ABSTRACTPressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting... more
ABSTRACTPressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water‐stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air‐seeding). Deduced xylem pressure sh...
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A new method is presented for measuring whole-shoot hydraulic conductance, K(T) (kg s(-1) MPa(-1)). The method was also used to determine other conductance values in maple (Acer saccharum Marsh.) stem segments of differing diameter... more
A new method is presented for measuring whole-shoot hydraulic conductance, K(T) (kg s(-1) MPa(-1)). The method was also used to determine other conductance values in maple (Acer saccharum Marsh.) stem segments of differing diameter including: K(h) (absolute conductance or conductance per unit pressure gradient, kg s(-1) m MPa(-1)), K(s) (specific conductance or K(h) per unit wood area, kg s(-1) m(-1) MPa(-1)), and LSC (leaf specific conductance or K(h) per unit leaf area, kg s(-1) m(-1) MPa(-1)). A regression of K(T) versus stem basal diameter, D (m), gave K(T) = 5.998 x 10(-2) D(1.402) (R(2) = 0.986 for D from 0.001 to 0.1 m) and a regression for leaf area, A(L) (m(2)), gave A(L) = 4.667 x 10(3) D(2.007) (R(2) = 0.981 for D from 0.001 to 0.3 m). More than 50% of the resistance to water flow in large shoots (0.1 m in diameter and 8 to 10 m long) was contained in branches less than 0.012 m in diameter, i.e., in the distal 1.5 m of branches. We used the regressions to predict the steady state difference in pressure potential, P, between the base of a shoot of diameter D and the average pressure potential at the apices of the shoot; the relation is given by P = 7.781 x 10(4) E D(0.605), where E is the average evaporative flux density (kg s(-1) m(-2)) in the leaves attached to the shoot. After comparing the predictions of this equation to field observations of E and leaf water potential and stomatal conductance, we concluded that the hydraulic conductance of large maple shoots is sufficiently low to prevent maximum stomatal conductance in maple leaves.
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The tempo of water efflux from single Fagus grandifolia leaves has been measured. The resistance to pressure-driven water efflux has been measured for normal leaves, Rs, and for leaves in which extracellular mesophyll spaces are... more
The tempo of water efflux from single Fagus grandifolia leaves has been measured. The resistance to pressure-driven water efflux has been measured for normal leaves, Rs, and for leaves in which extracellular mesophyll spaces are infiltrated with water, Rs*. The ratio Rs*/Rs is about 0.4. The resistance to water flow through the xylem, Rx, was also measured and found to be a small part of Rs, i.e., Rx/Rs = 0.08. The activation energies for water efflux from normal and infiltrated leaves are both about 26 ± 4 kJ/mol.After an analysis of our data, we conclude that the membrane hydraulic conductivity of F. grandifolia leaves is roughly 10−6 cm s−1 bar−1 (1 bar = 100 kPa) and that water travels the shortest path between the cell sap and the nearest xylem vessel, flowing in and out of mesophyll cells through the areas in contact between adjacent cells along the pathway.
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The physical implications of the various parameters obtained from a pressure-bomb study are explored and related to their possible ecological significance. Our analysis suggests that the original bulk osmotic pressure, the bulk osmotic... more
The physical implications of the various parameters obtained from a pressure-bomb study are explored and related to their possible ecological significance. Our analysis suggests that the original bulk osmotic pressure, the bulk osmotic pressure at incipient plasmolysis, and cell wall elasticity are closely associated with the extent to which a leaf can osmoregulate or conserve water within a certain range of water potential change in the environment and might therefore have certain adaptive value. The pressure–volume relation could be applied to predict changes in leaf water potential at various degrees of water loss in the field. The values of these various parameters were obtained from a pressure-bomb study on single leaves from a wide variety of species. The use of data from single leaves as compared with whole shoots is discussed.
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MA DIXON,* J. GRACEt & MT TYREE* *Botany Departtnent, University of Toronto, Toronto, Ontario, Canada 1V15S lAl, and fDepartment of Forestry & Natural Resources, University of... more
MA DIXON,* J. GRACEt & MT TYREE* *Botany Departtnent, University of Toronto, Toronto, Ontario, Canada 1V15S lAl, and fDepartment of Forestry & Natural Resources, University of Edinburgh, Edinbutgh, United Kingdotn EH9 3JU ... Received 25 April 1984; accepted for ...
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The theory of water transport across plant cell membranes is now reasonably satisfactory and straightforward, especially since the coupling of solute and water flows has been correctly introduced by the theory of irreversible... more
The theory of water transport across plant cell membranes is now reasonably satisfactory and straightforward, especially since the coupling of solute and water flows has been correctly introduced by the theory of irreversible thermodynamics. Adequate treatments of the theory can be found in the reviews of DAINTY (1963, 1969) and the book of SLATYER (1967).
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... Ibid. 42, 571-636. TYREE, MT, 1975 a. Some inconsistencies that arise when the Canny-Phillips model of phloem trans-location is applied to known "C-assimilate profiles in plant stems and petioles. Can. J. Bot. 53, 1128-31.... more
... Ibid. 42, 571-636. TYREE, MT, 1975 a. Some inconsistencies that arise when the Canny-Phillips model of phloem trans-location is applied to known "C-assimilate profiles in plant stems and petioles. Can. J. Bot. 53, 1128-31. 19756. ...
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The hydraulic architecture of trees is divided into segments of high and low hydraulic conductance. Segments of major stems are 50 to 1000 times more capable of supplying water to leaves down-stream than are minor branches. The functional... more
The hydraulic architecture of trees is divided into segments of high and low hydraulic conductance. Segments of major stems are 50 to 1000 times more capable of supplying water to leaves down-stream than are minor branches. The functional advantages of plant segmentation are: 1) to allow all leaves to compete on a more or less equal basis for water resources in the canopy of large trees and 2) to confine xylem cavitations and embolisms to the minor branches during times of water stress.
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Since 1972 several papers have appeared that deal with the mathematical modelling of Munch pressure flow systems. The first paper (Eschrich, Evert and Young, 1972) dealt with Munch pressure flow relaxation phenomena in man-made tubular... more
Since 1972 several papers have appeared that deal with the mathematical modelling of Munch pressure flow systems. The first paper (Eschrich, Evert and Young, 1972) dealt with Munch pressure flow relaxation phenomena in man-made tubular semi-permeable membranes. The next two papers treated models applicable to Munch pressure flow in sieve tubes. Young, Evert and Eschrich (1973) derived the mathematical formalism applicable to steady state translocation in sieve tubes for an arbitrary distribution of solute sources and sinks; while these authors discussed the qualitative behaviour of their equations they published no sample calculations. Christy and Ferrier (1973) published another paper at about the same time in which they independently derived a model that applied to Munch pressure flow for an arbitrary distribution of solute sources and sinks both in the steady state and in the time-dependent state; a number of sample calculations were published for sugar beet. Shortly thereafter, Tyree and Dainty (1975) and Tyree, Christy and Ferrier (1974) derived yet another steady state model that produced numerical results in substantial agreement with Christy and Ferrier (1973). This model had the advantage of being computationally simpler, and it was applied to obtain steady state solutions of translocation over long distances, e.g. 50 meters. Anderson (1974) appears to have independently arrived at a steady state model using the formalism of standing gradient osmotic flow which was applied first to a kind of Munch pressure flow system in animals (Diamond and Bossert, 1967).
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Plant water relations of nine woody species were studied in a lower montane rain forest in Panama. These data provide a partial test of the hypothesis that hydraulic architecture of lower montane species might limit transpiration and thus... more
Plant water relations of nine woody species were studied in a lower montane rain forest in Panama. These data provide a partial test of the hypothesis that hydraulic architecture of lower montane species might limit transpiration and thus leaf size or nutrient transport (as suggested by J. Cavelier and E. G. Leigh, respectively). Diurnal variation in leaf transpiration was closely correlated with changes in net radiation. Peak transpiration rates (7 10–5 kg s–1 m–2) were as high as peak transpiration rates from tropical lowland forests but mean daily water use [0.39 0.08 (SEM) kg m–2 day–1] were mostly lower than comparable data from tropical lowland forests. Thus transpiration rates are sufficiently high for sufficiently long periods to make it unlikely that nutrient transport is limited by transpiration. Another objective of this study was a comparison of two different methods to measure hydraulic conductance (Kh = flow rate per unit pressure gradient) and leaf specific conductance of stem segments (KL = Kh/leaf area distal to the segment). The results obtained with the traditional conductivity apparatus and the high pressure flow meter method, yielded similar results in six out of seven cases.
Research Interests: Environmental Science, High Pressure, Plant Biology, Rain forest, Hydraulic conductivity, and 13 moreTrees, Transpiration, Water Use, Forestry Sciences, Diurnal Variation, Montane Forest, Hydraulic Architecture, Plant Water Relations, Flow Rate, Leaf Area, Leaf Size, Pressure Gradient, and transpiration rate
An experimental introduction of pear thrips (Taeniothrips inconsequens Uzel), a major defoliator in sugar maple (Acer saccharum Marsh.) forests in northeastern North America, was conducted in a field plantation to determine if... more
An experimental introduction of pear thrips (Taeniothrips inconsequens Uzel), a major defoliator in sugar maple (Acer saccharum Marsh.) forests in northeastern North America, was conducted in a field plantation to determine if compensatory gas exchange occurs in response to feeding damage by this piercing-sucking insect. Sugar maple trees were enclosed in netting (167 micro m mesh) and pear thrips adults were introduced before leaf expansion in the spring. Pear thrips reduced whole-tree leaf area by approximately 23% and reduced leaf size (both mass and area) by 20% in the upper crown. Measurements of net CO(2) assimilation rate (A(net)) and stomatal conductance (g(s)) were made on tagged foliage that was later analyzed for stable carbon isotope composition (delta(13)C) to provide estimates of short- and long-term leaf water use efficiency (WUE). Pear thrips feeding reduced A(net) for fully expanded leaves by approximately 20%, although leaf chlorophyll content and leaf mass per unit area were apparently not affected. Comparison of A(net), g(s), instantaneous WUE and leaf delta(13)C between damaged and control trees as well as visibly undamaged versus moderately damaged foliage on pear thrips-infested trees indicated that there were no effects of pear thrips feeding damage on WUE or leaf delta(13)C. Long-term WUE among sugar maple trees in the field plantation, indicated by leaf delta(13)C analysis, was related to shorter-term estimates of leaf gas exchange behavior such as g(s) and calculated leaf intercellular CO(2) concentration (C(i)). We conclude that pear thrips feeding has no effect on leaf WUE, but at the defoliation levels in our experiment, it may reduce leaf A(net), as a result of direct tissue damage or through reduced g(s). Therefore, even small reductions in leaf A(net) by pear thrips feeding damage may have an important effect on the seasonal carbon balance of sugar maple when integrated over the entire growing season.
Research Interests: Photosynthesis, Plant Biology, Biology, Ecology, Medicine, and 7 moreTree Physiology, Sugar, Tree, Forestry Sciences, Thrips, Maple, and Pear
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SUMMARY The rate of rehydration of sunflower (Helianthus annuus L.) leaves was measured after dehydration (in air or in a pressure bomb) down to a relative water content of about 89 %. The results, when plotted as the log of the rate of... more
SUMMARY The rate of rehydration of sunflower (Helianthus annuus L.) leaves was measured after dehydration (in air or in a pressure bomb) down to a relative water content of about 89 %. The results, when plotted as the log of the rate of rehydration (mg min-1) against time, displayed an unusual shoulder in which a phase with a long half time was succeeded by a phase with a short half time. The rehydration kinetics can be explained by two cellular compartments in series in which the bulk elastic modulus increases substantially during rehydration. The first compartment might be associated with the phloem and bundle sheath cells. Water rehydrating the second compartment, consisting of the remainder of the leaf cells, must pass through the first compartment by a transcellular pathway.
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Abstract A model was developed to describe stomatal conductance as a function of photosynthetically active radiation, leaf water potential, leaf temperature, vapor pressure difference (leaf interior minus leaf surface) inAcer... more
Abstract A model was developed to describe stomatal conductance as a function of photosynthetically active radiation, leaf water potential, leaf temperature, vapor pressure difference (leaf interior minus leaf surface) inAcer saccharumMarsh. The model used a multiplicative approach and used a new method for determining the functional dependence of stomatal conductance on individual variables. The functions: (1) were expressed using a linear spline technique; (2) were constrained by very general mechanisms of stomatal action rather than specific analytical equations; and (3) were directly determined from data measured in the field. The spline functions were fit by least square estimation applied to data under uncontrolled field conditions. This is in contrast to the normal method of measuring functions from experiments conducted under controlled conditions. The data used for spline fitting were obtained in the field from more than 40 detached branches on about 30 days during various weather conditions throughout two summers (1990 and 1992). Our model predicted nearly 80% of the observed conditions throughout two summers (1990 and 1992). Our model predicted nearly 80% of the observed variation in leaf stomatal conductance and 90% of the variation in transpiration for leaves on mature trees and on excised branches when leaf water potential was >−2 MPa.
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... MT TYREE Botany School, University of Cambridge, England AND HT HAMMEL Physiological Research Laboratory, Scripps Instittdion of Oceanography, La Jolia, California 92037 Received 18 June 1971 ... Page 5. Tyree and HammelThe... more
... MT TYREE Botany School, University of Cambridge, England AND HT HAMMEL Physiological Research Laboratory, Scripps Instittdion of Oceanography, La Jolia, California 92037 Received 18 June 1971 ... Page 5. Tyree and HammelThe Pressure-bomb Technique. 271 ...
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A new technique is described for measuring oscillatory and diurnal changes in leaf thickness. Mature leaves of Populusdeltoides × nigra undergo diurnal or oscillatory changes in thickness of up to 20 μm in amplitude; thickness changes... more
A new technique is described for measuring oscillatory and diurnal changes in leaf thickness. Mature leaves of Populusdeltoides × nigra undergo diurnal or oscillatory changes in thickness of up to 20 μm in amplitude; thickness changes were measured to ±0.2 μm and appear to correlate with expected levels of leaf water stress.
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A reanalysis of the published thermodynamic transport coefficients has led to the conclusion that the contribution of Nitella cell walls to the electroosmotic efficiency of the wall – membrane complex in living Nitella cells has been... more
A reanalysis of the published thermodynamic transport coefficients has led to the conclusion that the contribution of Nitella cell walls to the electroosmotic efficiency of the wall – membrane complex in living Nitella cells has been incorrectly estimated. When the e-o efficiency observed in the living Nitella cell is less than 100 moles Faraday−1 the observed value is an overestimate if attributed to the membrane alone; when the e-o efficiency observed is over 100 moles Faraday−1 the value is an underestimate of the e-o in the membrane. A more detailed analysis of the salt concentration dependence of LPE (the electrokinetic coefficient) and of LPE2/LEELPP (the relative thermodynamic efficiency of electroosmosis) is given. It is found that LPE2/LEELPP reaches a peak efficiency (about 0.6) at about the saline concentration of pond water and xylem sap. It is concluded that the suggestion of Briggs that the electroosmosis observed in living Nitella cells may be a property of the cell w...
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We present a theoretical analysis of the events that occur while a plant enclosed in a pressure bomb evolves from one equilibrium balancing pressure to another. The initial rate of efflux from any one cell in response to a pressure... more
We present a theoretical analysis of the events that occur while a plant enclosed in a pressure bomb evolves from one equilibrium balancing pressure to another. The initial rate of efflux from any one cell in response to a pressure increment of ΔP equals (ALp)i ΔP, where (ALp)i is the surface area times hydraulic conductivity of the cell's semipermeable membrane(s). If the volume changes and pressure increment are small, the cell will approach equilibrium exponentially. The half time of the exponential process is governed by (ALp)iki, where ki is the cell constant (= the combined rate of change of osmotic and turgor pressure with the volume expressed from the cell). Experimental studies of the kinetics of water exchange between the symplast and apoplast of hemock (Tsuga canadensis) shoots in a pressure bomb reveal that the cells collectively behave as though they fall into three distinct populations which approach equilibrium with different half times.
Research Interests: Botany, Chemistry, Kinetics, Plant Biology, Biology, and 4 moreOsmotic pressure, Activation Energy, Tsuga, and Apoplast
Three techniques have been proposed for measuring hydrokinetic pressure gradients in the xylem of plants applying the Onsager equations of irreversible thermodynamics. In the first the same electrical gradient is applied in alternate... more
Three techniques have been proposed for measuring hydrokinetic pressure gradients in the xylem of plants applying the Onsager equations of irreversible thermodynamics. In the first the same electrical gradient is applied in alternate directions in a tree and the resulting current difference ΔI is measured. ΔI is directly proportional to the hydrokinetic pressure gradient, and by this method has been found in Eucalyptus to be about 0.13 atm m−1. Measurements of streaming potential gradients are also a linear function of the hydrokinetic pressure gradient and this has been calculated in maple to be between 0.6 to 1.2 atm m−1. A direct measurement of transpiration flow by tracer methods has also been considered.
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The temperature dependence of the balance pressure is reported for shoots of Tsuga canadensis at constant volume, i.e., when water is neither added to nor removed from the shoot. Since the balance pressure closely equals minus the water... more
The temperature dependence of the balance pressure is reported for shoots of Tsuga canadensis at constant volume, i.e., when water is neither added to nor removed from the shoot. Since the balance pressure closely equals minus the water potential, the temperature dependence of the balance pressure should reflect the combined temperature dependence of the osmotic and turgor pressures. Both the osmotic and the turgor pressures decline with decreasing temperature; frequently the turgor pressure declines 2 to 3 times more rapidly than the osmotic pressure, causing the balance pressure to rise with decreasing temperature. Only when the turgor pressure is zero (only beyond incipient plasmolysis) does the temperature dependence of the balance pressure closely follow the temperature dependence of the osmotic pressure; this occurs when the balance pressure equals or exceeds 24 bars.
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Transcellular osmosis measurements are combined with measurements of the kinetics of relative length change, when Chara corallina cells are transferred from artificial pond water to polyethylene glycol (mol wt 300 to 400) of higher... more
Transcellular osmosis measurements are combined with measurements of the kinetics of relative length change, when Chara corallina cells are transferred from artificial pond water to polyethylene glycol (mol wt 300 to 400) of higher osmotic pressure to yield the constant k, which relates relative volume change ΔV/V to relative length change Δl/l (ΔV/V = k Δl/l). The value of k is 3.5 and is temperature independent between 7 and 30 °C. Static and kinetic estimates of the bulk modulus, ε, indicate that ε is temperature independent and has a value between 500 to 650 bars at high turgor pressures. The value of ε declines as the turgor pressure declines. We show how ε and k are related to Young's modulus and to Poisson's ratio for anisotropic C. corallina cell walls and point out that the previous treatments of the problem are in error.
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Using the iterative model for the Münch pressure-flow mechanism derived by Christy and Ferrier (1973), the theoretical time-dependent behavior of the Münch pressure-flow system in a 15-m tree is computed in response to sinusoidal time... more
Using the iterative model for the Münch pressure-flow mechanism derived by Christy and Ferrier (1973), the theoretical time-dependent behavior of the Münch pressure-flow system in a 15-m tree is computed in response to sinusoidal time variation in sucrose loading and water potential. Our calculations predict a "concentration wave" moving down the transport path of the type reported by Huber et al. (1937) in Quercus borealis L. The "waves" move with a phase velocity (= 3.5 cm min−1) greatly in excess of the velocity of the sieve-tube solution (= 0.4 cm min−1). It seems very likely that Huber et al. (1937) measured a phase velocity and erroneously interpreted it as the true solution velocity.
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The pressure-bomb technique has been used to measure the kinetics of water exchange while a plant enclosed in a pressure bomb evolves from one equilibrium balance pressure to another. In earlier studies two observations were made. (1) The... more
The pressure-bomb technique has been used to measure the kinetics of water exchange while a plant enclosed in a pressure bomb evolves from one equilibrium balance pressure to another. In earlier studies two observations were made. (1) The kinetics of water exchange appeared to be described by an exponential process in which three populations of cells exchange water with apoplast independently of each other. (2) The temperature dependence of the tempo of water exchange yielded an activation energy of 25.9 ± 0.6 × 103 J/mol, which is higher than the activation energy for laminar flow of water in pipes (= 17 × 103 J/mol). These results have been repeated and a more careful analysis has been conducted involving infiltration of air spaces in leaves with water and the selective removal of the leaves. It now appears that the xylem network up to (but not including) the leaves contributes about two thirds of the resistance to water flow in whole shoots 15 to 40 g in fresh weight. Presumably ...
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The pressure bomb is being used to a much greater extent to measure some tissue – water relations parameters such as osmotic pressure, turgor pressure, and cell wall elasticity. Recently, Richards has developed a faster pressure-bomb... more
The pressure bomb is being used to a much greater extent to measure some tissue – water relations parameters such as osmotic pressure, turgor pressure, and cell wall elasticity. Recently, Richards has developed a faster pressure-bomb method of obtaining these and other parameters than the method used by Hammel and modified by us. In this paper, we compare the two methods and conclude that Richards’ method should not be used when accuracy is deemed important. The Richards method usually overestimates osmotic pressure by 0.2 MPa (= 2 bars) and sometimes by 0.8 MPa (= 8 bars).
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Leaf specific conductivities (LSC's) were measured on stem segments excised from various points within the canopy of eastern white cedar trees, Thuja occidentalis L. LSC is defined as the water flow rate (kilograms per second) through... more
Leaf specific conductivities (LSC's) were measured on stem segments excised from various points within the canopy of eastern white cedar trees, Thuja occidentalis L. LSC is defined as the water flow rate (kilograms per second) through a stem caused by a unit of pressure potential gradient (megapascals per metre) per unit leaf surface area supplied by the stem (square metres). LSC's were measured on stems of various diameters and were found to vary over a factor of 30 in magnitude from 1 × 10−5 kg s−1 m−1 MPa−1 for stems 1 mm in diameter to 3 × 10−4 kg s−1 m−1 MPa−1 for stems 100 mm in diameter. LSC was found to be related to stem diameter (D (millimetres)) by the following empirical formula: LSC = 9.58 × 10−6 × D0.727. LSC's measured on stem segments including a node had significantly lower LSC's than internodal stem segments of the same length. Various water relations parameters were measured on cedar trees on a diurnal basis including evaporative flux, leaf resista...
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Pressure–volume curves were constructed from well defined models and hypothetical shoots in which reasonable values of osmotic pressure and cell wall elastic moduli were specified for cell types of different relative volumes. The... more
Pressure–volume curves were constructed from well defined models and hypothetical shoots in which reasonable values of osmotic pressure and cell wall elastic moduli were specified for cell types of different relative volumes. The pressure–volume curves so obtained closely resembled those of real shoots and leaves. Comparing the bulk parameters obtained from analysis of the constructed pressure–volume curves with the values defined in the models allows us to examine the sources of error in their evaluation. The graphical values of the original bulk osmotic pressure and of the total volume of osmotic (symplasmic) water agreed very well with those defined; however, the osmotic pressure at incipient plasmolysis and the bulk elastic moduli estimated from the graph were generally lower than their actual values originally used in the models. We show that the apparent linear dependence of the bulk elastic modulus of sitka spruce reported by Hellkvist et al. (1974) may not reflect a similar ...
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We present data on a comparison of plasmodesmata in the nodal complexes of Chara corallina prepared for electron microscopy by chemical fixation of specimens at room temperature and those that have been freeze-substituted. The... more
We present data on a comparison of plasmodesmata in the nodal complexes of Chara corallina prepared for electron microscopy by chemical fixation of specimens at room temperature and those that have been freeze-substituted. The freeze-substitution technique was applied to intact nodal complexes with the internodal cells on either side intact and showed that most plasmodesmata in the cell walls joining a nodal cell with an internodal cell were free of any occluding substance. We feel that the nonoccluded plasmodesma is the in vivo condition and that previous results using chemical fixation at room temperature where occlusions were reported in Nitella translucens and Chara corallina are artifacts of preparation.
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Several papers purporting to demonstrate negative turgor pressure in plant cells under extreme water stress are reviewed. A reanalysis of the published data suggests the reports are fallacious.
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Translocation of uranin proceeds in the protoplasm of Tradescantia staminal hairs both in the acropetal and basipetal direction from a wound. A diffusion pattern is observed proportional to the square root of time. It is slower than... more
Translocation of uranin proceeds in the protoplasm of Tradescantia staminal hairs both in the acropetal and basipetal direction from a wound. A diffusion pattern is observed proportional to the square root of time. It is slower than physical diffusion in water. At any given time interval the number of cells traversed is almost equal whether transport is from the base or the tip. Consequently the distance from the base, where the cells are longer, is also longer. Translocation fits the calculation on a model in which the plasmodesmata in the interconnecting walls are almost occluded with solid obstructions or in which the plasmodesmata are under the influence of a more or less uniform negative charge. Resistance in the plasmodesmata seems to be the limiting factor in this case of symplasmic transport.
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The Scholander–Hammel pressure bomb has been used to measure ontogenetic and seasonal changes in π0 (the osmotic pressure of the symplasm at zero water potential), πp (the osmotic pressure of the symplasm at ‘incipient plasmolysis’), εmax... more
The Scholander–Hammel pressure bomb has been used to measure ontogenetic and seasonal changes in π0 (the osmotic pressure of the symplasm at zero water potential), πp (the osmotic pressure of the symplasm at ‘incipient plasmolysis’), εmax (the bulk elastic modulus near maximum turgor), and a number of other water relations parameters in single leaves of Acer saccharum and several species of Populus and in shoots of Tsuga canadensis and Picea abies. In newly emerged leaves of Acer, Populus, and Picea, π0, πp, and εmax are all small but rise rapidly with leaf development. These parameters stabilize at a maximum value or slowly increase with progress in season. In Acer, εmax declines shortly before senescence. In developing leaves, the water content reaches a maximum before the soluble solutes; this accounts for the low values of π0 and πp.In Tsuga π0 cycles through an annual maximum in winter and a minimum in summer. These changes may correlate with frost hardiness.
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A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the... more
A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the capillary equation. The model predicted the tempo of recovery from embolism in small diameter branches of woody plants with various xylem structures under different xylem water pressures. The model predicted the time required to recover conductivity in any position in the stem. Repeated iterative solution of the model for different situations yielded an empirical formula to calculate the time for complete recovery of conductivity in stems from a fully embolised initial state. The time, tp, is given by: where α is a temperature coefficient; D is the coefficient of diffusion of air in wood at 25°C; rcs is the ratio of the area of total conduit cross section to the stem cross section; Ψxp is the stem xylem pressure potential (Pa, where 0 Pa equals atmospheric pressure); τ is solution surface tension (0.072 N m−1); and Dc and Ds are diameters of the conduits and the stem, respectively (m). The equation is valid only when Ψxp > –4τ/Dc. The model predicts no recovery of conductivity when Ψxp≤–4τ/Dc. The model agreed with experiments.
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Research Interests: Chemistry, Medicine, Biological Sciences, Plant Physiology, Xylem, and 3 morePhloem, Xenobiotic, and Apoplast
Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K leaf) to irradiance. The effects of light and... more
Molecular and physiological studies in walnut (Juglans regia) are combined to establish the putative role of leaf plasma membrane aquaporins in the response of leaf hydraulic conductance (K leaf) to irradiance. The effects of light and temperature on K leaf are described. Under dark conditions, K leaf was low, but increased by 400% upon exposure to light. In contrast to dark conditions, K leaf values of light-exposed leaves responded to temperature and 0.1 mm cycloheximide treatments. Furthermore, K leaf was not related to stomatal aperture. Data of real-time reverse transcription-polymerase chain reaction showed that K leaf dynamics were tightly correlated with the transcript abundance of two walnut aquaporins (JrPIP2,1 and JrPIP2,2). Low K leaf in the dark was associated with down-regulation, whereas high K leaf in the light was associated with up-regulation of JrPIP2. Light responses of K leaf and aquaporin transcripts were reversible and inhibited by cycloheximide, indi...