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PubMed1994

Element concentrations in the xylem sap of Picea abies (L.) Karst. seedlings extracted by various methods under different environmental conditions.

We used a Scholander pressure chamber to assess the effects of various extraction methods under different environmental conditions on element concentrations in xylem sap of 3-year-old Picea abies (L.) Karst. seedlings. Sap from excised shoots contained higher element concentrations when extracted at low than at high over-pressures. When comparing plants differing in water status, we found that a high extraction over-pressure introduced a systematic error into the data. For example, in well-watered non-transpiring plants relative to unwatered transpiring plants, potassium concentrations were 70% higher in sap extracted at 0.1 MPa over-pressure, but only 10% higher in sap extracted at 1.0 MPa over-pressure. Moreover, treatment effects depended on the time of day when the sap was extracted. Increased water flux in transpiring plants relative to non-transpiring plants resulted in reduced xylem sap element concentrations when samples were collected after 9 h of transpiration, but not after 4 to 6 h of transpiration. Drought had little effect on xylem sap element concentrations, indicating that rates of element release into xylem conduits, element depletion by growing tissues, and water flow maintained a balance that may prevent nutrient stress during short-term drought.

Journal Article↗
PubMed1997

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).

Journal Article↗
PubMed1999

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.

Journal Article↗
PubMed2002

SAGE profiling and demonstration of differential gene expression along the axial developmental gradient of lignifying xylem in loblolly pine (Pinus taeda).

Wood formation has been studied extensively at the cellular and biochemical levels, but remains poorly understood with respect to gene expression and regulation. As a first step toward identifying genes specifically involved in wood formation and characterizing their roles in determining wood quality, serial analysis of gene expression (SAGE) was used to quantify gene expression in lignifying xylem from a single, 10-year-old loblolly pine (Pinus taeda L.). Two SAGE libraries were generated based on lignifying xylem isolated from either the upper (crown) or lower (base) portions of the trunk. Over 85,000 tags representing a maximum of 27,398 expressed genes were analyzed from the crown wood library, and more than 65,000 tags, representing a maximum of 25,983 expressed genes, were analyzed in the base wood library. Combining these data sets to reflect the sum of genes expressed in lignifying xylem, 150,855 tags were cataloged, representing a maximum of 42,641 different genes. Currently, this study represents the most extensive analysis of its kind in a higher plant and provides a quantitative description of the transcriptome representing the lignifying xylem of a 10-year-old loblolly pine.

DNA, Complementary↗
PubMed2004

Winter variation in xylem sap pH of walnut trees: involvement of plasma membrane H+-ATPase of vessel-associated cells.

We studied seasonal variation in xylem sap pH of Juglans regia L. Our main objectives were to (1) test the effect of temperature on seasonal changes in xylem sap pH and (2) study the involvement of plasma membrane H+-ATPase of vessel-associated cells in the control of sap pH. For this purpose, orchard-grown trees were compared with trees grown in a heated (> or = 15 degrees C) greenhouse. During autumn, sap pH was not directly influenced by temperature. A seasonal change in H+-ATPase activity resulting from seasonal variation in the amount of protein was measured in orchard-grown trees, whereas no significant seasonal changes were recorded in greenhouse-grown trees. Our data suggest that H+-ATPase does not regulate xylem sap pH directly by donating protons to the xylem, but by facilitating secondary active H+/sugar transport, among other mechanisms.

Hydrogen-Ion Concentration↗
PubMed1992

Hydraulic Conductivity Recovery versus Water Pressure in Xylem of Acer saccharum.

Experiments were conducted to determine the influence of stem diameter, xylem pressure potential, and temperature on the rate of recovery of hydraulic conductivity in embolized stems of Acer saccharum Marsh. Recovery of conductivity was accompanied by an increase in stem water content as water replaced air bubbles and bubbles dissolved from vessels into the surrounding water. The time required for stems to go from less than 3 to 100% hydraulic conductivity increased approximately with the square of the stem diameter and increased with decreasing xylem pressure potential. Recovery was halted when xylem pressure potential decreased below -6 kPa. Increasing xylem pressure from 13 to 150 kPa reduced the time for recovery by a factor of 4. Temperature had little influence on the rate of recovery of hydraulic conductivity. All of these results are in accord with a theory of bubble dissolution in which it is assumed that: (a) the rate of bubble dissolution is rate limited by diffusion of air from the bubbles to the outer surface of the stems, (b) the equilibrium concentration of gases in liquid in stems is determined by Henry's law at all air-water interfaces, (c) the equilibrium solubility concentration is determined only by the partial pressure of the gas in the gas phase and not directly by the liquid-phase pressure, and (d) the gas pressure of an entrapped air bubble in the lumen of a cell can never be less than atmospheric pressure at equilibrium.

Journal Article↗
PubMed2005

Do xylem fibers affect vessel cavitation resistance?

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P(min)), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b)(h)(2), where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r(2) = 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r(2) = 0.88), and P(min) (r(2) = 0.96). In contrast, cavitation resistance and P(min) were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b)(h)(2) (r(2) = 0.95), increased transverse fiber wall area (r(2) = 0.89), and decreased fiber lumen area (r(2) = 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.

Biomechanical Phenomena↗
PubMed1997

A Radial Concentration Gradient of Indole-3-Acetic Acid Is Related to Secondary Xylem Development in Hybrid Aspen.

The radial distribution pattern of indole-3-acetic acid (IAA) was determined across the developing tissues of the cambial region in the stem of hybrid aspen (Populus tremula L. x Populus tremuloides Michx). IAA content was measured in consecutive tangential cryo-sections using a microscale mass spectrometry technique. Analysis was performed with wild-type and transgenic trees with an ectopic expression of Agrobacterium tumefaciens IAA-biosynthetic genes. In all tested trees IAA was distributed as a steep concentration gradient across the developing tissues of the cambial region. The peak level of IAA was within the cambial zone, where cell division takes place. Low levels were reached in the region where secondary wall formation was initiated. The transgenic trees displayed a lower peak level and a wider radial gradient of IAA compared with the wild type. This alteration was related to a lower rate of cambial cell division and a longer duration of xylem cell expansion in the transgenic trees, resulting in a decreased xylem production and a larger fiber lumen area. The results indicate that IAA has a role in regulating not only the rate of physiological processes such as cell division, but also the duration of developmental processes such as xylem fiber expansion, suggesting that IAA functions as a morphogen, conveying positional information during xylem development.

Journal Article↗
PubMed2000

Compression wood-responsive proteins in developing xylem of maritime pine (Pinus pinaster ait.).

When a conifer shoot is displaced from its vertical position, compression wood (CW) is formed on the under side and can eventually return the shoot to its original position. Changes in cell wall structure and chemistry associated with CW are likely to result from differential gene/protein expression. Two-dimensional polyacrylamide gel electrophoresis of differentiating xylem proteins was combined with the physical characterization of wooden samples to identify and characterize CW-responsive proteins. Differentiating xylem was harvested from a 22-year-old crooked maritime pine (Pinus pinaster Ait.) tree. Protein extracted from different samples were revealed by high-resolution silver stained two-dimensional polyacrylamide gel electrophoresis and analyzed with a computer-assisted system for single spot quantification. Growth strain (GS) measurements allowed xylem samples to be classified quantitatively from normal wood to CW. Regression of lignin and cellulose content on GS showed that an increase in the percentage of lignin and a decrease of the percentage of cellulose corresponded to increasing GS values, i.e. CW. Of the 137 studied spots, 19% were significantly associated with GS effect. Up-regulated proteins included 1-aminocyclopropane-1-carboxylate oxidase (an ethylene forming enzyme), a putative transcription factor, two lignification genes (caffeic O-methyltransferase and caffeoyl CoA-O-methyltransferase), members of the S-adenosyl-L-methionine-synthase gene family, and enzymes involved in nitrogen and carbon assimilation (glutamine synthetase and fructokinase). A clustered correlation analysis was performed to study simultaneously protein expression along a gradient of gravistimulated stressed xylem tissue. Proteins were found to form "expression clusters" that could identify: (a) Gene product under similar control mechanisms, (b) partner proteins, or (c) functional groups corresponding to specialized pathways. The possibility of obtaining regulatory correlations and anticorrelations between proteins provide us with a new category of homology (regulatory homology) in tracing functional relationships.

Cellulose↗
PubMed2000

Effects of elevated [CO(2)] and nitrogen nutrition on cytokinins in the xylem sap and leaves of cotton.

We measured the level of xylem-derived cytokinins (CKs) entering a cotton leaf, and the CK levels in the same leaf, thus enabling xylem sap and foliar CKs to be compared concurrently. Although zeatin was the dominant CK in xylem sap, zeatin, dihydrozeatin, and N(6)-(2-isopentenyl) adenine were present in approximately equimolar levels in leaves. Elevated [CO(2)] (EC) has an effect on the levels of cytokinins in sap and leaf tissues. This effect was modulated by the two levels of root nitrogen nutrition (2 and 12 mM nitrate). Growth enhancement (70%) in EC over plants in ambient [CO(2)] (AC) was observed for both nitrogen nutrition treatments. Low-nitrogen leaves growing in EC exhibited photosynthetic acclimation, whereas there was no sign of photosynthetic acclimation in high-nitrogen grown leaves. Under these prevailing conditions, xylem sap and leaf tissues were obtained for CK analysis. Higher nitrogen nutrition increased the delivery per unit leaf area of CKs to the leaf at AC. EC caused a greater increase in CK delivery to the leaf at low nitrogen conditions (106%) than at high nitrogen conditions (17%). EC induced a significant increase in CK content in low-nitrogen leaves, whereas CK content in leaf tissues was similar for high-nitrogen leaves growing in AC and EC.

Carbon Dioxide↗
PubMed1970

The time course of xylem differentiation and its relation to deoxyribonucleic Acid synthesis in cultured coleus stem segments.

The relationship between DNA synthesis and wound xylem differentiation was investigated in cultured stem segments of Coleus blumei. The addition of 50 micrograms of indoleacetic acid per liter to the culture medium resulted in a 400 to 500% increase in the number of wound vessel members formed in 7 days. However, the time course of wound vessel member formation was similar in segments cultured in the presence and absence of auxin. In either case, no wound vessel members appeared before the 3rd day of culture, while the majority of wound vessel members appeared on the 4th and 5th days of culture. (3)H-Thymidine incorporation into DNA was used to measure changes in the DNA synthetic activity of the tissues during the culture period. Comparatively little (3)H-thymidine incorporation occurred during the 1st day of culture. Maximum (3)H-thymidine incorporation was observed on the 2nd day of culture, 2 days before the peak period of xylem differentiation. The rate of incorporation of (3)H-thymidine into DNA decreased with increasing time in culture after the 2nd day. Auxin at 50 micrograms per liter had no effect on the time course of (3)H-thymidine incorporation, although somewhat more (3)H-thymidine was incorporated into DNA throughout the culture period in the presence of auxin. The magnitude of this effect was small when compared to the effect of auxin on xylem differentiation. The antimetabolite 5-fluorodeoxyuridine was shown to block DNA synthesis in the cultured stem segments. When the tissues were isolated on media containing 10(-6)m 5-fluorodeoxyuridine, wound vessel member differentiation was inhibited by approximately 80%, in both the presence and absence of auxin. Thymidine at 10(-5)m completely overcame the 5-fluorodeoxyuridine inhibition of wound vessel member formation. 5-Fluorodeoxyuridine was effective in blocking xylogenesis only when this substance was supplied to the tissues during the early part of the culture period. 5-Fluorodeoxyuridine had no effect on xylem differentiation when it was applied after the 3rd day of culture.

Journal Article↗
PubMed1976

Different mass transfer rates of labeled sugars and tritiated water in xylem vessels and their dependency on metabolism.

Solutions of (14)C-sugars in tritiated water or solutions of (14)C- and (3)H-sugars were perfused by gravity through the xylem vessels of excised tomato internodes (Lycopersicon esculentum) mostly during 2 hours.Mass flow of a solution in plant vessels is found not to be in conflict with different mass transfer rates of sugar and water molecules. It is explained by individual lateral escape rates from the vessel for each single compound. The escape of tritiated water can be ascribed to diffusion, while the escape of sugars is apparently linked to the metabolism of the surrounding parenchyma cells (Q(10) sucrose uptake = 3.5). Lateral escape rates of sugars from the xylem vessels are in the proportion of sucrose-glucose-fructose (2.4:1.0:0.6). The accessibility of xylem parenchyma cells and their differential permeability to sugars control the longitudinal mass transfer of sugars in the xylem vessels.As sucrose and glucose do not compete for uptake from the vessels by the contiguous cells, separate uptake systems for both may be postulated.

Journal Article↗
PubMed1977

Effect of azetidine 2-carboxylic Acid on ion uptake and ion release to the xylem of excised barley roots.

Azetidine 2-carboxylic acid (AZ) was used as an analog of proline to investigate further the relationship between protein synthesis and ion transport. AZ does not inhibit protein assembly, but the proteins formed are ineffective as enzymes. At relatively low concentrations (50 muM) AZ was a potent inhibitor of release of ions to the xylem of excised roots of barley (Hordeum vulgare L.) and intact plants. Uptake to the root was also inhibited but to a lesser degree. A procedure was introduced for estimating unidirectional fluxes from measurements of net tracer uptake, net transport to the xylem, and net efflux from the roots. It was shown that inhibition of release to the xylem was not caused by reduction in influx at the plasmalemma or to stimulation of influx to the vacuoles. It was suggested that AZ was acting on the process of release from symplast to the xylem. The action of AZ is compared with similar effects on ion transport produced by p-fluorophenylalanine, cycloheximide, and abscisic acid.

Journal Article↗
PubMed1980

Xylem Transport of 1-Aminocyclopropane-1-carboxylic Acid, an Ethylene Precursor, in Waterlogged Tomato Plants.

Waterlogging is known to cause an increase in ethylene synthesis in the shoot which results in petiole epinasty. Evidence has suggested that a signal is synthesized in the anaerobic roots and transported to the shoot where it stimulates ethylene synthesis. Experimental data are presented showing that 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, serves as the signal. Xylem sap was collected from detopped tomato plants (Lycopersicon esculentum Mill. cv. VFN8). ACC in the sap was quantitated by a sensitive and specific assay, and its tentative chemical identity verified by paper chromatography. ACC levels in both roots and xylem sap increased markedly in response to waterlogging or root anaerobiosis. The appearance of ACC in the xylem sap of flooded plants preceded both the increase in ethylene production and epinastic growth, which were closely correlated. Plants flooded and then drained showed a rapid, simultaneous drop in ACC flux and ethylene synthesis rate. ACC supplied through the cut stem of tomato shoots at concentrations comparable to those found in xylem sap caused epinasty and increased ethylene production. These data indicate that ACC is synthesized in the anaerobic root and transported to the shoot where it is readily converted to ethylene.

Journal Article↗
PubMed1981

Metal Complexation in Xylem Fluid : I. CHEMICAL COMPOSITION OF TOMATO AND SOYBEAN STEM EXUDATE.

Xylem fluid was analyzed for numerous solutes to characterize chemically the sap as a medium for forming and transporting metal complexes. The stem exudate was collected hourly for 8 hours from topped 31-day-old soybean (Glycine max L. Merr.) and 46-day-old tomato (Lycopersicon esculentum Mill.) plants grown in normal (0.5 micromolar) and Za-phytotoxic nutrient solutions. Soybean plants were grown in the normal and high-Zn solutions for 24 days; tomato plants were grown for 32 days. The exudate was analyzed for seven organic acids, 22 amino acids, eight inorganic solutes, apparent ionic strength, and pH. Significant changes in many solutes occurred over the 8-hour sampling period. These fluctuations depended on plant species, individual solute, and Zn treatment, and demonstrated that extrapolation of xylem-fluid analyses to whole-plant xylem sap is valid only for sap samples collected shortly after topping a plant. Exudate pH decreased over the 8-hour period for both species; exudate ionic strength increased for tomato and decreased for soybean. At the normal-Zn treatment (0 to 1 hour), the highest acid micromolar concentrations in soybean exudate were: asparagine, 2,583; citric, 1,706; malic, 890; and malonic, 264. Under the same conditions, the highest acid micromolar concentrations in tomato exudate were: maleic, 1,206; malic, 628; glutamine, 522; citric, 301; and asparagine, 242. Cysteine and methionine were above detection limits only in soybean exudate. Zinc phytotoxicity caused significant changes in many solutes. The analyses reported here provide a comprehensive data base for further studies on metal-complex equilibria in xylem fluid.

Journal Article↗
PubMed1984

Introduction of xylem differentiation in lactuca by ethylene.

Evidence was obtained to support the hypothesis that ethylene is involved in xylem differentiation in primary pith explants of Lactuca sativa L. cv Romaine cultured in vitro. Xylem elements differentiated when explants were supplied indole-3-acetic acid (IAA) in combination with either the ethylene biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC), the ethylene-releasing agent 2-chloroethylphosphonic acid (CEPA), or kinetin. In contrast, no xylem elements differentiated in the presence of IAA, kinetin, ACC, or CEPA alone, or when kinetin was supplied together with ACC or CEPA. These results show that ethylene will substitute qualitatively for cytokinin during auxin-induced xylogenesis, and suggest that both ethylene and auxin are required for xylem differentiation in Lactuca.

Journal Article↗
PubMed1988

Do woody plants operate near the point of catastrophic xylem dysfunction caused by dynamic water stress? : answers from a model.

We discuss the relationship between the dynamically changing tension gradients required to move water rapidly through the xylem conduits of plants and the proportion of conduits lost through embolism as a result of water tension. We consider the implications of this relationship to the water relations of trees. We have compiled quantitative data on the water relations, hydraulic architecture and vulnerability of embolism of four widely different species: Rhizophora mangle, Cassipourea elliptica, Acer saccharum, and Thuja occidentalis. Using these data, we modeled the dynamics of water flow and xylem blockage for these species. The model is specifically focused on the conditions required to generate ;runaway embolism,' whereby the blockage of xylem conduits through embolism leads to reduced hydraulic conductance causing increased tension in the remaining vessels and generating more tension in a vicious circle. The model predicted that all species operate near the point of catastrophic xylem failure due to dynamic water stress. The model supports Zimmermann's plant segmentation hypothesis. Zimmermann suggested that plants are designed hydraulically to sacrifice highly vulnerable minor branches and thus improve the water balance of remaining parts. The model results are discussed in terms of the morphology, hydraulic architecture, eco-physiology, and evolution of woody plants.

Journal Article↗
PubMed1988

Mechanism of water stress-induced xylem embolism.

We investigated the hypothesis that water stress-induced xylem embolism is caused by air aspirated into functional vessels from neighboring embolized ones (e.g. embolized by physical damage) via pores in intervessel pit membranes. The following experiments with sugar maple (Acer saccharum Marsh.) support the hypothesis. (a) Most vessels in dehydrating stem segments embolized at xylem pressures < -3 megapascals; at this point the pressure difference across intervessel pits between air-filled vessels at the segment's ends and internal water-filled vessels was >3 megapascals. This same pressure difference was found to be sufficient to force air across intervessel pits from air injection experiments of hydrated stem segments. This suggests air entry at pits is causing embolism in dehydrating stems. (b) Treatments that increased the permeability of intervessel pits to air injection also caused xylem to embolize at less negative xylem pressures. Permeability was increased either by perfusing stems with solutions of surface tension below that of water or by perfusion with a solution of oxalic acid and calcium. The mechanism of oxalic-calcium action on permeability is unknown, but may relate to the ability of oxalate to chelate calcium from the pectate fraction of the pit membrane. (c) Diameter of pores in pit membranes measured with the scanning electron microscope were within the range predicted by hypothesis (</=0.4 micrometer).

Journal Article↗