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1.
The dimorphic exodermis of the root of onion (Allium cepa L.) consists of long and short cells, both of which have Casparian bands. The long cells and some of the short cells also have suberin lamellae. The proportion of short cells with lamellae increases with distance from the root tip and with plant age, but is not influenced by drought stress. In young regions of onion roots, characterized by a mature endodermis and an immature exodermis, the plasmalemma surface area that can be contacted by the soil solution is 90·9 mm2 per mm length of root, i.e. the sum of the plasmalemma surface areas of the epidermis, immature exodermis, cortical parenchyma and endodermis external to the Casparian band. This is reduced to 14·5–14·7 mm2 by the development of a Casparian band in the exodermis, which cuts off access to the cortical parenchyma, and by the development of suberin lamellae, which cut off access to the plasmalemmae of the long and some of the short cells of the exodermis. Death of all the epidermal cells, a consequence of drought, further reduces this area to 0·205–0·0183 mm2, i.e. the area of the outer tangential plasmalemmae of the short cells without suberin lamellae. In this condition, the root's capacity for ion uptake should be reduced but its capacity to resist water loss to the soil should be increased. 相似文献
2.
Rice plants were grown hydroponically and roots were prepared for light and electron microscopy using standard techniques. The roots are bounded by an epidermis, exodermis, and fibrous layer. The exodermis has a suberin lamella along its inner tangential wall. The fibrous layer is composed of thick-walled lignified cells with little pitting. The cortical parenchyma is compact when young, but expands and separates to form a zone of cell walls and air spaces in a spoked arrangement. Supporting columns of living parenchyma cells are occasionally present, particularly near lateral roots. The endodermis is typical for grasses with Casparian strips, suberin lamellae, and tertiary state walls with numerous pits. The pericycle and pith become sclerified. Protoxylem elements alternate with protophloem in the young root; later, early metaxylem, late metaxylem, and metaphloem proliferate. The exodermis, fibrous layer, lacunate cortex, and endodermis appear to present a formidable barrier to radial ion movement in the mature portions of the root. 相似文献
3.
Growing tree roots are characteristically brown with white tips. The browning process, which occurs as the white region matures, has often been attributed to the deposition of suberin in various tissues. However, in pouch-grown tree seedlings of jack pine (Pinus banksiana Lamb.) and eucalyptus (Eucalyptus pilularis Sm.), browning was not linked to suberization but was caused by the deposition of condensed tannins in the walls of all cells external to the stele. Therefore, we propose using the term “tannin zone” to refer to this region of the root. Vitality tests indicated that the cells of the epidermis and cortex were alive in white regions but were dead in brown regions. Following sequential treatment with berberine hemisulfate and potassium thiocyanate, the cortical walls external to the endodermal Casparian band were full of berberine thiocyanate crystals, indicating that they were permeable to berberine. These walls should also be permeable to water and ions, which have smaller molecular dimensions than the tracer dye. Based on the anatomy and permeability of the tannin zone, we predict that its capacity for ion uptake would be reduced compared to the white zone because of a reduced absorptive plasmalemma surface area. In jack pine, some uptake could be effected by the passage cells of the endodermis. The tannin zone should be even less absorptive in eucalyptus because the exodermis remains an apoplastic barrier and the endodermis lacks passage cells. It is difficult to predict the difference between the tannin and white zones with respect to water uptake. Death of the cells external to the endodermis would reduce the resistance of the root to water movement, but deposition of tannins would increase it. The deposition of suberin lamellae in increasing numbers of endodermal cells may also retard water flow. The anatomy and physiological properties of the tannin zone are unique from those of the distal, white zone and the proximal, cork-clad zone. 相似文献
4.
Root Endodermis and Exodermis: Structure, Function, and Responses to the Environment 总被引:9,自引:0,他引:9
Daryl E. Enstone Carol A. Peterson Fengshan Ma 《Journal of Plant Growth Regulation》2003,21(4):335-351
Roots of virtually all vascular plants have an endodermis with a Casparian band, and the majority of angiosperm roots tested also have an exodermis with a Casparian band. Both the endodermis and exodermis may develop suberin lamellae and thick, tertiary walls. Each of these wall modifications has its own function(s). The endodermal Casparian band prevents the unimpeded movement of apoplastic substances into the stele and also prevents the backflow of ions that have moved into the stele symplastically and then were released into its apoplast. In roots with a mature exodermis, the barrier to apoplastic inflow of ions occurs near the root surface, but prevention of backflow of ions from the stele remains a function of the endodermis. The suberin lamellae protect against pathogen invasion and possibly root drying during times of stress. Tertiary walls of the endodermis and exodermis are believed to function in mechanical support of the root, but this idea remains to be tested. During stress, root growth rates decline, and the endodermis and exodermis develop closer to the root tip. In two cases, stress is known to induce the formation of an exodermis, and in several other cases to accelerate the development of both the exodermis and endodermis. The responses of the endodermis and exodermis to drought, exposure to moist air, flooding, salinity, ion deficiency, acidity, and mechanical impedance are discussed. 相似文献
5.
Daryl E. Enstone Carol A. Peterson Fengshan Ma 《Journal of Plant Growth Regulation》2002,21(4):335-351
Roots of virtually all vascular plants have an endodermis with a Casparian band, and the majority of angiosperm roots tested also have an exodermis with a Casparian band. Both the endodermis and exodermis may develop suberin lamellae and thick, tertiary walls. Each of these wall modifications has its own function(s). The endodermal Casparian band prevents the unimpeded movement of apoplastic substances into the stele and also prevents the backflow of ions that have moved into the stele symplastically and then were released into its apoplast. In roots with a mature exodermis, the barrier to apoplastic inflow of ions occurs near the root surface, but prevention of backflow of ions from the stele remains a function of the endodermis. The suberin lamellae protect against pathogen invasion and possibly root drying during times of stress. Tertiary walls of the endodermis and exodermis are believed to function in mechanical support of the root, but this idea remains to be tested. During stress, root growth rates decline, and the endodermis and exodermis develop closer to the root tip. In two cases, stress is known to induce the formation of an exodermis, and in several other cases to accelerate the development of both the exodermis and endodermis. The responses of the endodermis and exodermis to drought, exposure to moist air, flooding, salinity, ion deficiency, acidity, and mechanical impedance are discussed. 相似文献
6.
Soybean root suberin: anatomical distribution, chemical composition, and relationship to partial resistance to Phytophthora sojae 总被引:1,自引:0,他引:1
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Thomas R Fang X Ranathunge K Anderson TR Peterson CA Bernards MA 《Plant physiology》2007,144(1):299-311
Soybean (Glycine max L. Merr.) is a versatile and important agronomic crop grown worldwide. Each year millions of dollars of potential yield revenues are lost due to a root rot disease caused by the oomycete Phytophthora sojae (Kaufmann & Gerdemann). Since the root is the primary site of infection by this organism, we undertook an examination of the physicochemical barriers in soybean root, namely, the suberized walls of the epidermis and endodermis, to establish whether or not preformed suberin (i.e. naturally present in noninfected plants) could have a role in partial resistance to P. sojae. Herein we describe the anatomical distribution and chemical composition of soybean root suberin as well as its relationship to partial resistance to P. sojae. Soybean roots contain a state I endodermis (Casparian bands only) within the first 80 mm of the root tip, and a state II endodermis (Casparian bands and some cells with suberin lamellae) in more proximal regions. A state III endodermis (with thick, cellulosic, tertiary walls) was not present within the 200-mm-long roots examined. An exodermis was also absent, but some walls of the epidermal and neighboring cortical cells were suberized. Chemically, soybean root suberin resembles a typical suberin, and consists of waxes, fatty acids, omega-hydroxy acids, alpha,omega-diacids, primary alcohols, and guaiacyl- and syringyl-substituted phenolics. Total suberin analysis of isolated soybean epidermis/outer cortex and endodermis tissues demonstrated (1) significantly higher amounts in the endodermis compared to the epidermis/outer cortex, (2) increased amounts in the endodermis as the root matured from state I to state II, (3) increased amounts in the epidermis/outer cortex along the axis of the root, and (4) significantly higher amounts in tissues isolated from a cultivar ('Conrad') with a high degree of partial resistance to P. sojae compared with a susceptible line (OX760-6). This latter correlation was extended by an analysis of nine independent and 32 recombinant inbred lines (derived from a 'Conrad' x OX760-6 cross) ranging in partial resistance to P. sojae: Strong negative correlations (-0.89 and -0.72, respectively) were observed between the amount of the aliphatic component of root suberin and plant mortality in P. sojae-infested fields. 相似文献
7.
Effects of exposure to humid air on epidermal viability and suberin deposition in maize (Zea mays L.) roots 总被引:4,自引:1,他引:3
When the basal zones of 4-d-old hydroponically grown maize ( Zea mays L. cv. Seneca Horizon) roots were exposed to moist air for 2 d, the development of both endodermis and exodermis was affected. In the endodermis, Casparian bands enlarged and more cells developed suberin lamellae. The most striking effect was seen in the exodermis. In submerged controls, only 4% of the cells had Casparian bands, whereas in root regions exposed to air, 93% developed these structures. Similarly, in submerged roots 11% of the exodermal cells had either developing or mature suberin lamellae compared with 92% in the air-treated region. The majority of epidermal cells remained alive in the zone exposed to air. Some cell death had occurred earlier in the experiment when the seedlings were transferred from vermiculite to hydroponic culture. The precise stimulus(i) associated with the air treatment which led to accelerated development in both endodermis and exodermis is as yet unknown. 相似文献
8.
《Aquatic Botany》1987,27(4):309-322
Root structure of the seagrass Zostera capensis Setchell was investigated by light and electron microscopy. Roots possess conspicuous root hairs which greatly increase the surface area available for absorption. Exodermal cells abutting root-hair bases possess transfer cell characteristics. The strategic location of these cells suggests that they participate in absorptive and/or transfer processes between the epidermis and cortex. Vascular parenchyma cells within the stele also possess transfer cell features. Wall ingrowths of these cells about xylem elements, sieve tubes, companion cells and other vascular parenchyma cells, suggesting that they play a role in absorptive and/or transfer processes between the stele and cortex. Apoplastic barriers in the form of suberin lamellae and Casparian bands occur in walls of both the exodermis and endodermis. However, plasmodesmata perforate the suberin lamellae in these walls, and a symplastic pathway can be traced from the root hairs to vascular parenchyma transfer cells contiguous with conducting elements of the stele. The occurrence of wall ingrowths adjacent to xylem elements implies that transfer processes occur between vascular transfer cells and xylem. Although reduced, xylem could therefore play a role in transport. Structural evidence obtained in this study supports the role of the roots in absorptive processes and shows pathways available for transport from the water column to the conducting tissues of the root. 相似文献
9.
Radial hydraulic conductivity along developing onion roots 总被引:24,自引:0,他引:24
Although most studies have shown that water uptake varies along the length of a developing root, there is no consistent correlation of this pattern with root anatomy. In the present study, water movement into three zones of onion roots was measured by a series of mini-potometers. Uptake was least in the youngest zone (mean hydraulic conductivity, Lpr = 1.5 x 10(-7) +/- 0.34 x 10(-7) m MPa-1 s-1; +/- SE, n = 10 roots) in which the endodermis had developed only Casparian bands and the exodermis was immature. Uptake was significantly greater in the middle zone (Lpr = 2.4 x 10(-7) +/- 0.43 x 10(-7) m MPa-1 s-1; +/- SE, n = 10 roots) which had a mature exodermis with both Casparian bands and suberin lamellae, and continued at this level in the oldest zone in which the endodermis had also developed suberin lamellae (Lpr = 2.8 x 10(-7) +/- 0.30 x 10(-7) m MPa-1 s-1; +/- SE, n = 10 roots). Measurements of the hydraulic conductivities of individual cells (Lp) in the outer cortex using a cell pressure probe indicated that this parameter was uniform in all three zones tested (Lp = 1.3 x 10(-6) +/- 0.01 x 10(-6) m MPa-1 s-1; +/- SE, n = 60 cells). Lp of the youngest zone was lowered by mercuric chloride treatment, indicating the involvement of mercury-sensitive water channels (aquaporins). Water flow in the older two root zones measured by mini-potometers was also inhibited by mercuric chloride, despite the demonstrated impermeability of their exodermal layers to this substance. Thus, water channels in the epidermis and/or exodermis of the older regions were especially significant for water flow. The results of this and previous studies are discussed in terms of two models. The first, which describes maize root with an immature exodermis, is the 'uniform resistance model' where hydraulic resistances are evenly distributed across the root cylinder. The second, which describes the onion root with a mature exodermis, is the 'non-uniform resistance model' where resistances can be variable and are concentrated in a certain layer(s) on the radial path. 相似文献
10.
11.
Summary Onion (Allium cepa L. cv. Ebeneezer) roots from vermiculite culture were examined with transmission electron microscopy to detect the plasmodesmata in all tissues. In young root regions, plasmodesmata linked all living cells together in all directions. In old zones, the plasmodesmatal connections of the endodermis to its neighbor tissues were not interrupted by later suberin lamella and cellulosic wall deposition. Moreover, plasmodesmata in the fully mature endodermis usually exhibited a large central cavity. In the exodermis, however, upon deposition of suberin lamellae in long cells, all plasmodesmata that initially linked them to their adjacent cells were severed. Afterwards, the long cells lost the capability of forming wound pit callose and their protoplasts began to degenerate. The mature exodermal layer was symplastically bridged to its neighbors only by the short (passage) cells that lacked suberin lamellae. Compared to the long cells, the short cells not only had thicker cytoplasm surrounding their central vacuoles but also a higher density of mitochondria and rough endoplasmic reticulum, consistent with an active involvement in the transport processes of the root. The above results were obtained by an improved, extended transmission electron microscopy procedure devised to analyze plasmodesmata in cells with suberin lamellae. By prefixing root tissues in glutaraldehyde and acrolein, all cells were well preserved. Postfixation was carried out in osmium tetroxide at a low concentration (0.5%). Following dehydration in acetone and transfer to propylene oxide, infiltration with Spurr's resin was accomplished by incubating samples in the accelerator-free mixture for 4 days, then infiltrating samples in the accelerator-amended mixture for additional 4 days.Abbreviations IE
immature exodermis
- ME
mature exodermis
- TBO
toluidine blue O
- TEM
transmission electron microscopy 相似文献
12.
Patterns of infection withFusarium culmorum (W G Smith) Saccardo were observed in seedling roots of barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), maize (Zea mays L.) and asparagus (Asparagus officinalis L). Apical regions of the main roots were not infected. Since penetration into the root occurred several days after inoculation and the roots were growing during the experiment, these regions had apparently not been in existence long enough to be infected. In older regions of barley, wheat and asparagus, hyphae entered through the tips of lateral roots. In barley and wheat, which had not developed any suberin lamellae in their subepidermal layer, infection occurred randomly over the remainder of the root. In maize, the fungus penetrated the epidermis at many sites but did not breach the exodermis in which all cells possessed both Casparian bands and suberin lamellae. Maize roots, therefore, sustained only minimal infections. In asparagus, the fungus grew through the short (passage) cells but never the long cells of the exodermis. In doing so, it penetrated cells possessing Casparian bands but lacking suberin lamellae. The results support the hypothesis that suberin lamellae provide effective barriers to the growth ofF. culmorum hyphae. 相似文献
13.
Neil E. Robbins II Charlotte Trontin Lina Duan José R. Dinneny 《Plant physiology》2014,166(2):551-559
The root endodermis is characterized by the Casparian strip and by the suberin lamellae, two hydrophobic barriers that restrict the free diffusion of molecules between the inner cell layers of the root and the outer environment. The presence of these barriers and the position of the endodermis between the inner and outer parts of the root require that communication between these two domains acts through the endodermis. Recent work on hormone signaling, propagation of calcium waves, and plant-fungal symbiosis has provided evidence in support of the hypothesis that the endodermis acts as a signaling center. The endodermis is also a unique mechanical barrier to organogenesis, which must be overcome through chemical and mechanical cross talk between cell layers to allow for development of new lateral organs while maintaining its barrier functions. In this review, we discuss recent findings regarding these two important aspects of the endodermis.Soil contains water and dissolved nutrients needed for plant growth, but also holds pathogens and toxic compounds that can be detrimental to the plant. The root system, which is directly in contact with soil particles, can integrate environmental cues to adjust its development in order to optimize nutrient (Péret et al., 2011; Lynch, 2013) and water uptake (Cassab et al., 2013; Lynch, 2013; Bao et al., 2014) or avoid regions of high salinity (Galvan-Ampudia et al., 2013). Once anchored in the soil, roots must deal with the constraints of their local environment and develop specific barriers to balance uptake of nutrients, water, and interactions with symbionts with protection against detrimental biotic and abiotic factors.In young roots, these barriers are mainly formed by the deposition of hydrophobic polymers such as lignin and suberin within the primary cell wall of the endodermis, which separates the pericycle from the cortex (Fig. 1), and of the exodermis, which lies between the cortex and the epidermis (Nawrath et al., 2013). Although formation of an exodermis is species dependent, the endodermis is a distinguishing figure of extant vascular plants (Raven and Edwards, 2001). Within this layer, two barriers (i.e. the Casparian strip and the suberin lamellae) are sequentially deposited and regulate water and nutrient movements between the inner and outer parts of the root. In this review, we discuss how the presence of these two major endodermal barriers affects communication between the different cell layers of the root. We focus on recent articles highlighting the importance of the endodermis in this communication during various biological and developmental processes.Open in a separate windowFigure 1.Endodermal barriers affect radial movement of water and solutes through the root. A, At the root tip, to move from the soil to the outer tissues of the root and then into the stele, water and solute molecules can use either the apoplastic (black lines), symplastic (dotted lines), or transcellular (dashed lines) pathways. B, The deposition of the Casparian strip in the endodermis prevents the free apoplastic diffusion of molecules between the outer part and the inner part of the root forcing molecules to pass through the symplast of endodermal cells. C, The deposition of suberin lamellae prevents the uptake of molecules from the apoplast directly into the endodermis forcing molecules to enter the symplast from more outer tissue layers. Suberin deposition is also likely to prevent the backflow of water and ions out of the stele. Passage cells are unsuberized and may facilitate the uptake of water and nutrients in older parts of the root. Cor, Cortex; End, endodermis; Epi, epidermis; Peri, pericycle; Vasc, vasculature. Figure redrawn and modified from Geldner et al. (2013). 相似文献
14.
Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development) 总被引:3,自引:1,他引:2
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The hydraulic architecture of developing onion (Allium cepa L. cv Calypso) roots grown hydroponically was determined by measuring axial and radial hydraulic conductivities (equal to inverse of specific hydraulic resistances). In the roots, Casparian bands and suberin lamellae develop in the endodermis and exodermis (equal to hypodermis). Using the root pressure probe, changes of hydraulic conductivities along the developing roots were analyzed with high resolution. Axial hydraulic conductivity (Lx) was also calculated from stained cross-sections according to Poiseuille's law. Near the base and the tip of the roots, measured and calculated Lx values were similar. However, at distances between 200 and 300 mm from the apex, measured values of Lx were smaller by more than 1 order of magnitude than those calculated, probably because of remaining cross walls between xylem vessel members. During development of root xylem, Lx increased by 3 orders of magnitude. In the apical 30 mm (tip region), axial resistance limited water transport, whereas in basal parts radial resistances (low radial hydraulic conductivity, Lpr) controlled the uptake. Because of the high axial hydraulic resistance in the tip region, this zone appeared to be "hydraulically isolated" from the rest of the root. Changes of the Lpr of the roots were determined by measuring the hydraulic conductance of roots of different length and referring these data to unit surface area. At distances between 30 and 150 mm from the root tip, Lpr was fairly constant (1.4 x 10-7 m s-1 MPa-1). In more basal root zones, Lpr was considerably smaller and varied between roots. The low contribution of basal zones to the overall water uptake indicated an influence of the exodermal Casparian bands and/or suberin lamellae in the endodermis or exodermis, which develop at distances larger than 50 to 60 mm from the root tip. 相似文献
15.
On the basis of recent results with young primary maize roots, a model is
proposed for the movement of water across roots. It is shown how the
complex, 'composite anatomical structure' of roots results in a 'composite
transport' of both water and solutes. Parallel apoplastic, symplastic and
transcellular pathways play an important role during the passage of water
across the different tissues. These are arranged in series within the root
cylinder (epidermis, exodermis, central cortex, endodermis, pericycle
stelar parenchyma, and tracheary elements). The contribution of these
structures to the root's overall radial hydraulic resistance is examined.
It is shown that as soon as early metaxylem vessels mature, the axial
(longitudinal) hydraulic resistance within the xylem is usually not
rate-limiting. According to the model, there is a rapid exchange of water
between parallel radial pathways because, in contrast to solutes such as
nutrient ions, water permeates cell membranes readily. The roles of
apoplastic barriers (Casparian bands and suberin lamellae) in the root's
endo- and exodermis are discussed. The model allows for special
characteristics of roots such as a high hydraulic conductivity (water
permeability) in the presence of a low permeability of nutrient ions once
taken up into the stele by active processes. Low root reflection
coefficients indicate some apoplastic by-passes for water within the root
cylinder. For a given root, the model explains the large variability in the
hydraulic resistance in terms of a dependence of hydraulic conductivity on
the nature and intensity of the driving forces involved to move water. By
switching the apoplastic path on or off, the model allows for a regulation
of water uptake according to the demands from the shoot. At high rates of
transpiration, the apoplastic path will be partially used and the hydraulic
resistance of the root will be low, allowing for a rapid uptake of water.
On the contrary, at low rates of transpiration such as during the night or
during stress conditions (drought, high salinity, nutrient deprivation),
the apoplastic path will be less used and the hydraulic resistance will be
high. The role of water channels (aquaporins) in the transcellular path is
in the fine adjustment of water flow or in the regulation of uptake in
older, suberized parts of plant roots lacking a substantial apoplastic
component. The composite transport model explains how plants are designed
to optimize water uptake according to demands from the shoot and how
external factors may influence water passage across
roots. 相似文献
16.
Serenoa repens (Bartr.) Small is a palm native to the southeastern USA. It is a common understory plant in pine communities
on both acid sands and alkaline limestone. Roots have only primary growth and range in thickness from 8.0 mm (first order
roots from the stem) to 0.8–2.9 mm (ultimate roots of third to fifth order). The thickest roots occur at soil depths >20 cm;
fine roots (<1.2 mm) occur at all depths (1–60 cm). Some second and third order roots are negatively geotropic and grow up
to the mineral soil surface. The epidermis of all roots has a thick, eventually lignified outer wall. Except for the thinnest,
all roots have a single-layered, thick-walled exodermis, which is first suberized and later lignified. Root hairs are never
present. A hypodermis composed of several layers of lignified cells (up to 8-cells-thick) is next to the exodermis and forms
the outer cortex. Radial series of thin walled and slightly lignified cells sporadically occur in the outer cortex of the
thinnest roots, but there are no passage cells in the exodermis, which is continuous. The remaining inner cortex is composed
of unlignified parenchyma with air canals and a completely lignosuberized endodermis in old roots. Passage cells were seen
the the endodermis of the some of the thinnest roots. Arbuscular mycorrhizal (AM) fungi occur in the outer one-third of the
cortical parenchyma adjacent to the hypodermis. Fungal coils, arbuscules and vesicles are found most frequently in the thinnest
roots, but also occur sporadically in all root orders. Cells a few mm from the apical meristem are sometimes colonized. At
sites of appressoria, coils of AM hyphae occur within an epidermal cell and exodermal and hypodermal cells beneath. Intercellular
hyphae with intracellular branch arbuscules (Arum-type) are common in the inner cortex. There is evidence of a dieback of
the highest order roots during the winter dry season. Profiles of soil and roots have the highest density of AM spores in
the surface 10 cm layer. Total AM spore density ranged from 130 to 1100 spores per 50 g soil in different samples. Glomus
spp. dominated followed by Gigaspora spp. The findings are related to a more general understanding of growth and AM colonization
in long-lived roots of tropical woody monocotyledons. Palm roots, in particular, are slow growing and are protected by massive
hypodermal layers.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
The exodermis of Iris germanica roots is multiseriate. Its outermost layer matures first with typical Casparian bands and suberin lamellae. But as subsequent layers mature, the Casparian band extends into the tangential and anticlinal walls of their cells. Compared with roots in which the endodermis represents the major transport barrier, the multiseriate exodermis (MEX) was expected to reduce markedly radial water and solute transport. To test this idea, precocious maturation of the exodermis was induced with a humid air gap inside a hydroponic chamber. Hydraulic conductivity (Lp(pc)) was measured on completely submerged roots (with an immature exodermis) and on air-gap-exposed root regions (with two mature exodermal layers) using a pressure chamber. Compared with regions of roots with no mature exodermal layers, the mature MEX reduced Lp(pc) from 8.5×10(-8) to 3.9×10(-8) m s(-1) MPa(-1). Puncturing the MEX increased Lp(pc) to 19×10(-8) m s(-1) MPa(-1), indicating that this layer constituted a substantial hydraulic resistance within the root (75% of the total). Alternatively, a root pressure probe was used to produce pressure transients from which hydraulic conductivity was determined, but this device measured mainly flow through the endodermis in these wide-diameter roots. The permeability of roots to NaCl and ethanol was also reduced in the presence of two mature MEX layers. The data are discussed in terms of the validity of current root models and in terms of a potential role for I. germanica MEX during conditions of drought and salt stress. 相似文献
18.
19.
Evidence for symplastic involvement in the radial movement of calcium in onion roots 总被引:3,自引:0,他引:3
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The pathway of Ca(2+) movement from the soil solution into the stele of the root is not known with certainty despite a considerable body of literature on the subject. Does this ion cross an intact, mature exodermis and endodermis? If so, is its movement through these layers primarily apoplastic or symplastic? These questions were addressed using onion (Allium cepa) adventitious roots lacking laterals. Radioactive Ca(2+) applied to the root tip was not transported to the remainder of the plant, indicating that this ion cannot be supplied to the shoot through this region where the exodermis and endodermis are immature. A more mature zone, in which the endodermal Casparian band was present, delivered 2.67 nmol of Ca(2+) mm(-1) treated root length d(-1) to the transpiration stream, demonstrating that the ion had moved through an intact endodermis. Farther from the root tip, a third zone in which Casparian bands were present in the exodermis as well as the endodermis delivered 0.87 nmol Ca(2+) mm(-1) root length d(-1) to the transpiration stream, proving that the ion had moved through an unbroken exodermis. Compartmental elution analyses indicated that Ca(2+) had not diffused through the Casparian bands of the exodermis, and inhibitor studies using La(3+) and vanadate (VO(4)(3-)) pointed to a major involvement of the symplast in the radial transport of Ca(2+) through the endodermis. It was concluded that in onion roots, the radial movement of Ca(2+) through the exodermis and endodermis is primarily symplastic. 相似文献
20.
The exodermis: a variable apoplastic barrier. 总被引:29,自引:0,他引:29
E Hose D T Clarkson E Steudle L Schreiber W Hartung 《Journal of experimental botany》2001,52(365):2245-2264
The exodermis (hypodermis with Casparian bands) of plant roots represents a barrier of variable resistance to the radial flow of both water and solutes and may contribute substantially to the overall resistance. The variability is a result largely of changes in structure and anatomy of developing roots. The extent and rate at which apoplastic exodermal barriers (Casparian bands and suberin lamellae) are laid down in radial transverse and tangential walls depends on the response to conditions in a given habitat such as drought, anoxia, salinity, heavy metal or nutrient stresses. As Casparian bands and suberin lamellae form in the exodermis, the permeability to water and solutes is differentially reduced. Apoplastic barriers do not function in an all-or-none fashion. Rather, they exhibit a selectivity pattern which is useful for the plant and provides an adaptive mechanism under given circumstances. This is demonstrated for the apoplastic passage of water which appears to have an unusually high mobility, ions, the apoplastic tracer PTS, and the stress hormone ABA. Results of permeation properties of apoplastic barriers are related to their chemical composition. Depending on the growth regime (e.g. stresses applied) barriers contain aliphatic and aromatic suberin and lignin in different amounts and proportion. It is concluded that, by regulating the extent of apoplastic barriers and their chemical composition, plants can effectively regulate the uptake or loss of water and solutes. Compared with the uptake by root membranes (symplastic and transcellular pathways), which is under metabolic control, this appears to be an additional or compensatory strategy of plants to acquire water and solutes. 相似文献