Specific Hydraulic Conductivity Research Articles

A biphasic, anisotropic model of the aortic wall.

A biphasic, anisotropic elastic model of the aortict wall is developed and compared to literature values of experimental measurements of vessel wall radii, thickness, and hvdraulic conductivity as a function of intraluminal pressure. The model gives good predictions using a constant wall modulus for pressures less than 60 mmHg, but requires a strain-dependent modulus for pressures greater than this. In both bovine and rabbit aorta, the tangential modulus is found to be approximately 20 times greater than the radial modulus. These moduli lead to predictions that, when perfused in a cylindrical geometry, the aortic volume and its specific hydraulic coonductivity are relatively independent of perfusion pressure, in agreement with experimental measurements. M, the parameter that relates specific hydraulic conductivy, to tissue dilation, is found to be a positive quantity correcting a previous error in the literature.

Interspecific variability of δ13C among trees in rainforests of French Guiana: functional groups and canopy integration.

The interspecific variability of sunlit leaf carbon isotope composition (δ13C), an indicator of leaf intrinsic water-use efficiency (WUE, CO2 assimilation rate/leaf conductance for water vapour), was investigated in canopy trees of three lowland rainforest stands in French Guiana, differing in floristic composition and in soil drainage characteristics, but subjected to similar climatic conditions. We sampled leaves with a rifle from 406 trees in total, representing 102 species. Eighteen species were common to the three stands. Mean species δ13C varied over a 6.0‰ range within each stand, corresponding to WUE varying over about a threefold range. Species occurring in at least two stands displayed remarkably stable δ13C values, suggesting a close genetic control of species δ13C. Marked differences in species δ13C values were found with respect to: (1) the leaf phenology pattern (average δ13C=-29.7‰ and -31.0‰ in deciduous-leaved and evergreen-leaved species, respectively), and (2) different types of shade tolerance defined by features reflecting the plasticity of growth dynamics with respect to contrasting light conditions. Heliophilic species exhibited more negative δ13C values (average δ13C=-30.5‰) (i.e. lower WUE) than hemitolerant species (-29.3‰). However, tolerant species (-31.4‰) displayed even more negative δ13C values than heliophilic ones. We could not provide a straightforward ecophysiological interpretation of this result. The negative relationship found between species δ13C and midday leaf water potential (Ψwm) suggests that low δ13C is associated with high whole tree leaf specific hydraulic conductance. Canopy carbon isotope discrimination (Δ A ) calculated from the basal area-weighed integral of the species δ13C values was similar in the three stands (average Δ A =23.1‰), despite differences in stand species composition and soil drainage type, reflecting the similar proportions of the three different shade-tolerance types among stands.

Quercus species differ in water and nutrient characteristics in a resource-limited fall-line sandhill habitat.

We compared co-occurring mature Quercus laevis Walt. (turkey oak), Q. margaretta Ashe (sand post oak) and Q. incana Bartr. (bluejack oak) trees growing in resource-limited sandhill habitats of the southeastern United States for water and nutrient characteristics. The Quercus spp. differed in their distribution along soil water and nutrient gradients, and in their access to and use of water, even though the study year was wetter than average with no mid-season drought. Quercus laevis had the greatest access to soil water (least negative pre-dawn water potential, psi(pd)) and the most conservative water-use strategy based on its relatively low stomatal conductance (g(s)), high instantaneous water-use efficiency (WUE), least negative midday water potential (psy(md)) and high leaf specific hydraulic conductance (K(L)). Quercus margaretta had the least conservative water-use characteristics, exhibiting relatively high g(s), low instantaneous WUE, most negative psi(md), and low K(L). Quercus margaretta also had a low photosynthetic nitrogen-use efficiency (PNUE), but a high leaf phosphorus concentration. Quercus incana had the poorest access to soil water, but intermediate water-use characteristics and leaf nutrient characteristics more similar to those of Q. laevis. There were no species differences for photosynthesis (A), leaf nitrogen on an area basis, or seasonally integrated WUE (delta13C). Both A and g(s) were positively correlated for each species, but A and g(s) were generally not correlated with psi(pd), psi(md) or delta psi(pd-md). Although we found differences in resource use and resource status among these sandhill Quercus spp., the results are consistent with the interpretation that they are generally drought avoiders. Quercus laevis may have an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with the other Quercus spp.

Xylem conductivity and vulnerability to cavitation of ponderosa pine growing in contrasting climates.

We examined the effects of increased transpiration demand on xylem hydraulic conductivity and vulnerability to cavitation of mature ponderosa pine (Pinus ponderosa Laws.) by comparing trees growing in contrasting climates. Previous studies determined that trees growing in warm and dry sites (desert) had half the leaf/sapwood area ratio (A(L)/A(S)) and more than twice the transpiration rate of trees growing in cool and moist sites (montane). We predicted that high transpiration rates would be associated with increased specific hydraulic conductivity (K(S)) and increased resistance to xylem cavitation. Desert trees had 19% higher K(S) than montane trees, primarily because of larger tracheid lumen diameters. Predawn water potential and water potential differences between the soil and the shoot were similar for desert and montane trees, suggesting that differences in tracheid anatomy, and therefore K(S), were caused primarily by temperature and evaporative demand, rather than soil drought. Vulnerability to xylem cavitation did not differ between desert and montane populations. A 50% loss in hydraulic conductivity occurred at water potentials between -2.61 and -2.65 MPa, and vulnerability to xylem cavitation did not vary with stem size. Minimum xylem tensions of desert and montane trees did not drop below -2.05 MPa. Foliage turgor loss point did not differ between climate groups and corresponded to mean minimum xylem tensions in the field. In addition to low A(L)/A(S), high K(S) in desert trees may provide a way to increase tree hydraulic conductivity in response to high evaporative demand and prevent xylem tensions from reaching values that cause catastrophic cavitation. In ponderosa pine, the flexible responses of A(L)/A(S) and K(S) to climate may preclude the existence of significant intraspecific variation in the vulnerability of xylem to cavitation.

Enhanced water relations of residual foliage following defoliation in Populus tremuloides

Stomatal conductance and leaf water potential of aspen (Populus tremuloides Michx.) were measured in response to defoliation intensity, both in the field and under controlled conditions. There was evidence of increased stomatal conductance in trees with 50 and 98% defoliation, but no change in leaf water potential. Under controlled conditions, stomatal conductance and rate of photosynthesis were measured under high and low vapour pressure deficits (VPD). Under high VPD, overall stomatal conductance and rates of photosynthesis were greatly reduced. However, in both VPD treatments, there was evidence of increased stomatal conductance and compensatory photosynthesis following defoliation. These findings may be due to increases in leaf specific hydraulic conductance following defoliation.Key words: defoliation, stomatal conductance, leaf water potential, compensatory photosynthesis.

Dynamic changes in petiole specific conductivity in red maple (Acer rubrum L.), tulip tree (Liriodendron tulipifera L.) and northern fox grape (Vitis labrusca L.)

ABSTRACTDiurnal variation in petiole specific hydraulic conductivity and simultaneous measurements of leaf water potential were recorded in red maple, tulip tree and fox grape. Petiole specific conductivity was determined from in situ measurements of water flow into the distal (leaf‐bearing) end of an attached petiole as a function of applied hydrostatic pressure and petiole dimensions. The hydraulic properties of the petiole dominated the measurements, indicating that this technique can be used for rapid estimates of petiole hydraulic conductivity. There was a significant decrease in petiole specific conductivity associated with increasingly more negative leaf water potentials in maple and tulip tree, but not in grape. Petiole specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem sap was under tension. The recovery of petiole conductivity during the afternoon suggests that hydraulic conductivity reflects a dynamic balance between a loss of hydraulic conductivity with increasing water stress, and its restoration as tension within the xylem decreases. Three experimental manipulations were applied to red maple and tulip tree to examine the sensitivity of diurnal changes in petiole conductivity to various physiological perturbations. Both phloem girdling and application of HgCl2 to the transpiration stream resulted in a marked decrease in the degree to which petiole specific conductivity recovered as xylem tension relaxed during the afternoon. Delivery of a surfactant to the xylem, however, did not significantly alter the relation between leaf water potential and petiole hydraulic conductivity.

Reverse phenology and dry‐season water uptake byFaidherbia albida(Del.) A. Chev. in an agroforestry parkland of Sudanese west Africa

1.Faidherbia (Acacia) albidais a multipurpose tree widely distributed in semiarid Africa, notably in agroforestry parklands. It is in leaf during the dry season and defoliated during the rainy season, displaying therefore a peculiar reverse phenology. The related water‐use strategy, including leaf water potential, sapflow, hydraulic conductance and depth of uptake, were monitored on adult trees in a Sudanese west‐African parkland.2.Despite a severe drought in the superficial soil layers, the predawn leaf water potential ofF. albidadropped only toca.– 0·5 MPa during the end of the dry season, indicating only a moderate water stress.3.Radial trunk growth ceased before the end of the dry season and could have been affected by the moderate drought stress. However, leafiness remained constant during the dry season. Leaf shedding occurred after the first rains and was probably independent of drought.4.Faidherbia albidadisplayed large transpiration rates under favourable conditions but the ratio of sapflow to Penman evapotranspiration and the soil‐to‐leaf specific hydraulic conductance decreased severely towards the end of the dry season.5.Roots ofF. albidawere distributed through the weathered rock, down to a depth of 7 m, and vanished in the vicinity of a permanent water‐table. The isotopic composition of oxygen in the xylem sap (δ18O) remained very close to the values recorded in the water‐table during the course of the year. Phreatophytism thus explained the maintenance of growth and transpiration during the dry season. Nevertheless, during early rains, δ18O of sap switched towards the composition of the superficial soil layers, indicating facultative phreatophytism.6.Reverse phenology, low density and depth of water uptake ofF. albidaindicated a low competition with annual crops for water; the fraction of annual rainfall used by the trees was estimated to remain below 5%.

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.

Regulation of water loss in populations of Populus trichocarpa: the role of stomatal control in preventing xylem cavitation.

Variations in resistance to drought-induced xylem cavitation, xylem air-entry points, stomatal behavior, and hydraulic conductivity were measured in four populations of Populus trichocarpa Torr. & A. Gray collected along an east-west humidity and temperature gradient in Washington State, USA. Xylem air-entry points were less negative in trees from moist environments (-0.71 and -1.32 MPa in the Hoh and Nisqually populations, respectively) than in trees from dry environments (-1.55 and -1.67 MPa in the Palouse and Yakima populations, respectively). Xylem cavitation in response to experimental drought was consistent with air-injection measures of xylem air-entry points for a given population. Populations vulnerable to cavitation also exhibited higher stem specific hydraulic conductivities and limited stomatal control compared with resistant populations. Populations exhibiting vulnerability to cavitation and limited stomatal control desiccated more rapidly during drought compared with resistant populations. This study provides evidence of interpopulation variation in resistance to drought-induced xylem cavitation, stomatal behavior, and hydraulic conductivity within Populus trichocarpa.

Analysis and Testing of a Concentric-Disk Tension Infiltrometer

Tension infiltrometers are widely used to measure soil hydraulic properties at soil potentials near saturation in the field. Current methods of hydraulic property analysis using tension infiltrometers rely on secondary measurements of soil water content andlor use of a specific hydraulic conductivity (K(h)) form. In this paper, design criteria and method of application for a concentric disk (CD) infiltrometer are proposed, which can be used to measure saturated and nearly saturated hydraulic conductivities (K(h 0 )) and matric flux potential (Φ(h 0 )) in the field without secondary measurements or explicit reliance on a specific conductivity form. Using measured pairs of K(h 0 ) and Φ(h 0 ), a parameter estimation method is suggested to allow extrapolation of K(h) to lower than measured soil water potentials. The design of the infiltrometer and the accuracy of parameter estimation were tested against results of numerical simulations. We found that an outer-disk radius of 150 mm was sufficient to establish a 50-mm-radius inner-disk of one-dimensional (I-D) infiltration under a wide range of simulated conditions. Identification of input K(h ) functions using the suggested method of analysis was excellent, although errors were introduced by nonsteady-state infiltration data. A field test of a CD infiltrometer yielded a promising comparison with K(h) data generated by the instantaneous profile method, an alternative approach but one that is more labor intensive. The advantage of the CD infiltrometer is that, in addition to providing a rapid, direct estimate of K(h 0 ), the results are not conditional upon an assumed form of K(h) and they may be used to estimate the parameters of a K(h) form of choice.

Seasonal embolism and xylem vulnerability in deciduous and evergreen Mediterranean trees influenced by proximity to a carbon dioxide spring.

We investigated how proximity to natural CO(2) springs affected the seasonal patterns of xylem embolism in Quercus ilex L., Quercus pubescens Willd., Fraxinus ornus L., Populus tremula L. and Arbutus unedo L., which differ in leaf phenology and wood anatomy. Xylem embolism was evaluated in both artificially dehydrated branches and in hydrated apical branches collected at monthly intervals during a 20-month sampling period. Initial specific hydraulic conductivity was also evaluated. We found species-dependent differences in xylem hydraulic properties in response to elevated CO(2) concentration. Populus tremula was the most embolized and A. unedo was the least embolized of the species examined. Effects of elevated CO(2) were significant in Q. pubescens, P. tremula and A. unedo, whereas the overall response to elevated CO(2) was less evident in F. ornus and Q. ilex. Specific hydraulic conductivity differed among species but not between sites, although the interaction between species and site was significant. Differences in xylem vulnerability between trees growing near to the CO(2) spring and those growing in control areas were small. Although differences in hydraulic properties in response to elevated CO(2) concentration were small, they may be of great importance in determining future community composition in Mediterranean-type forest ecosystems. The possible causes and ecological significance of such differences are discussed in relation to elevated CO(2) concentration and other environmental conditions.

Stomatal conductance, growth and root signaling in Betula pendula seedlings subjected to partial soil drying.

Seedlings of Betula pendula Roth were grown with their root systems separated between two soil compartments. Four treatments were imposed: (i) adequate irrigation in both compartments (WW, controls); (ii) adequate irrigation in one compartment and drought in the other compartment (WD); (iii) drought in both compartments (DD); and (iv) half of the root system severed and the remainder kept well-watered (root excision, RE). Predawn leaf water potential, stomatal conductance, soil-to-leaf specific hydraulic conductance, and root and leaf growth decreased in DD-treated seedlings, which also displayed severe leaf shedding (30% loss in leaf area). The DD treatment also resulted in increased concentrations of abscisic acid (ABA) and its glucose ester in the xylem sap of roots and shoots compared to concentrations in control seedlings (about 200 versus 20 nM). Despite the difference in xylem sap concentrations, total ABA flux to the shoots was similar in the two treatments (1-2 pmol ABA m(-2) leaf area s(-1)) as a result of reduced transpiration in the DD-treated seedlings. Compared with root growth in control plants, root growth increased in the RE-treated plants and decreased in the drying compartment of the WD treatment; however, the RE and WD treatments only slightly reduced leaf expansion, and had no detectable effects on shoot water relations or ABA concentrations of the root and shoot xylem sap. We conclude that short-term soil water depletion affecting only 50% of the root system does not cause a measurable stress response in birch shoots, despite root growth cessation in the fraction of drying soil.

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 μm mean diameter) and perforation plates are reticulate. This leads to specific and leaf specific hydraulic conductivities that are amongst the lowest recorded for angiosperms (ks0.87 kg m−1MPa−1s−1; kl3.28×10−5kg m−1MPa−1s−1, stem diameter 3 mm). Hydraulic conductivities decrease with increasing pressure gradient. Transpiration rates in well watered plants were moderate to low, generating xylem water potentials of -1 to -2 MPa. Acoustic emissions indicated extensive cavitation events that were initiated at xylem water potentials of -2 to -3 MPa. The desiccation-tolerant nature of the tissue permits this species to survive this interruption of the water supply. On rewatering the roots pressures that were developed were low (2.4 kPa). However capillary forces were demonstrated to be adequate to account for the refilling of xylem vessels and re-establishment of hydraulic continuity even when water was under a tension of -8 kPa. During dehydration and rehydration cycles stems showed considerable shrinking and swelling. Unusual knob-like structures of unknown chemical composition were observed on the outer surface of xylem vessels. These may be related to the ability of the stem to withstand the mechanical stresses associated with this shrinkage and swelling.

Growth and Seasonal Utilisation of Water and Nutrients by Banksia prionotes

Banksia prionotes Lindley is a fire-sensitive, fast-growing tree of nutrient-impoverished deep sands of south-western Australia. Its root system is dimorphic, comprising proteoid root-bearing, lateral roots absorbing superficially concentrated nutrients during the wet winter season and a single main sinker (tap) root extending down to the water table. Shoot extension commences in early summer coincident with drying of topsoil and ceases at the end of summer upon initiation of inflorescences and resting buds. Shoot growth utilises nutrients accumulated the previous and earlier wet seasons and current photosynthate formed at the expense of ground water abstracted by the sinker root. Rooting morphologies of differently aged trees are described and yearly changes in dry matter distribution between leaves, trunk and parts of root systems are related to dry matter gain and foliage area. The seasonality of nutrient uptake by proteoid, lateral and sinker roots and nutrient translocation in shoots is assessed by xylem and phloem sap analyses. Specific hydraulic conductivities of xylem of sinker roots are considerably higher than in lateral roots and higher again than in trunk xylem. The differences involved relate to vessel lengths and diameters and proportional transectional areas devoted to conducting tissues. Seasonal changes in dependence on ground water as opposed to recent rain are estimated using deuterium : hydrogen natural abundance ratios of water extracted from xylem of lateral roots, tap root and trunk. Relationships between water stress and timing and progress of extension growth of shoots are studied using carbon isotope natural abundance ratios of new leaf dry matter and abscisic acid levels in xylem and phloem sap.

Hydraulic adjustment of maple saplings to canopy gap formation.

The leaf-specific hydraulic conductivity (K L) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A L/A S) and the specific hydraulic conductivity of xylem tissue (K S). In environments with high atmospheric vapor pressure deficit (VPD), K L may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K L and lower A L/A S than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K S increased with sapling size for both species. In A. rubrum, K S did not differ between the two environments but lower A L/A S (P=0.05, ANCOVA) led to higher K L for gap-grown saplings (P < 0.05, ANCOVA). In A. pensylvanicum, neither K S, A L/A S, nor KL differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with K L (r 2=0.75, P < 0.05). A hypothetical large A. pensylvanicum sapling (2 m tall) had 2.4 times higher K L and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r 2=0.69, P < 0.05). Calculations indicated that small A.pensylvanicum saplings (low K L) could not transpire at the rate of large saplings (high K L) without reaching theoretical thresholds for xylem embolism induction. The coordination between K L and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The K S of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing K L. However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in K L and K S with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment.

Apoplasmic and Protoplasmic Water Transport through the Parenchyma of the Potato Storage Organ.

Stationary volume fluxes through living and denatured parenchyma slices of the potato (Solanum tuberosum L.) storage organ were studied to estimate the hydraulic conductivity of the cell wall and to evaluate the significance of water transport through protoplasts, cell walls, and intercellular spaces. Slices were placed between liquid compartments, steady-state fluxes induced by pressure or concentration gradients of low- and high-molecular-mass osmotica were measured, and water transport pathways were distinguished on the basis of their difference in limiting pore size. The protoplasts were the dominating route for osmotically driven water transport through living slices, even in the case of a polymer osmoticum that is excluded from cell walls. The specific hydraulic conductivity of the cell wall matrix is too small to allow a significant contribution of the narrow cell wall bypass to water flow through the living tissue. This conclusion is based on (a) ultrafilter coefficients of denatured parenchyma slices, (b) the absence of a significant difference between ultrafilter coefficients of the living tissue slices for osmotica with low and high cell wall reflection coefficients, and (c) the absence of a significant interaction (solvent drag) between apoplasmic permeation of mannitol and the water flux caused by a concentration difference of excluded polyethylene glycol. Liquid-filled intercellular spaces were the dominating pathways for pressure-driven volume fluxes through the parenchyma tissue.

Ecophysiological Aspects of the Woody Root HemiparasiteSantalum acuminatum(R. Br.) A. DC and its Common Hosts in South Western Australia

The water and nitrogen relationships of the xylem-tapping root hemiparasite, quandong (Santalum acuminatum) and its principal hosts were examined at a series of sites in native coastal heathlands of south west Australia. Assessments based on densities of above-ground biomass, ground cover and frequencies of haustoria on host roots indicated that woody N2fixers (legumes andAllocasuarina) were principal hosts ofSantalum. δ15N values for shoot dry matter of component species suggested these N2fixers were strongly (70% or more) dependent on atmospheric N and thatSantalumderived N principally from these species. Structural studies of haustoria showed the interface with host xylem to be almost entirely comprised of parenchymatous tissue. No luminal continuities were observed between xylem conducting elements of the partners. Formation and functional life of haustoria were closely coordinated with seasonal growth of hosts, with some haustoria surviving summer and overlapping functionally with new ones establishing in the next autumn. Transpiration and photosynthetic rates of the parasite were consistently less and water use efficiencies very similar to those of the principal hostAcacia rostellifera. δ13C values of foliage ofSantalumand this host were similar, but large variations in δ13C values for above-ground dry matter of parasite and hosts between study sites prevented evaluations of water stress or water-use-efficiency based on carbon isotope discrimination. Specific hydraulic conductivities of roots ofSantalumwere consistently lower than those ofAcacia, a finding consistent with more conservative water use by the parasite than the host.

Biomechanical and hydraulic determinants of tree structure in Scots pine: anatomical characteristics.

The development of anatomical, hydraulic and biomechanical properties in Scots pine (Pinus sylvestris L.) stems aged 7 to 59 years was followed. The hydraulic diameter and length of tracheids increased with age to a maximum at 15 and 35 years, respectively. Number of tracheids per unit of sapwood area decreased with age to a minimum of 500-600 tracheids mm(-2). Variations in specific hydraulic conductivity and Young's modulus of stems were associated with variation in anatomical properties. Over the time sequence considered, hydraulic and mechanical properties were positively related to each other and followed a similar developmental pattern, with no suggestion of a trade-off between the two. For most of the tree's life-cycle, heartwood made only a small contribution to whole-section mechanical stiffness because of its location close to the flexural neutral axis, and because of the presence of juvenile wood.

Xylem cavitation and loss of hydraulic conductance in western hemlock following planting.

Following planting, western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings experience water stress and declining xylem pressure potential (Psi(x)). Low Psi(x) can result in xylem cavitation and embolism formation, causing a decline in hydraulic conductance. This study focused on the relationship between Psi(x), xylem cavitation and transpiration (E) of newly planted seedlings. Leaf specific hydraulic conductance (k(AB)) declined from 0.56 to 0.09 mmol m(-2) s(-1) MPa(-1) over a 9-day period. Stomatal conductance (g(s)) declined from 143.5 to 39.15 mmol m(-2) s(-1) over the same period without an associated change in environmental conditions. A vulnerability profile indicated a 30% loss in hydraulic conductivity when seedlings experienced a Psi(x) between -2.5 and -3.0 MPa. A Psi(x) of -4.0 MPa led to a complete loss of conductivity. We conclude that following planting, western hemlock seedlings often experience Psi(x) values that are low enough to cause xylem cavitation and a decline in k(AB).

Effects of parasitism by a mistletoe on the structure and functioning of branches of its host

ABSTRACTInfestation of Acacia acuminata by the xylem‐tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.