Publications (29)94.61 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: The continuity of the xylem water columns was studied on 17- to 23-m tall birch trees (trunk diameter about 23 cm; first branching above 10 m) all year round. Fifty-one trees were felled, and 5-cm thick slices or 2-m long boles were taken at regular, relatively short intervals over the entire height of the trees. The filling status of the vessels was determined by (i) xylem sap extraction from trunk and branch pieces (using the gas bubble-based jet-discharge method and centrifugation) and from trunk boles (using gravity discharge); (ii) (1)H nuclear magnetic resonance imaging of slice pieces; (iii) infusion experiments (dye, (86)Rb(+), D(2)O) on intact trees and cut branches; and (iv) xylem pressure measurements. This broad array of techniques disclosed no evidence for continuous water-filled columns, as postulated by the Cohesion-Tension theory, for root to apex directed mass transport. Except in early spring (during the xylem refilling phase) and after extremely heavy rainfall during the vegetation period, cohesive/mobile water was found predominantly at intermediate heights of the trunks but not at the base or towards the top of the tree. Similar results were obtained for branches. Furthermore, upper branches generally contained more cohesive/mobile water than lower branches. The results suggest that water lifting occurs by short-distance (capillary, osmotic and/or transpiration-bound) tension gradients as well as by mobilisation of water in the parenchymatic tissues and the heartwood, and by moisture uptake through lenticels.
- [Show abstract] [Hide abstract] ABSTRACT: Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.
- [Show abstract] [Hide abstract] ABSTRACT: Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σr) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σr assumed values of the order of about 0·2–0·4. The σr values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m−2 s−1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σr= 1. Further increase of the light intensity to about 400 μmol m−2 s−1 resulted in a slight but significant decrease of the σr values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m−2 s−1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σr values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m−2 s−1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σr values of the respective glycophytes, the σc values of the root cortical cells of wheat and barley were close to unity, whereas σc for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m−2 s−1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σr values, the σc values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.
- [Show abstract] [Hide abstract] ABSTRACT: The water supply to leaves of 25 to 60 m tall trees (including high-salinity-tolerant ones) was studied. The filling status of the xylem vessels was determined by xylem sap extraction (using jet-discharge, gravity-discharge, and centrifugation) and by (1)H nuclear magnetic resonance imaging of wood pieces. Simultaneously, pressure bomb experiments were performed along the entire trunk of the trees up to a height of 57 m. Clear-cut evidence was found that the balancing pressure (P(b)) values of leafy twigs were dictated by the ambient relative humidity rather than by height. Refilling of xylem vessels of apical leaves (branches) obviously mainly occurred via moisture uptake from the atmosphere. These findings could be traced back to the hydration and rehydration of mucilage layers on the leaf surfaces and/or of epistomatal mucilage plugs. Xylem vessels also contained mucilage. Mucilage formation was apparently enforced by water stress. The observed mucilage-based foliar water uptake and humidity dependency of the P(b) values are at variance with the cohesion-tension theory and with the hypothesis that P(b) measurements yield information about the relationships between xylem pressure gradients and height.
- [Show abstract] [Hide abstract] ABSTRACT: The ion-selective xylem probe is a new tool that allows for on-line recording of ion activities in the xylem sap when water is under tension. Ion-selective electrodes are combined with a xylem pressure probe additionally measuring the electrical potential in a vessel. Using a K+-selective probe, xylem K+ in maize roots was recorded simultaneously with the xylem pressure and the trans-root potential (the electrical potential in the root xylem with respect to the ambient medium) under conditions of varying light irradiation and K+ supply. Furthermore, a new pH-selective xylem probe is presented. The impact of the new xylem probes for studies on plant nutrition is discussed.
- [Show abstract] [Hide abstract] ABSTRACT: Herbaceous plants exhibit complex reaction patterns as a response to changes in environmental factors such as light intensity, relative humidity, temperature and root water-supply. The reactions of volume flows and hydrostatic pressures within the intact plant can only be described in an authentic way if minimal- and non-invasive methods are used for investigation. This review article highlights some aspects on the correlation of xylem volume flows and corresponding hydrostatic pressures, with special emphasis put on the role of the water supply to the roots. From these studies it can be concluded that the tissue cells play an important role in determining and maintaining xylem pressures.
- [Show abstract] [Hide abstract] ABSTRACT: Since its introduction in the late 19th century, the so-called cohesion theory has become widely accepted as explaining the mechanism of the ascent of sap. According to the cohesion theory, the minimum standing vertical xylem tension gradient should be 0·01 MPa m−1. When transpiration is occurring, frictional resistances are expected to make this gradient considerably steeper. The results of numerous pressure chamber measurements reported in the literature are generally regarded as corroborating the cohesion theory. Nevertheless, several reports of pressure chamber measurements in tall trees appear to be incompatible with predictions of the cohesion theory. Furthermore, the pressure chamber is an indirect method for inferring xylem pressure, which, until recently, has not been validated by comparison against a direct method. The xylem pressure probe provides a means of testing the validity of the pressure chamber and other indirect techniques for estimating xylem pressure. We discuss here the results of concurrent measurements made with the pressure chamber and the xylem pressure probe, particularly recent measurements made at the top of a tall tropical tree during the rainy season. These measurements indicate that the pressure chamber often substantially overestimates the tension previously existing in the xylem, especially in the partially dehydrated tissue of droughted plants. We also discuss other evidence obtained from classical and recent approaches for studying water transport. We conclude that the available evidence derived from a wide range of independent approaches warrants a critical reappraisal of tension-driven water transport as the exclusive mechanism of long-distance water transport in plants.
- [Show abstract] [Hide abstract] ABSTRACT: Volume changes of human T-lymphocytes (Jurkat line) exposed to hypotonic carbohydrate-substituted solutions of different composition and osmolality were studied by videomicroscopy. In 200 mOsm media the cells first swelled within 1-2 min and then underwent regulatory volume decrease (RVD) to their original isotonic volume within 10-15 min. RVD also occurred in strongly hypotonic 100 mOsm solutions of di- and trisaccharides (trehalose, sucrose, raffinose). In contrast to oligosaccharide media, 100 mOsm solutions of monomeric carbohydrates (glucose, galactose, inositol and sorbitol) inhibited RVD. The complex volumetric data were analyzed with a membrane transport model that allowed the estimation of the hydraulic conductivity and volume-dependent solute permeabilities. We found that under slightly hypotonic stress (200 mOsm) the cell membrane was impermeable to all carbohydrates studied here. Upon osmolality decrease to 100 mOsm, the membrane permeability to monomeric carbohydrates increased dramatically (apparently due to channel activation caused by extensive cell swelling), whereas oligosaccharide permeability remained very poor. The size-selectivity of the swelling-activated sugar permeation was confirmed by direct chromatographic measurements of intracellular sugars. The results of this study are of interest for biotechnology, where sugars and related compounds are increasingly being used as potential cryo- and lyoprotective agents for preservation of rare and valuable mammalian cells and tissues.
- [Show abstract] [Hide abstract] ABSTRACT: A micro-Raman spectroscopy approach was used for the direct in situ characterization of lipid bodies in the water-conducting branch xylem of an African resurrection plant and three deciduous European tree species. Because of average diameters of at least 1 microm, the lipid bodies of all investigated species proved to be easily accessible by this technique. All vesicle-forming xylem lipids were identified as fatty acid esters, which may correspond to phospholipids. Whereas in the resurrection plant saturated lipids were dominant, the lipid bodies of the European trees consisted of highly unsaturated fatty acids. A comparison of the spectra of lipid droplets of lime obtained in situ and from isolated xylem sap revealed slightly different signatures. This finding suggests that micro-Raman spectroscopy may be used to detect modifications of the chemical composition of biological substances as a result of the extraction mode.
- [Show abstract] [Hide abstract] ABSTRACT: Summary • Lipids play a crucial role in the maintenance of the structural and functional integ- rity of the water-conducting elements and cells of the resurrection plant Myrotham- nus ﬂabellifolia during complete dehydration. • Lipid composition, mobility and distribution within the internodal and nodal xylem regions (including short shoots and leaves) were investigated in the presence and absence of water by using various nuclear magnetic resonance (NMR) spectroscopy and imaging techniques differing greatly in the level of spatial resolution and acqui- sition of lipid parameters. • Signiﬁcant ﬁndings include: a discontinuity in the branch xylem between an inner zone where no water moves and an outer zone where the water moves; the blocking of water movement in the inner zone by lipids that are not dispersed by water, and the facilitation of water advance in the xylem elements and pits of the outer zone by water-dispersed lipids; the relative impermeability of leaf trace xylem to the rehy- drating water and, hence, the relative hydraulic isolation of the leaves. • These results elucidated part of the strategy used by the resurrection plant to cope with extreme drought and to minimize transpirational water loss upon hydration. Key words: Myrothamnus ﬂabellifolia, xylem reﬁlling, lipid lining, phospholipid, leaf trace, NMR imaging and spectroscopy, diffusion, transmission electron microscopy. © New Phytologist (2003) 159: 487–505
- [Show abstract] [Hide abstract] ABSTRACT: The resurrection plant Myrothamnus flabellifolia has the ability to recover from repeated prolonged and extreme desiccation cycles. During the dry state the inner walls of the xylem vessels seemed to be covered, at least partly, by a lipid film as shown by Sudan III and Nile Red staining. The lipid film apparently functioned as an ‘internal cuticle’ which prevented the adjacent parenchyma ray cells from complete water loss. The hydrophobic nature of the inner xylem walls was supported by the finding that benzene ascended as rapidly as water in the xylem of dry Myrothamnus branches. On watering, numerous lipid bodies were found in the water-conducting vessels, presumably formed from the lipid film and/or from lipids excreted from the adjacent living cells into the vessels. The presence of lipid bodies within the vessels, as well as the hydrophobic properties of the inner xylem walls, could explain the finding that the xylem pressure of hydrated, well watered plants (measured both under laboratory and field conditions with the xylem pressure probe) never dropped below c. −0.3 MPa and that cavitation occurred frequently at low negative xylem pressure values (−0.05 to −0.15 MPa). The xylem pressure of M. flabellifolia responded rapidly and strongly to changes in relative humidity and temperature, but less obviously to changes in irradiance (which varied between 10 and c. 4000 μmol m m−2 s−1). The morphological position of the stomata in the leaves could explain the extremely weak and slow response of the xylem pressure of this resurrection plant to illumination changes. Stomata were most abundant in the furrows, and were thus protected from direct sunlight. Simultaneous measurements of the cell turgor pressure in the leaf epidermal cells (made by using the cell turgor pressure probe) revealed that the xylem and the cell turgor pressure dropped in a ratio of 1:0.7 on changes in the environmental parameters, indicating a quite close hydraulic connection and, thus, water equilibrium between the xylem and cellular compartments. An increase in irradiance of c. 700 μmol m−2 s−1 resulted in a turgor pressure decrease from 0.63 to 0.48 MPa. Correspondingly, the cell osmotic pressure increased from 1.03 to 1.22 MPa. From these values and by assuming water equilibrium, the osmotic pressure of the xylem sap was estimated to be 0.25–0.4 MPa. This value seems to be fairly high but may, however, be explained by the reduction of the water volume within the vessels due to the floating lipid bodies.
Conference Paper: Raman spectroscopic investigation on lipid bodies in xylem elements
- [Show abstract] [Hide abstract] ABSTRACT: The acropetal water refilling kinetics of the dry xylem of branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia were determined with high temporal resolution by observation of light refraction at the advancing water front and the associated recurving of the folded leaves. To study the effect of gravity on water rise, data were acquired for cut upright, horizontal and inverted branches. Water rise kinetics were also determined with hydrostatic and osmotic pressure as well as at elevated temperatures (up to 100 degrees C) under laboratory conditions and compared with those obtained with intact (rooted) and cut branches under field conditions. Experiments in which water climbed under its capillary pressure alone, showed that the axial flow occurred only in a very few conducting elements at a much higher rate than in many of the other ones. The onset of transpiration of the unfolded and green leaves did not affect the rise kinetics in the 'prominent' conducting elements. Application of pressure apparently increased the number of elements making a major contribution to axial xylem flow. Analysis of these data in terms of capillary-pressure-driven water ascent in leaky capillaries demonstrated that root pressure, not capillary pressure, is the dominant force for rehydration of rooted, dry plants. The main reasons for the failure of capillary forces in xylem refilling were the small, rate-limiting effective radii of the conducting elements for axial water ascent (c. 1 micrometer compared with radii of the vessels and tracheids of c. 18 micrometers and 3 micrometers, respectively) and the very poor wetting of the dry walls. The contact (wetting) angles were of the order of 80 degrees and decreased on root or externally applied hydrostatic pressure. This supported our previous assumption that the inner walls of the dry conducting elements are covered with a lipid layer that is removed or disintegrates upon wetting. Consistent with this, potassium chloride and, particularly, sugars exerted an osmotic pressure effect on axial water climbing (reflection coefficients > zero, but small). Although the osmotically active solutes apparently suppressed radial water spread through the tissue to the leaf cells, they reduced the axial water ascent rather than accelerating it as predicted by the theory of capillary-driven water rise in leaky capillaries. Killing cells by heat treatment and removal of the bark, phelloderm, cortex and phloem also resulted in a reduction of the axial rise rate and final height. These observations demonstrated that radial water movement driven by the developing osmotic and turgor pressure in the living cells was important for the removal of the lipid layer from the walls of those conducting elements that were primarily not involved in water rise. There is some evidence from field measurements of the axial temperature gradients along rooted branches that interfacial (Marangoni) streaming facilitated lipid removal (under formation of vesicle-like structures and lipid bodies) upon wetting. Grant numbers: 50WB 9643, Zi 99/9-1.
- [Show abstract] [Hide abstract] ABSTRACT: The axial and radial refilling with water of cut dry branches (up to 80 cm tall) of the resurrection plant Myrothamnus flabellifolia was studied in both acro- and basipetal directions by using 1H-NMR imaging. NMR measurements showed that the conducting elements were not filled simultaneously. Axial water ascent occurred initially only in a cluster of a very few conducting elements. Refilling of the other conducting elements and of the living cells was mainly achieved by radial extraction of water from these initial conducting elements. With time, xylem elements in a few further regions were apparently refilled axially. Radial water spread through the tissue occurred almost linearly with time, but much faster in the acropetal than in the basipetal direction. Application of hydrostatic pressure (up to 16 kPa) produced similar temporal and spatial radial refilling patterns, except that more conducting elements were refilled axially during the first phase of water rise. The addition of raffinose to the water considerably reduced axial and radial spreading rates. The polarity of water climbing was supported by measurements of the water rise in dry branches using the 'light refraction'(and, sometimes, the 'leaf recurving') method. Basipetal refilling of the xylem conduit exhibited biphasic kinetics; the final rise height did not exceed 20-30 cm. Three-cm-long branch pieces also showed a directionality of water climbing, ruling out the possibility that changes in the conducting area from the base to the apex of the branches were responsible for this effect. The polarity of water ascent was independent of gravity and also did not change when the ambient temperature was raised to c. 40 degrees C. At external pressures of 50-100 kPa the polarity disappeared, with basipetal and acropetal refill times of the xylem conduit of tall branches becoming comparable. Refilling of branches dried horizontally (with a clinostat) or inverted (in the direction of gravity) showed a pronounced reduction of the acropetal water rise to or below basipetal water climbing level (which was unaffected by this treatment). Unlike water, benzene and acetone climbing showed no polarity. In the case of benzene, the rise kinetics (including the final heights) were comparable with those measured acropetally for water, whereas with acetone the rise height was less. Transmission electron microscopy of dry branches demonstrated that the inner surfaces of the conducting tracheids and vessels were lined with a continuous osmiophilic (lipid) layer, as postulated by the kinetic analysis and light microscopy studies. The thickness of the layer varied between 20 and 80 nm. The parenchymal and intervessel pits as well as numerous tracheid corners contained opaque inclusions, presumably also consisting of lipids. Electron microscopy of rehydrated plants showed that the lipid layer was either thinned or had disintegrated and that numerous vesicle-like structures and lipid bodies were formed (together with various intermediate structural elements). These, many other data and the physical-chemical literature imply that several (radial) driving forces (such as capillary condensation, Marangoni forces, capillary, osmotic and turgor pressure forces) operate when a few conducting elements become axially refilled with water. These forces apparently lead to an avalanche-like radial refilling of most of the conducting elements and living cells, and thus to the removal of the 'internal cuticle' and of the hydrophobic inclusions in the pits. The polarity of water movement presumably results from high resistances in the basipetal direction, which are created by local gradients in the thickness of the lipid film as a result of draining under gravity in response to drought. There are striking similarities in morphology and function between the xylem-lining lipid film and the lung surfactant film lining the pulmonary air spaces of mammals.
- [Show abstract] [Hide abstract] ABSTRACT: : Flow-sensitive NMR imaging and pressure probe techniques were used for measuring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipremnum aureum. Selection of tall specimens allowed continuous and simultaneous measurements of all parameters at various distances from the root under diurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow velocity. Concomitant multiple-probe insertions along the plant shoot revealed xylem and turgor pressure gradients with changing magnitudes due to environmental changes and plant orientation (upright, apex-down, or horizontal). The data suggest that in upright and - to a lesser extent - in horizontal plants the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, longitudinal cellular osmotic pressure gradients exist. Due to the tight hydraulic coupling of the xylem and the tissue cells these gradients represent (besides the transpiration-induced tension in the xylem) an additional tension component for anti-gravitational water movement from the roots through the vessels to the apex.
- [Show abstract] [Hide abstract] ABSTRACT: Many diseases are closely tied to deficient or subnormal metabolic and secretory cell functions. Milder forms of these diseases can be managed by a variety of treatments. However, it is often extremely difficult or even impossible to imitate the moment-to-moment fine regulation and the complex roles of the hormone, factor or enzyme that is not sufficiently produced by the body. Immunoisolated transplantation is one of the most promising approaches to overcome the limitations of current treatments. Non-autologous (transformed) cell lines and allogeneic and xenogeneic cells/tissues that release the therapeutic substances are enclosed in immunoprotective microcapsules. The microcapsules avoid a lifetime of immunosuppressive therapy while excluding an immune response in the host. Research in this direction has shown the feasibility of microcapsules based on hydrogels (particularly of alginate) for transplantation of non-autologous cells and tissue fragments. Numerous technical accomplishments of the immunoisolation method have recently made possible the first successful long-term clinical applications. However, realizing the potential of immunoisolated therapy requires the use of several factors that have received limited attention in the past but are important for the formulation of hydrogel-based immunoisolation systems that are highly versatile, potentially economical and can gain medical approval.
- [Show abstract] [Hide abstract] ABSTRACT: Transplantation studies with immunoisolated foreign cells/tissues (encapsulated in Ba2+-cross-linked alginate) show that several obligatory requirements have to be met before this immunoisolation technique can be used for routine clinical trials. We present chemical procedures and technical developments which could address and solve the current problems and limitations of the microencapsulation technique. Large-scale production of highly purified (biocompatible) alginate is possible and this material does not evoke any foreign body reactions under implantation conditions. Atomic force microscopy (AFM) studies suggest that geometric inhomogeneities may lead to immunological reactivity of the cross-linked alginate. Material traces released from fibroblasts, macrophages and/or lymphocytes migrating over surfaces can also be studied by AFM and may initiate the primary foreign body reactions. Long-term stability of the alginate beads can be improved by incorporating proteins or (medically approved) perfluorocarbons during the cross-linking process and by subsequent treatment with 6 mM sulphate. Long-term stability and local oxygen supply can be monitored in vivo using non-invasive 19F-nuclear magnetic resonance imaging (MRI) of perfluorocarbon-loaded alginate beads. These improvements and developments allow clinical trials with allogenic (and xenogenic) tissue by alginate immunoisolation.
University of Wuerzburg
Würzburg, Bavaria, Germany
- • Department of Biotechnology and Biophysics
- • Institute of Physical and Theoretical Chemistry