Biophysical Model of Xylem Conductance in Tracheids of the Fern Pteris vittata

Calkin, H. W., Gibson, A. C. and Nobel, P. S. 1986. Biophysicalmodel of xylem conductance in tracheids of the fern Pteris vittata.—J.exp. Bot. 37: 1054–1064. Water movement in the xylem is often analysed with the Hagen-Poiseuilleequation, which applies to capillaries of specific diameters.However, the predicted hydraulic conductances per unit length(K_h) are generally much higher than measured values and importantanatomical details, such as the pits of tracheids, are ignored.Here, a previous model based on the Hagen-Poiseuille analysisfor water flow in the stipes of Pteris vittata is improved byincorporating the actual lumen transectional shape (usuallyelliptical or ovate) and the tapering that occurs at the endsof its tracheids, as well as using a better method for analysingthe electrical circuit analogues for the pits (pit cavitiesplus pit membranes). The measured K_h was similar to that predictedby the Hagen-Poiseuille equation for narrow stipes with theirsmall tracheids, but was only about half the measured K^h forlarge stipes. Correcting for the actual shape changed K^h 2-to 3-fold for tracheids with elliptic and ovate transections.For the smaller diameter tracheids, most of the flow resistancewas from the lumens but for the larger tracheids most was fromthe pit membranes. For all stipes the pit cavities accountedfor 12–22% of the total resistance. When the pit membraneswere partially digested away with cellulase, K^h increased about66%, consistent with the deduced resistance of this part ofthe pathway. The present model incorporating realistic anatomicaldetails allowed reasonable predictions of the hydraulic conductanceper unit length over a wide size range of stipes for this fern. Key words: Hydraulic conductance, pit, tracheid, xylem     

Structural analysis of K<Superscript>+</Superscript> dependence in <Emphasis Type="SmallCaps">l</Emphasis>-asparaginases from <Emphasis Type="Italic">Lotus japonicus</Emphasis>

Wood is composed of various types of cells and each type of cell has different structural and functional properties. However, the temporal and spatial diversities of cell wall components in the cell wall between different cell types are rarely understood. To extend our understanding of distributional diversities of cell wall components among cells, we investigated the immunolabeling of mannans (O-acetyl-galactoglucomannans, GGMs) and xylans (arabino-4-O-methylglucuronoxylans, AGXs) in ray cells and pits. The labeling of GGMs and AGXs was temporally different in ray cells. GGM labeling began to be detected in ray cells at early stages of S₁ formation in tracheids, whereas AGX labeling began to be detected in ray cells at the S₂ formation stage in tracheids. The occurrence of GGM and AGX labeling in ray cells was also temporally different from that of tracheids. AGX labeling began to be detected much later in ray cells than in tracheids. GGM labeling also began to be detected in ray cells either slightly earlier or later than in tracheids. In pits, GGM labeling was detected in bordered and cross-field pit membranes at early stages of pit formation, but not observed in mature pits, indicating that enzymes capable of GGM degradation may be involved in pit membrane formation. In contrast to GGMs, AGXs were not detected in pit membranes during the entire developmental process of bordered and cross-field pits. AGXs showed structural and depositional variations in pit borders depending on the developmental stage of bordered and cross-field pits.     

Modelling the hydrodynamic resistance of bordered pits

Previous studies of the hydrodynamics of plant stems have shown that resistance to flow through bordered pits on the side walls of tracheids makes up a significant proportion of their total resistance, and that this proportion increases with tracheid diameter. This suggests a possible reason why tracheids with a diameter above around 100 microm have failed to evolve. This possibility has been investigated by obtaining an estimate for the resistance of a single pit, and incorporating it into analytical models of tracheid resistance and wood resistivity. The hydrodynamic resistance of the bordered pits of Tsuga canadensis was investigated using large-scale physical models. The importance of individual components of the pit were investigated by comparing the resistance of models with different pore sizes in their pit membrane, and with or without the torus and border. The estimate for the resistance of a real bordered pit was 1.70x10(15) Pa s m(-3). Resistance of pits varied with morphology as might be predicted; the resistance was inversely proportional to the pore size to the power of 0.715; removing the torus reduced resistance by 28%, while removal of the torus and border together reduced it by 72%. It was estimated that in a 'typical tracheid' pit resistance should account for 29% of the total. Incorporating the results into the model for the resistivity of wood showed that resistivity should fall as tracheid diameter increases. However, to minimize resistance wider tracheids would also need to be proportionally much longer. It is suggested that the diameter of tracheids in conifers is limited by upper limits to cell length or cell volume. This limitation is avoided by angiosperms because they can digest away the ends of their cells to produce long, wide vessels composed of many short cells.     

Xylem of early angiosperms: Nuphar (Nymphaeaceae) has novel tracheid microstructure1

SEM studies of xylem of stems of Nuphar reveal a novel feature, not previously reported for any angiosperm. Pit membranes of tracheid end walls are composed of coarse fibrils, densest on the distal (outside surface, facing the pit of an adjacent cell) surface of the pit membrane of a tracheid, thinner, and disposed at various levels on the lumen side of a pit membrane. The fibrils tend to be randomly oriented on the distal face of the pit membrane; the innermost fibrils facing the lumen take the form of longitudinally oriented strands. Where most abundantly present, the fibrils tend to be disposed in a spongiform, three-dimensional pattern. Pores that interconnect tracheids are present within the fibrillar meshwork. Pit membranes on lateral walls of stem tracheids bear variously diminished versions of this pattern. Pits of root tracheids are unlike those of stems in that the lumen side of pit membranes bears a reticulum revealed on the outer surface of the tracheid after most of the thickness of a pit membrane is shaved away by the sectioning process. No fibrillar texturing is visible on the root tracheid pits when they are viewed from the inside of a tracheid. Tracheid end walls of roots do contain pores of various sizes in pit membranes. These root and stem patterns were seen in six species representing the two sections of Nuphar, plus one intersectional hybrid, as well as in one collection of Nymphaea, included for purposes of comparison. Differences between root and stem tracheids with respect to microstructure are consistent in all species studied. Microstructural patterns reported here for stem tracheid pits of Nymphaeaceae are not like those of Chloranthaceae, Illiciaceae, or other basal angiosperms. They are not referable to any of the patterns reported for early vascular plants. The adaptational nature of the pit membrane structure in these tracheids is not apparent; microstructure of pit membranes in basal angiosperms is more diverse than thought prior to study with SEM.     

Direct measurements of intervessel pit membrane hydraulic resistance in two angiosperm tree species

The hydraulic resistance of pit membranes was measured directly in earlywood vessels of Fraxinus americana and Ulmus americana. The area-specific resistance of pit membranes (r(mem)) was higher than modeled or measured values obtained previously for hardwood species, with r(mem) of 5.24 × 10(3) MPa·s·m(-1) for Fraxinus and 2.56 × 10(3) MPa·s·m(-1) for Ulmus. The calculated resistance of pit canals was three orders of magnitude below total pit resistance indicating that pit membranes contributed the majority of resistance. Scanning electron microscopy indicated that pit membranes of Ulmus were thinner and more porous than those of Fraxinus, consistent with the difference in r(mem) between the species. Measurements of average vessel diameter and length and area of wall overlap with neighboring vessels were used to partition the vascular resistance between vessel lumen and pit membrane components. Pit membrane resistance accounted for 80% of the total resistance in Fraxinus and 87% in Ulmus in 2-yr-old branch sections. However, measurements of vessel dimensions in the trunk suggest that the division of resistance between pit membrane and lumen components would be closer to co-limiting in older regions of the tree. Thus, pit membrane resistance may be of greater relative importance in small branches than in older regions of mature trees.     

Bordered pit structure and function determine spatial patterns of air-seeding thresholds in xylem of Douglas-fir (Pseudotsuga menziesii; Pinaceae) trees

The air-seeding hypothesis predicts that xylem embolism resistance is linked directly to bordered pit functioning. We tested this prediction in trunks, roots, and branches at different vertical and radial locations in young and old trees of Pseudotsuga menziesii. Dimensions of bordered pits were measured from light and scanning electron micrographs, and physiological data were from published values. Consistent with observations, calculations showed that earlywood tracheids were more resistant to embolism than latewood tracheids, mainly from earlywood having stretchier pit membranes that can distend and cover the pit aperture. Air seeding that occurs in earlywood appears to happen through gaps between the torus edge and pit border, as shown by the similar calculated pressures required to stretch the membrane over the pit aperture and to cause embolism. Although bordered pit functioning was correlated with tracheid hydraulic diameter, pit pore size and above all pit aperture constrained conductivity the most. From roots to branches and from the trunk base to higher on the trunk, hydraulic resistance of the earlywood pit membrane increased significantly because of a decrease in the size of the pit aperture and size and number of margo pores. Moreover, overall wood conductivity decreased, in part due to lower pit conductivity and a decrease in size and frequency of pits. Structural and functional constraints leading to the trade-off of efficiency against safety of water transport were also demonstrated at the individual pit level, with a positive correlation between pit membrane resistance on an area basis and the pressure differential required to cause membrane stretching, a characteristic that is essential for pit aspiration.     

Vertical and radial profiles in tracheid characteristics along the trunk of Douglas-fir trees with implications for water transport

The main stems of three young Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirbel) Franco) trees were dissected to obtain samples of secondary xylem from internodes axially along the trunk and radially within each internode. From these samples, measurements were obtained of tracheid diameter, length, the number of inter-tracheid pits per tracheid, and the diameter of the pit membranes. In addition, samples were obtained along the trunks of three old growth trees and also a small sample of roots for measurement of tracheid diameter. A gradient was apparent in all measured anatomical characters vertically along a sequence among the outer growth rings. These gradients arose not because of a gradient vertically along the internodes, but because of the strong gradients present at each internode among growth rings out from the pith. Tracheid characteristics were correlated: wider and longer tracheids had more numerous pits and wider pits, such that total pit area was about 6% of tracheid wall area independent of tracheid size. A stem model combining growth rings in parallel and internodes in series allowed for estimates of whole trunk conductance as a function of tree age. Conductance of the stem (xylem area specific conductivity) declined during the early growth of the trees, but appeared to approach a stable value as the trees aged.     

Ba2+-sensitive potassium permeability of the apical membrane in newt kidney proximal tubule

Summary The apical membrane K⁺ permeability of the newt proximal tubular cells was examined in the doubly perfused isolated kidney by measuring the apical membrane potential change (V _a change) during alteration of luminal K⁺ concentration and resultant voltage deflections caused by current pulse injection into the lumen.V _a change/decade for K⁺ was 50 mV at K⁺ concentration higher than 25mm, and the resistance of the apical membrane decreased bt 58% of control when luminal K⁺ concentration was increased from 2.5 to 25mm. Ba²⁺ (1mm in the lumen) reducedV _a change/decade to 24 mV and increased the apical membrane resistance by 70%. These data support the view that Ba²⁺-sensitive K⁺ conductance exists in the apical membrane of the newt proximal tubule. Furthermore, intracellular K⁺ activity measured by K⁺-selective electrode was 82.4 ± 3.6 meq/liter, which was higher than that predicted from the Nernst equation for K⁺ across both cell membranes. Thus, it is concluded that cell K⁺ passively diffuses, at least in part, through the K⁺ conductive pathway of the apical membrane.     

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (<Emphasis Type="Italic">Fagus sylvatica</Emphasis> L.) and sessile oak [<Emphasis Type="Italic">Quercus petraea</Emphasis> (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψ_pd) and midday water potential were measured in transpiring (Ψ_leaf) and non-transpiring leaves (Ψ_xyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (g_s) and sap flow (F_d) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (K_h) and whole tree hydraulic conductance (K_hsf) were calculated by using the simple analogy of the Ohm’s law. K_h was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψ^l_xyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψ^u_xyl) at the uppermost canopy. Predawn water potential (Ψ_pd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (K_h) decreased progressively as water stress increases. Minimum values of K_h and K_hsf were recorded when Ψ_pd was lower. However, Q. petraea showed higher K_h than F. sylvatica for the same Ψ_pd. The decrease in K_h with water stress was mainly linked to its fall from the soil to the lowermost canopy (K_srs). Nevertheless, a significant resistance in the petiole–leaf lamina (K_pl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in K_h was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (g_s) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.     

Effects of a severe drought on Quercus ilex radial growth and xylem anatomy

We assessed the response of Quercus ilex subsp. ballota to the severe summer drought recorded in 1994 in NE Spain through the study of changes in radial growth and wood anatomy. We selected a coppice stand in the Iberian Peninsula, which is characterized by a Mediterranean climate under continental influence. We measured internode length, tree-ring width, mean and maximum vessel diameter, and vessel density for 1981–1997. The annual predicted hydraulic conductance (K_h) was calculated following Hagen-Poisseuille's law. We compared the tree-ring width, vessel diameter and K_h of Q. ilex subsp. ballota and co-existing ring-porous oaks (Q. faginea, Q. pyrenaica) for a dry summer (1994) and a wet summer (1997). To evaluate the drought-resistance of xylem for Q. ilex subsp. ballota (dominant under continental conditions) and Q. ilex subsp. ilex (dominant in mild areas) we determined vulnerability curves. Dimensionless indices of internode length, tree-ring width, and vessel density were compared with climatic data (monthly total precipitation and mean temperature) using correlation analyses. Internode length, tree-ring width, K_h, and mean and maximum vessel diameter declined in 1994. According to vulnerability curves, Q. ilex subsp. ballota showed a greater drought resistance than Q. ilex subsp. ilex. During the year of growth, we found a positive influence of January and June–August precipitation on the internode length, tree-ring width, and vessel density. The response of Q. ilex subsp. ballota radial-growth to summer drought was comparable to that of Q. faginea latewood. Overall, growth and wood anatomy of Q. ilex subsp. ballota showed a plastic response to drought.     

Effects of antidiuretic hormone on cellular conductive pathways in mouse medullary thick ascending limbs of Henle: I. ADH increases transcellular conductance pathways

Summary This paper reports experiments designed to assess the relations between net salt absorption and transcellular routes for ion conductance in single mouse medullary thick ascending limbs of Henle microperfusedin vitro. The experimental data indicate that ADH significantly increased the transepithelial electrical conductance, and that this conductance increase could be rationalized in terms of transcellular conductance changes. A minimal estimate (G _c ^min ) of the transcellular conductance, estimated from Ba⁺⁺ blockade of apical membrane K⁺ channels, indicated thatG _c ^min was approximately 30–40% of the measured transepithelial conductance. In apical membranes, K⁺ was the major conductive species; and ADH increased the magnitude of a Ba⁺⁺-sensitive K⁺ conductance under conditions where net Cl^– absorption was nearly abolished. In basolateral membranes, ADH increased the magnitude of a Cl^– conductance; this ADH-dependent increase in basal Cl^– conductance depended on a simultaneous hormone-dependent increase in the rate of net Cl^– absorption. Cl^– removal from luminal solutions had no detectable effect onG _e , and net Cl^– absorption was reduced at luminal K⁺ concentrations less than 5mm; thus apical Cl^– entry may have been a Na⁺,K⁺,2Cl^– cotransport process having a negligible conductance. The net rate of K⁺ secretion was approximately 10% of the net rate of Cl^– absorption, while the chemical rate of net Cl^– absorption was virtually equal to the equivalent short-circuit current. Thus net Cl^– absorption was rheogenic; and approximately half of net Na⁺ absorption could be rationalized in terms of dissipative flux through the paracellular pathway. These findings, coupled with the observation that K⁺ was the principal conductive species in apical plasma membranes, support the view that the majority of K⁺ efflux from cell to lumen through the Ba⁺⁺-sensitive apical K⁺ conductance pathway was recycled into cells by Na⁺,K⁺,2Cl^– cotransport.     

WOOD ANATOMY OF BUBBIA (WINTERACEAE), WITH COMMENTS ON ORIGIN OF VESSELS IN DICOTYLEDONS

Quantitative and qualitative features of wood anatomy are reported for ten collections of seven species of Bubbia. Variations on the basic plan for Winteraceae can be interpreted in terms of taxonomic and ecological distinctions. Tracheid length is correlated with plant size and habit: tracheids are shortest in shrubs. Tracheid wall thickness and ray cell wall thickness distinguish species. Ray cell procumbency and multiseriate ray width increase with age. Growth rings occur only in a species from stream margins. SEM studies reveal absence of a warty layer within tracheids. Helical thickenings are absent. Presence of these two features in Pseudowintera may be correlated with the cool temperate habitats of that genus. Overlap areas of tracheids in Bubbia show various degrees of scalariform pitting, ranging from none (B. semecarpoides) to abundant presence (B. balansae). Perforation-like pits in tracheids of the latter prove, with SEM studies, to have pit membranes containing porosities less than 1 μm in diameter. Scalariform pitting on overlap areas is absent in earlier secondary xylem and increases during later secondary xylem. Scalariform lateral wall pitting can occur in abnormally wide tracheids formed after pauses in cambial activity. These facts show that primitive dicotyledon woods like those of Bubbia can activate genetic information for scalariform end wall patterns and lateral wall pitting such as primitive vessels show without the intervention of paedomorphosis. Paedomorphosis in dicotyledon woods is held still to apply only to special herbaceous and herblike growth forms, not to primarily woody plants. Progenesis (in xylem, loss of secondary xylem) is not held to be necessary to account for the scalariform patterns seen in tracheary elements of primitive dicotyledons. Reasons are given for rejection of the hypothesis that Winteraceae and other woody dicotyledons (Amborella, Sarcandra, Tetracentron, Trochodendron) are secondarily vesselless.     

ELECTRON MICROSCOPY OF WOOD OF CALLIXYLON AND CORDAITES

Replicas and ultrathin sections of the wood of two Paleozoic genera, Callixylon and Cordaites, were examined with the electron microscope. The pattern of wall layering of Callixylon closely resembles that of extant plants. An electron-dense compound middle lamella markedly thickened at the corners of cells, a thin, electron-transparent S₁ layer of the secondary wall, and a thick, electron-dense, partially decayed S₂ layer of the secondary wall are evident in transverse sections of tracheids. No S₃ layer seems to be present. The structure of the bordered pit-pairs of Callixylon is described in detail. The slitlike outer pit apertures are conspicuously narrower and shorter than the inner pit apertures. Both sections and replicas of the bordered pit-pairs display pit membranes lacking tori. Microfibrillar structure is obscure in both sections and replicas of Callixylon wood. Replicas of the bordered pits of Cordaites wood are very similar to those of Callixylon. Pit membranes lack tori, and microfibrillar structure is not very discernible. Knowledge about the evolution of the torus is summarized. It is postulated that the type of pit membrane of Callixylon and Cordaites, which is very homogeneous in structure and lacks a torus, represents a primitive condition among gymnosperms from which structurally more complex pit membranes and the torus later evolved.     

Changes of pH of solutions during perfusion through stem segments: further evidence for hydrogel regulation of xylem hydraulic properties?

Changes in hydraulic conductivity (K_h) and pH were measured in stem segments of laurel (Laurus nobilis L.) during perfusion with iso-osmotic solutions of KCl, NaCl and sucrose. Sucrose had no effect on K_h while 100 mM NaCl or KCl induced up to 22 and 35 % increase of K_h with respect to deionized water, respectively. Increases in K_h were accompanied by a sharp drop in pH from 6.0 (inlet solution) to 5.0 (outlet solution). The same effect was observed with both KCl and NaCl solutions but not in the case of sucrose. Also, similar changes of K_h and pH were observed for stems killed after immersion in hot water. Our results might provide further evidence for ion-mediated regulation of xylem hydraulic conductivity based on the hydrogel properties of pectins at the pit membrane level.     

Xylem of early angiosperms: Novel microstructure in stem tracheids of Barclaya (Nymphaeaceae)

Pit membranes of stem tracheids of all recognized species of Barclaya, an Indomalaysian genus of Nymphaeaceae, were studied with scanning electron microscopy (SEM). Pit membranes of the tracheids are composed of two thick layers, both constructed of fibrils much larger than those of tracheary elements of angiosperms other than Nymphaeaceae. The outer (distal) layer, which comprises the continuous primary wall around the tracheids, is spongiform, perforated by porosities of relatively uniform size, and confined to or most prominent on end walls of stem tracheids. The second layer consists of thick widely spaced fibrils that are oriented axially and are laid down proximally (facing the cell lumen) to the first (outer) layer, although continuous with it. These axial fibrils are attached at their ends to the pit cavities. This peculiar microstructure is not known outside Nymphaeaceae except in Brasenia and Cabomba (Cabombaceae, Nymphaeales), and has not been previously described for Barclaya. The longitudinally oriented threads and strands in perforation plates of secondary xylem of wood and stems of a variety of primitive woody angiosperms (e.g., Illicium) are not homologous to the pit membrane structure observed in stem tracheids of Barclaya, which, like other Nymphaeaceae, has only primary xylem and no perforation plates. The tracheid microstructure reported here is different from pit structures observed in any other group of vascular plants, living or fossil. The tracheid stem microstructures of Barclaya and other Nymphaeaceae appear to be a synapomorphy of Nymphaeaceae and Cabombaceae, and need further study with respect to ultrastructure 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 (g_s), intercellular CO₂ concentration (C_i) and the petiole vessel lumen diameters (D_v), but positively correlated with wood density. Pit morphology was only negatively correlated with sapwood‐ and leaf‐specific hydraulic conductivity and D_v. Pit density was positively correlated with g_s, C_i and D_v, 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.     

Calcium-independent K+-selective channel from chromaffin granule membranes

Summary Intact adrenal chromaffin granules and purified granule membrane ghosts were allowed to fuse with acidic phospholipid planar bilayer membranes in the presence of Ca²⁺ (1 mm). From both preparations, we were able to detect a large conductance potassium channel (ca. 160 pS in symmetrical 400 mm K⁺), which was highly selective for K⁺ over Na⁺ (P _k/P _Na = 11) as estimated from the reversal potential of the channel current. Channel activity was unaffected by charybdotoxin, a blocker of the [Ca²⁺] activated K⁺ channel of large conductance. Furthermore, this channel proved quite different from the previously described channels from other types of secretory vesicle preparations, not only in its selectivity and conductance, but also in its insensitivity to both calcium and potential across the bilayer. We conclude that the chromaffin granule membrane contains a K⁺-selective channel with large conductance. We suggest that the role of this channel may include ion movement during granule assembly or recycling, and do not rule out events leading to exocytosis.     

Neither xylem collapse,cavitation, or changing leaf conductance drive stomatal closure in wheat

Identifying the drivers of stomatal closure and leaf damage during stress in grasses is a critical prerequisite for understanding crop resilience. Here, we investigated whether changes in stomatal conductance (g_s) during dehydration were associated with changes in leaf hydraulic conductance (K_leaf), xylem cavitation, xylem collapse, and leaf cell turgor in wheat (Triticum aestivum). During soil dehydration, the decline of g_s was concomitant with declining K_leaf under mild water stress. This early decline of leaf hydraulic conductance was not driven by cavitation, as the first cavitation events in leaf and stem were detected well after K_leaf had declined. Xylem vessel deformation could only account for <5% of the observed decline in leaf hydraulic conductance during dehydration. Thus, we concluded that changes in the hydraulic conductance of tissues outside the xylem were responsible for the majority of K_leaf decline during leaf dehydration in wheat. However, the contribution of leaf resistance to whole plant resistance was less than other tissues (<35% of whole plant resistance), and this proportion remained constant as plants dehydrated, indicating that K_leaf decline during water stress was not a major driver of stomatal closure.     

Distinctive tracheid microstructure in stems of Victoria and Euryale (Nymphaeaceae)

Scanning electron microscopy (SEM) photographs of thick sections from liquid‐preserved stems of Victoria cruziana and Euryale ferox show accretions of coarse fibrils on pit membranes of tracheids. The first‐deposited fibrils are randomly orientated; on top of them (facing the tracheid lumina) are axially orientated coarse fibrils. The two systems are interconnected. Axially orientated fibrils were more extensively observed in Euryale than in Victoria and tips of fibrils in Euryale extend over the pit apertures onto secondary wall surfaces. Tracheid–parenchyma interfaces bear rudimentary coarse fibrils on the tracheid side. End walls of Victoria tracheids have highly porose pit membranes, thinner and less complex than those of the lateral intertracheid walls. The structures reported in Victoria and Euryale are consistent with those concurrently reported for stems of other Nymphaeaceae. Although also present in Cabombaceae, the coarse fibrils are otherwise not reported for stems of angiosperms and are not yet reported in roots of any species. Pit membrane remnants in perforation plates of various woody dicotyledons represent a nonhomologous phenomenon. The accretions of coarse fibrils in stem tracheids of Nymphaeaceae do not appear to enhance conduction, although they do contain porosities interconnecting tracheids. Removal of pit membrane remnants from perforation plates of primitive dicotyledon woods by hydrolysis does, on the contrary, suggest conduction enhancement. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 52–57.     

Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.