Pressure and flow relations in vascular bundles of the tomato plant

In the tomato plant water flows through primary xylem in accordance with Poiseuille's law. This relation and the analogy between Poiseuille's and Ohm's law were employed to calculate rates of flow and differences in pressure within vascular bundles when transpiration rates from individual leaves were known. The resistance of vascular bundles to flow was calculated from a modification of Poiseuille's law and from measurements of vessels in all bundles. The rates of flow in all bundles were derived from a set of simultaneous linear equations of flow, written to correspond with the nature of the vascular network. Values of the difference in pressure associated with flow in bundles were derived from resistances and flow rates in individual bundles. These agreed substantially with values observed in a comparable plant.

In large bundles, vessels occur in a frequency distribution that is approximately normal with respect either to the logarithms of their radii or to the fourth power of their radii. The largest vessels in a bundle transport most of the water when they are functioning.

The tomato plant contains 2 types of vascular bundle. The large bundles of the stem form a network by joining above each node in combinations of 2 at a time. The small bundles of the stem and petiolar bundles are independent of other bundles from their origins at junctions to their termini. The small bundles offer high resistance to flow, whereas the resistance of large bundles is low. The average conductance of large bundles decreases from the base to the apex of the stem. That of small vascular bundles remains low and more or less constant throughout the plant.

Only a small difference in pressure is required to maintain flow in large bundles. For lower leaves, the driving pressure required to move water to the base of a petiole is considerably less than that which moves water through petioles. The difference in pressure that maintains flow increases steadily for successively higher nodes. However, the pressure that drives flow to leaves is not always greater for higher leaves than for intermediate ones. For the plant examined, the highest leaves required a smaller amount of energy to move water from the ground than intermediate leaves did. This was also true of the power expended in moving water to individual leaves.

In the large network bundles, significant cross transfer of flow occurs at junction points from one bundic to another. Because of the interconnections between large bundles. pressure and flow relations are apparently not greatly altered when localized dysfunction occurs in the vessels of large bundles. In small, independent bundles, a localized dysfunction in vessels produces a significant effect on pressure and flow relations.

     

Can cell walls bending round xylem vessels control water flow?

Vascular bundles of petioles below wilted leaves of Nymphoides peltata (S.G. Gmel. O. Kuntze) were frozen intact and freeze-fractured for electron microscopy. Cell walls in them appeared drawn in against the helical thickenings of xylem vessels. By contrast, walls round vessels which had been frozen in vascular bundles below turgid leaves, and walls round vessels which had been fixed, embedded and sectioned, were straight or bulged outwards slightly. Walls bulged outwards slightly also from cut vessels filled with sucrose solution before freezing. Movement of vessel walls could produce the clicks audible when water cavitates in vessels, and might explain a variable resistance to the flow of water through plants.     

Foliar water supply of tall trees: evidence for mucilage-facilitated moisture uptake from the atmosphere and the impact on pressure bomb measurements

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.     

Short communication. Comparative measurements of xylem pressure in transpiring and non-transpiring leaves by means of the pressure chamber and the xylem pressure probe

Simultaneous measurements were made with the xylem pressure probe on exposed, transpiring leaves and with the Scholander pressure chamber on both transpiring and covered, non-transpiring leaves of sugarcane and maize plants. Xylem tensions inferred from pressure chamber balancing pressures on non-transpiring leaves were similar to those measured directly with the xylem pressure probe in transpiring leaves. However, tensions inferred with the pressure chamber on transpiring leaves that were placed in plastics bags just prior to excision were up to 0.6 MPa greater than those measured concurrently with the xylem pressure probe. These findings suggest that relatively large differences in water potential between the xylem and bulk leaf tissue can exist during periods of rapid transpiration, and they confirm that the balance pressure of an excised, previously transpiring leaf is only a measure of the bulk average equilibrium leaf water potential and not of the true xylem pressure that existed prior to excision.Key words: Cohesion-Tension theory, xylem pressure probe, pressure chamber, xylem tension.      

Water Flow Through Cotton Roots in Relation to Xylem Anatomy

The effect of root anatomy on water flow was studied in 7-d-oldcotton (Gossypium hirsutum L.) seedlings grown in solution culture.The total water flux of the intact root system was measuredusing a pressure chamber. Then successive terminal root sectionswere removed at 2,6,10 and 12 cm behind the root tip and theflux was remeasured after each successive cut was made. Xylemdevelopment at different distances behind the root apex wasstudied with a microscope using sections cut free-hand and stainedwith toluidine blue. Water flux increased with the removal ofsuccessive terminal root sections and this coincided with thedegree of basipetal primary xylem development. The large increasein water flux at 10 to 12 cm was associated with secondary xylemdevelopment and increased xylem vessel number. A comparison of water flow and xylem anatomy between roots withtetrarch (Stoneville 506 and Deltapine 41) and pentarch (T25strain) vascular bundle arrangements showed no significant differencesin the measured values of water flux for the primary root. Waterflux, estimated using Poiseuille's equation and measured xylemdimensions, was greater for the tetrarch roots, primarily becauseof the larger diameter of individual vessel elements. The increasednumber of vessel elements in the pentarch primary root of T25did not result in any apparent decrease in axial resistanceto water flow. Key words: Gossypium hirsutum L., roots, vascular bundle, xylem water flux, xylem     

Water Transport in Impaired Leaf Vein Systems

Abstract: The subject of our investigation was the water regime of broad bean leaves ( Vicia faba L.), especially after having mechanically severed parts of the leaf blade and the leaf venation. Under moderate conditions, 18 - 22 °C temperature and 50 - 70 % relative humidity, the leaves remained viable even after extensive damage. Only if more than 90 % of the xylem cross sectional area of a leaf was severed, the leaf wilted. Lesser damage to the xylem cross-sectional area only resulted in a reduced rate of transpiration and assimilation, compared to intact leaves. The cuts in larger veins were bypassed into small or even very small veins, as shown by xylem transport of dyes. In intact leaves, small veins have a negligible task in long-distance transport. Here, however, transport velocity in small veins was severalfold increased compared to the measurement of transport velocity in veins of the same size in intact leaves. Thereby, water transport to leaf areas distal from the cut was ensured.     

XYLEM ANATOMY,WATER FLOW,AND HYDRAULIC CONDUCTANCE IN THE FERN CYRTOMIUM FALCATUM

Xylem anatomy and water relations were studied in holly fern (Cyrtomium falcatum, Aspidiaceae) to determine the details of the pathway for water flow through an entire plant and the influence of tracheid number and lumen diameter on water flow. Each leaf has two adaxial traces and an abaxial trace, which are supplied by diarch adventitious roots attached to the dictyostele of the rhizome near the leaf base. Anatomical observations and dye experiments showed that each adaxial bundle vascularizes the approximately seven pinnae on its side of a leaf. An abaxial bundle is intermittently connected to an adaxial bundle as well as other abaxial bundles, forming a minor vascular pathway between the bundles of the leaf axis. Changes in both number and diameter of tracheids result in an acropetal decrease in hydraulic conductance per unit length along the rachis, although tracheid number locally increases when the trace for a pinna is produced in an adaxial bundle. Water flow was determined from the transpiration distal to the point in question or by forcing a solution through an axis with applied pressure. The water potential gradient along the plant axis was quite constant, indicating that hydraulic conductance per unit length varied with leaf area to be supplied. About 40% of the overall water potential drop occurred from the rachis into the pinnae, which reflected factors controlling water potential gradients in the lamina and not a very low conductance in the petiolule xylem. Hydraulic conductances calculated using the Hagen-Poiseuille equation and tracheid diameters were generally double those of measured conductances. Since the values tended to vary by a constant factor, tracheid number and diameter may largely control water flow in the xylem.     

Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. I. The concentration of cations, anions, amino acids in, and pH of, the xylem sap

Sunflower plants (Helianthus annuus L.) were subjected to soil drying with their shoots either kept fully turgid using a Passioura-type pressure chamber or allowed to decrease in water potential. Whether the shoots were kept turgid or not, leaf conductance decreased below a certain soil water content. During the soil drying, xylem sap samples were taken from individual intact and transpiring plants. Xylem sap concentrations of nitrate and phosphate decreased with soil water content, whereas the concentrations of the other anions (SO₄² and Cl^?) remained unaltered. Calcium concentrations also decreased. Potassium, magnesium, manganese and sodium concentrations stayed constant during soil drying. In contrast, the pH, the buffering capacity at a pH below 5 and the cation/anion ratio increased after soil water content was lowered below a certain threshold. Amino acid concentration of the xylem sap increased with decreasing soil water content. The effect of changes in ion concentrations in the xylem sap on leaf conductance is discussed.     

The mechanisms of refilling of xylem conduits and bleeding of tall birch during spring

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.     

Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.     

Ionic control of the lateral exchange of water between vascular bundles in tomato

Ions can enhance water flow through the xylem via changes in the hydraulic resistance at border pit membranes. Because flow between adjacent xylem vessels occurs primarily via bordered pit fields, it is hypothesized that xylem sap ion concentrations would affect lateral movement of water more than longitudinal flow. Using tomato as a model system, evidence is presented for ion-mediated changes in xylem hydraulic resistance and the lateral transport of water. Water flow between adjacent xylem bundles increased by approximately 50% in the presence of ions while longitudinal flow only increased by approximately 20%. However, the enhancement of lateral exchange due to ions was magnified by the presence of a pressure difference between vascular bundles. These results indicate that the degree of nutrient-sharing among sectors of a plant may depend on both nutrient concentration and the availability of water in the root zone.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

Effect of Interruption of Flow Path on Stomatal Conductance of Abies amabilis

Wounding of root or stem water conduction systems or coolingof roots in Abies amabilis produced rapid stomatal closure independentof evaporative demand or leaf water potential. The responsealso occurred in a branch if its xylem was only partially cut,but did not occur if the branch was completely severed. Removingpart of the root system or cooling the roots produced the sameeffect as partial severing of the stem. The speed and uniformityof stomatal closure indicated that the stimulus was physical,linked to water flux in the xylem, and not caused by releaseof a chemical stimulus at the point of xylem flow disruption.The results suggested that stomatal closure could be rapidlyinduced with a change in the flux of water through the soil-plant-atmospherecontinuum. Key words: Capacitance, Stomata, Xylem water flux, Xylem wounding     

Vulnerability of xylem vessels to cavitation in sugar maple. Scaling from individual vessels to whole branches

The relation between xylem vessel age and vulnerability to cavitation of sugar maple (Acer saccharum Marsh.) was quantified by measuring the pressure required to force air across bordered pit membranes separating individual xylem vessels. We found that the bordered pit membranes of vessels located in current year xylem could withstand greater applied gas pressures (3.8 MPa) compared with bordered pit membranes in vessels located in older annular rings (2.0 MPa). A longitudinal transect along 6-year-old branches indicated that the pressure required to push gas across bordered pit membranes of current year xylem did not vary with distance from the growing tip. To understand the contribution of age-related changes in vulnerability to the overall resistance to cavitation, we combined data on the pressure thresholds of individual xylem vessels with measurements of the relative flow rate through each annual ring. The annual ring of the current year contributed only 16% of the total flow measured on 10-cm-long segments cut from 6-year-old branches, but it contributed more than 70% of the total flow when measured through 6-year-old branches to the point of leaf attachment. The vulnerability curve calculated using relative flow rates measured on branch segments were similar to vulnerability curves measured on 6-year-old branches (pressure that reduces hydraulic conductance by 50% = 1.6-2.4 MPa), whereas the vulnerability curve calculated using relative flow rates measured on 6-year-old branches were similar to ones measured on the extension growth of the current year (pressure that reduces hydraulic conductance by 50% = 3.8 MPa). These data suggest that, in sugar maple, the xylem of the current year can withstand larger xylem tensions than older wood and dominates water delivery to leaves.     

植物体内水分长距离运输的生理生态学机制

植物体内长距离水分运输是植物生理生态学研究中的一个重要问题,长期为植物生理学家和生理生态学家所关注。木质部探针技术的问世,掀起了近年来植物生理学界最为激烈的一场争论。提出了已经有100多年,风行40年的内聚力-张力(Cohesion-Tension, C-T)学说受到质疑。随后维护派和质疑派围绕木质部探针技术、压力室技术(C-T理论的主要支撑实验技术)的可靠性展开辩论。进一步从物理学原理和各种实验上就C-T理论的3个支柱(木质部导管或管胞中巨大的张力、沿树高的压力梯度、连续水柱)进行争论。这场争论似暂告一段落,C-T理论没有被推翻,但仍留有问题期待以后的研究。     

Intact plant magnetic resonance imaging to study dynamics in long-distance sap flow and flow-conducting surface area

Due to the fragile pressure gradients present in the xylem and phloem, methods to study sap flow must be minimally invasive. Magnetic resonance imaging (MRI) meets this condition. A dedicated MRI method to study sap flow has been applied to quantify long-distance xylem flow and hydraulics in an intact cucumber (Cucumis sativus) plant. The accuracy of this MRI method to quantify sap flow and effective flow-conducting area is demonstrated by measuring the flow characteristics of the water in a virtual slice through the stem and comparing the results with water uptake data and microscopy. The in-plane image resolution of 120 x 120 microm was high enough to distinguish large individual xylem vessels. Cooling the roots of the plant severely inhibited water uptake by the roots and increased the hydraulic resistance of the plant stem. This increase is at least partially due to the formation of embolisms in the xylem vessels. Refilling the larger vessels seems to be a lengthy process. Refilling started in the night after root cooling and continued while neighboring vessels at a distance of not more than 0.4 mm transported an equal amount of water as before root cooling. Relative differences in volume flow in different vascular bundles suggest differences in xylem tension for different vascular bundles. The amount of data and detail that are presented for this single plant demonstrates new possibilities for using MRI in studying the dynamics of long-distance transport in plants.     

Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue

Xylem maturation in elongating leaf blades of tall fescue ( Festuca arundinacea ) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region in which most of the longitudinal water movements occurred in the LMX. The axial hydraulic conductivity ( K _h) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conductivity ( K _t) computed from the diameter of individual inner vessels. K _t was proportional to K _h throughout the leaf, but K _t was about three times K _h. The changes in K _h and K _t along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, K _h was about 0.30±0.07 mmol s⁻¹ mm MPa⁻¹. Beyond that region, K _h rapidly increased with metaxylem element maturation to a maximum value of 5.0±0.3 mmol s⁻¹ mm MPa⁻¹, 105 mm from the leaf base. It then decreased to 3.5±0.2 mmol s⁻¹ mm MPa⁻¹ near the leaf tip. The basal expanding region was observed to restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.     

高羊茅叶片表皮蜡质含量与其抗旱性的关系

以14个高羊茅品种为试验材料,在田间试验中对干旱高温胁迫下的叶片表皮蜡质含量、净光合速率、蒸腾速率、气孔导度、胞间CO2浓度等生理指标测定分析。结果表明,干热胁迫下高羊茅品种间的叶片表皮蜡质含量和水分利用效率均存在极显著差异(P<0.01);叶片蜡质含量与综合抗旱性和水分利用效率的等级相关系数分别为0.78(P<0.01)和0.68(P<0.01);蜡质含量越高的品种,其叶片气孔导度和胞间CO2浓度越低,水分利用效率越高,但所有品种的水分利用效率绝对值都较低。研究发现,在干热胁迫时,高羊茅叶片表皮蜡质可通过对气孔导度的调节来减少气孔蒸腾,提高水分利用效率,最终提高其抗旱性;表皮蜡质含量可以作为高羊茅品种抗旱性鉴定的一个新指标。     

Direct measurement of xylem pressure in leaves of intact maize plants. A test of the cohesion-tension theory taking hydraulic architecture into consideration

The water relations of maize (Zea mays L. cv Helix) were documented in terms of hydraulic architecture and xylem pressure. A high-pressure flowmeter was used to characterize the hydraulic resistances of the root, stalk, and leaves. Xylem pressure measurements were made with a Scholander-Hammel pressure bomb and with a cell pressure probe. Evaporation rates were measured by gas exchange and by gravimetric measurements. Xylem pressure was altered by changing the light intensity, by controlling irrigation, or by gas pressure applied to the soil mass (using a root pressure bomb). Xylem pressure measured by the cell pressure probe and by the pressure bomb agreed over the entire measured range of 0 to −0.7 MPa. Experiments were consistent with the cohesion-tension theory. Xylem pressure changed rapidly and reversibly with changes in light intensity and root-bomb pressure. Increasing the root-bomb pressure increased the evaporation rate slightly when xylem pressure was negative and increased water flow rate through the shoots dramatically when xylem pressure was positive and guttation was observed. The hydraulic architecture model could predict all observed changes in water flow rate and xylem. We measured the cavitation threshold for oil- and water-filled pressure probes and provide some suggestions for improvement.