Regulation of xylem sap flow in an evergreen, a semi-deciduous, and a deciduous Meliaceae species from the Amazon

The significance of phenological characteristics, stomatal conductance of the leaves, and stem water storage fluctuations for the regulation of xylem sap flow in an evergreen (Carapa guianensis Aubl.), in a semi-deciduous (Swietenia macrophylla King), and in a deciduous (Cedrela odorata L.) Meliaceae species was studied in a 7-year-old plantation near Manaus, Brazil. The study responds to the increasing demand for knowledge on the water relations of highly exploited timber trees of the Amazon. Xylem sap flow measurements indicated that the daily sap flow of Carapa (3.8 l day^-1 tree^-1 to 16.4 l day^-1 tree^-1) exceeded the daily sap flow of Swietenia (2.4 l day^-1 tree^-1 to 7.0 l day^-1 tree^-1) and Cedrela (1.6 l day^-1 tree^-1 to 11.6 l day^-1 tree^-1) during the entire year, although the highest flux densities were measured in Cedrela. The decrease in xylem sap flow observed in periods with low soil water potentials and high atmospheric vapor saturation deficits was more pronounced in the deciduous (Cedrela) and semi-deciduous species (Swietenia) than the evergreen species (Carapa). Carapa, which has the highest daily sap flow, had the highest biomass and sapwood portion. The high flux densities measured in Cedrela most likely result from the large earlywood vessels in this species. The seasonal variation of xylem sap flow of the three species was correlated with the stomatal conductance of the leaves measured by infiltration experiments. Stem water storage fluctuations in Carapa and Swietenia were predominantly due to transpiration; in Cedrela it was predominantly due to evaporative water loss on the stem surface during dry periods.     

Xylem Hydraulic Characteristics, Water Relations and Wood Anatomy of the Resurrection PlantMyrothamnus flabellifoliusWelw.

Myrothamnus flabellifoliusWelw. is a desiccation-tolerant (‘resurrection’)plant with a woody stem. Xylem vessels are narrow (14 µmmean diameter) and perforation plates are reticulate. This leadsto specific and leaf specific hydraulic conductivities thatare amongst the lowest recorded for angiosperms (k_s0.87 kg m^-1MPa^-1s^-1;k_l3.28x10^-5kg m^-1MPa^-1s^-1, stem diameter 3 mm). Hydraulic conductivitiesdecrease with increasing pressure gradient. Transpiration ratesin well watered plants were moderate to low, generating xylemwater potentials of -1 to -2 MPa. Acoustic emissions indicatedextensive cavitation events that were initiated at xylem waterpotentials of -2 to -3 MPa. The desiccation-tolerant natureof the tissue permits this species to survive this interruptionof the water supply. On rewatering the roots pressures thatwere developed were low (2.4 kPa). However capillary forceswere demonstrated to be adequate to account for the refillingof xylem vessels and re-establishment of hydraulic continuityeven when water was under a tension of -8 kPa. During dehydrationand rehydration cycles stems showed considerable shrinking andswelling. Unusual knob-like structures of unknown chemical compositionwere observed on the outer surface of xylem vessels. These maybe related to the ability of the stem to withstand the mechanicalstresses associated with this shrinkage and swelling.Copyright1998 Annals of Botany Company cavitation, desiccation, hydraulic conductivity, refilling, resurrection plant, root pressure, xylem anatomy,Myrothamnus flabellifolius     

Stem radius changes and their relation to stored water in stems of young Norway spruce trees

Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (O_s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as O_s was >-2.3ǂ.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When O_s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which - in the experiment with stem segments - was indicated by an increasing ratio between (V and (W. We assume that for O_s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies O_s rarely falls below -2.3ǂ.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.     

Water relations of coastal and estuarine Rhizophora mangle: xylem pressure potential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (P_x), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (?_sw) surrounding the roots of R. mangle was c. -2.6LJ.52᎒^-3 and -0.4Lj.13᎒^-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (O_L) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (?_x) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P_x (calculated by subtracting ?_x from covered O_L) were about 1 MPa more positive than O_L determined on freely transpiring leaves. The differences in O_L between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.     

The effects of cambial age and position within the stem on specific conductivity in Douglas-fir (Pseudotsuga menziesii) sapwood

Specific conductivity (k_s, m²s^-1MPa^-1) describes the permeability of xylem and is determined by all aspects of xylem anatomy that create resistance to the flow of water. Here we test the hypothesis that k_s is a function of radial and vertical position within the stem, rather than solely a function of cambial age (ring number from the pith), by measuring k_s on samples excised from 35-year-old Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco] trees at six heights and two or three radial positions. Sapwood k_s decreased from the cambium to the heartwood boundary, and the difference between outer and inner sapwood increased with height in the tree. Beneath the live crown, inner sapwood had 80-90% the k_s of outer sapwood, but only 55% just 10 m higher in the stem (about 10 nodes down from the tree top). Outer sapwood k_s peaked near the base of the crown and declined toward both the base and top of the stem. These patterns can be explained by two superimposed effects: the effect of cambial age on the dimensions of tracheids as they are produced, and the effect of xylem aging, which may include accumulation of emboli and aspiration of bordered pits. Tracheid lumen diameter and earlywood and latewood density and width, all factors known to vary with cambial age, were measured on different trees of the same age and from the same stand. Lumen diameter increased with cambial age, whereas the proportion of latewood and growth ring density increased after an initial decrease in the first 5 years. Our results suggest that the effect of cambial age on xylem anatomy is not sufficient to explain variation in k_s. Instead, physical position (both vertical and radial) in the stem and cambial age must be considered as determinants of conductivity.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

Freezing in Conifer Xylem: I. PRESSURE CHANGES AND GROWTH VELOCITY OF ICE

To determine whether freezing causes wide-spread cavitationin the xylem of freezing trees, pressure and temperature weremeasured inside freezing conifer sapwood blocks. Pressure risesof up to 3.3 MPa were recorded and average radial growth velocitiesof ice were between 1.75 and 2.3 µm s^–l. These growthvelocities of ice are less than the minimum growth velocityfor bubble nucleation during freezing. To complement this experimental study finite difference modelsof freezing in a single tracheid and freezing in an idealizedtree stem were constructed. The single tracheid model predictspressure rises similar to those measured experimentally. Thismodel also predicts that 5% to 8% of water in a tracheid lumenmigrates out of the tracheid during freezing. The tree stemmodel predicts growth velocities of ice three times faster thanthe values measured experimentally. These results are compared with previous contradictory theoriesof freezing in conifers. Key words: Freezing, xylem     

Stem water transport and freeze-thaw xylem embolism in conifers and angiosperms in a Tasmanian treeline heath

The effect of freezing on stem xylem hydraulic conductivity and leaf chlorophyll a fluorescence was measured in 12 tree and shrub species from a treeline heath in Tasmania, Australia. Reduction in stem hydraulic conductivity after a single freeze-thaw cycle was minimal in conifers and the vessel-less angiosperm species Tasmannia lanceolata (Winteraceae), whereas mean loss of conductivity in vessel-forming angiosperms fell in the range 17-83%. A positive linear relationship was observed between percentage loss of hydraulic conductivity by freeze-thaw and the average conduit diameter across all 12 species. This supports the hypothesis that large-diameter vascular conduits have a greater likelihood of freeze-thaw cavitation because larger bubbles are produced, which are more likely to expand under tension. Leaf frost tolerances, as measured by a 50% loss of maximum PSII quantum yield, varied from -6 to -13°C, indicating that these species were more frost-sensitive than plants from northern hemisphere temperate forest and treeline communities. There was no evidence of a relationship between frost tolerance of leaves and the resilience of stem water transport to freezing, suggesting that low temperature survival and the resistance of stem water transport to freezing are independently evolving traits. The results of this study bear on the ecological importance of stem freezing in the southern hemisphere treeline zones.     

Repeated freeze–thaw cycles induce embolism in drought stressed conifers (Norway spruce,stone pine)

Freezing and thawing lead to xylem embolism when gas bubbles caused by ice formation expand during the thaw process. However, previous experimental studies indicated that conifers are resistant to freezing-induced embolism, unless xylem pressure becomes very negative during the freezing. In this study, we show that conifers experienced freezing-induced embolism when exposed to repeated freeze-thaw cycles and simultaneously to drought. Simulating conditions at the alpine timberline (128 days with freeze-thaw events and thawing rates of up to 9.5 K h(-1) in the xylem of exposed twigs during winter), young trees of Norway spruce [Picea abies (L.) Karst.] and stone pine (Pinus cembra L.) were exposed to 50 and 100 freeze-thaw cycles. This treatment caused a significant increase in embolism rates in drought-stressed samples. Upon 100 freeze-thaw cycles, vulnerability thresholds (50% loss of conductivity) were shifted 1.8 MPa (Norway spruce) and 0.8 MPa (stone pine) towards less negative water potentials. The results demonstrate that freeze-thaw cycles are a possible reason for winter-embolism in conifers observed in several field studies. Freezing-induced embolism may contribute to the altitudinal limits of conifers.     

Xylem embolism in response to freeze-thaw cycles and water stress in ring-porous, diffuse-porous, and conifer species

Vulnerability to xylem embolism by freeze-thaw cycles and water stress was quantified in ring-porous (Quercus gambelii Nutt.), diffuse-porous (Populus tremuloides Michx., Betula occidentalis Hook.), and conifer species (Abies lasiocarpa Nutt., Juniperus scopulorum Sarg.). Embolism was measured by its reduction of xylem hydraulic conductivity; it was induced by xylem tension (water-stress response) and by a tension plus a freeze-thaw cycle (freeze response). Conifers showed little (Juniperus) or no (Abies) freeze response even to repeated cycles. In contrast, Quercus embolized more than 90% by freezing at tensions below 0.2 MPa, whereas similar embolism without freezing required tensions above 4.5 MPa. Diffuse-porous trees (Betula, Populus) showed an intermediate freeze response. The magnitude of the freeze response was correlated with conduit volume but occurred at higher tensions than predicted from theory. Large early-wood vessels (2.8 × 10⁻⁹ m³) in oak were most vulnerable to embolism by freezing, small vessels in Populus and Betula were intermediate (approximately 7 × 10⁻¹¹ m³), and tracheids in conifers (about 3 × 10⁻¹³ m³) were most resistant. The same trend was found within a stem: embolism by freeze-thawing occurred preferentially in wider conduits. The water-stress response was not correlated with conduit volume; previous work indicates it is a function of interconduit pit membrane structure. Native embolism levels during winter corroborated laboratory results on freezing: Quercus embolized 95% with the first fall freeze, Populus and Betula showed gradual increases to more than 90% embolism by winter's end, and Abies remained below 30%.     

Relations between efficiency of water transport and duration of leaf growth in some deciduous and evergreen trees

Shoot and leaf growth rate as well as shoot hydraulic conductance per unit leaf area (K_SL) were measured on three evergreen (Viburnum tinus L., Prunus laurocerasus L., Laurus nobilis L.) and three deciduous (Corylus avellana L., Juglans regia L., Castanea sativa L.) trees growing under the same environmental conditions. The times required to complete shoot growth (27 days for P. laurocerasus to 51 days for V. tinus) and leaf expansion (24 days for C. sativa to 42 days for C. avellana) were very different among the studied species. These species also differed in K_SL that ranged between 1.5 and 3.5 e-4 kg s^-1 m^-2 MPa^-1 in C. avellana and C. sativa, respectively, with intermediate values recorded in the other species. A strong, negative and statistically significant correlation was found to exist between K_SL and the time required for complete leaf expansion. This suggests that duration of leaf growth is shortened by the high hydraulic efficiency of the shoot. In contrast, no statistically significant relationship was found to exist between K_SL and shoot growth rate. Whether a high leaf growth rate can be interpreted as advantageous to plants or it is only an epiphenomenon of the high efficiency in the vertical water transport is discussed.     

Influence of xylem development on axial hydraulic conductance within Prunus root systems

The effect of secondary growth on the distribution of the axial hydraulic conductance within the Prunus root system was investigated. Secondary growth resulted in a large increase in both the number (from about 10 to several thousand) and diameter of xylem vessels (from a few micrometres to nearly 150 µm). For fine roots (<3 mm), an increase in root diameter was correlated with a slight increase in the number of xylem vessels and a large increase in their diameter. Conversely, for woody roots, an increase in root diameter was associated with a dramatic increase in the number of xylem vessels, but little or no change in vessel diameter. The theoretical axial conductivity (Kh, m⁴.s^-1.MPa^-1) of root segments was calculated with the Poiseuille-Hagen equation from measurements of vessel diameter. Kh measured using the tension-induced technique varies over several orders of magnitude (7.4᎒^-11 to 5.7᎒^-7 m⁴.s^-1.MPa^-1) and shows large discrepancies with theoretical calculated Kh. We concluded that root diameter is a pertinent and useful parameter to predict the axial conductance of a given root, provided the root type is known. Indeed, the relationship between measured Kh and root diameter varies according to the root type (fine or woody), due to differences in the xylem produced by secondary growth. Finally, we show how the combination of branching pattern and axial conductance may limit water flow through root systems. For Prunus, the main roots do not appear to limit water transfer; the axial conductance of the main axes is at least 10% higher than the sum of the axial conductance of the branches.     

Refilling of Embolized Vessels in Young Stems of Laurel. Do We Need a New Paradigm?

Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than −1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N₂ to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than −1 MPa. We have concluded that none of the existing paradigms explains the results.     

Mechanism of freeze-induced embolism in Fagus sylvatica L.

 The mechanism of freeze stress-induced embolism in Fagus sylvatica L. branches was analyzed under controlled conditions. Excised branches were exposed to successive freeze-thaw cycles in temperature controlled chambers. Thermocouples were placed on the bark to detect sap freezing exotherms. The degree of xylem embolism was estimated after each cycle by the loss of hydraulic conductivity. After one freeze-thaw cycle the degree of embolism was found to decrease with xylem specific hydraulic conductivity, small apical shoots being more susceptible to embolism. Exotherms revealed that distal shoots were freezing first and exuded sap as a result of water expansion. The lower water content in apical shoots upon freezing probably induced higher sap tensions which promoted air bubble expansion and vessel cavitation preferentially near the apices. When the decrease in water content was experimentally prevented, embolism developed to a lesser extent. The higher vulnerability of shoot apices may protect the rest of the branch from winter damage. Received: 29 May 1998 / Accepted: 15 August 1998     

伊犁河谷退化野果林中新疆野苹果茎的水力结构

为揭示伊犁河谷退化野果林中新疆野苹果植株个体的死亡机理,对比研究3种长势(Ⅰ级,20%枯枝率;Ⅱ级,40%~60%枯枝率;Ⅲ级,>80%枯枝率)新疆野苹果茎的水力结构特征的差异以及水分运输有效性和安全性及其影响因素。结果表明: 随野果林退化程度的增加,3种长势的新疆野苹果茎的边材比导水率和叶比导水率呈下降趋势,但差异不显著;傍晚时的栓塞程度、栓塞脆弱性呈显著增加的趋势,其中Ⅰ、Ⅱ和Ⅲ级长势个体的导水率损失50%时木质部水势分别为-1.87、-1.35和-0.53 MPa;黎明前和正午叶水势、水力安全边际均为Ⅰ级>Ⅱ级>Ⅲ级;与水力学相关的木质部解剖结构以及枝叶性状在3种长势个体之间存在显著差异。相关性分析表明,新疆野苹果茎木质部水分运输的有效性和安全性之间存在弱的权衡关系。新疆野苹果茎的水力结构变化伴随着新疆野果林的衰退过程。野果林的退化会加剧新疆野苹果木质部导管的栓塞化程度,降低其抵抗空穴化栓塞的能力,新疆野苹果面临水力失衡的威胁更大。     

Carnivorous feeding and respiration of the Arctic under-ice amphipod Gammarus wilkitzkii

. The dominant Arctic under-ice amphipod Gammarus wilkitzkii consumes a wide range of food items. The carnivorous feeding activity and energy budget of this large species were studied using three different approaches. Maximum potential ingestion rates I_max estimated from an allometric function taken from the literature and based on body mass were 2.1ǂ.4% of body carbon day^-1. Based on respiration measurements, the specific ingestion rates required to meet metabolic demands were lower (1.4ǂ.4% of body carbon day^-1). Feeding experiments, in which co-occurring pelagic calanoid (Calanus hyperboreus) or sympagic harpacticoid (Halectinosoma sp.) copepods were offered as prey, yielded actual ingestion rates of 8.0LJ.6% of body carbon day^-1 and 0.1ǂ.1% of body carbon day^-1, respectively. These results indicate that predatory feeding on pelagic copepods may constitute an important food source for G. wilkitzkii. Abundances of G. wilkitzkii at the ice underside (median: 1.6 ind. m^-2), Calanus spp. in the upper metre below the ice (2.6 ind. m^-3), and Halectinosoma sp. in the lowermost 2-3 cm of the ice (393.5 ind. m^-2) were determined from several multi-year pack-ice floes in the northern Greenland Sea and Fram Strait. Potential predation impact of G. wilkitzkii was estimated by combining information on ingestion rates with population densities. It was very high on Calanus spp. in the under-ice water layer (61.5% of the under-ice standing stock day^-1), but comparatively low on Halectinosoma sp. in the bottom of the ice (3.8% of standing stock day^-1). The observation of G. wilkitzkii preying on pelagic copepods in the under-ice water layer represents a hitherto unknown but obviously significant process and a new direction in the cryo-pelagic coupling in the Arctic marine ecosystem.     

Effect of drought on leaf gland secretion of the mangrove Avicennia germinans L.

This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil ? (, soil N MPa) during the wet season was -0.95ǂ.05 and -2.12ǂ.08 at low and high salinity sites, respectively. During the dry season, these values decreased to -3.24ǂ.09 at low salinity and to -5.75ǂ.06 at high salinity. Consequently, predawn and midday plant water potential were lowered during drought at both sites. The rates of secretion (mmol m^-2 h^-1 ) increased during drought from 0.91ǂ.12 during the wet season to 1.93ǂ.12 at low salinity, and from 1.69ǂ.12 during the wet season to 2.81ǂ.15 at high salinity. Conversely, stomatal conductance (g_s) was lowered by both salinity and drought. As xylem osmolality increased during drought, secretion tended to rise exponentially, and g_s decayed hyperbolically. Thus, a trade-off is obtained between enhancement in salt secretion and control of water loss suggested by g_s.     

Laurel forests in Tenerife, Canary Islands

The general wood structure, vessel size and distribution along the stem xylem radius and in petioles were studied in Laurus azorica trees living in a Tenerife laurel forest. The fractions of volume occupied by dry matter, water and air in percentage of wood fresh volume were also studied. The wood showed a diffuse-porous structure, with solitary vessels or vessels somewhat clustered in small radially oriented groups. Vessels had a diameter ranging from 20 to 130 µm. This diameter was minimal close to the pith, increased more than 2-fold with age, and reached its maximum width close to the cambium. Vessel density decreased from 36 vessels mm^-2 near the pith to about 13 vessels mm^-2 near the cambium. Accordingly, the lumen area was small in young xylem close to the pith (0.0015 mm²), reaching a value 5 times larger (0.007 mm²) near the cambium than in the centre of the stem. Lumen area of vessels in petioles was about 1.5% of petiole cross-sectional area and thus much lower than in stems. Mean hydraulic diameter of these vessels was about 20 µm, and mean vessel density about 136 per petiole. There were only small differences in proportions of dry matter, water and air along stem radius. The relevance of each one of these fractions in the wood is discussed as evidence of the possible existence of a number of embolized vessels dispersed in the total functional cross-sectional area of the xylem.     

Shoot orientation affects vessel size, shoot hydraulic conductivity and shoot growth rate in Vitis vinifera L.

Vitis vinifera L. plants were grown in containers and each plant's single shoot was orientated upwards or downwards. Some plants were trained first upwards, then downwards, then again upwards (N-shaped plants). Vegetative growth was reduced in plants trained downwards compared to that in upward and N-shaped plants. Shoot growth rate slowed in downward shoot portions, but only after the apex had grown downwards for at least 10 internodes. Shoot hydraulic conductivity k _h, measured after elimination of xylem embolisms, was lower in downward than in upward plants. In N-shaped plants k _h was higher in the upward-growing shoot portions, and lower in the central, downward-growing portion. Shoot- and leaf-specific conductivities were also lower in downward than in upward shoot portions. Xylem cross-sectional area and xylem structure (number of wedges, number of vessels per unit xylem area) differed little in the three orientations. In contrast, vessel diameter and the sum of vessel cross-sectional areas were significantly smaller in downward than in upward shoot portions. These differences could explain the reduction in conductivity observed in the downward-orientated shoot portions. The measurements taken on N-shaped plants showed that the decreases in k _h and in vessel size were a result of shoot orientation, not shoot bending.