wall resistance of tracheids with torus-margo pits. Instead, vessels gain conducting efficiency over tracheids by achieving wider maximum diameters. End-walls contributed 56-64% to total xylem resistance in both conduit types, indicating that length limits conducting efficiency. Tracheid dimensions may be more limited by unicellularity and the need to supply strength to homoxylous wood than by the need to protect against cavitation. In contrast, the greater size of the multicellular vessel is facilitated by fibers that strengthen heteroxylous wood. Vessel dimensions may be most limited by the need to restrict intervessel pitting and cavitation by air-seeding. Stressful habitats that promote narrow vessels should favor coexistence of conifers and angiosperms. The evolution of vessels in angiosperm wood may have required early angiosperms to survive a phase of mechanic and hydraulic instability.     

Seasonal and perennial changes in the distribution of water in the sapwood of conifers in a sub-frigid zone

An analysis was made of progressive changes in patterns of cavitation in the sapwood of three species of conifer (Larix kaempferi, Abies sachalinensis, and Picea jezoensis) that were growing in a sub-frigid zone. In all three conifers, all tracheids of the newly forming outermost annual ring were filled with water or cytoplasm during the period from May to August. However, many tracheids in the transition zone from earlywood to latewood lost water in September, presumably through drought-induced cavitation. Cavitated tracheids tended to be continuously distributed in a tangential direction. Subsequently, some earlywood tracheids of the outermost annual ring lost water during the period from January to March. This was associated with freeze-thaw cycles. In the second and third annual rings from the cambium of all three conifers, the lumina of most tracheids in the transition zone from earlywood to latewood contained no water. In contrast, some latewood tracheids near the annual ring boundary and many earlywood tracheids retained water in their lumina. The third annual ring had more cavitated tracheids than the second annual ring. Our observations indicated that cavitation progressed gradually in the tracheids of the conifers and that they were never refilled once cavitation had occurred. The region involved in water transport in conifers did not include the entire sapwood and differed among annual rings.     

Hydraulic efficiency compromises compression strength perpendicular to the grain in Norway spruce trunkwood

The aim of this study was to investigate bending stiffness and compression strength perpendicular to the grain of Norway spruce (Picea abies (L.) Karst.) trunkwood with different anatomical and hydraulic properties. Hydraulically less safe mature sapwood had bigger hydraulic lumen diameters and higher specific hydraulic conductivities than hydraulically safer juvenile wood. Bending stiffness (MOE) was higher, whereas radial compression strength lower in mature than in juvenile wood. A density-based tradeoff between MOE and hydraulic efficiency was apparent in mature wood only. Across cambial age, bending stiffness did not compromise hydraulic efficiency due to variation in latewood percent and because of the structural demands of the tree top (e.g. high flexibility). Radial compression strength compromised, however, hydraulic efficiency because it was extremely dependent on the characteristics of the “weakest” wood part, the highly conductive earlywood. An increase in conduit wall reinforcement of earlywood tracheids would be too costly for the tree. Increasing radial compression strength by modification of microfibril angles or ray cell number could result in a decrease of MOE, which would negatively affect the trunk’s capability to support the crown. We propose that radial compression strength could be an easily assessable and highly predictive parameter for the resistance against implosion or vulnerability to cavitation across conifer species, which should be topic of further studies.     

Water transport properties of seven woody species from the semi-arid Mu Us Sandy Land,China

Maintenance of water transport is very important for plant growth and survival. We studied seven woody species that inhabit the semi-arid Mu Us Sandy Land, China, to understand their strategies for maintaining hydraulic function. We evaluated water transport properties, including cavitation resistance, hydraulic recovery, and water loss regulation by stomatal control, which are associated with xylem structural and leaf physiological traits. We also discussed the water-use characteristics of these species by comparing them with those of species in other regions. Species with tracheids had higher levels of xylem resistance to cavitation and a smaller midday transpiration rate than the other species studied. Although species with vessels were less resistant to cavitation, some recovered hydraulic conductivity within 12 h of rehydration. Species with xylem tracheids could maintain their hydraulic function through resistance to cavitation and by relaxing xylem tension. Although species with vessels had less resistant xylem, they could maintain hydraulic function through hydraulic recovery even when xylem dysfunction occurred. Additionally, the species studied here were less resistant to cavitation than species in semi-arid environments, and equally or less resistant than species in humid environments. Rather than allow hydraulic dysfunction due to drought-induced dehydration, they may develop water absorption systems to avoid or recover quickly from hydraulic dysfunction. Thus, not only stem cavitation resistance to drought but also stem–root coordination should be considered when selecting plants for the revegetation of arid regions.     

Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection

Plant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry. Root and stem wood were sampled from conifers belonging to the Pinaceae, Cupressaceae, Podocarpaceae, and Araucariaceae. Hydraulic resistivity of tracheids decreased with increasing tracheid diameter and width, with 64 ± 4% residing in the end wall pitting regardless of tracheid size or phylogenetic affinity. This end-wall percentage was consistent with a near-optimal scaling between tracheid diameter and length that minimized flow resistance for a given tracheid length. There was no evidence that tracheid size and hydraulic efficiency were constrained by the role of the pits in protecting against cavitation by air-seeding. An increase in pit area resistance with safety from cavitation was observed only for species of the northern hemisphere (Pinaceae and Cupressaceae), but this variable was independent of tracheid size, and the increase in pit resistance did not significantly influence tracheid resistance. In contrast to recent work on angiosperm vessels, protection against air-seeding in conifer tracheids appears to be uncoupled from conduit size and conducting efficiency.     

Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers

Background and Aims In temperate regions, trees undergo annual cycles of cambial growth, with periods of cambial activity and dormancy. Environmental factors might regulate the cambial growth, as well as the development of cambial derivatives. We investigated the effects of low temperature by localized cooling on cambial activity and latewood formation in two conifers, Chamaecyparis obtusa and Cryptomeria japonica.Methods A plastic rubber tube that contained cooled water was wrapped around a 30-cm-wide portion of the main stem of Chamaecyparis obtusa and Cryptomeria japonica trees during seasons of active cambium. Small blocks were collected from both cooled and non-cooled control portions of the stems for sequential observations of cambial activity and for anatomical measurements of cell morphology by light microscopy and image analysis.Key Results The effect of localized cooling was first observed on differentiating tracheids. Tracheids narrow in diameter and with significantly decreased cambial activity were evident 5 weeks after the start of cooling in these stems. Eight weeks after the start of cooling, tracheids with clearly diminished diameters and thickened cell walls were observed in these stems. Thus, localized low temperature induced narrow diameters and obvious thickening of secondary cell walls of tracheids, which were identified as latewood tracheids. Two months after the cessation of cooling, a false annual ring was observed and cambium became active again and produced new tracheids. In Cryptomeria japonica, cambial activity ceased earlier in locally cooled portions of stems than in non-cooled stems, indicating that the cambium had entered dormancy sooner in the cooled stems.Conclusions Artificial cooling of stems induced latewood formation and cessation of cambial activity, indicating that cambium and its derivatives can respond directly to changes in temperature. A decrease in the temperature of the stem is a critical factor in the control of cambial activity and xylem differentiation in trees.     

Hydraulic acclimation to shading in boreal conifers of varying shade tolerance

The purpose of this study was to determine how shading affects the hydraulic and wood‐anatomical characteristics of four boreal conifers (Pinus banksiana, Pinus contorta, Picea glauca and Picea mariana) that differ in shade tolerance. Plants were grown in an open field and under a deciduous‐dominated overstory for 6 years. Sapwood‐ and leaf‐area specific conductivity, vulnerability curves, and anatomical measurements (light and scanning electron microscopy) were made on leading shoots from six to nine trees of each treatment combination. There was no difference in sapwood‐area specific conductivity between open‐grown and understory conifers, although two of four species had larger tracheid diameters in the open. Shaded conifers appeared to compensate for small diameter tracheids by changes in pit membrane structure. Scanning electron microscopy revealed that understory conifers had thinner margo strands, greater maximum pore size in the margo, and more torus extensions. All of these trends may contribute to inadequate sealing of the torus. This is supported by the fact that all species showed increased vulnerability to cavitation when grown in the understory. Although evaporative demand in an understory environment is low, a rapid change into fully exposed conditions could be detrimental for shaded conifers.     

A new method for vulnerability analysis of small xylem areas reveals that compression wood of Norway spruce has lower hydraulic safety than opposite wood

Compression wood is formed at the underside of conifer twigs to keep branches at their equilibrium position. It differs from opposite wood anatomically and subsequently in its mechanical and hydraulic properties. The specific hydraulic conductivity (k_s) and vulnerability to drought‐induced embolism (loss of conductivity versus water potential ψ) in twigs of Norway spruce [Picea abies (L.) Karst.] were studied via cryo‐scanning electron microscope observations, dye experiments and a newly developed ‘Micro‐Sperry’ apparatus. This new technique enabled conductivity measurements in small xylem areas by insertion of syringe cannulas into twig samples. The hydraulic properties were related to anatomical parameters (tracheid diameter, wall thickness). Compression wood exhibited 79% lower k_s than opposite wood corresponding to smaller tracheid diameters. Vulnerability was higher in compression wood despite its narrower tracheids and thicker cell walls. The P₅₀ (ψ at 50% loss of conductivity) was ?3.6 MPa in opposite but only ?3.2 MPa in compression wood. Low hydraulic efficiency and low hydraulic safety indicate that compression wood has primarily a mechanical function.     

Structure-function constraints of tracheid-based xylem: a comparison of conifers and ferns

The ferns comprise one of the most ancient tracheophytic plant lineages, and occupy habitats ranging from tundra to deserts and the equatorial tropics. Like their nearest relatives the conifers, modern ferns possess tracheid-based xylem but the structure-function relationships of fern xylem are poorly understood. Here, we sampled the fronds (megaphylls) of 16 species across the fern phylogeny, and examined the relationships among hydraulic transport, drought-induced cavitation resistance, the xylem anatomy of the stipe, and the gas-exchange response of the pinnae. For comparison, the results are presented alongside a similar suite of conifer data. Fern xylem is as resistant to cavitation as conifer xylem, but exhibits none of the hydraulic or structural trade-offs associated with resistance to cavitation. On a conduit diameter basis, fern xylem can exhibit greater hydraulic efficiency than conifer and angiosperm xylem. In ferns, wide and long tracheids compensate in part for the lack of secondary xylem and allow ferns to exhibit transport rates on a par with those of conifers. We suspect that it is the arrangement of the primary xylem, in addition to the intrinsic traits of the conduits themselves, that may help explain the broad range of cavitation resistance in ferns.     

Modelling of the hygroelastic behaviour of normal and compression wood tracheids

Compression wood conifer tracheids show different swelling and stiffness properties than those of usual normal wood, which has a practical function in the living plant: when a conifer shoot is moved from its vertical position, compression wood is formed in the under part of the shoot. The growth rate of the compression wood is faster than in the upper part resulting in a renewed horizontal growth. The actuating and load-carrying function of the compression wood is addressed, on the basis of its special ultrastructure and shape of the tracheids. As a first step, a quantitative model is developed to predict the difference of moisture-induced expansion and axial stiffness between normal wood and compression wood. The model is based on a state space approach using concentric cylinders with anisotropic helical structure for each cell-wall layer, whose hygroelastic properties are in turn determined by a self-consistent concentric cylinder assemblage of the constituent wood polymers. The predicted properties compare well with experimental results found in the literature. Significant differences in both stiffness and hygroexpansion are found for normal and compression wood, primarily due to the large difference in microfibril angle and lignin content. On the basis of these numerical results, some functional arguments for the reason of high microfibril angle, high lignin content and cylindrical structure of compression wood tracheids are supported.     

Cambium Destruction in Conifers Caused by Pinewood Nematodes

Percentage and rate of mortality in 2-4-year-old conifers depended upon the numbers of pinewood nematodes Bursaphelenchus xylophilus inoculated into their stems. In addition, percentage of conifer mortality was greater for spring inoculations when cambial activity was greater than for late summer and fall inoculations. Gross and histological examination of stems revealed destruction of the cambial layer, including fusiform and ray intitials and their derivatives. These data suggest that cambial and ray destruction causes tree death through blockage of tracheids by gas, oleoresin, or metabolites from dying ray tissues.     

Water stress induced cavitation and embolism in some woody plants

A comparison was made of the relative vulnerability of xylem conduits to cavitation and embolism in three species [ Thuja occidentalis L., Tsuga canadensis (L.) Carr. and Acer saccharum Marsh.]. Waterlogged samples of wood were air dehydrated while measuring relative water loss, loss of hydraulic conductance, cumulative acoustic emissions (= cavitations) and xylem water potential. Most cavitation events and loss of hydraulic conductance occurred while water potential declined from – 1 to –6 MPa. There were differences in vulnerability between species. Other people have hypothesized that large xylem conduits (e.g. vessels) should be more vulnerable to cavitations than small conduits (e.g. tracheids). Our findings are contrary to this hypothesis. Under water stress, the vessel bearing wood retained water better than tracheid bearing wood. However, within a species large conduits were more prone to cavitation than small conduits.     

Enhancing aniline blue fluorescent staining of cell wall structures.

Prior staining with the periodic acid-Schiff reaction, toluidine blue O, Congo red, or Calcofluor White M2R New, or reduction by NaBH4 do not interfere with aniline blue-induced fluorescence of sieve plates, new cell walls, pit fields or tracheids in compression wood of conifers. Detail of such fluorescent structures is improved by these treatments because of increased contrast, reduced flare, and a quenching of autofluorescence.     

Enhancing Aniline Blue Fluorescent Staining of Cell Wall Structures

Prior staining with the periodic acid-Schiff reaction, toluidine blue O, Congo red, or Calcofluor White M2R New, or reduction by NaBH₄ do not interfere with aniline blue-induced fluorescence of sieve plates, new cell walls, pit fields or tracheids in compression wood of conifers. Detail of such fluorescent structures is improved by these treatments because of increased contrast, reduced flare, and a quenching of autofluorescence.     

Xylem function of arid-land shrubs from California, USA: an ecological and evolutionary analysis

Xylem traits were examined among 22 arid-land shrub species, including measures of vessel dimensions and pit area. These structural measures were compared with the xylem functional traits of transport efficiency and safety from cavitation. The influence of evolution on trait relationships was examined using phylogenetic independent contrasts (PICs). A trade-off between xylem safety and efficiency was supported by a negative correlation between vessel dimensions and cavitation resistance. Pit area was correlated with cavitation resistance when cross species data were examined, but PICs suggest that these traits have evolved independently of one another. Differences in cavitation resistance that are not explained by pit area may be related to differences in pit membrane properties or the prevalence of tracheids, the latter of which may alter pit area through the addition of vessel-to-tracheid pits or through changes in xylem conduit connectivity. Some trait relationships were robust regardless of species ecology or evolutionary history. These trait relationships are likely to be the most valuable in predictive models that seek to examine anatomical and functional trait relationships among extant and fossil woods and include the relationship among hydraulic conductivity and vessel diameter, between vessel diameter and vessel length, and between hydraulic conductivity and wood density.     

Immunolabelling of intervessel pits for polysaccharides and lignin helps in understanding their hydraulic properties in Populus tremula?×?alba

Background and Aims The efficiency and safety functions of xylem hydraulics are strongly dependent on the pits that connect the xylem vessels. However, little is known about their biochemical composition and thus about their hydraulic properties. In this study, the distribution of the epitopes of different wall components (cellulose, hemicelluloses, pectins and lignins) was analysed in intervessel pits of hybrid poplar (Populus tremula × alba).Methods Immunogold labelling with transmission electron microscopy was carried out with a set of antibodies raised against different epitopes for each wall polysaccharide type and for lignins. Analyses were performed on both immature and mature vessels. The effect of sap ionic strength on xylem conductance was also tested.Key Results In mature vessels, the pit membrane (PM) was composed of crystalline cellulose and lignins. None of the hemicellulose epitopes were found in the PM. Pectin epitopes in mature vessels were highly concentrated in the annulus, a restricted area of the PM, whereas they were initially found in the whole PM in immature vessels. The pit border also showed a specific labelling pattern, with higher cellulose labelling compared with the secondary wall of the vessel. Ion-mediated variation of 24 % was found for hydraulic conductance.Conclusions Cellulose microfibrils, lignins and annulus-restricted pectins have different physicochemical properties (rigidity, hydrophobicity, porosity) that have different effects on the hydraulic functions of the PM, and these influence both the hydraulic efficiency and vulnerability to cavitation of the pits, including ion-mediated control of hydraulic conductance. Impregnation of the cellulose microfibrils of the PM with lignins, which have low wettability, may result in lower cavitation pressure for a given pore size and thus help to explain the vulnerability of this species to cavitation.     

Quantification of compression wood severity in tracheids of Pinus radiata D. Don using confocal fluorescence imaging and spectral deconvolution

Confocal fluorescence microscopy was used to examine the spectral characteristics of lignin autofluorescence in secondary cell walls of normal and compression wood from Pinus radiata. Using UV excitation, fluorescence spectra of normal and compression wood sections showed significant differences, especially in the outer secondary cell wall of tracheids, with a shift in maxima from violet to blue wavelengths between normal and compression wood. A comparison of normal wood, mild and severe compression wood, showed that the wavelength shift was intermediate in the mild compression wood compared to the severe compression wood, thus offering the possibility of quantifying the severity by measuring ratios of fluorescence at violet and blue wavelengths. Fluorescence induced by blue light, rather than UV, was less well differentiated amongst wood types. Spectral deconvolution indicated the presence of a minimum of five discrete lignin fluorophores in the cell walls of both normal and compression wood tracheids. Comparison with lignin model compounds suggest that the wavelength shift may correspond in part to increased levels of p-hydroxy type lignin in the compression wood samples. The combination of confocal fluorescence imaging and related spectral deconvolution therefore offers a novel technique for characterising cell wall lignin in situ.     

Effects of Flooding, Tilting of Stems, and Ethrel Application on Growth, Stem Anatomy and Ethylene Production of Pinus densiflora Seedlings

Flooding of soil, tilting of seedlings, application of ethrelto stems, and combinations of these treatments, variously alteredthe rate of growth and stem anatomy of 2-year-old Pinus densifloraseedlings. Either flooding or tilting increased stem diametergrowth and induced formation of abnormal xylem. Whereas floodingdecreased the rate of dry weight increment of roots and needlesand increased growth of bark tissues, tilting of stems did not.However, tilting decreased the rate of height growth, stimulatedtracheid production, and induced formation of well-developedcompression wood with rounded, thick-walled tracheids, witha high lignin content but without an S3 layer in the tracheidwall. Ethylene appeared to have an important regulatory rolein stimulating growth of bark tissues as shown by thicker barkin flooded seedlings or those treated with ethrel. Ethyleneappeared to have a less important role in regulating formationof compression wood. Flooding increased the ethylene contentsof stems and induced formation of rounded, thick-walled tracheids.However, these tracheids lacked such features of well-developedcompression wood tracheids as a thick S2 layer, high lignincontent, and absence of an S3 layer. Furthermore, applicationof ethrel to vertical stems greatly increased their ethylenecontents but did not induce formation of well-developed compressionwood. Furthermore, ethrel application blocked development ofcertain characteristics of compression wood when applied totilted seedlings. For example an S3 wall layer was absent intracheids of tilted seedlings but present in tracheids of tilted,ethrel-treated seedlings. Also lignification of tracheids wasincreased on the under side of tilted stems, but reduced intilted, ethrel-treated seedlings, further de-emphasizing a directrole of ethylene in the formation of compression wood. Ethreltreatment induced formation of longitudinal resin ducts in thexylem whereas flooding or tilting of stems did not. Key words: Pinus densiflora, xylogenesis, reaction wood, compression wood, lignification, ethrel, ethylene