Late Maturation of Large Metaxylem Vessels in Soybean Roots: Significance for Water and Nutrient Supply to the Shoot

The large, late metaxylem (LMX) in the roots of soybean beginsdevelopment in the centre of the stele after lignification ofthe early metaxylem poles. Subsequent maturation of the firstappearing LMX elements is gradual. They were never mature inthe 8-d-old seedlings examined. In 10 to 15-d-old plants thefirst LMX matured to open vessels at a mean of 17 cm proximalto the root tip. Additional LMX vessels developed in more proximalregions of the roots and these also matured gradually. Based on calculations from relative vessel diameters, the potentialflow of xylem sap in a single central LMX vessel is 50 timesthat in the total of all the early metaxylem (EMX) vessels ofa typical primary root of soybean. There was a marked dependence of relative leaf area on the lengthof primary root with open LMX vessels. This may result fromthe predicted increased water and nutrient flow to the shoot,facilitated by the opening of the large vessels. It is suggestedthat, as in maize, the living LMX elements may function in ionaccumulation. Dicotyledonous roots, soybean, Glycine max, xylem vessels, xylem maturation, water conduction     

Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions.The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.     

The water status of the roots of soil-grown maize in relation to the maturity of their xylem

The long delayed maturation of the late metaxylem of maize ( Zea mays ) roots imposes a high-resistance barrier between the immature apices and the negative water potential of the leaves. These apices (20+ cm) bear strongly adhering soil sheaths to within 0.5 to 2 cm of the distal end. It was hypothesized that the sheathed immature apices should show less response to transpiration stress than bare regions. Measurements were made of the relative water content (RWC) of the sheathed and bare zones of the axile roots, both at different ages of the plant, and early and late in the day's transpiration. Sheathed roots maintained a steady RWC of about 83% irrespective of age or transpiration. Bare roots had RWCs of about 63% in the morning, but this fell to 55% in the afternoon. The first-order branches on the bare roots in the morning had still lower values of RWC, near 50%. Plots of RWC against water potential were indistinguishable for the three root types. It is concluded that the immature apices are indeed relatively isolated from the fluctuating tensions in the stem xylem, and that these tensions reduce the water content of bare roots and their branches to low values.     

The Effect of Soil Compaction on Differentiation of Late Metaxylem in Common Bean (Phaseolus vulgaris L.)

Recent studies among several plant species have shown that maturationof the largest vessels in primary xylem of roots occurs muchlater than is commonly assumed. These results have importantimplications for studies of water and nutrient uptake sincethe condition of the vessels, termed late metaxylem (LMX), mighthave a large effect on the potential conductivity of the xylem.To determine whether this phenomenon occurred in common bean(Phaseolus vulgaris L.), patterns of root xylem differentiationwere studied in young bean plants. Soil bulk density was variedin one trial to determine whether differentiation of LMX wassensitive to the growing medium. Vessels of LMX lost cell contentsand-became functional conduits between 100 and 150 mm from theroot apex. Increasing soil bulk density caused the zone of maturationof LMX to shift toward the root apex, but this zone was nevercloser than 67 mm. In the region where the primary root increasedin diameter as it merged with the hypocotyl, a zone was foundwhere vessels increased in number, had a reduced diameter, andwere arranged in a ring, the normal tetrarch arrangement ofthe xylem being lost. Potential conductivity in this zone wasconsiderably less than in zones with conventional large LMXvessels, so the zone appears to present an important restrictionto water transport from the root to the shoot. Thus, while thephenomenon of late maturation of LMX occurs in common bean,its significance in transport of water from roots to shootsis unclear Phaseolus vulgaris L., common bean, metaxylem, soil compaction, roots, anatomy     

Living Vessel Elements in the Late Metaxylem of Sheathed Maize Roots

The two types of nodal roots of field-grown maize, sheathedand bare, were found to have such different water conductivitiesthat an investigation of the anatomy of their large metaxylemvessels was made. While the vessels of the bare roots were openfor scores of centimetres, those of the sheathed roots werefound to be not vessels but developing vessel elements, withcross walls at 1 mm intervals, and protoplasts. The cross wallsbetween the elements had several unique histochemical properties.Previous investigators have often failed to find the cross wallsbecause they are very easily dislodged during the usual methodsof tissue preparation. They are best identified by microdissectionof fresh xylem. The living elements persist in the late metaxylemup to 20 – 30 cm from the tip. As the roots become longerthan this both the cross walls and the soil sheaths disappearand there is a transition to a bare root with open vessels inthe proximal region. The soil sheath persists a little longerthan the cross walls. The two types are thus stages in a developmentalsequence through which all nodal roots pass. A fundamental differencebetween the two types is in their water status, since the estimatedconductive capacity of a bare root is about 100 times greaterthan that of a sheathed root. These observations point to theneed for a reassessment of the published work on transport ofions into the xylem of grass roots through a reinvestigationof the ‘maturity’ of their xylem vessels. Grass roots, dimorphic roots, ion secretion to xylem, soil sheaths, xylem vessels, xylem differentiation, water conduction, Zea mays L     

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field-grown maize ( Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X-ray microanalysis in unetched fracture faces of bulk, frozen-hydrated pieces of sheathed roots, and found to be in the range 150–400 m M . Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 m M . It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.     

Solute concentrations in xylem sap along vessels of maize primary roots at high root pressure

The elemental composition of xylem sap has been determined by cryo-analytical microscopy in situ along vessels in the roots of maize plants frozen intact while root pressure was high. The only chemical element (including carbon) present in significant concentrations in the vessels was potassium at 11 mM and 15 mM in the late (LMX) and early (EMX) metaxylem, respectively. There was no gradient of [K] along the vessels, which each run the length of the mature proximal end of the roots. At the distal end of each vessel, in the oldest still living vessel, which each run the length of the mature proximal end of the roots. At the distal end of each vessel, in the oldest still living vessel elements, there was sharp rise in [K] to 110 mM and 130 mM in the LMX and EMX, respectively.     

Development of water conducting capacity in the root systems of young plants of corn and some other c4 grasses

Development of the primary and early nodal roots was studied in Zea mays L., Zea mexicana (Schrad.) Reeves & Mangelsd., Sorghum bicolor (L.) Moench., and Sorghum sudanese (Piper) Stapf. in relation to shoot development. In all the types studied all roots reached lengths of about 30 centimeters before the late metaxylem (LMX) was open, and young plants with total root lengths of around 100 centimeters had almost no open LMX. On average, corn seedlings with up to 36 square centimeters of leaf had no open LMX. The name “immature apices” is suggested for such long but not fully functional roots. In plants up to 50 days old a fairly constant proportion of less than half the total root length had open LMX. A pilot study of stomatal resistance on days of high evaporative demand suggested that young seedlings may show higher resistance than older plants in the afternoon. Estimates of longitudinal permeability of corn roots with only early metaxylem vessels open indicate very steep gradients of water potential would develop under such conditions.     

Effects of Temperature on the Development of Metaxylem in Primary Wheat Roots and Its Hydraulic Consequence

The effects of different temperatures on the development ofmetaxylem were studied in the primary seminal root of winterwheat (Triticum aestivum L.) seedlings. Xylem development wasstudied microscopically at different distances behind the rootapex after safranin staining to reveal lignification. Diameter of the central late metaxylem (LMX) and its proportionto the stele cross-sectional area increased in the acropetaldirection. Diameter of the LMX and stele decreased with an increasein growing temperature. Numbers of early metaxylem (EMX) wereseven, seven and six at 10, 20 and 30 C, respectively. EMXwas lignified much more rapidly than the LMX along the seminalroot axes. Lignification of xylem elements commenced furthertowards the root apex at the higher temperatures. The LMX vesselsof the roots grown at the higher temperature had thicker secondarywalls. The relative conductivity of seminal roots, calculated fromPoiseuille's equation, decreased as growing temperature increased.In a drought-prone environment where wheat plants rely heavilyon stored soil water, a lowered axial conductivity in the rootswould be advantageous. The plants would tend to conserve waterduring vegetative growth for use during the critical periodsof flowering and grain-filling. Breeders selecting wheat plants for altered LMX diameters shouldcontrol temperatures during primary root development, sectionthe roots at the same distance from the tip and be aware thatcross walls may exist in the LMX for up to 30 cm from the tip. Wheat, Triticum aestivum L., roots, xylem development, hydraulic conductivity, temperature     

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.Abbreviations and Symbols CPP cell pressure probe - EMX early metaxylem - LMX Late metaxylem - P_c cell turgor - P_r root pressure - RPP root pressure probe - t_1/2,c half-time of water exchange across a single cell - t_1/2 half-time of water exchange across multiple cells We thank Antony Matista for his expert assistance in the construction and modification of instruments. The work was supported by grant DCB8802033 from the National Science Foundation and grant 91-37100-6671 from USDA, and by the award of a Feodor Lynen-Fellowship from the Alexander von Humboldt-Foundation (Germany) to J.F.     

The Developmental Anatomy of the Seminal Root of Wheat

Wheat embryo usually gives rise to five seminal roots in matured caryopsls, although, the sixth root might develop in some cases. The first one is known as the primary root. Primary root emerged early, and its primodium was distinctly originated from the proembryo and could be gradually identified as three layers of initials. Lateral seminal roots emerged later from the embryonic axis in pairs, and originated from the surrouding cells of the procambium. Differentiation of lateral roots was much more vigorous than that of the first seminal root (primary root), and, its mother cells of metaxylem vessel appeared soon, Lateral seminal roots usually had more metaxylem vessels. In short, only the first root is the primary root, the lateral seminal roots are adventitious in nature, since their structures are similar to those of other adventitious roots.     

Accumulation of high levels of potassium in the developing xylem elements in roots of soybean and some other dicotyledons

Summary Potassium concentrations have been determined by cryo-analytical scanning microscopy in vacuoles of cells of the roots of soybean and six other dicotyledons. Developing vessel elements accumulate the highest concentration of potassium in any cell type in these roots, and those of the secondary xylem have more (median 190 mM normalized to 120%) than those of either the early (median 100 mM, normalized 80%) or late metaxylem (median 110 mM, normalized 100%). Potassium concentration in these developing vessels always exceeds those of their adjacent parenchyma (60–80%), which in turn are higher than those in cells of the cortex (30–40%), including the endodermis. Potassium concentration in the vessels increases during their development until cell death and maturation when it drops dramatically (normalized 4–12%). Developing vessel elements are clearly major sites of potassium accumulation in roots and need to be considered in any models of ion uptake, accumulation, circulation, or exudation.Abbreviations EDX energy dispersive X-ray microanalysis - EMX early metaxylem - K elemental potassium - LMX late metaxylem - CSEM cryo-scanning electron microscopy - SX secondary xylem Dedicated to the memory of Professor John G. Torrey     

小麦种子根的发育解剖

小麦胚胎发育过程中通常形成5条幼根(少数可形成6条),这些根统称为种子根,中间最先发生的为初生根.初生根的原基在胚胎发育的早期就在胚轴的一侧发生,原基细胞由不规则到规则排列。侧生种子根的原基在胚胎发育后期才出现,通常成对发生,并且是由胚轴上的节(盾片节和胚芽鞘节)维管束外方的细胞形成。侧生种子根的发育明显较初生根的快,分化能力也较强,后生木质部导管母细胞出现早,数目较多.因此,小麦胚胎发育过程中从胚轴上形成的这些侧生的种子根,形态上,仍应看作是一些不定根,其结构特征与后来形成须根系的不定根的比较近似。     

Anatomy of Seedling Roots of Tropical Maize (Zea mays L.) Cultivars at Low Water Supply

Establishment of maize seedlings can be difficult at low soilmoisture content. Anatomy of root metaxylem vessels may influencethe capacity for water transport and respective genotypic differencesmight be useful for selection purposes. To test this, six tropicalmaize (Zea mays L.) cultivars were grown in large PVC tubescontaining a sandy substrate at 5% (M5) and 10% (M10) moisturecontents for 2 weeks. The percentage changes in root diametersdue to M5 was similar for most cultivars but differed for mainroot types. Root diameters were not consistently related tometaxylem structure, but in a few cases, thin roots had smallerdiameter metaxylem vessels. The M5 treatment reduced the numberof late metaxylem vessels of primary roots by about 0 to 20%,while effects on nodal roots were slight. Generally, the ratioof cross-sectional areas between late and early metaxylem vesselsincreased from primary to seminal and nodal roots. Within thecultivar Tuxpefio this ratio was much reduced by M5. A few cultivarsmaintained the combined cross-sectional areas of metaxylem vesselsat M5 in some main root types, but only one cultivar could achievethis for the total of cross-sectional areas of metaxylem vessels,calculated over all root axes, by increasing the number of seminaland nodal roots. These anatomical traits seemed to be mostlyconstitutive with limited response to an actual environment,but they could be decisive for the suitability of a cultivarto an environment with frequent water shortages during seedlingestablishment. Key words: Metaxylem vessels, water stress, tropical maize     

Effect of heavy metals and temperature on the oxalate oxidase activity and lignification of metaxylem vessels in barley roots

The effect of Cd on oxalate oxidase (OxO) activity and its localisation were analysed in barley root. In Cd-treated roots OxO activity was strongly induced in the region 2–4 mm behind the root tip and in the area toward the root base. In situ analyses showed that Cd-induced OxO activity was localised to the cell wall (CW) of early metaxylem vascular bundles and surrounding parenchyma cells and was accompanied by lignification of metaxylem vessels. OxO activation was also observed during treatment with other heavy metals (HMs), salt treatment and at elevated non-optimal temperature. In contrast to HM activation of OxO and lignification, high temperature and NaCl indeed activated OxO but did not induce lignification of metaxylem vessels. These results suggest that oxalate oxidase as an H₂O₂-generating enzyme is activated in response to several stresses, however the ectopic lignification of metaxylem vessels is activated specifically by HMs. This HM-induced premature root xylogenesis due to ectopic lignification of metaxylem vessels probably causes shortening of the root elongation zone and therefore a reduction in root growth.     

Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

A new approach to study dynamic interactions between transpiration and xylem pressure in intact plants is presented. Pressure probe measurements were preformed in living (immature) late metaxylem of maize roots rather than in adjacent mature xylem. This eliminated technical limitations related to the measurement of negative pressures. Water relations of single cells showed that turgor and volumetric elastic modulus were significantly larger in living metaxylem than in cortical cells; hydraulic conductivity was similar in both types of root cells. Increasing transpiration induced an immediate decrease of xylem pressure, and vice versa. Turgor in the living metaxylem could be continuously recorded for more than 1 h. The relationship between xylem pressure and transpiration yielded a root hydraulic resistance of 1.3 x 10⁹ MPa s m^-3. Control experiments indicated that the response of living xylem in the positive pressure range essentially paralleled that of mature root xylem in the negative range. In mature xylem, pressures as low as -0.55 MPa were recorded for short periods (several minutes). Several tests verified that the pressure probe was in contact with mature xylem during the measurements of tensions. The results demonstrate convincingly that transpiration generates an effective driving force for water uptake in roots, a central feature of the cohesion theory.Key words: Hydraulic conductivity, negative pressure, root development, turgor, water transport, Zea mays.      

Effect of heavy metals and temperature on the oxalate oxidase activity and lignification of metaxylem vessels in barley roots

The effect of Cd on oxalate oxidase (OxO) activity and its localisation were analysed in barley root. In Cd-treated roots OxO activity was strongly induced in the region 2–4 mm behind the root tip and in the area toward the root base. In situ analyses showed that Cd-induced OxO activity was localised to the cell wall (CW) of early metaxylem vascular bundles and surrounding parenchyma cells and was accompanied by lignification of metaxylem vessels. OxO activation was also observed during treatment with other heavy metals (HMs), salt treatment and at elevated non-optimal temperature. In contrast to HM activation of OxO and lignification, high temperature and NaCl indeed activated OxO but did not induce lignification of metaxylem vessels. These results suggest that oxalate oxidase as an H₂O₂-generating enzyme is activated in response to several stresses, however the ectopic lignification of metaxylem vessels is activated specifically by HMs. This HM-induced premature root xylogenesis due to ectopic lignification of metaxylem vessels probably causes shortening of the root elongation zone and therefore a reduction in root growth.     

Drought-induced changes in soil contact and hydraulic conductivity for roots of Opuntia ficus-indica with and without rhizosheaths

Water movement between roots and soil can be limited by incomplete root–soil contact, such as that caused by air gaps due to root shrinkage, and can also be influenced by rhizosheaths, composed of soil particles bound together by root exudates and root hairs. The possible occurrence of air gaps between the roots and the soil and their consequences for the hydraulic conductivity of the root–soil pathway were therefore investigated for the cactus t Opuntia ficus-indica, which has two distinct root regions: a younger, distal region where rhizosheaths occur, and an older, proximal region where roots are bare. Resin-embedded sections of roots in soil were examined microscopically to determine root–soil contact for container-grown plants kept moist for 21 days, kept moist and vibrated to eliminate air gaps, droughted for 21 days, or droughted and vibrated. During drought, roots shrank radially by 30% and root–soil contact in the bare root region of nonvibrated containers was reduced from 81% to 31%. For the sheathed region, the hydraulic conductivity of the rhizosheath was the least limiting factor and the root hydraulic conductivity was the most limiting; for the bare root region, the hydraulic conductivity of the soil was the least limiting factor and the hydraulic conductivity of the root–soil air gap was the most limiting. The rhizosheath, by virtually eliminating root–soil air gaps, facilitated water uptake in moist soil. In the bare root region, the extremely low hydraulic conductivity of the root–soil air gap during drought helped limit water loss from roots to a drier soil.     

Is Xylem Embolism and Refilling Involved in the Rapid Wilting and Recovery of Plants Following Root Cooling and Rewarming?

Abstract: Rapid wilting and subsequent rapid recovery of the shoots of plants whose roots are cooled and rewarmed (first described by Sachs, 1860^[23]), has been investigated by cryo-scanning electron microscopy. Squash plants began to wilt within 5 min and were completely wilted 1 h after their roots were placed in nutrient solution at 4C. Recovery began in 5 min and was complete by 45 min when the roots were returned to solution at 22C. Some stomata on the abaxial leaf surface remained fully or partially open in the wilted plants and transpiration continued at a low level. Both control and wilted plants had the same proportion (60%) of large root vessels partially or totally gas-filled, showing that the supply of water was not limited by the reduction of axial hydraulic conductance due to vessel embolism. However, only 10% of these embolized vessels in the wilted plants contained any liquid, compared to ≅ 80% of similar vessels in control and recovered plants. This is visual evidence of reduced radial hydraulic conductance into the vessels in the cold roots, and that this reduced conductance, together with still open-stomata, produces wilting. These effects were reversed by rewarming.     

Subcellular localization of mineral deposits in the roots of wheat (Triticum aestivum L.)

Summary Mineral distribution in the roots of wheat (Triticum aestivum L. cv. Wheaton) was investigated using X-ray microanalysis of bulk frozen hydrated roots in SEM and of freeze substituted sections in TEM. Results obtained using the two methods agreed reasonably well. A total often elements were detected: Na, Mg, Si, P, S, Cl, K, Ca, Mn, and Fe. Of these Si, P, Ca, and Mn were incorporated into biomineralized structures. Silica was deposited in the endodermal walls in the older parts of the root. Silicon was also detected in the large central metaxylem lumina in the basal zone of the root, and in the smaller peripheral metaxylem and the immediately contiguous pericycle and outer parenchyma cells bridging the small metaxylem vessels to the endodermal layer. In the basal zone of the root some of the inner cortical cells contained intracellular electron opaque deposits. These were associated with the cell walls, had non-opaque inclusions and microanalysis revealed that they consisted of calcium, phosphorus and manganese.Abbreviations A apical zone of root - M midzone of root - B basal zone of root - SEM scanning electron microscope - TEM transmission electron microscope