Distribution of lead in mycorrhizal and non-mycorrhizal Norway spruce seedlings

Non-mycorrhizal spruce seedlings (Picea abies Karst.) and spruce seedlings colonized with Lactarius rufus (Scop.) Fr. or two strains of Paxillits involutus (Batsch) Fr. were grown in an axenic silica sand culture system with frequently renewed nutrient solution. After successful mycorrhizal colonization, the seedlings were exposed to 1 μM PbCI₂ for 19 weeks. The degree of infection in all of the mycorrhizal treatments approached 100% during the experiment and was not affected by exposure to Pb. However, the number of root tips per root dry weight and the shoot: root ratio, both in the non-mycorrhizal and the mycorrhizal seedlings, had decreased after the 19 week treatment with PbCl₂ Using X-ray microanalysis, the distribution and concentration of Pb in the tissues of mycorrhizal and non-mycorrhizal root tips were compared. In the mycorrhizae of seedlings exposed to Pb no significant accumulation of Pb in the hyphal mantle or in fungal cell walls of the Hartig net were detected. Lead accumulated primarily in the cortex cell walls both of non-mycorrhizal and mycorrhizal root tips. No significant difference of Pb concentrations in root cortex cell walls of non-mycorrhizal and mycorrhizal seedlings was found; except for seedlings colonized with Paxillus involutus strain 537. However, at the endodermis no effect of mycorrhizal fungal colonization on the Pb tissue concentration was detected. The presence of the fungal sheath did not prevent Pb from reaching the root cortex. The endodermis acted as a barrier to Pb radial transport in both mycorrhizal and non-mycorrhizal seedling roots.     

Stage-dependent border cell and carbon flow from roots to rhizosphere

Rising CO(2) levels in the atmosphere have drawn attention to the important role of soil in sequestering carbon. This project goal was to quantify soil carbon deposition associated with border cell release and exudation from root growth zones. Carbon was measured with a Carlo Erba C/N analyzer in soil from the rhizosphere of mature grasses and, in separate experiments, in soil collected around root growth zones. Root border cells in "rhizosphere soil" (silica sand) were counted using a compound microscope after soil sonication and extraction with surfactant. For sand-grown Bromus carinatus, Zea mays, and Cucumis sativus, young seedlings (with roots shorter than 2 cm) released thousands of border cells, while older root tips released only hundreds. For a variety of native annual and perennial grasses and invasive annual grasses (Nassella pulchra, B. carinatus, B. diandrus, B. hordeaceus, Vulpia microstachys, Aegilops triuncialis, Lolium multiflorum, Zea mays), the rhizosphere of mature root systems contained between 18 and 32 μg C g(-1) sand more than that of the unplanted controls. Spatial analysis of the rhizosphere around the cucumber growth zone confirmed C enrichment there. The root tip provided C to the rhizosphere: 4.6 μg C in front of the growing tip, with the largest deposition, 20.4 μg C, to the rhizosphere surrounding the apical 3 mm (root cap/meristem). These numbers from laboratory studies represent the maximum C that might be released during flooding in soils. Scaling up from the organ scale to the field requires a growth analysis to quantify root tip distributions in space and time.     

Enzymatic adaptations to arsenic-induced oxidative stress in Zea mays and genotoxic effect of arsenic in root tips of Vicia faba and Zea mays

Agronomic plant species may display physiological and biochemical responses to oxidative stress caused by heavy metals and metalloids. Zea mays plants were grown hydroponically for eight days at different concentrations of As (0, 134 and 668 μM) and at different pH (4, 7 and 9). Metabolic variations in response to As toxicity were measured using physiological parameters and antioxidant enzymatic activities. A significant decrease in SOD activity was observed in the leaves and roots of Z. mays with the majority of As treatments. As decreased G-POX activity less in leaves than in roots. An increase in the concentration of As increased APX activity in leaves and roots, except As(V) at pH 4 and pH 9 in the leaves and As(III) at pH 9 in the roots, when there was a significant decrease in APX activity at low As concentrations. After exposure to As(V), CAT activity was the same as in the control. As(III) led to an increase in CAT activity in leaves and to a decrease in roots. With increasing concentrations of As(III), CAT activity increased in both leaves and roots whatever the pH. To obtain more detailed knowledge on the effects of arsenate and arsenite exposure on Vicia faba and Z. mays, root meristems were also examined. Roots were fed hydroponically with 134, 334, 534 and 668 μM arsenate or arsenite and 4 × 10(-3)M of maleic hydrazide as positive control, at three different pH. Physiological parameters, the mitotic index and micronuclei frequencies were evaluated in root meristems. At all three pH, the highest As(V) and As(III) concentrations induced a substantial modification in root colour, increased root thickness with stiffening, and reduced root length. High concentrations also caused a significant decrease in the mitotic index, and micronucleus chromosomic aberrations were observed in the root meristems of both species.     

In situ distribution and speciation of toxic copper, nickel, and zinc in hydrated roots of cowpea

The phytotoxicity of trace metals is of global concern due to contamination of the landscape by human activities. Using synchrotron-based x-ray fluorescence microscopy and x-ray absorption spectroscopy, the distribution and speciation of copper (Cu), nickel (Ni), and zinc (Zn) was examined in situ using hydrated roots of cowpea (Vigna unguiculata) exposed to 1.5 μm Cu, 5 μm Ni, or 40 μm Zn for 1 to 24 h. After 24 h of exposure, most Cu was bound to polygalacturonic acid of the rhizodermis and outer cortex, suggesting that binding of Cu to walls of cells in the rhizodermis possibly contributes to the toxic effects of Cu. When exposed to Zn, cortical concentrations remained comparatively low with much of the Zn accumulating in the meristematic region and moving into the stele; approximately 60% to 85% of the total Zn stored as Zn phytate within 3 h of exposure. While Ni concentrations were high in both the cortex and meristem, concentrations in the stele were comparatively low. To our knowledge, this is the first report of the in situ distribution and speciation of Cu, Ni, and Zn in hydrated (and fresh) plant tissues, providing valuable information on the potential mechanisms by which they are toxic.     

Brassinosteroids and iron plaque affect arsenic and cadmium uptake by rice seedlings grown in hydroponic solution

Brassinosteroids (Brs) have drawn wide attention due to their protective role against toxicity of heavy metals in plants. To better understand the role of Br in arsenic (As) and cadmium (Cd) uptake by rice plants, a hydroponic experiment was conducted to investigate the combined effect of 24-epibrassinolide (Br24) or 28-homobrassinolide (Br28) and iron plaque (IP) on As and Cd uptake and accumulation in rice seedlings. Six-week-old seedlings were sprayed with 0.2 or 0.02 μM Br24 or Br28 and grown in nutrient solution for 3 d, and then 20 or 60 mg Fe²⁺ dm^-3 (Fe20 and Fe60) was used to induce root IP formation for 3 d. These seedlings with or without Br and with or without IP were exposed to solution containing 0.5 mg dm^-3 As^III or Cd for 9 d. The results showed that rice growth decreased when Br24 were applied, but it increased when combination of Br24 and IP was applied. Fe concentrations in dithionite-citratebicarbonate (DCB) extracts were increased after 0.2 or 0.02 μM Br24 application in the absence of IP, but decreased by Br24 in the presence of IP. In the absence of IP, As and Cd content in leaves was significantly reduced by 0.02 μM Br24 and 0.2 μM Br28, respectively. The As content in leaves was also reduced by the combination of 0.02 and 0.2 μM Br28 and IP, and the Cd content in leaves was reduced by the combined effect of 0.2 μM Br24 and IP. These results indicate that Br24 and Br28 could impede As and Cd accumulation, and the interactions between Br and IP may have a potential in restricting the transport of As and Cd into rice shoots.     

Zonal responses of sensitive vs. tolerant wheat roots during Al exposure and recovery

Aluminium (Al) irreversibly inhibits root growth in sensitive, but not in some tolerant genotypes. To better understand tolerance mechanisms, seedlings from tolerant ('Barbela 7/72' line) and sensitive ('Anahuac') Triticum aestivum L. genotypes were exposed to AlCl(3) 185 μM for: (a) 24 h followed by 48 h without Al (recovery); (b) 72 h of continuous exposure. Three root zones were analyzed (meristematic (MZ), elongation (EZ) and hairy (HZ)) for callose deposition, reserves (starch and lipids) accumulation, endodermis differentiation and tissue architecture. Putative Al-induced genotoxic or cytostatic/mytogenic effects were assessed by flow cytometry in root apices. Tolerant plants accumulated less Al, presented less root damage and a less generalized callose distribution than sensitive ones. Starch and lipid reserves remained constant in tolerant roots but drastically decreased in sensitive ones. Al induced different profiles of endodermis differentiation: differentiation was promoted in EZ and HZ, respectively, in sensitive and tolerant genotypes. No ploidy changes or clastogenicity were observed. However, differences in cell cycle blockage profiles were detected, being less severe in tolerant roots. After Al removal, only the 'Barbela 7/72' line reversed Al-induced effects to values closer to the control, mostly with respect to callose deposition and cell cycle progression. We demonstrate for the first time that: (a) cell cycle progression is differently regulated by Al-tolerant and Al-sensitive genotypes; (b) Al induces callose deposition >3 cm above root apex (in HZ); (c) callose deposition is a transient Al-induced effect in tolerant plants; and (d) in HZ, endodermis differentiation is also stimulated only in tolerant plants, probably functioning in tolerant genotypes as a protective mechanism in addition to callose.     

Influence of sulphur on arsenic accumulation and metabolism in rice seedlings

The influence of sulphur on the accumulation and metabolism of arsenic in rice was investigated. Rice seedlings were grown in nutrient solutions with low sulphate (1.8 μM SO₄²⁻) or high sulphate (0.7 mM SO₄²⁻) for 12 or 14 d, before being exposed to 10 μM arsenite or arsenate for 2 or 1 d, respectively. In the arsenite exposure treatment, low sulphate-pretreated rice accumulated less arsenite than high sulphate pretreated plants, but the arsenite concentrations in shoots of low sulphate pretreated rice were higher than those of high sulphate pretreated. In the arsenate exposure treatment, the low sulphate pre-treatments also resulted in less arsenite accumulation in rice roots. Sulphur deprivation in nutrient solution decreased the concentrations of non-protein thiols in rice roots exposed to either arsenite or arsenate. The low sulphate-pretreated plants had a higher arsenic transfer factor than the high sulphate-pretreated plants. The results suggest that rice sulphate nutrition plays an important role in regulating arsenic translocation from roots to shoots, possibly through the complexation of arsenite-phytochelatins.     

Radial transport of water and abscisic acid (ABA) in roots of Zea mays under conditions of nutrient deficiency

Radial water (J(V)) and abscisic acid (ABA) flows (J(ABA)) through maize root seedlings have been investigated under different conditions of nutrient deficiency. Whereas J(V) was reduced under nitrogen deficiency, potassium deficiency stimulated J(V). A substantial increase of J(ABA) was observed in roots kept under potassium deficiency. The observed changes of J(V) might have resulted from changed barrier properties of the endodermis. Nitrogen and potassium deficiency also caused an accumulation of endogenous ABA in root tissues. Under all conditions studied, except under K(+)-deficiency, external ABA (100 nM) caused an increase of J(V). The data of this study were used to analyse the relations between internal and endogenous root ABA, J(V), and J(ABA). The internal ABA of root tissues was positively correlated with J(V) and was highly significant (P <0.001 for internal and P=0.03 for endogenous root ABA) within the range 2-300 pmol g(-1) FW. It was also highly positively correlated to the radial ABA flows. There was also a highly positive correlation between J(V) and J(ABA). The data of this study indicate, for the first time, the relations between internal ABA, water, and ABA flows. Independent of treatment with external ABA, an ABA transport by solvent drag across the endodermis is confirmed.     

Effects of exposure to humid air on epidermal viability and suberin deposition in maize (Zea mays L.) roots

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.     

Gravistimulation changes the accumulation pattern of the CsPIN1 auxin efflux facilitator in the endodermis of the transition zone in cucumber seedlings

Cucumber (Cucumis sativus) seedlings grown in a horizontal position develop a specialized protuberance (or peg) on the lower side of the transition zone between the hypocotyl and the root. This occurs by suppressing peg formation on the upper side via a decrease in auxin resulting from a gravitational response. However, the gravity-stimulated mechanism of inducing asymmetric auxin distribution in the transition zone is poorly understood. The gravity-sensing tissue responsible for regulating auxin distribution in the transition zone is thought to be the endodermal cell. To characterize the gravity-stimulated mechanism, the auxin efflux facilitator PIN-FORMED1 (CsPIN1) in the endodermis was identified and the localization of CsPIN1 proteins during the gravimorphogenesis of cucumber seedlings was examined. Immunohistochemical analysis revealed that the accumulation pattern of CsPIN1 protein in the endodermal cells of the transition zone of cucumber seedlings grown horizontally differed from that of plants grown vertically. Gravistimulation for 30 min prompted changes in the accumulation pattern of CsPIN1 protein in the endodermis as well as the asymmetric distribution of auxin in the transition zone. Furthermore, 2,3,5-triiodobenzoic acid inhibited the differential distribution of auxin as well as changes in the accumulation pattern of CsPIN1 in the endodermis of the transition zone during gravistimulation. These results suggest that the altered pattern of CsPIN1 accumulation in the endodermis in response to gravistimulation influences lateral auxin transport through the endodermis, resulting in asymmetric auxin distribution in the transition zone.     

Tissue zinc distribution in maize seedling roots and its action on growth

Zinc (Zn) distribution over tissues and organs of maize (Zea mays L.) seedlings and its action on root growth, cell division, and cell elongation were studied. Two-day-old seedlings were incubated in the 0.25-strength Hoagland solution containing 2 or 475 μM Zn(NO₃)₂. Zn toxicity was assessed after the inhibition of primary root increment during the first and second days of incubation. The content of Zn was determined by atomic absorption spectrometry in the apical (the first centimeter from the root tip) and basal (the third centimeter from the kernel) root parts. Zn distribution in various tissues was studied by histochemical methods, using a metallochromic indicator zincon and fluorescent indicator Zinpyr-1 and light and confocal scanning fluorescent light microscopy, respectively. To evaluate Zn effects on growth processes, the average length of the meristem; the length of fully elongated cells; the number of meristematic cells in the cortex row; and duration of the cell cycle were measured. When the Zn concentration in the solution was high, the Zn content per weight unit was higher in the basal root part due to its accumulation in lateral root primordial. Zn was also accumulated in both the meristem apoplast and cell protoplasts. In the basal and middle root parts, Zn was detected essentially in all tissues predominantly in the apoplast. Zn inhibited both cell division and elongation. Under Zn influence, the size of the meristem and the number of meristematic cells decreased, which was determined by an increase in the cell cycle duration. The length of the fully elongated cells was also reduced. A comparison of Zn distribution and growth-suppressing activity with other heavy metals studied earlier allows a conclusion that toxic action of heavy metals is mainly determined by physical and chemical properties of their ions and specific patterns of their transport and distribution. As a result, two basic processes determining root growth, e.g., cell division and elongation, could be affected differently.     

Meristem-Specific Suppression of Mitosis and a Global Switch in Gene Expression in the Root Cap of Pea by Endogenous Signals

Two functionally distinct sets of meristematic cells exist within root tips of pea (Pisum sativum): the root apical meristem, which gives rise to the body of the root; and the root cap meristem, which gives rise to cells that differentiate progressively through the cap and separate ultimately from its periphery as border cells. When a specific number of border cells has accumulated on the root cap periphery, mitosis within the root cap meristem, but not the apical meristem, is suppressed. When border cells are removed by immersion of the root tip in water, a transient induction of mitosis in the root cap meristem can be detected starting within 5 min. A corresponding switch in gene expression throughout the root cap occurs in parallel with the increase in mitosis, and new border cells begin to separate from the root cap periphery within 1 h. The induction of renewed border cell production is inhibited by incubating root tips in extracellular material released from border cells. The results are consistent with the hypothesis that operation of the root cap meristem and consequent turnover of the root cap is self-regulated by a signal from border cells.     

Influence of a weak DC electric field on root meristem architecture

BACKGROUND AND AIMS: Electric fields are an important environmental factor that can influence the development of plants organs. Such a field can either inhibit or stimulate root growth, and may also affect the direction of growth. Many developmental processes directly or indirectly depend upon the activity of the root apical meristem (RAM). The aim of this work was to examine the effects of a weak electric field on the organization of the RAM. METHODS: Roots of Zea mays seedlings, grown in liquid medium, were exposed to DC electric fields of different strengths from 0.5 to 1.5 V cm(-1), with a frequency of 50 Hz, for 3 h. The roots were sampled for anatomical observation immediately after the treatment, and after 24 and 48 h of further undisturbed growth. KEY RESULTS: DC fields of 1 and 1.5 V cm(-1) resulted in noticeable changes in the cellular pattern of the RAM. The electric field activated the quiescent centre (QC): the cells of the QC penetrated the root cap junction, disturbing the organization of the closed meristem and changing it temporarily into the open type. CONCLUSIONS: Even a weak electric field disturbs the pattern of cell divisions in plant root meristem. This in turn changes the global organization of the RAM. A field of slightly higher strength also damages root cap initials, terminating their division.     

Root structure and cellular chloride,sodium and potassium distribution in salinized grapevines

X-ray microanalysis was used to study the patterns of K+, Na+ and Cl- accumulation in salinized (25 mm NaCl) and non-salinized grapevine (Vitis) roots. The aim was to determine whether NaCl affects patterns of Cl- accumulation differentially in the roots of a Cl--excluding genotype and a non-excluding genotype. Two regions of fibrous roots were analysed: (1) a region 2-3 mm basipetal to the root tip; and (2) a region of the root 10-12 mm basipetal to the root tip where the outermost layer is the hypodermis. The ion contents of the hypodermis, cortex, endodermis and pericycle vacuoles were analysed. Data were also collected from the cytoplasm of the endodermal and pericycle cells. The analyses showed that the ion profiles of the hypodermis and the endodermis were significantly different from those of the cortex and pericycle. The hypodermis and endodermis had higher K+ and lower Na+ and Cl- than surrounding cells. Some changes due to salinity such as increased K+ concentrations in the hypodermis were also noted. Chloride concentrations did not differ between the genotypes in the hypodermis, across the cortex or in the endodermis, but were higher in the pericycle of the excluder in comparison with the non-excluding genotype. However, K+/Na+ ratios of the cortex and endodermis were higher in the excluder. The pericycle cells exhibited the greatest ability to sequester Na+ and Cl- in vacuoles. Overall the data show cell-type-specific ion accumulation patterns and small but significant differences were found between genotypes. The possibility that these accumulation patterns arise from differences in uptake properties of cell types and/or result from the spatial distribution of the cell types along the competing symplastic and apoplastic ion transport pathways across the root is discussed.     

Expressing ScACR3 in rice enhanced arsenite efflux and reduced arsenic accumulation in rice grains

Arsenic (As) accumulation in rice grain poses a serious health risk to populations with high rice consumption. Extrusion of arsenite [As(III)] by ScAcr3p is the major arsenic detoxification mechanism in Saccharomyces cerevisiae. However, ScAcr3p homolog is absent in higher plants, including rice. In this study, ScACR3 was introduced into rice and expressed under the control of the Cauliflower mosaic virus (CaMV) 35S promoter. In the transgenic lines, As concentrations in shoots and roots were about 30% lower than in the wild type, while the As translocation factors were similar between transgenic lines and the wild type. The roots of transgenic plants exhibited significantly higher As efflux activities than those of the wild type. Within 24 h exposure to 10 μM arsenate [As(V)], roots of ScACR3-expressing plants extruded 80% of absorbed As(V) to the external solution as As(III), while roots of the wild type extruded 50% of absorbed As(V). Additionally, by exposing the As-containing rice plants to an As-lacking solution for 24 h, about 30% of the total As derived from pre-treatment was extruded to the external solution by ScACR3-expressing plants, while about 15% of As was extruded by wild-type plants. Importantly, ScACR3 expression significantly reduced As accumulation in rice straws and grains. When grown in flooded soil irrigated with As(III)-containing water, the As concentration in husk and brown rice of the transgenic lines was reduced by 30 and 20%, respectively, compared with the wild type. This study reports a potential strategy to reduce As accumulation in the food chain by expressing heterologous genes in crops.     

Cellular analysis of UV-B-induced barley root subapical swelling

UV-B irradiation of barley (Hordeum vulgare L.) roots (1 W/m², 15 min) or leaves (3 W/m², 3.3 h) and also one-day-long root incubation in the Knop solution supplemented with 1–4 μM ABA, 1 mM salicylic acid, 16 μM ionomycin, or 0.1 mM colchicine induced growth retardation and subapical root swelling. All factors, except for colchicine, initiated growth of root hairs on the surface of swellings and suppressed their initiation and growth in more basal root region. During the first hour after unilateral root UV-B irradiation, their growth sharply retarded and hydraulic conductivity of membranes in the rhizodermis of growth zone rose 1.5-fold. In 2.5 h, root tips bent toward the source of irradiation. In 4.5 h, the ratio of longitudinal to transverse root extensibility in the root growth zone reduced twofold. In 8 h, root diameter in the subapical zone increased and root hairs appeared in this zone and attained 300 μm in length. In a day after irradiation, on unirradiated root side, meristematic cells continued to divide and grow, although at a much lower rate. On the irradiated root side, the cells of the rhizodermis and outer cortex ceased to divide and produced vacuoles. Vacuolation did not occur in the cells of the quiescent center and a distal part of the meristem. The lower part of the elongation zone swelled due to cortical cell expansion (except for the endodermis) in both irradiated and unirradiated root sides. It is supposed that cortical microtubule randomization plays an important role in the changed anisotropy of cell wall extensibility and cytosolic calcium is involved in this process. The role of oxidative stress and hormonal shifts in the development of subapical root swelling and root hair formation caused by UV-B radiation is discussed.     

DISTRIBUTION AND RELATIONSHIP OF CELL DIVISION AND MATURATION EVENTS IN PISUM SATIVUM (FABACEAE) SEEDLING ROOTS

Pea roots have open apical organization, where discrete initial cells do not exist. Differentiation of all tissues occurs in cylinders and vascular sectors that blend gradually with each other. This study reports the distribution of dividing cells and their relationship to maturation events in the 2 mm root tip, and in the 8–10 and 18–20 mm segments. Up to 200 μm from the root body/cap junction, cell division is uniformly distributed throughout all meristem regions. By 350 to 500 μ, xylem tracheary elements and cells of the pith parenchyma and middle cortex have stopped dividing. At this level cell division is almost entirely restricted to two cylinders, one composed of the inner root cap, the epidermis, and the outer cortex (outer cortex cylinder) and another composed of cells of the inner cortex, the pericycle and vascular tissue (inner cortex cylinder). When the protophloem matures, all cells in the phloem sector of the inner cortex cylinder, including the 1 layered pericycle, the endodermis and the phloem parenchyma, stop dividing. The 3–4 layered pericycle in the xylem sectors continues dividing until about 10 mm from the body/cap junction following the maturation of the protoxylem tracheary elements.     

Apical organization and maturation of the cortex and vascular cylinder inArabidopsis thaliana (Brassicaceae) roots

Developmental and physiological studies of roots are frequently limited to a post-germination stage. In Arabidopsis, a developmental change in the root meristem architecture during plant ontogenesis has not previously been studied and is addressed presently. Arabidopsis thaliana have closed root apical organization, in which all cell file lineages connect directly to one of three distinct initial tiers. The root meristem organization is dynamic and changes as the root ages from 1 to 4 wk post-germination. During the ontogeny of the root, the number of cells within the root apical meristem (RAM) increases and then decreases due to changes in the number of cortical layers and number of cell files within a central cylinder. The architecture of the initial tiers also changes as the root meristem ages. Included in the RAM's ontogeny is a pattern associated with the periclinal divisions that give rise to the middle cortex and endodermis; the three-dimensional arrangement of periclinally dividing derivative cells resembles one gyre of a helix. Four- or 5-wk-old roots exhibit a disorganized array of vacuolated initial cells that are a manifestation of the determinate nature of the meristem. Vascular cambium is formed via coordinated divisions of vascular parenchyma and pericycle cells. The phellogen is the last meristem to complete its development, and it is derived from pericycle cells that delineate the outer boundary of the root.     

Branch Roots of Zea. V. Structural Features that may Influence Water and Nutrient Transport

First-order branch roots of field-grown Zea mays L. were examined by optical and electron microscopy. They were small-scale versions of nodal roots except for the usual retention of a live epidermis throughout their length. The Casparian strips and suberized lamellae of hypodermis and endodermis developed closer to the root tip than reported for main roots (in the zone 0.5 to 5.5 cm from the tip for the hypodermis, and 0.5 to 4 cm for the endodermis), in branches retaining an apical meristem. The hydrophobic deposits were in place to the distal ends of determinate branches. All hydrophobic deposits were fully formed before the late metaxylem elements were mature. Gaps in the suberized lamellae of both hypodermis and endodermis may permit apoplastic diffusion of solutes through these layers. Pit frequency in the outer tangential walls of the hypodermis and endodermis was 0.3 per 100 μm², and 0.6 to 0.7 per 100 μm², respectively, in both branch and main roots. Numbers of plasmodesmata per pit in the branches were 60 and 30 in the hypodermis and endodermis, respectively. Water fluxes from published data were used to calculated the possible flux through plasmodesmata on a symplastic path. Values up to 0.2 pl h^?1 for the hypodermis and twice this for the endodermis were obtained.