Xylem Resistance Research Articles

Developmental control of xylem hydraulic resistances and vulnerability to embolism in Fraxinus excelsior L.: impacts on water relations

The hydraulic properties and leaf gas exchanges of Fraxinus excelsior L. branches differing by their age and their vertical crown position, but in comparable ambient air conditions (vapour pressure deficit and global radiation) were compared. The variations in leaflet water potential ψleaflet, leaflet stomatal conductance and transpiration rate, E, were small between different branches of the same crown. Whole branch hydraulic resistances (r branch ), and partitioning between leaf (r leaf ) and xylem resistance (r xylem ) were assessed with a high pressure flowmeter. r leaf represented 90% and 10% of r branch for upper and lower crown branches, respectively. The changes resulted from increases in r xylem caused by the formation of short shoot internodes mostly located in secondary axes. However, leaf area-specific branch resistances (r branch =r branch x LA) were nearly constant throughout the crown. This was consistent with the vertical variations in ψ leaflet because r branch x E represents the water potential drop from the trunk to the leaves. Because r xylem was higher, lower ψxylem values were predicted in lower crown rachises. However, rachises from lower crown branches were less vulnerable to embolism than in upper branches ( ψxylem at onset of embolism, ψ cav, were -3 and -2MPa, respectively). It was concluded that r xylem increased with branch age, but r* branch remained constant because LA decreased. As a consequence, E was maximized and ψxylem remained above ψcav, This suggested that, in Fraxinus, leaf gas exchanges and leaf areas were coupled with xylem hydraulic capacities probably through a control of bud activity.

Hydraulic architecture ofAcer saccharumandA. rubrum: comparison of branches to whole trees and the contribution of leaves to hydraulic resistance

Hydraulic resistance to water flow was measured in the xylem and leaves of above-ground portions of Acer saccharum and Acer rubrum for trees with trunk diameters (D) ranging from 0.02 to 0.2 m. Resistance (area basis) to water flow in leaves and petioles was 24 and 13×10 3 MPa m 2 s kg -1 for A. saccharum and A. rubrum, respectively. Leaf area of whole trees was proportional to D 1.82 . Absolute xylem resistance (MPa s kg -1 ) was proportional to D -1.64 . So xylem resistance (area basis) was proportional to D 0.2 , which was not significantly different from a zero dependence (D 0 ) for D≤0.2 m

Resistance to Haustorial Development of Two Mistletoes, Amyema preissii (Miq.) Tieghem and Lysiana exocarpi (Behr.) Tieghem ssp. Exocarpi (Loranthaceae), on Host and Nonhost Species

Seeds of two mistletoe species, Amyema preissii and Lysiana exocarpi, were inoculated onto host and nonhost species, and haustorial development was examined. Resistance to haustorial penetration was observed in the bark and xylem. Bark resistance took one or all of three forms: (1) mechanical resistance; (2) production of wound periderm; and (3) abnormal changes in host tissues surrounding the mistletoe haustorium. Xylem resistance included physical constraints on haustorial growth and abnormal changes in host tissues surrounding the haustorium. Bark resistance to A. preissii occurred on all nonhost species, and mistletoe haustoria failed to penetrate the bark of all nonhost species except Geijera linearifolia. In the latter case, A. preissii haustoria reached host xylem, but further development was hindered by xylem resistance. No resistance was recorded on Acacia nyssophylla, the host species of A. preissii. Bark resistance to L. exocarpi was observed in haustoria growing on the two major host species, Heterodendrum oleifolium and Myoporum platycarpum, and haustorial development on these two species was delayed for up to 3 mo compared with other host species. While no bark resistance was observed in haustoria growing on the three nonhost species, Exocarpos aphyllus, Eucalyptus oleosa, and G. linearifolia, all three species showed resistance by the xylem to haustorial development. No xylem resistance, however, was recorded on the three host species, A. nyssophylla, H. oleifolium, and M. platycarpum.

Resistance to water flow in xylem of Picea abies (L.) Karst. trees grown under contrasting light conditions

The relative hydraulic conductivity (k) of xylem and resistance (R) to water flow through trunk, primary roots and branches in Picea abies trees growing under contrasting light conditions were investigated. The xylem permeability to water was measured by forcing 10 mM water solution of KC1 through excised wood specimens. From the values of k, the sapwood transverse area and the length of conducting segments, R of the whole trunk, branches and roots was calculated. The relative conductivity of xylem in open-grown trees exceeded that of shade-grown trees by 1.4–3.1 times, while k was closely correlated with the hydraulically effective radius (Re) of the largest tracheids (R2 was 0.85–0.94 for open- and 0.51–0.79 for shade-grown trees). Because of both a low k and a smaller sapwood area in shade-grown trees the resistance to water movement through their trunk, roots and branches was many times higher. The distribution of R between single segments of the water-conducting pathway differed considerably in trees from different sites. At high water status the largest share of the total resistance in open- as well as shade-grown trees resides in the apical part of the trunk. The contribution of the branches to total xylem resistance is supposed to increase with developing water deficit.

Histological and Physical Factors Affecting Digestibility of Forages

AbstractFactors that limit forage quality are complex and interactive. Structural characteristics that limit forage breakdown are primarily highly lignified support tissues like sclerenchyma and xylem. However, the presence of chemical barriers (likely the low molecular weight phenolic compounds) within “living” cell walls, such as the parenchyma bundle sheath and epidermis, prevents fiber degradation in these tissues. The presence of high proportions of these tissues in leaves of warm‐season grasses plus growth in hotter climates, which tends to speed maturity, combine to limit breakdown of warm‐season grass leaves compared to cool‐season species. Legume leaflets in warm‐ and cool‐season species can be readily degraded due to a high proportion of mesophyll. However, leaflets of certain species or cultivars with high tannin concentrations are poorly degraded, with mesophyll and vascular bundles comprising the residue. Grass stems have a more rigid structure than leaves, with the non‐digestible epidermis, sclerenchyma ring, and vascular tissue occupying about 30% of the tissues in the cross sections. In addition, mature grass stems have a poorly degraded parenchyma that contributes to a rigid structure in the residue. Parenchyma in legume stems is totally degraded, leaving a residue of a hollow cylinder surrounded by a uniformly lignified ring. Possibly, variations between structures of mature grass and legume stems contribute to observed differences in forage quality for these forage types. Rumen bacteria and fungi both physically associate with the more recalcitrant forage tissues. Fungi are better able to colonize the lignocellulosic tissues and can partially degrade the most resistant xylem tissues. Research avenues for improving forage quality are: (i) to enhance the ability of anaerobic fungi to weaken lignocellulosic barriers, (ii) to reduce the amount of non‐degradable barriers, particularly in stems, by plant breeding, and (iii) to improve by plant breeding the rate and extent of slowly or partially degraded tissues in leaves.

Sequential Leaf Senescence and Correlatively Controlled Increases in Xylem Flow Resistance

In bean, Phaseolus vulgaris L. (Contender), the directly measured hydraulic resistance of the xylem pathway between roots and primary leaf pulvinal junctions increased rapidly and progressively from 21 to 28 days after planting. These increases in xylem resistance (+390%) were specifically located in the pulvinal junction of the primary leaf. Moreover, they occurred just prior to the onset of primary leaf yellowing. Developmental increases in xylem hydraulic flow resistance and stomatal resistance, as well as subsequent primary leaf yellowing, were completely prevented by detopping the shoots above the primary leaves at 21 days. Thus, the onset of these senescence-associated symptoms was correlatively controlled. In short-term investigations of the mechanisms involved, flow between petiole and cut tip of excised leaves was rapidly reduced by infiltration of 20 picomoles of soluble dextran into the xylem. Moreover, imbibition of approximately 120 picomoles of dextran by excised leaves increased stomatal resistances. A programmed secretion of hormonal concentrations of similar polysaccharides into specific xylem sections in vivo might provide a mechanism for regulating the partitioning of essential xylem supplies between leaves, thus inducing sequential leaf senescence.

CORN RESPONSE TO RESTRICTED NODAL ROOT GROWTH WITH RELEVANCE TO ZERO TILLAGE

In Ontario, corn (Zea mays L.) yields with zero tillage are 10–15% lower than those with conventional tillage. Slower growth with zero tillage usually begins at the four-or five-leaf stage and continues until the 10- to 12-leaf stage. We hypothesized that restriction of nodal root development occurs with zero tillage and causes the reductions in shoot growth and final yield. Two greenhouse experiments were conducted in which nodal root development of corn plants was restricted by soil compaction or dry soil while the seminal roots grew in flowing nutrient culture. Compaction reduced nodal root length by 54%, and dry soil reduced it by 90% at the 10-leaf stage. Shoot dry weight at the 12-leaf stage was significantly reduced by dry soil but not by compaction. Leaf water potential and stomatal conductance at the 12-leaf stage were reduced by dry soil despite a negligible drop in pressure potential across the mesocotyl. Dry soil reduced shoot growth in terms of plant height after the eight-leaf stage. It was concluded that restriction of nodal root growth in zero tillage systems probably would not account for the reduced yields. Key words: Corn, Zea mays L., growth regulator, seminal roots, mesocotyl, xylem resistance

An analysis of resistance to water flow through wheat and tall fescue leaves during pressure chamber efflux experiments

Abstract. This is a physical analysis of water movement in wheat (Triticum) and tall fescue (Festuca arundinacea) leaves placed in the Scholander pressure chamber. It takes into account the efflux resistances of water movement through the xylem and water flow across the cell membranes. Xylem resistance was estimated using Poiseuille's law.Leaves which had been pressurized in the chamber were embedded, sectioned, examined under a light microscope and photographed. Cells were intact but distorted and xylem vessels were intact. Cells in portions of the blade squeezed by the chamber sealing grommet were crushed, but xylem vessels remained intact.By applying pressure several tenths of a megapascal in excess of the balance pressure, water was forced from each leaf through the severed end which protruded from the chamber. Efflux curves were drawn by plotting the total water expressed as a function of time after the pressure increase. Water efflux from the shortest wheat leaf lasted only 10 min while efflux from the longest continued for up to 40 min. The efflux from a tall fescue leaf which was rehydrated and cut to a shorter length was much more rapid than efflux from the original leaf.Experiments combined with mathematical analysis suggested that the effect of leaf length on efflux is related to a high resistance to water flow through vascular bundles. Xylem resistance would be sufficient to produce this effect if it were 10 times greater than that predicted by Poiseuille's law. Both the observations of water flow from the cut end of the leaf and the mathematical model suggested very little water flows from bundles with vessels of diameter less than 12 μm. The apparent explanation is high resistance to water flow through these small diameter vessels.

Xylem Embolism in the Palm Rhapis Excelsa

Xylem failure via gas embolism (cavitation) was investigated in Rhapis excelsa (Palmae). Embolism was detected using measurements of xylem flow resistance in excised stems and petioles: a decrease in resistance after the removal of flow-impeding embolisms by a pressure treatment indicated their previous presence in the axis. Results suggested that Rhapis avoids serious damage from embolism in at least four ways. 1) Xylem pressure potentials reached embolism-inducing levels (c. -2.90 MPa) only during prolonged drought. 2) When embolism did occur, it was confined to leaf xylem; stem xylem, most critical to shoot survival, remained fully functional. This is due in part to hydraulic architecture: 70 to 85% of shoot xylem resistance is in the leaf, and thus xylem pressures are much lower in leaves than stems. 3) Even during prolonged drought, the amount of embolism is probably limited by complete stomatal closure, which occurred at xylem pressure potentials of -3.20 ± 0.18 MPa. 4) Embolism is potentially reversible during prolonged rains, since embolism dissolved within 5 h at zero pressure (atmospheric), and xylem pressure potential can reach zero during extended rain.

Distribution of Xylem Resistance to Water Flow in Stems and Branches of Hardwood Species

Distribution of Xylem Resistance to Water Flow in Stems and Branches of Hardwood Species

A model for the water relations, photosynthesis, and expansive growth of crops

A conceptual model was developed for studying the integrated effects of soil, crop, and climatic conditions on the expansive growth, photosynthesis, and water use of agricultural crops. The plant was represented by water storage capacity, leaf area index, and osmotic, turgor, and total leaf water potentials. Water loss from the leaves was based on Monteith's combination equation for vapor flux. The resulting gradient between leaf and root water potentials caused a liquid water flux through the xylem resistance. The water potential gradient between root xylem and bulk soil causes water flux as the system departs from thermodynamic equilibrium. Resistances to water flux into the root depended on soil hydraulic conductivity, root density, and root radial resistance as defined by Taylor and Klepper (1978). Failure of the root system to supply water rapidly enough to replace water lost by the leaves resulted in decreased turgor and osmotic potentials. Stomatal control and expansive growth of leaves was a function of leaf turgor pressure. The model was used to simulate 10‐day drying conditions for sand and clay soils and for various combinations of root densities, root zone depths, and potential evapotranspiration rates. Diurnal and total daily values of transpiration and photosynthesis predictions suggest that soil root zone depth and soil hydraulic conductivities are very important factors in determining the maintenance of favorable turgor potential in the plant. Results also suggest that the relationship between daily transpiration and soil water potential is not unique but highly dependent on soil properties. The model is intended for developing simpler relationships for specific soils for describing the effects of various atmospheric conditions on plant growth processes such as transpiration, photosynthesis, and growth.

SIMULATED FLOW THROUGH THE ROOT XYLEM

Flow through the xylem of a complex root system is simulated to measure xylem resistance to water flow, and to determine whether or not this resistance is significant. Equations are set up that model xylem water flow as if it were flow through thin tubes. The root characteristics needed to solve these equations are obtained from an earlier work. Xylem water flow is first considered for a single main root and its laterals. It decreases with age of the root or increase of root length, and it increases with the addition of laterals. The advantage of an increasing number of main roots is also shown. Finally, flow through the plant crown is compared to Thornthwaite and Blaney-Criddle evapotranspirational estimates. Less than 10 millibars of water pressure is needed to overcome xylem resistance, and, therefore, the resistance is considered negligible.

The water relations of hemlock (Tsuga canadensis). V. The localization of resistances to bulk water flow

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 leaves near the cut basal end of the shoot experience a smaller xylem resistance than leaves near the apex. The kinetics of water exchange from water in the air spaces of infiltrated shoots indicates that there is a barrier at the air–water interface of leaves that equals the normal shoot resistance to bulk water flow. The activation energy for water flow through the xylem alone was measured to be about 17 × 103 J/mol. Although the leaves contribute a measurable amount to the overall resistance to water flow, the mathematical description of the system is much more complicated than previously supposed.

Water relations of Fusarium wilt in tomato

The possible causes of wilting in tomato plants infected with Fusarium oxysporum f.sp. lycopersici were examined. Determinations of leaf water potential and solute potential showed that the wilting was due to water stress. The diffusive resistance of leaves to water vapor loss in infected plants was as high as or higher than the resistance in healthy plants at a given leaf water potential, and it was concluded that an alteration in transpirational behavior did not cause water stress to occur in infected plants. Measurements of water flow through excised root systems indicated that infection did not increase the resistance of roots to water flow. When water was forced through stem segments the resistance of infected segments was found to be several times the resistance of healthy segments. However, accurate estimation of xylem resistance in infected plants was impossible by this technique because the resistance of infected segments decreased markedly as water flow occurred. Apparently, a major portion of the resistance in infected xylem can be attributed to material that was removed by abnormally high rates of water flow. In general, the results confirm the hypothesis that a high xylem resistance to water flow is the sole cause of the wilting which characterizes Fusarium wilt.

Resistance to Water Movement in Tomato Plants infected with Fusarium

PATHOGENS causing vascular wilt diseases grow in the xylem tissues of their host and induce severe wilting of the foliage, possibly by causing an impediment in the movement of water in the xylem1,2. Measurements of water flux through stem segments have indicated that the xylem resistance of infected plants may be increased twenty-fold3,4. Xylem resistance is normally so low, however, that such an increase would not be expected to result in wilting5. Furthermore, there has been no clear demonstration that a decrease in water potential is associated with the observed wilting. The measurements of transpiration and leaf water potential reported here show that Fusarium oxysporum Schlect. f.sp. lycopersici (Sacc.) Snyd. and Hans. causes an increase in the xylem resistance of tomato (Lycopersicon esculentum Mill.) sufficient to account for leaf wilting.