Relationships between structural development and the absorption of ions by the root system of Cucurbita pepo

Groups of multi-cellular passage cells in the root exodermis of Echinochloa crus-galli varieties lack not only suberin lamellae but also lignin deposits

Passage cells are frequently found in the exodermis and the endodermis of the roots. Because passage cells lack an apoplastic diffusion barrier, they are thought to provide pathways for the transport of nutrients and the entrance of endomycorrhizal fungi. Exodermal passage cells possess Casparian strips but not suberin lamellae. So far, exodermal passage cells have not been associated with a particular internal structure. In some wetland plants, the outer part of the root (i.e., epidermis, exodermis, and sclerenchyma) of emerging lateral root primordia has an oxygen leaky zone called a window. The exodermis at the window site also lacks suberin lamellae, but it remains unclear whether the exodermis at the window site also lacks Casparian strips. Here, we report that several of the exodermal cells in the window of Echinochloa crus-galli grown under aerated or deoxygenated stagnant agar nutrient solution also lack lignin, which is a major constituent of Casparian strips. The sclerenchyma cells that form part of the window also lacked lignin deposits. Sites at which lateral root primordia developed were highly permeable to an apoplastic tracer (periodic acid). These observations indicate that windows consist of a novel type of passage cell at the exodermis that lacks lignin as well as suberin lamellae.

Root zone-specific localization of AMTs determines ammonium transport pathways and nitrogen allocation to shoots

In plants, nutrient provision of shoots depends on the uptake and transport of nutrients across the root tissue to the vascular system. Nutrient delivery to the vasculature is mediated via the apoplastic transport pathway (ATP), which uses the free space in the cell walls and is controlled by apoplastic barriers and nutrient transporters at the endodermis, or via the symplastic transport pathway (STP). However, the relative importance of these transport routes remains elusive. Here, we show that the STP, mediated by the epidermal ammonium transporter 1;3 (AMT1;3), dominates the radial movement of ammonium across the root tissue when external ammonium is low, whereas apoplastic transport controlled by AMT1;2 at the endodermis prevails at high external ammonium. Then, AMT1;2 favors nitrogen (N) allocation to the shoot, revealing a major importance of the ATP for nutrient partitioning to shoots. When an endodermal bypass was introduced by abolishing Casparian strip (CS) formation, apoplastic ammonium transport decreased. By contrast, symplastic transport was increased, indicating synergism between the STP and the endodermal bypass. We further establish that the formation of apoplastic barriers alters the cell type–specific localization of AMTs and determines STP and ATP contributions. These results show how radial transport pathways vary along the longitudinal gradient of the root axis and contribute to nutrient partitioning between roots and shoots.

Radial transport of nutrients from the soil to the vascular system of plant roots occurs via the symplastic transport pathway (STP) and apoplastic transport pathway (ATP). Nutrients move along the STP when crossing the plasma membrane of outer cells and moving to xylem through the cytoplasmic continuum formed by plasmodesmata. Nutrients following the ATP, in turn, initially move passively through the extracellular space but are eventually taken up by endodermal cells, in which Casparian strips (CSs) prevent further apoplastic movement. We assessed the contribution of these transport pathways to radial transport in roots and nutrient provision to shoots by expressing cell type–specific ammonium transporters in a CS-defective mutant. Our study reveals that i) symplastic transport is more efficient at low external ammonium supply; ii) when endodermal cells become sealed by the deposition of suberin lamellae, the expression of ammonium transporters shifts to cortical cells; and iii) apoplastic transport depends on a functional apoplastic barrier at the endodermis, favoring nitrogen (N) partitioning to shoots at high external ammonium.

Diffusible repression of cytokinin signalling produces endodermal symmetry and passage cells

In vascular plants, the root endodermis surrounds the central vasculature as a protective sheath that is analogous to the polarized epithelium in animals, and contains ring-shaped Casparian strips that restrict diffusion. After an initial lag phase, individual endodermal cells suberize in an apparently random fashion to produce 'patchy' suberization that eventually generates a zone of continuous suberin deposition. Casparian strips and suberin lamellae affect paracellular and transcellular transport, respectively. Most angiosperms maintain some isolated cells in an unsuberized state as so-called 'passage cells', which have previously been suggested to enable uptake across an otherwise-impermeable endodermal barrier. Here we demonstrate that these passage cells are late emanations of a meristematic patterning process that reads out the underlying non-radial symmetry of the vasculature. This process is mediated by the non-cell-autonomous repression of cytokinin signalling in the root meristem, and leads to distinct phloem- and xylem-pole-associated endodermal cells. The latter cells can resist abscisic acid-dependent suberization to produce passage cells. Our data further demonstrate that, during meristematic patterning, xylem-pole-associated endodermal cells can dynamically alter passage-cell numbers in response to nutrient status, and that passage cells express transporters and locally affect the expression of transporters in adjacent cortical cells.

Permeability of Plant Young Root Endodermis to Cu Ions and Cu-Citrate Complexes in Corn and Soybean

The non-selective apoplastic passage of Cu and Cu-citrate complexes into the root stele of monocotyledonous corn and dicotyledonous soybean was investigated using an inorganic-salt-precipitation technique. Either Cu ions or Cu-citrate complexes were drawn into root through the apoplast from the root growth medium, and K4[Fe(CN)6] was subsequently perfused through xylem vessels or the entire root cross section. Based on microscopic identification of the reddish-brown precipitates of copper ferrocyanide in the cell walls of the xylem of corn and soybean roots, Cu(2+) passed through the endodermal barrier into the xylem of both species. When the solution containing 200 μM CuSO4 and 400 μM sodium citrate (containing 199.98 μM Cu-citrate, 0.02 μM Cu(2+)) was drawn via differential pressure gradients into the root xylem while being perfused with K4[Fe(CN)6] through the entire root cross-section, reddish-brown precipitates were observed in the walls of the stele of soybean, but not corn root. However, when a CuSO4 solution containing 0.02 or 0.2 μM free Cu(2+) was used, no reddish-brown precipitates were detected in the stele of either of the two plants. Results indicated that endodermis was permeable to Cu-citrate complexes in primary roots of soybean, but not corn. The permeability of the endodermal barrier to the Cu-citrate complex may vary between dicotyledonous and monocotyledonous plants, which has considerable implications for chelant-enhanced phytoextraction.

Glyphosate effects on plant mineral nutrition, crop rhizosphere microbiota, and plant disease in glyphosate-resistant crops

Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

Genome-wide transcriptome dissection of the rice root system: implications for developmental and physiological functions

The root system is a crucial determinant of plant growth potential because of its important functions, e.g. uptake of water and nutrients, structural support and interaction with symbiotic organisms. Elucidating the molecular mechanism of root development and functions is therefore necessary for improving plant productivity, particularly for crop plants, including rice (Oryza sativa). As an initial step towards developing a comprehensive understanding of the root system, we performed a large-scale transcriptome analysis of the rice root via a combined laser microdissection and microarray approach. The crown root was divided into eight developmental stages along the longitudinal axis and three radial tissue types at two different developmental stages, namely: epidermis, exodermis and sclerenchyma; cortex; and endodermis, pericycle and stele. We analyzed a total of 38 microarray data and identified 22,297 genes corresponding to 17,010 loci that showed sufficient signal intensity as well as developmental- and tissue type-specific transcriptome signatures. Moreover, we clarified gene networks associated with root cap function and lateral root formation, and further revealed antagonistic and synergistic interactions of phytohormones such as auxin, cytokinin, brassinosteroids and ethylene, based on the expression pattern of genes related to phytohormone biosynthesis and signaling. Expression profiling of transporter genes defined not only major sites for uptake and transport of water and nutrients, but also distinct signatures of the radial transport system from the rhizosphere to the xylem vessel for each nutrient. All data can be accessed from our gene expression profile database, RiceXPro (http://ricexpro.dna.affrc.go.jp), thereby providing useful information for understanding the molecular mechanisms involved in root system development of crop plants.

Arabidopsis eIF5A3 influences growth and the response to osmotic and nutrient stress

AteIF5A3, one of three genes encoding eukaryotic translation initiation factor 5A (eIF5A) in Arabidopsis thaliana, and corresponding genes PdeIF5A3 from Populus deltoides (eastern cottonwood) and SleIF5A4 from Solanum lycopersicum (tomato) were constitutively over-expressed in A. thaliana. The resultant transgenic plants exhibited enhanced vegetative and reproductive growth. Indeed, the increase in seed yield relative to empty vector controls for the PdeIF5A3 over-expressing plants ranged from 50% to 300% depending on the line. The PdeIF5A3 over-expressing plants also exhibited enhanced fitness when exposed to osmotic and nutrient (N, P and K) stress. The spatial localization of AteIF5A3 was visualized by confocal microscopy using transgenic plants expressing P(AteIF5A3) :GFP-AteIF5A3. GFP fluorescence reflecting expression of AteIF5A3 was detectable in the phloem, particularly companion cells, of roots, stems and leaves, in the epidermal cells of the root tip, in the columella cells of the root cap and in the chalazal tissue of fertilized ovules, which all play a pivotal role in nutrient or hormone translocation. Thus, AteIF5A3 appears to be involved in supporting growth and to play a regulatory role in the response of plants to sub-lethal osmotic and nutrient stress.

Salt affects plant Cd-stress responses by modulating growth and Cd accumulation

Cadmium contamination is a serious environmental problem for modern agriculture and human health. Salinity affects plant growth and development, and interactions between salt and cadmium have been reported. However, the molecular mechanisms of salinity-cadmium interactions are not fully understood. Here, we show that a low concentration of salt alleviates Cd-induced growth inhibition and increases Cd accumulation in Arabidopsis thaliana. Supplementation with low concentrations of salt reduced the reactive oxygen species level in Cd-stressed roots by increasing the contents of proline and glutathione and down-regulating the expression of RCD1, thereby protecting the plasma membrane integrity of roots under cadmium stress. Salt supplementation substantially reduces the Cd-induced elevation of IAA oxidase activity, thereby maintaining auxin levels in Cd-stressed plants, as indicated by DR5::GUS expression. Salt supply increased Cd absorption in roots and increased Cd accumulation in leaves, implying that salt enhances both Cd uptake in roots and the root-to-shoot translocation of Cd. The elevated Cd accumulation in plants in response to salt was found to be correlated with the elevated levels of phytochelatin the expression of heavy metal transporters AtHMA1-4, especially AtHMA4. Salt alleviated growth inhibition caused by Cd and increased Cd accumulation also was observed in Cd accumulator Solanum nigrum.

Root suberin forms an extracellular barrier that affects water relations and mineral nutrition in Arabidopsis

Though central to our understanding of how roots perform their vital function of scavenging water and solutes from the soil, no direct genetic evidence currently exists to support the foundational model that suberin acts to form a chemical barrier limiting the extracellular, or apoplastic, transport of water and solutes in plant roots. Using the newly characterized enhanced suberin1 (esb1) mutant, we established a connection in Arabidopsis thaliana between suberin in the root and both water movement through the plant and solute accumulation in the shoot. Esb1 mutants, characterized by increased root suberin, were found to have reduced day time transpiration rates and increased water-use efficiency during their vegetative growth period. Furthermore, these changes in suberin and water transport were associated with decreases in the accumulation of Ca, Mn, and Zn and increases in the accumulation of Na, S, K, As, Se, and Mo in the shoot. Here, we present direct genetic evidence establishing that suberin in the roots plays a critical role in controlling both water and mineral ion uptake and transport to the leaves. The changes observed in the elemental accumulation in leaves are also interpreted as evidence that a significant component of the radial root transport of Ca, Mn, and Zn occurs in the apoplast.

The root system is a highly specialized plant organ that works to get both water and essential mineral nutrients from the changing chemically and physically complex environment of the soil. Roots do this by both controlling the uptake of water and essential mineral ions, as well as regulating their movement to the central vascular system of the plant for long distance transport to the shoot. To allow the cellular control of water and mineral ion uptake and transport via specialized transport proteins, plant roots contain a waxy layer of suberin that acts to seal connections between cells, preventing uncontrolled leakage of water and mineral ions between cells. By screening thousands of mutant A. thaliana plants, we were able to identify a plant with elevated levels of suberin in the root. Using this mutant, we were able to uncover the importance of suberin in sealing connections between root cells to regulate water movement through the plant and accumulation of various essential and nonessential minerals in leaves, including sodium, sulfur, potassium, calcium, manganese, zinc, arsenic, selenium, and molybdenum.

Improved understanding of hyperaccumulation yields commercial phytoextraction and phytomining technologies

This paper reviews progress in phytoextraction of soil elements and illustrates the key role of hyperaccumulator plant species in useful phytoextraction technologies. Much research has focused on elements which are not practically phytoextracted (Pb); on addition of chelating agents which cause unacceptable contaminant leaching and are cost prohibitive; and on plant species which offer no useful phytoextraction capability (e.g., Brassica juncea Czern). Nickel phytoextraction by Alyssum hyperaccumulator species, which have been developed into a commercial phytomining technology, is discussed in more detail. Nickel is ultimately accumulated in vacuoles of leaf epidermal cells which prevents metal toxicity and provides defense against some insect predators and plant diseases. Constitutive up-regulation of trans-membrane element transporters appears to be the key process that allows these plants to achieve hyperaccumulation. Cadmium phytoextraction is needed for rice soils contaminated by mine wastes and smelter emissions with 100-fold more soil Zn than Cd. Although many plant species can accumulate high levels of Cd in the absence of Zn, when Cd/Zn>100, only Thlaspi caerulescens from southern France has demonstrated the ability to phytoextract useful amounts of Cd. Production of element-enriched biomass with value as ore or fertilizer or improved food (Se) or feed supplement may offset costs of phytoextraction crop production. Transgenic phytoextraction plants have been achieved for Hg, but not for other elements. Although several researchers have been attempting to clone all genes required for effective hyperaccumulation of several elements, success appears years away; such demonstrations will be needed to prove we have identified all necessary processes in hyperaccumulation.

Evidence for symplastic involvement in the radial movement of calcium in onion roots

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.

Mechanisms and Control of Nutrient Uptake in Plants

This review is a distillation of the vast amount of physiological and molecular data on plant membrane transport, to provide a concise overview of the main processes involved in the uptake of mineral nutrients in plants. Emphasis has been placed on transport across the plasma membrane, and on the primary uptake from soil into roots, or in the case of aquatic plants, from their aqueous environment. Control of uptake has been mainly considered in terms of local effects on the rate of transport and not in terms of long-distance signaling. The general picture emerging is of a large array of membrane transporters, few of which display any strong selectivity for individual nutrients. Instead, many transporters allow low-affinity uptake of several different nutrients. These features, plus the huge number of potential transporter genes that has been revealed by sequencing of plant genomes, raise some interesting questions about their evolution and likely function.

The pathways of calcium movement to the xylem

Calcium is an essential plant nutrient. It is acquired from the soil solution by the root system and translocated to the shoot via the xylem. The root must balance the delivery of calcium to the xylem with the need for individual root cells to use [Ca2+]cyt for intracellular signalling. Here the evidence for the current hypothesis, that Ca2+ travels apoplastically across the root to the Casparian band which it then circumvents via the cytoplasm of the endodermal cell, is critically reviewed. It is noted that, although Ca2+ channels and Ca2+-ATPases are present and could catalyse Ca2+ influx and efflux across the plasma membrane of endodermal cells, their transport capacity is unlikely to be sufficient for xylem loading. Furthermore, there seems to be no competition, or interactions, between Ca2+, Ba2+ and Sr2+ for transport to the shoot. This seems incompatible with a symplastic pathway involving at least two protein-catalysed transport steps. Thus, a quantity of purely apoplastic Ca2+ transport to the xylem is indicated. The relative contributions of these two pathways to the delivery of Ca2+ to the xylem are unknown. However, the functional separation of symplastic Ca2+ fluxes (for root nutrition and cell signalling) and apoplastic Ca2+ fluxes (for transfer to the shoot) would enable the root to fulfil the demand of the shoot for calcium without compromising intracellular [Ca2+]cyt signals. This is also compatible with the observed correlation between transpiration rate and calcium delivery to the shoot.

Roots and the delivery of solutes to the xylem

The structural features of the pathways followed by solutes and water are described. The porous nature of the cell walls comprising the apoplasm is described and the difficulties in verifying the passage of water through different parts of the apoplasm are discussed. The endoderm is of ubiquitous occurrence and has two invariant characteristics, a girdle-like wall thickening, the Casparian band, and the attachment of the plasma membrane to the band. Suggestions are made concerning the constraints placed on the passage of materials in the stele by these structures. The hypodermis is also a very common structure which shares a number of properties seen in the endodermis. The implications of an apoplasmic barrier in the hypodermis are discussed. The plasmodesmata are the key structural feature of the symplasmic pathway and recent information makes it clear that the size of the pores in the neck region can vary with the physiological state and position of tissues. The symplasmic pathway seems not to be interrupted by structural developments which make the endodermis an apoplasmic barrier of high resistance. Recent information from transpiring plants indicates that the turgor pressure in cortical cells increases centripetally: there is, therefore an outwardly directed hydrostatic pressure gradient. The implications of these new findings for water and solute flows in the symplast are considered. The final step in the radial transfer of materials is their release into the xylem. There is evidence that stelar tissues contain an H+-translocating ATPase whose activity can be influenced by physiological factors. It is pointed out that there may be major changes in the concentration of K+in xylem sap during a day-night cycle which may influence the polarization of the cell membranes of xylem parenchyma and the opening of ion-channels. The xylem elements themselves are not always fully conductive, even when their final diameter has been reached. The protoplasts and cross walls may be more persistent than is usually assumed, especially in soil-grown roots. Because of the low activity of Ca2+in the cytoplasm and the discontinuity of compartments within cells which contain abundant free Ca2+, this ion probably moves radially primarily by diffusion in the apoplasm. The transfer of Ca+2across the endodermis is shown to depend on the activity of Ca2+ATPase in the plasma membrane of the stelar side of the endodermis, emphasising once again the epithelial nature of this cell layer.

Suberin lamellae in the hypodermis of maize (Zea mays) roots; development and factors affecting the permeability of hypodermal layers

The development of suberin lamellae in the hypodermis of Zea mays cv. LG 11 was observed by electron microscopy and the presence of suberin inferred from autoliuorescence and by Sudan black B staining in nodal (adventitious) and primary (seminal) root axes. Suberin lamellae were evident at a distance of 30-50 mm from the tip of roots growing at 20°C and became more prominent with distance from the tip. Both oxygen deficiency and growth at 13°C produced shorter roots in which the hypodermis was suberized closer to the root tip. There were no suberin lamellae in epidermal cells or cortical collenchyma adjacent to the hypodermis. Plasmodesmata were not occluded by the suberin lamellae: there were twice as many of them in the inner tangential hypodermal wall (1,14 μn^-2 ) as in the junction between the epidermis and hypodermis (0.54 μm^-2 ). Water uptake by seminal axes (measured by micropotometry) was greater at distances more than 100 mm from the root lip than in the apical zone where the hypodermis was unsuberized. In the more mature zones of roots grown at 13°C rates of water uptake were greater than in roots grown at 20°C even though hypodermal suberization was more marked. Sleeves of epidermal/hypodermal cells (plus some accessory collenchyma) were isolated from the basal 60 mm of nodal axes by enzymatic digestion (drisclase). The roots were either kept totally immersed in culture solution or had the basal 50 mm exposed to moist air above the solution surface. In both treatments the permeabilities to tritiated water and ₈₆ Rb were low (circa 10^-5 mms^-1 ) in sleeves isolated from the extreme base. In roots grown totally immersed, however, the permeability of sleeves increased 10 to 50-fold over a distance of 40 mm. In roots exposed to moist air the permeability remained at a low level until the point where the root entered the culture solution and then increased rapidly (> 50-fold in a distance of 8 mm). Growth of roots in oxygen depleted (5% O₂ ) solutions promoted the development of extensive cortical aerenchymas. These developments were not associated with any reduction in permeability of sleeves isolated from the basal 40 mm of the axis. It was concluded that the presence of suberin lamellae in hypodermal walls does not necessarily indicate low permeability of cells or tissues to water or solutes. The properties of the walls (lamellae?) can be greatly changed by exposure to moist air, perhaps due to increased oxygen availability.

STRUCTURAL ASPECTS OF THE PRIMARY TISSUES OF THE CUCURBITA PEPO L. ROOT WITH SPECIAL REFERENCE TO THE PHLOEM

The primary phloem and associated tissues in the root of Cucurbita pepo L. were examined by light and transmission electron microscopy to provide information on the feasibility of symplastic transport from the sieve-tube members to the cortex in this organ. The structure, distribution and frequency of cytoplasmic connections between the various cell types of the root are reported as well as the cytological characteristics of the various cells. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections: plasmodesmata (simple and branched) between parenchymatous elements; pore-plasmodesmata between sieve-tube members and parenchymatous elements (companion, phloem parenchyma or pericycle cells in the Cucurbita root); and sieve-area pores between contiguous sieve elements. The plasmodesmata associated with secondary- and tertiary-state endodermal cells are not constricted by the suberin lamellae. Differences in the frequency of cytoplasmic connections between the various cell types of the primary phloem, pericycle and ground tissue do occur with the highest frequency between sieve-tube members and companion cells. The results indicate that the structure of the C. pepo root may be compatible with a symplastic pathway of phloem unloading and transport to the cells of the ground tissue.

Tomato responses to ammonium and nitrate nutrition under controlled root-zone pH

Tomato (Lycopersicon esculentum L. Mill. 'Vendor') plants were grown for 21 days in flowing solution culture with N supplied as either 1.0 mM NO3- or 1.0 mM NH4+. Acidity in the solutions was automatically maintained at pH 6.0. Accumulation and distribution of dry matter and total N and net photosynthetic rate were not affected by source of N. Thus, when rhizosphere acidity was controlled at pH 6.0 during uptake, either NO3- or NH4+ can be used efficiently by tomato. Uptake of K+ and Ca2+ were not altered by N source, but uptake of Mg2+ was reduced in NH4(+)-fed plants. This indicates that uptake of Mg2+ was regulated at least partially by ionic balance within the plant.

Calcium-oxalate crystals and crystal cells in determinate root nodules of legumes

Early reports of the presence of calciumoxalate crystals in the cortices ofPhaseolus vulgaris root nodules have been confirmed. Crystals were found in all six genera examined (Cajanus, Desmodium, Glycine, Lespedeza, Phaseolus, Vigna) that have determinate nodules and export ureides. They were absent from six genera examined that have indeterminate nodules and export amides. The possible physiological significance of these structures is discussed.

Fine structure of isolated and non-isolated potato tuber periderm

Cell walls of the periderm of native potato tuber (Solanum tuberosum L. cv. Primura) consist of a primary wall, a suberized secondary wall and a tertiary wall. With a mixture of pectinase and cellulase intact periderm membranes can be isolated. Isolation does not affect fine structure. It is suggested that the lignin in the middle lamellae and primary walls prevents the enzymes from digesting pectinaceous materials and cellulose. In specimens fixed with OsO4, the suberized walls appear as alternating electrondense and electron-lucent lamellae. This lamellar architecture is not altered by extraction with chloroform. Therefore, the current view that the electronlucent lamellae consist of soluble lipids (waxes) can no longer be maintained. It is argued that the lamellation is a property of the suberin itself, and the suberized wall consists of alternating layers of suberins differing in polarity. A hypothesis of suberin assembly from sub-units is advanced and the subunits are shown for the first time.

Simultaneous uptake and translocation of magnesium and calcium in barley (Hordeum vulgare L.) roots

The patterns of uptake and translocation of magnesium in different regions of the root are very similar to those of calcium. Once the endodermis has become suberized translocation of either ion to the shoot is greatly reduced and it is concluded that magnesium, like calcium, appears to move across the root cortex largely in the free space.