Pit Aperture Research Articles

Hydraulic consequences of enzymatic breakdown of grapevine pit membranes.

Xylella fastidiosa (Xf) is the xylem-dwelling bacterial agent associated with Pierce's disease (PD), which leads to significant declines in productivity in agriculturally important species like grapevine (Vitis vinifera). Xf spreads through the xylem network by digesting the pit membranes (PMs) between adjacent vessels, thereby potentially changing the hydraulic properties of the stem. However, the effects of Xf on water transport vary depending on the plant host and the infection stage, presenting diverse outcomes. Here, we investigated the effects of polygalacturonase, an enzyme known to be secreted by Xf when it produces biofilm on the PM surface, on stem hydraulic conductivity, and PM integrity. Experiments were performed on six grapevine genotypes with varying levels of PD resistance, with the expectation that PM resistance to degradation by polygalacturonase may play a role in PD resistance. Our objective was to study a single component of this pathosystem in isolation to better understand the mechanisms behind reported changes in hydraulics, thereby excluding the biological response of the plant to the presence of Xf in the vascular system. PM damage only occurred in stems perfused with polygalacturonase. Although the damaged PM area was small (2%-9% of the total pit aperture area), membrane digestion led to significant changes in the median air-seeding thresholds, and most importantly, shifted frequency distribution. Finally, enzyme perfusion also resulted in a universal reduction in stem hydraulic conductivity, suggesting the development of tyloses may not be the only contributing factor to reduced hydraulic conductivity in infected grapevine.

Analysis of the Structure and Hydraulic Function of Bordered Pits Using the Lattice Boltzman Method

Fluid flow between adjacent tracheids is realized through bordered pits in the xylem of conifers. The pit has an extremely small size and a highly complex structure. This paper presents a mesoscopic analytical method for the relationship between the pit structure and its hydraulic characteristics through mathematical modeling using the lattice Boltzmann method (LBM) and curved boundary treatment. Mongolian Scots pine were selected as the research subject of this study, and the bordered pit structure parameters was collected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the original geometric features were maintained for direct modeling analysis. The model revealed the relationship between various components of the bordered pit and liquid flow velocity/resistance, indicating that margo is the main factor affecting flow resistance. Further anatomical investigation separately analyzed the influence of change in a single factor, including pit diameter, pit aperture diameter, pit depth, torus diameter, and margo thickness, on the overall flow and pressure drop to confirm the importance of various factors in this relationship. Additionally, the influence of pore size and pore location distribution in the margo on the flow rate and pressure drop was further analyzed quantitatively. The results showed that the flow rate through individual pores is the result of the combined effect of pore area and radial position of the pore in the margo. Our study promotes the research and application of the mesoscopic model LBM in simulating flow conditions in the complex flow field of pits, which realizes the numerical analysis of the flow field model based on individualized real bordered pits. In comparison with the classical macroscopic model, the accuracy and effectiveness of the proposed model are proved. This research can provide a promising method for analyzing the physiological and ecological functions of conifer and realizing the efficient utilization of wood resources.

Cavitation resistance of peduncle, petiole and stem is correlated with bordered pit dimensions in Magnolia grandiflora

Variation in resistance of xylem to embolism among flowers, leaves, and stems strongly influences the survival and reproduction of plants. However, little is known about the vulnerability to xylem embolism under drought stress and their relationships to the anatomical traits of pits among reproductive and vegetative organs. In this study, we investigated the variation in xylem vulnerability to embolism in peduncles, petioles, and stems in a woody plant, Magnolia grandiflora. We analyzed the relationships between water potentials that induced 50% embolism (P50) in peduncles, petioles, and stems and the conduit pit traits hypothesized to influence cavitation resistance. We found that peduncles were more vulnerable to cavitation than petioles and stems, supporting the hypothesis of hydraulic vulnerability segmentation that leaves and stems are prioritized over flowers during drought stress. Moreover, P50 was significantly correlated with variation in the dimensions of inter-vessel pit apertures among peduncles, petioles and stems. These findings highlight that measuring xylem vulnerability to embolism in reproductive organs is essential for understanding the effect of drought on plant reproductive success and mortality under drought stress.

Effects of Microwave Treatment on Microstructure of Chinese Fir

Microwave (MW) treatment is an effective method to increase refractory wood permeability, thereby reducing drying time and defects. The extent of modification depends on the damage extent of the wood microstructure. In this study, MW intensities of 43 kWh/m3 (low intensity) and 57 kWh/m3(high intensity) were adopted to treat Chinese fir lumber. Microstructural changes in wood samples were observed using scanning electron microscopy (SEM) and pore structure was characterized using mercury intrusion porosimetry (MIP). Results were as follows: After low-intensity MW treatment, parts of the bordered pit membranes in tracheids were damaged, and micro-fibrils on the margo were ruptured, while the torus basically remained intact. Micro-cracks were observed at both ends of the cross-field pit apertures, propagating to the cell walls of tracheids. The middle lamellar between ray parenchyma cells and longitudinal tracheids cracked, and the width of cracks was in the range of 1–25 μm. After high-intensity MW treatment, damage to the wood microstructure was more severe than that in the low-intensity MW treatment, with macro-cracks having a width range of 100–130 μm being generated. In addition, on the fracture surface of macro-cracks, the bordered pit membranes in tracheids fell off, cross-field pit membranes disappeared and the ray parenchyma cells were seriously damaged, exhibiting fracture of the tracheid walls. Both low-intensity and high-intensity MW treatment can increase the pore diameter corresponding to the margo capillaries (peak value increased from 674.7 nm to 831.8 nm and 1047.6 nm, respectively). The number of pores in the tracheid lumen diameter range also significantly increased. These results provide a theoretical support forMW treatment processes’ improvement and high-value utilization of Chinese fir.

Pit and pit aperture dimensions in plantation-grown Douglas fir as affected by local growth conditions and height in stem

Abstract This study assessed whether allometric scaling applied to pit sizes in stems of Douglas fir. Pit and pit aperture diameters were measured in xylem from stems of four plantation-grown Douglas fir (Pseudotsuga menziesii) trees from each of two different sites in Denmark. One site had fertile soil, the other had poor soil fertility. Three different heights were accessed for each tree, and 40 pits were measured per height. Results showed that pit diameter varied between 17 and 24 μm and decreased significantly with increasing height above ground. Representing the position in the tree as a power function of distance from the top of the tree (L0.2) rather than the height above ground improved the model fit for pit diameter as expressed by the R2 value. However, the pit diameter relationship was found to be significantly affected by site, suggesting that anatomic dimensions relate not only to tree size but also to growth conditions. This would imply that even though pit size supposedly has a strong biophysical determination, some xylem plasticity may still be induced by environmental factors.

Elevational adaptation of morphological and anatomical traits by Sakhalin fir (Abies sachalinensis)

In Sakhalin fir trees from nine different source elevation provenances, we found genetic differentiation of traits related to mechanical reinforcement, hydraulic efficiency, and photosynthetic capacity. Climatic conditions change with elevation and trees must cope with the resulting variation in stresses. Thus, trees may differentiate into elevational ecotypes with genetic-based variations in morphological and physiological traits. To explore genetically differentiated traits related to elevational adaptation, needles and stems were analyzed in 43-year-old Sakhalin fir [Abies sachalinensis (Fr. Schm.) Masters] trees which derived from nine source elevations (230–1250 m above sea level) and grown in a nursery plantation at 230 m above sea level. Trees from a high-elevation provenance showed greater mechanical reinforcement in needles and stems. Needles from high-elevation provenances were shorter and thicker, and developed more sclerenchyma in transfusion tissue. Shorter and thicker stems and larger reaction wood portions were also found. Moreover, needles and stems from high-elevation provenance trees also exhibited xylem traits associated with higher hydraulic efficiency and lower hydraulic safety. In the midrib xylem, the theoretical conductivity was greater due to higher number of tracheids. Pit architecture of stem-xylem tracheid indicated a higher hydraulic efficiency, but lower hydraulic safety due to larger pit apertures. Furthermore, high-elevation provenance trees exhibited a thicker bark, which may reduce water losses and act as a water reservoir in winter. Leaf nitrogen content and stomata number per needle were higher in high-elevation provenance trees, both of which were related to high photosynthetic capacity. Overall, the data suggested genetic differentiation of traits related to various trade-offs and optimization for mechanical resistance, hydraulic efficiency, and photosynthetic capacity at high elevation in Sakhalin fir.

Uncovering the ultrastructure of ramiform pits in the parenchyma cells of bamboo [Phyllostachys edulis (Carr.) J. Houz.

Abstract Pits are the main transverse channels of intercellular liquid transport in bamboo. Ramiform pits are a special type of simple pit with two or more branches. However, little is known about the morphology and physiological functions of ramiform pits. The anatomy of plants can provide important evidence for the role of cells. To better understand the ultrastructure and the structure-function relationship of ramiform pits, their characteristics need to be investigated. In this study, both qualitative and quantitative features of ramiform pits were studied using field-emission environmental scanning electron microscopy (FE-ESEM). The samples included the native structures and the replica structures obtained by resin castings. The results show that the ramiform pits have a diverse morphology that can be divided into main categories: type I (the primary branches) and type II (the secondary branches). The distribution of ramiform pits is different in ground parenchyma cells (GPCs) and vascular parenchyma cells (VPCs). The number, the pit aperture diameter and the pit canal length of ramiform pits in the VPCs were, respectively, greater (3-fold), larger (2–3-fold) and shorter (1.3-fold) than those in the GPCs.

Influence of Cambial Age and Axial Height on the Spatial Patterns of Xylem Traits in Catalpa bungei, a Ring-Porous Tree Species Native to China

Studying how cambial age and axial height affects wood anatomical traits may improve our understanding of xylem hydraulics, heartwood formation and axial growth. Radial strips were collected from six different heights (0–11.3 m) along the main trunk of three Manchurian catalpa (Catalpa bungei) trees, yielding 88 samples. In total, thirteen wood anatomical vessel and fiber traits were observed usinglight microscopy (LM) and scanning electron microscopy (SEM), and linear models were used to analyse the combined effect of axial height, cambial age and their interaction. Vessel diameter differed by about one order of magnitude between early- and latewood, and increased significantly with both cambial age and axial height in latewood, while it was positively affected by cambial age and independent of height in earlywood. Vertical position further had a positive effect on earlywood vessel density, and negative effects on fibre wall thickness, wall thickness to diameter ratio and length. Cambial age had positive effects on the pit membrane diameter and vessel element length, while the annual diameter growth decreased with both cambial age and axial position. In contrast, early- and latewood fiber diameter were unaffected by both cambial age and axial height. We further observed an increasing amount of tyloses from sapwood to heartwood, accompanied by an increase of warty layers and amorphous deposits on cell walls, bordered pit membranes and pit apertures. This study highlights the significant effects of cambial age and vertical position on xylem anatomical traits, and confirms earlier work that cautions to take into account xylem spatial position when interpreting wood anatomical structures, and thus, xylem hydraulic functioning.

Comparison of metaxylem vessels and pits in four sympodial bamboo species

The anatomical morphologies of vessel elements and pits of bamboo plants are unique, however, intensive research about vessel elements and pits in bamboo species is very scarce. The vessel elements and pits of four sympodial bamboo species were analyzed by light microscopy and environmental scanning electron microscopy (ESEM). Results show that the length and width of vessel elements were significantly different across bamboo species. The simple (main type), scalariform, and reticulate perforation plates were discovered on the end of vessel elements. The four species also displayed distinct pit forms. Characteristics of vessel elements, perforation plates, and the shape and size of pit apertures were examined separately for their potential relationship of bamboo structure and function.

The effects of intervessel pit characteristics on xylem hydraulic efficiency and photosynthesis in hemiepiphytic and non-hemiepiphytic Ficus species.

Xylem vulnerability to cavitation and hydraulic efficiency are directly linked to fine-scale bordered pit features in water-conducting cells of vascular plants. However, it is unclear how pit characteristics influence water transport and carbon economy in tropical species. The primary aim of this study was to evaluate functional implications of changes in pit characteristics for water relations and photosynthetic traits in tropical Ficus species with different growth forms (i.e. hemiepiphytic and non-hemiepiphytic) grown under common conditions. Intervessel pit characteristics were measured using scanning electron microscopy in five hemiepiphytic and five non-hemiepiphytic Ficus species to determine whether these traits were related to hydraulics, leaf photosynthesis, stomatal conductance and wood density. Ficus species varied greatly in intervessel pit structure, hydraulic conductivity and leaf physiology, and clear differences were observed between the two growth forms. The area and diameter of pit aperture were negatively correlated with sapwood-specific hydraulic conductivity, mass-based net assimilation rate, stomatal conductance (gs ), intercellular CO2 concentration (Ci ) and the petiole vessel lumen diameters (Dv ), but positively correlated with wood density. Pit morphology was only negatively correlated with sapwood- and leaf-specific hydraulic conductivity and Dv . Pit density was positively correlated with gs , Ci and Dv , but negatively with intrinsic leaf water-use efficiency. Pit and pit aperture shape were not significantly correlated with any of the physiological traits. These findings indicate a significant role of pit characteristics in xylem water transport, carbon assimilation and ecophysiological adaptation of Ficus species in tropical rain forests.

Vestured Pits in Wood of Onagraceae: Correlations with Ecology, Habit, and Phylogeny

All Onagraceae for which data are available have vestured pits on vessel-to-vessel pit pairs. Vestures may also be present in some species on the vessel side of vessel-to-ray pit pairs. Herbaceous Onagraceae do not have fewer vestures, although woods with lower density (Circaea L. and Oenothera L.) have fewer vestures. Some Onagraceae from drier areas tend to have smaller vessel pits, and on that account may have fewer vestures (Epilobium L. and Megacorax S. Gonzalez & W. L. Wagner). Pit apertures as seen on the lumen side of vessel walls are elliptical, occasionally oval, throughout the family. Vestures are predominantly attached to pit aperture margins. As seen from the outer surfaces of vessels, vestures may extend across the pit cavities. Vestures are usually absent or smaller on the distal portions of pit borders (except for Ludwigia L., which grows consistently in wet areas). Distinctive vesture patterns were observed in the several species of Lopezia Cav. and in Xylonagra Donn. Sm. & Rose. Vestures spread onto the lumen-facing vessel walls of Ludwigia octovalvis (Jacq.) P. H. Raven. Although the genera are presented here in the sequence of a recent molecular phylogeny of Onagraceae, ecology and growth forms are more important than evolutionary relationships with respect to abundance, degree of grouping, and morphology of vestured pits. Designation of vesture types is not warranted based on the distribution of named types in Onagraceae and descriptive adjectives seem more useful, although more data on vesturing in the family are needed before patterns of diversity and their extent can be fully ascertained. Vestures are less common and may have a more complicated genetic basis than helical thickenings in vessels, but may be a more effective form of water column maintenance.

Quantitative Anatomical Characteristics of Compression Wood, Lateral Wood, and Opposite Wood in the Stem Wood of Ginkgo biloba L.

Quantitative aspects were investigated and compared for anatomical characteristics among compression wood (CW), lateral wood (LW), and opposite wood (OW) in the stem wood of Ginkgo biloba. Characteristics of each part were observed in the 5th, 10th, and 15th to 20th growth rings via optical and scanning electron microscopy. The crystalline characteristics were measured using an X-ray diffractometer. The microfibril angles (MFAs) were measured using the iodine method applied to the tangential section, as well as from the slopes of the pit apertures. The CW and OW showed similar tracheid lengths in the 5th and 10th growth rings; however, the CW was the shortest in the 15th to 20th growth rings. The CW showed the highest ray height and lowest ray number in each growth ring. The MFAs of the CW were greater than those in the LW and OW from both measurement methods. The MFAs obtained from the iodine method were smaller than that of the MFAs obtained from the pit aperture measurements. The CW had the lowest relative crystallinity in each growth ring, whereas the crystal width of the CW was the smallest in the 5th and 15th to 20th growth rings.

Root xylem characteristics and hydraulic strategies of species co-occurring in semi-arid Australia

Xylem traits such as xylem vessel size can influence the efficiency and safety of water transport and thus plant growth and survival. Root xylem traits are much less frequently examined than those of branches despite such studies being critical to our understanding of plant hydraulics. In this study, we investigated primary lateral and sinker roots of six co-occurring species of semi-arid Australia. Two species are restricted to a floodplain, two were sampled only from the adjacent sand plain, and two species co-occur in both habitats. We assessed root wood density, xylem traits (i.e., vessel diameter, fibre and vessel wall thickness), outer pit aperture diameter and calculated theoretical hydraulic conductivity and vessel implosion resistance. We hypothesized that (1) roots have larger xylem vessel diameters and lower wood density than branches of the same species and that (2) there is an inverse correlation between theoretical sapwood hydraulic conductivity and vessel implosion resistance for roots. Variation in root wood density was explained by variations in xylem vessel lumen area across the different species (r2 = 0.73, p = 0.03), as hypothesized. We rejected our second hypothesis, finding instead that the relationship between theoretical hydraulic conductivity and vessel implosion resistance was not maintained in roots of all of our studied species, in contrast to our previous study of branches from the same species. Xylem traits were found to depend upon habitat and eco-hydrological niche, with the groupings including (i) arid-adapted shrubs and trees with shallow lateral roots (Acacia aneura and Psydrax latifolia), (ii) trees restricted to the floodplain habitat, both evergreen (Eucalyptus camaldulensis) and deciduous (Erythrina vespertilio) and (iii) evergreen trees co-occurring in both floodplain and adjacent sand plain habitats (Corymbia opaca and Hakea sp.).

Robustness of xylem properties in conifers: analyses of tracheid and pit dimensions along elevational transects.

In alpine regions, tree hydraulics are limited by low temperatures that restrict xylem growth and induce winter frost drought and freezing stress. While several studies have dealt with functional limitations, data on elevational changes in functionally relevant xylem anatomical parameters are still scarce. In wood cores of Pinus cembra L. and Picea abies (L.) Karst. trunks, harvested along five elevational transects, xylem anatomical parameters (tracheid hydraulic diameter dh, wall reinforcement (t/b)2), pit dimensions (pit aperture Da, pit membrane Dm and torus Dt diameters) and respective functional indices (torus overlap O, margo flexibility) were measured. In both species, tracheid diameters decreased and (t/b)2 increased with increasing elevation, while pit dimensions and functional indices remained rather constant (P. cembra: Dt 10.3 ± 0.2 μm, O 0.477 ± 0.005; P. abies: Dt 9.30 ± 0.18 μm, O 0.492 ± 0.005). However, dh increased with tree height following a power trajectory with an exponent of 0.21, and also pit dimensions increased with tree height (exponents: Dm 0.18; Dt 0.14; Da 0.11). Observed elevational trends in xylem structures were predominantly determined by changes in tree size. Tree height-related changes in anatomical traits showed a remarkable robustness, regardless of the distributional ranges of study species. Despite increasing stress intensities towards the timberline, no adjustment in hydraulic safety at the pit level was observed.

Conifer tracheids resolve conflicting structural requirements: Data, hypotheses, questions

The nature of conduction involves movement of a liquid (under tension or pressure) through a solid (cell walls necessary to direct the liquid and provide mechanical strength). The numerous consequences of the liquid/solid nature of the conductive interface in plants can be viewed as a series of conflicting requirements that are resolved by various mechanisms. For example, the types of mechanical strength conferred by thicker cell walls (latewood) run counter to optimal conduction (earlywood). Conflict resolution situations are examined with light microscopy and SEM to show in detail not merely conflicting requirements but the various types of resolution in various conifers. Abies is presented as exemplary of a cool temperate conifer with numerous aspects to earlywood/latewood structure. Tropical conifers (Araucaria) present different compromises; the riparian conifer Dacrydium guillauminii has only earlywood; the parasitic conifer Parasitaxus has only latewood. Particular conifers have only some of the features by which latewood differs from earlywood. Cell dimorphism is only one aspect of resolution of conflicting requirements; others include modifications in pit size, shape, and density; the nature of the pit membrane; the nature of the pit cavity, pit border and pit aperture; and surface relief (warty layer) of the tracheid wall. The invention of coniferous bordered pits involves a circular shape, so that tension on the margo strands is equal, and thus the pit can be closed. These factors and margo pore maximization necessitate expending a large amount of space to pits in earlywood, the strength of which is thereby lessened and must be compensated by greater wall strength in latewood. The paper concludes with a series of twenty features which represent resolutions of conflicting requirements in terms of anatomical structure. Wood physiological literature is integrated with the anatomical observations.

Vascular development in very young conifer seedlings: Theoretical hydraulic capacities and potential resistance to embolism.

Conifers have the highest rates of mortality during their first year, often attributed to water stress; yet, this tree life stage is the least studied in terms of hydraulic properties. Previous work has revealed correlations between xylem anatomy to both hydraulic transport capacity and resistance to hydraulic dysfunction. In this study, we compared xylem anatomical and plant functional traits of Pseudotsuga menziesii, Larix occidentalis, and Pinus ponderosa seedlings over the first 10 wk of growth to evaluate potential maximum hydraulic capabilities and resistance to drought-induced embolism. We hypothesized that, based on key functional traits of the xylem, predicted xylem embolism resistance of the species will reflect their previously determined drought tolerances with L. occidentalis, P. menziesii, and P. ponderosa in order of least to most embolism-resistant xylem. Xylem and pit anatomical characteristics and additional hydraulic-related functional traits were compared at five times during the first 10 wk of growth using confocal laser scanning microscopy (CLSM). Based on thickness to span ratio, torus to pit aperture overlap, and torus thickness, primary xylem appeared to be not only more hydraulically conductive but also less embolism-resistant than secondary xylem. By week 10, P. menziesii was predicted to have the most embolism-resistant xylem followed by P. ponderosa and L. occidentalis. Theoretical measurements suggest that hydraulic transport capacities and vulnerability to embolism varied for each species over the first 10 wk of growth; thus, the timing of germination and onset of limited soil moisture is critical for growth and survival of seedlings.

What the Penaeaceae alliance (Myrtales) tells us about the nature of vestured pits in xylem

Molecular studies indicate that Penaeaceae, Oliniaceae, and the monospecific families Alzateaceae and Rhynchocalycaceae form a clade of Myrtales. Of these four families, Penaeaceae have tracheids with vestured pits, whereas the others have septate fibers lacking vestures; all have vestured pits in vessels. Tracheid presence in Penaeaceae may be related to the arid South African habitats of the family. Presence of vestures on tracheids in families with vestured vessel pits is one indication that imperforate elements are tracheids and are conductive cells, whereas fiber-tracheids and libriform fibers are non-conductive. Tracheids occur widely in angiosperms and may be plesiomorphies or apomorphies. Combretaceae, the first branch of the Myrtales clade, has a great diversity of vesture features in vessels compared to the Penaeaceae alliance families. Alzatea has vestures that spread over the inside of the vessels, whereas in most taxa of the alliance, vestures are confined to the pit cavities and pit apertures. Vestures in the alliance tend to be globular in shape, and are bridged together by strands of wall material. Lignotubers and roots in Penaeaceae have vestures much like those in stems. Only a few species and genera (notably Alzatea) of the alliance have vesture features the pattern of which correlates with the current taxonomic system. Vestured pits should be viewed from the inside surface of vessels as well as the outer surface, and although sectional views of vestured pits are infrequent, they are very informative. Studies that explore diversity from one order or family to another are needed and offer opportunities for understanding the evolutionary significance of this feature.

Fungal decay in Permian Glossopteridalean stem and root wood from Antarctica

Evidence of fungal decay is frequently encountered in silicified wood. However, studies focusing on fossil fungal wood degradation remain rare. A characteristic pattern of degradation and decay symptoms congruent with present-day white pocket rot occur in Late Permian silicified glossopteridalean stem and root wood (Australoxylon sp.) from Skaar Ridge, Antarctica. Co-occurring with the decay symptoms are fungal hyphae with clamp connections. Hyphae usually progress through the pit apertures, but some may also penetrate tracheid walls. The individual wall layers in some of the infected tracheids are separated from each other, apparently forming appositions. Small, opaque bodies (? arthropod coprolites) occur in some of the decay pockets. The abundance of infected specimens among the silicified woods from Skaar Ridge suggests that white pocket rot fungi were important decomposers in late Paleozoic high-latitude forest ecosystems.

Persistent Supercooling of Reproductive Shoots Is Enabled by Structural Ice Barriers Being Active Despite an Intact Xylem Connection.

Extracellular ice nucleation usually occurs at mild subzero temperatures in most plants. For persistent supercooling of certain plant parts ice barriers are necessary to prevent the entry of ice from already frozen tissues. The reproductive shoot of Calluna vulgaris is able to supercool down to below -22°C throughout all developmental stages (shoot elongation, flowering, fruiting) despite an established xylem conductivity. After localization of the persistent ice barrier between the reproductive and vegetative shoot at the base of the pedicel by infrared differential thermal analysis, the currently unknown structural features of the ice barrier tissue were anatomically analyzed on cross and longitudinal sections. The ice barrier tissue was recognized as a 250 μm long constriction zone at the base of the pedicel that lacked pith tissue and intercellular spaces. Most cell walls in this region were thickened and contained hydrophobic substances (lignin, suberin, and cutin). A few cell walls had what appeared to be thicker cellulose inclusions. In the ice barrier tissue, the area of the xylem was as much as 5.7 times smaller than in vegetative shoots and consisted of tracheids only. The mean number of conducting units in the xylem per cross section was reduced to 3.5% of that in vegetative shoots. Diameter of conducting units and tracheid length were 70% and 60% (respectively) of that in vegetative shoots. From vegetative shoots water transport into the ice barrier must pass pit membranes that are likely impermeable to ice. Pit apertures were about 1.9 μm x 0.7 μm, which was significantly smaller than in the vegetative shoot. The peculiar anatomical features of the xylem at the base of the pedicel suggest that the diameter of pores in pit membranes could be the critical constriction for ice propagation into the persistently supercooled reproductive shoots of C. vulgaris.

TRACHEID AND PIT ANATOMY VARY IN TANDEM IN A TALL SEQUOIADENDRON GIGANTEUM TREE

Across land plants there is a general pattern of xylem conduit diameters widening towards the stem base thus reducing the accumulation of hydraulic resistance as plants grow taller. In conifers, xylem conduits consist of cells with closed end-walls and water must flow through bordered pits imbedded in the side walls. As a consequence both cell size, which determines the numbers of walls that the conductive stream of water must cross, as well as the characteristics of the pits themselves, crucially affect total hydraulic resistance. Because both conduit size and pit features influence hydraulic resistance in tandem, we hypothesized that features of both should vary predictably with one another. To test this prediction we sampled a single tall (94.8 m) Sequoiadendron giganteum tree (giant sequoia), collecting wood samples from the most recent annual ring progressively downwards from the tree top to the base. We measured tracheid diameter and length, number of pits per tracheid, and the areas of pit apertures, tori, and margos. Tracheid diameter widened from treetop to base following a power law with an exponent (tracheid diameterstem length slope) of approximately 0.20. A similar scaling exponent was found between tracheid length and distance from tree top. Additionally, pit aperture, torus, and margo areas all increased (again with a power of ~0.20) with distance from tree top, paralleling the observed variation in tracheid diameter and length. Pit density scaled isometrically with tracheid length. Within individual tracheids, total permeable area of pits, measured as the sum of the margo areas, scaled isometrically with lumen area. Given that pores of the margo membrane are believed to increase in parallel with membrane area, from a strictly anatomical perspective, our results support the interpretation that pit resistance remains a relatively constant fraction of total resistance along the hydraulic pathway.