Xylem Cavitation in Nodes and Internodes of Whole Chorisia insignis H.B. et K. Plants Subjected to Water Stress: Relations Between Xylem Conduit Size and Cavitation

Measurements of cavitation occurring in xylem conduits of differentstem parts in whole Chorisia insignis H.B. et. K. plants subjectedto water stress are reported. Pre-stressed plants were shownto undergo cavitation over 10 times greater than watered ones.The most vulnerable parts of plants were one-year-old twigswhere cavitation reached a peak of over 50 acoustic emissions(AE) min^–1 while in two-year-old twigs AE min^–1were about one half this value. Stem zones were found wherecavitation was typically very low even during water stress:these were one-year-old nodes and junctions where branches meet.Measurements of the inside diameters of xylem conduits and distributionof conduit ends in stem parts where AE were detected, showedthat nodes have a significantly larger percentage of narrowxylem conduits than internodes. Similar ‘constricted zones’were found injunctions with respect to two-year-old twigs. Hereabout 50 per cent of the xylem conduits were as narrow as 20to 50 µm in diameter. The distribution of xylem conduitends show about 3 per cent of them ending in the nodes and 1per cent in the internodes of one-year-old twigs. About 11.6per cent of xylem conduits end in the junctions and about ahalf in two-year-old internodes. Our data would give furtherexperimental evidence to the functional concept of ‘plantsegmentation’ into zones (internodes) more efficient inwater conduction, i.e. with wider xylem conduits but more vulnerableto cavitation and others (nodes and junctions) with oppositecharacteristics. Chorisia insignis, acoustic emissions, water stress, nodes, internodes, xylem conduit size, vessel ends     

SEASONAL OCCURRENCE OF XYLEM EMBOLISM IN SUGAR MAPLE (ACER SACCHARUM)

Xylem embolism, the reduction of water flow by air-filled vessels, was measured in a stand of 5- to 8-year-old sugar maple (Acer saccharum Marsh.) saplings growing in a nursery bed in northwestern Vermont. Embolism was quantified as percentage loss in hydraulic conductivity of trunk and branch segments relative to maximum values obtained by removing air from vessels by repeated high pressure (173 kPa) perfusions. Ten segments per tree were cut from 6 trees for each of 11 measurement periods spaced at roughly monthly intervals from May 1986 to June 1987. During the 1986 growing season, embolism increased significantly from 11 to 31% in the larger branches and trunk (segment diameter #8805;0.5 cm), but remained at about 10% in twigs (segment diameter <0.5 cm). This was unexpected because the greatest water stess and thus potential for embolism occurs in twigs. During the winter, embolism increased throughout the trees and the trend with diameter was reversed; by February, small twigs were 84% embolized vs. 69% for larger branches and trunk. Dye perfusions showed that winter embolism in trunks was localized on the south side; this may have resulted from water loss by sublimation or evaporation in the absence of water uptake. Beginning in late March, embolism decreased throughout the trees to approximately 20% in June. This decrease was associated with positive xylem pressure of at least 16 kPa which may have originated in the roots, because weather conditions at the time were unfavorable for the generation of stem pressures characteristic of Acer species in early spring.     

The Role of Osmotic Potential in Spring Sap Flow of Mature Sugar Maple Trees (Acer saccharum Marsh)

Field measurements of xylem sap osmotic and pressure potentialwere made on sugar maple trecs (Acer saccharum Marsh) duringthe winter and spring of 3 years to determine whether the hydrostaticpressure was osmotically generated. Sap osmotic potential waslow enough to account for the hydrostatic pressure but the dynamicsof its diurnal behaviour indicated that osmotic potential wasnot directly responsible for hydrostatic pressure. The diurnalcourse of hydrostatic pressure showed definite peaks but osmoticpotential often did not. The magnitude of the diurnal changesin hydrostatic pressure was approximately 0·15 MPa whereasthe changes in osmotic potential were only 0·05 MPa.Because the sap osmoticum is primarily sucrose, and starch isstored in the xylem throughout the tree, the temperature dependenceof the sucrose-starch interconversion system was investigated.More active amylase was formed in maple twigs after incubationat 0°C and 4°C than at –3, 6 or 15°C. Therate of starch hydrolysis by maple amylase increased with temperature,reaching a maximum at approximately 45°C. There was somestarch hydrolysis at –3°C. The starch hydrolysis systemthus indicated no critical role for temperature fluctuationsabout 0°C. Starch was found to be densely stored in therays of the trunk and twigs and around the central pith in thetwigs. Key words: Acer succhmum Marsh, Osmotic potential, Xylem sap pressure     

Improvement to the air-injection technique to estimate xylem vulnerability to cavitation

Several techniques have been developed to quantify the degree of embolism of the xylem using hydraulic conductance. Although there have been several improvements to these techniques, their reliability is still questionable and many technical pitfalls persist. We are proposing here a manometric approach to improve the accuracy of xylem cavitation measurement by the original air-injection technique which uses twigs exposed to pressurized air to cause cavitation. The measured parameter is air bubble production (P _b) caused by xylem cavitation in birch (Betula pendula Roth) twigs from which the percent increase in bubble production is calculated to quantify xylem cavitation. Data produced by three different methods (bench-drying, air-injection, and manometric approach) are compared. Xylem vulnerability curves (VCs) constructed by the reference and reliable bench-drying technique and the manometric approach show similar sigmoid “S” shape, but a small anomaly appeared in the VC constructed by the original air-injection technique. The xylem pressure inducing 50% of embolism (P ₅₀) was the same with the three techniques. Furthermore, there was a strong positive correlation between the estimators of xylem cavitation measured by the three different methods. For its reliability, precision and ease we recommend the manometric technique as an improved version of the original hydraulic air-injection method.     

Three Different Methods for Measuring Xylem Cavitation and Embolism: A Comparison

Three different methods for measuring xylem embolism due towater cavitation were compared—the acoustic method, thehydraulic method and the anatomical method. Young plants ofCeratonia siliqua L. were water stressed for 9, 16 and 23 d. Xylem cavitation was detected by counting ultrasound (100–300kHz) acoustic emissions (AE) from 1-year-old twigs (acousticmethod). Xylem embolism was detected by measuring the loss ofhydraulic conductivity of twigs of the same age (hydraulic method).The blockage of single xylem conduits was detected by perfusingSafranin into the xylem of 1-year-old twigs of stressed plantsand measuring the number and the diameters of non-conductingxylem conduits, under the microscope (anatomical method). It was noted that: (a) the number of AE and the loss of conductivityincreased with the water stress applied; (b) a linear relationseemed to exist between the number of AE and the loss of conductivity,suggesting that the AE counted could be only (or mainly) producedin the xylem conduits; (c) the vulnerability of the xylem conduitsto embolism was a direct function of their diameter; and (d)the measured loss of conductivity was of the same order of magnitudeas the theoretical one. The three methods gave fairly similar results. Nonetheless,they are not alternative to one another in that: (a) the acousticmethod allows continuous recordings to be made but does notprovide information about the actual damage suffered by plants;(b) the hydraulic method is very informative but destructive;and (c) the anatomical method is very useful both in phytogcographicaland in genetic improvement studies. Ceratonia siliqua L., Carob tree, water stress, xylem embolism, acoustic method, hydraulic method, anatomical method     

Three Different Methods for Measuring Xylem Cavitation and Embolism: A Comparison

Three different methods for measuring xylem embolism due towater cavitation were compared—the acoustic method, thehydraulic method and the anatomical method. Young plants ofCeratonia siliqua L. were water stressed for 9, 16 and 23 d. Xylem cavitation was detected by counting ultrasound (100–300kHz) acoustic emissions (AE) from 1-year-old twigs (acousticmethod). Xylem embolism was detected by measuring the loss ofhydraulic conductivity of twigs of the same age (hydraulic method).The blockage of single xylem conduits was detected by perfusingSafranin into the xylem of 1-year-old twigs of stressed plantsand measuring the number and the diameters of non-conductingxylem conduits, under the microscope (anatomical method). It was noted that: (a) the number of AE and the loss of conductivityincreased with the water stress applied; (b) a linear relationseemed to exist between the number of AE and the loss of conductivity,suggesting that the AE counted could be only (or mainly) producedin the xylem conduits; (c) the vulnerability of the xylem conduitsto embolism was a direct function of their diameter; and (d)the measured loss of conductivity was of the same order of magnitudeas the theoretical one. The three methods gave fairly similar results. Nonetheless,they are not alternative to one another in that: (a) the acousticmethod allows continuous recordings to be made but does notprovide information about the actual damage suffered by plants;(b) the hydraulic method is very informative but destructive;and (c) the anatomical method is very useful both in phytogeographicaland in genetic improvement studies. Ceratonia siliqua L, Carob tree, water stress, xylem embolism, acoustic method, hydraulic method, anatomical method     

Studies of Frost Hardiness in Woody Plants. II. Effect of Temperature on Hardening

The effect of temperature on hardening was studied at temperatures ranging from 0° to −20° using twigs of willow and poplar. In October and in late April when the twigs are not very frost hardy, hardening at 0° produced a considerable increase in their frost hardiness, although the effectiveness of hardening at 0° decreased with a decrease in the environmental temperature. In twigs which could withstand continuous freezing without injury, hardening at −3° to −5° was most effective in increasing the frost hardiness of the twigs. Below −20°, only negligible increase was observed either in frost hardiness or sugar content.

The rate of starch to sugar conversion differed remarkably in different twig tissues. The starch in xylem was more slowly converted to sugar than that in the cortex. The optimum temperature for converting starch into sugar during frost hardening was also found to be −3° to −5°. In addition, the greater the effectiveness of the hardening treatment, the greater the rate of conversion from starch to sugar. The frost hardiness of a twig is closely related to the sugar content of the twig, especially in the xylem.

     

The mechanism of freezing injury in xylem of winter apple twigs

In acclimated winter twigs of Haralson apple (Pyrus Malus L.), a lag in temperature during cooling at a constant rate was observed at about −41 C by differential thermal analysis. The temperature at which this low temperature exotherm occurred was essentially unaffected by the cooling rate. During thawing there was no lag in temperature (endotherm) near the temperature at which the low temperature exotherm occurred, but upon subsequent refreezing the exotherm reappeared at a somewhat higher temperature when twigs were rewarmed to at least −5 C before refreezing. These observations indicate that a small fraction of water may remain unfrozen to as low as −42 C after freezing of the bulk water in stems. The low temperature exotherm was not present in twigs freeze-dried to a water content below 8.5% (per unit fresh weight), but it reappeared when twigs were rehydrated to 20% water. When freeze-dried twigs were ground to a fine powder prior to rehydration, no exotherm was observed. Previous work has shown that the low temperature exotherm arises from xylem and pith tissues, and that injury to living cells in these tissues invariably occurs only when twigs are cooled below, but not above the temperature of the low temperature exotherm. This study revealed that the low temperature exotherm resulted from the freezing of a water fraction, that the freezing of this water was independent of the freezing of the bulk water, that the exotherm was associated with some gross structural feature but not the viability of the tissue, and that injury to living cells in the xylem and pith was closely and perhaps causally related to the initial freezing of this water.     

Root Xylem Influence on the Water Relations and Drought Resistance of Rice

In order to determine the importance of root axial resistanceto water flow for drought resistance of rice (Oryza sativa L.)aseries of glasshouse and growth chamber studies was conductedfrom 1985 to 1986. A preliminary study surveyed root distributionand histological characteristics of six cultivars grown in aerobicsoil (20x20x90cm boxes) under well–watered ormoisturedeficit conditions. Subsequently, four experiments were conductedwith plants grown in culture solution. Our results demonstratethat plant breeders can use root thickness as a selection indexfor xylem size for root diameters up to about 1–2 mm.Usingthe Poiseuille–Hagen Law for water movement in capillaries,rice root axial resistance explained differences in leaf waterpotential and transpiration when only one cultivar was used,but did not explain differences among cultivars. Thus, increasingroot xylem vessel radii probably will not directly increasedrought resistance. Key words: Rice (Oryza sativa), roots, xylem characteristics, drought resistance     

Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.     

Uptake of water and solutes through twigs of Picea abies (L.) Karst

Summary Uptake of water and magnesium chloride solution was investigated through the outer surface of twigs of Picea abies (L.) Karst. Water uptake was determined by using pressure/volume (P/V) curves of the twigs as a basis for calculation to avoid problems of superficial extraneous water. When water was sprayed on bark and needles of 3- to 7-year-old twigs at a xylem water potential of -1.00 MPa, they absorbed as much as 80 mm³ water in 200 min/g twig dry weight as the twig water potential recovered to -0.15 MPa. With fluorescent dyes, pathways for absorption of water and solutes through the twig bark were found, particularly through the radially orientated ray tissue. In addition to uptake by mass flow, magnesium could also diffuse along a concentration gradient from the twig surface into the xylem. In the field, the magnitude of these uptake processes would depend on the concentration of elements deposited by atmospheric precipitation, the concentration gradient between the plant surface and the xylem sap, the xylem water potential and the intensity and duration of each precipitation event.     

Modelling non-uniform soil water uptake by a single plant root

The assumption of uniform water flow to the root or uniform water potential at the root surface was shown by Hainsworth and Aylmore (1986, 1989) to be erroneous. The present paper demonstrates how the non-uniform uptake of water by a single root can be modeled. Differential equations are numerically solved to describe simultaneous water movement in the plant and in the soil. In the plant, boundary conditions are the water potentials at the root surface (Ψ_s) and in the xylem at the root base (Ψ_b). A set of difference equations describe the flow of water radially through the cortex to the xylem and in the xylem axially upwards to the base. For calculating the water flow in the soil and the values of Ψ_s, i.e. the boundary conditions for flow in the root, the finite element method (FEM) is used, the boundary conditions being the flux of water into the plant root and the zero flow across the wall, bottom and surface of a hypothetical soil cylinder surrounding the root. ei]Section editor: B E Clothier     

Hydraulic Parameters Measured in 1-Year-Old Twigs of some Mediterranean Species with Diffuse-Porous Wood: Changes in Hydraulic Conductivity and Their Possible Functional Significance

We report measurements of hydraulic conductivity of Vitis oinifera,Oleo europaea and Populus deltoides 1-year-old twigs. Singleserial internodes were tested for the volume flow rate whichwas related to: (a) the xylem tissue cross-sectional area, (b)the vessel lumina cross-sectional area and (c) the leaf surfacearea supplied by a given stem section. From this, whole xylemhydraulic conductivity (L_x), vessel lumina hydraulic conductivity(L_xv) and leaf specific conductivity (LSC) were calculated.All the three parameters turned out to be linearly related toeach other. This is kcause: (a) the leaf surface area (A₁) waslinearly related to xylem cross-sectional area (A_x and (b) theratio of the vessel lumina cross-sectional area (A_xv) to xylemcross-sectional area (A_x) was approximately constant along thetwigs. Moreover, the hydraulic conductivity of twig segmentswhere buds grow most actively (distal internodes in V. viniferaand proximal ones in O. europaea) was much lower than in therest of the twigs. A possible role played by these ‘constricted’twig regions is discussed. Key words: Changes, Hydraulic conductivity, Stem     

The Import and Redistribution of Several Cations and Anions in Tomato Leaves

Wolterbeek, H. Th. and De Bruin, M. 1986. The import and redistributionof several cations and anions in tomato leaves.—J. exp.Bot. 37: 331–340. The upward movements in the xylem and redistribution from theleaf of Na⁺ , K⁺ , Rb⁺, Cs⁺ and four anions were examined insub-systems of tomato plants (Lycopersicon esculentum, Mill.cv. Tiny Tim). There was a delay with respect to the redistributionof newly imported elements from the source leaf of about 16–20h for all four alkali ions. This is considerably less than theapparent delay for the anions Sb(SO₄)^– WO₄^2– Mo₇O₂₄^6–and AsO₄^3– The prolonged delay for the anions is suggestedto be a consequence of metabolic transformation in the leaf.Reduction of the source-sink activity ratio did not decreasethe delay period from the source leaf, but apparently causedincreased Na⁺ transfer from the xylem. It is concluded thatthe application of a detailed mathematical descnption of upwardelement movement has considerable potential possibilities forunderstanding circulation of nutrients in the plant. Key words: Alkali ions, anions, xylem, phloem, redistribution, tomato     

Adaptive Growth Responses to Osmotic Stress of Hypocotyl Sections of Vigna unguiculata: Roles of the Xylem Proton Pump and IAA

The growth responses to osmotic stress of hypocotyl sectionsof Vigna unguiculata were studied by the xylem perfusion method.Hypocotyl sections shrank upon exposure to osmotic stress. Sectionsshowed no adaptive responses to osmotic stress when they werein an IAA-depleted condition as a result of perfusion with solutionsthat lacked IAA for 3–4 h. The correlation between thegrowth rate and the membrane potential of the xylem/symplastboundary (V^px) was very limited in the absence of IAA. By contrast,hypocotyl sections showed distinct adaptive responses to osmoticstress after perfusion with solutions that contained 10 µMIAA. In the presence of IAA, V^px increased in the negative directionand growth resumed in spite of the osmotic stress. The growthrate was closely correlated with the xylem membrane potential.Hyper-polarization of the membranes of the xylem/symplast boundaryalways preceded the recovery of growth under osmotic stress.It appears that IAA is essential for the adaptive recovery ofgrowth under osmotic stress and, moreover, that the xylem protonpump plays an indispensable role in modulating the growth ofhypocotyl sections. This result confirms prediction of an earliersimulation study using the apoplast canal model [Katou and Furumoto(1986) Protoplasma 133: 174, Katou and Enomoto (1991) PlantCell Physiol. 32: 343]. (Received June 27, 1996; Accepted October 28, 1996)     

Water storage in the wood and xylem cavitation in 1-year-old twigs of Populus deltoides Bartr.

The possible role of water expelled from cavitated xylem conduits in the rehydration of water-stressed leaves has been studied in one-year-old twigs of populus deltoides Bartr. Twigs were dehydrated in air. At desired values of leaf water potential (Ψ_l) (between near full turgor and -1.62 MPa), twigs were placed in black plastic bags for 1–2h. Leaf water content was measured every 3–5 min before bagging and every 10 min in the dark. Hydraulic conductivity and xylem cavitation were measured both in the open and in the dark. Cavitation was monitored as ultrasound acoustic emissions (AE). A critical Ψ_l value of -0.96 MPa was found, at which AE increased significantly while the leaf water deficit decreased by gain of water. Since the twigs were no longer attached to roots, it was concluded that water expelled from cavitated xylem conduits was transported to the leaves, thus contributing to their rehydration. Xylem cavitation is discussed in terms of a ‘leaf water deficit buffer mechanism’, under not very severe water stress conditions.     

The Occurrence of Peroxide in a Perennial Plant, Populus gelrica

A large amount of peroxide was found in twigs of poplar, Populus gelrica, which was grown in the field under natural conditions. The peroxide found in xylem and living bark was about 1.2 and 0.5 μmoles per gram dry weight sample, respectively, and served as a substrate both for catalase and cytochrome c peroxidase.     

Variation in gene transcription and protein for key enzymes of carbohydrate metabolism in poplar xylem with respect to growth phase

We compared gene expression levels for enzymes of carbohydrate metabolism in the twig xylem of two Populus species with the seasonal levels of starch and soluble sugars (sucrose, glucose, and fructose) and relative levels of the enzymes. Plants of Populus deltoides Bartr. ex Marsh and P. balsamifera L., 3–4 years old, were grown outside in Lubbock, TX, USA in 43 L pots. The xylem in the middle portion of the twigs was sampled during the dormant period (November–February), at bud break (for P. balsamifera), and during the growth flush (April–July). The gene expression for ADP-glucose pyrophosphorylase (AGPase), sucrose synthase (SuSy), and sucrose-phosphate synthase (SPS) generally coincided with the levels of the carbohydrates in whose metabolism these enzymes are involved. Gene expression for AGPase and its protein levels were high when the xylem starch content was high (growing period). However, P. balsamifera maintained high AGPase levels in dormant and growing twigs, unlike P. deltoides whose dormant twigs had low AGPase and low gene expression. Compared to growing twigs, gene expression for SuSy and SPS and their protein levels were higher in dormant twigs when soluble sugar content was higher. No down-regulation of these genes appears to occur when pools of the associated carbohydrates are high. Contrary to our expectation, the gene expression for β-amylase was highest in growing twigs when starch content was high. High β-amylase gene expression in growing twigs may be involved in maintaining a sufficient level of soluble sugars for growth through possibly controlling the extent of starch accumulation.     

Ion-mediated enhancement of xylem hydraulic conductivity is not always suppressed by the presence of Ca2+ in the sap

The physiological significance of ion-mediated enhancement of xylem hydraulic conductivity (K(h)) in planta has recently been questioned. The phenomenon has been suggested to be an artefact caused by the use of deionized water as a reference fluid during measurements of the impact of different ions on K(h). In the present study, ion-mediated changes in K(h) were measured in twigs of five woody species during perfusion with 25 mM KCl compared with different reference fluids like deionized water, a commercial mineral water containing different ions (including 0.5 mM Ca(2+)), and a 1 mM CaCl(2) solution. Both fully hydrated twigs and twigs with about 50% loss of hydraulic conductivity due to cavitation-induced embolism were tested. Adding 25 mM KCl to the three reference fluids caused K(h) to increase by about 20%. The KCl-mediated increase of K(h) was even larger (up to 100%) in embolized twigs. The presence of Ca(2+) in the reference solution decreased, but not suppressed, the KCl-mediated enhancement of K(h) in fully hydrated twigs of three species, but not in the other two species tested. Ca(2+) did not affect the K(h) response to KCl in embolized twigs. These data suggest that the recently reported suppression of the 'ionic effect' by the presence of calcium in the xylem sap is not a general phenomenon and that ion-mediated changes of K(h) may play a role in planta partially to compensate for cavitation-induced loss of xylem hydraulic conductivity.     

Rapid increase in deep supercooling of xylem parenchyma

Malus pumila Mill. twigs were collected from September through December and stored at 5°C until the low temperature exotherms of the xylem were determined by differential thermal analysis. During the differential thermal analysis, cooling was interrupted, and temperatures of 5 to −18°C were held for 0.4 to 10 hours before cooling to −50°C was resumed. Control twigs were cooled to −50°C without interruption. Holding the twigs at 1.3 to −5°C shifted the start of the low temperature exotherm from about −20 to −30°C. Slightly higher (2.6°C) and lower (−10°C) temperatures were occasionally effective. The shift began within 20 to 30 minutes and increased progressively to 150 minutes. The acclimation was reversibly inhibited by N₂ atmosphere.