Secondary Growth in Bougainvillea

The anomalous secondary growth was studied in roots and stemsof two species of Bougainvillea. The anomalous cambia arisesuccessively in centrifugal order, each originating among thederivatives of the preceding cambium. Each cambium layer functionsbidirectionally producing xylem towards the inside of the axisand phloem towards the outside. The sequence of production ofvascular cells is the following: (1) conjunctive tissue andxylem fibres towards the inside; (2) phloem towards the outside;(3) additional xylem with vessels towards the inside and additionalphloem towards the outside. The new cambia arise outside theoldest phloem cells of a given increment. This phloem may benonfunctional and crushed at that time. The phloem and the xylemdifferentiate from radially seriated derivatives produced sequentiallyby tangential divisions in the cambium. Divisions among thephloem initials and growth readjustments in the differentiatingxylem obscure the radial seriation to a moderate extent.     

Changes in the structure ofArabidopsis thaliana during female development of the plant-parasitic nematodeHeterodera schachtii

Summary The beet cyst nematodeHeterodera schachtii is able to establish a feeding structure (syncytium) in the vascular tissue of roots and shoots ofArabidopsis thaliana. Histological and ultrastructural studies were performed to assess plant responses during the development of juvenile females under monoxenic conditions. After destructively invading a root the nematode selects and pierces a single procambial cell with its stylet and transforms it into an initial syncytial cell (ISC) by secretory activity. The first most obvious changes in the ISC occur in the vacuolar system and at the wall. Differentiation of a central vacuole is impeded resulting in the formation of numerous small vacuoles. Multivesicular and paramural bodies are formed. An electron translucent material is deposited on the cell wall. Partial dissolution of the cell wall leads to the formation of a syncytium. At the juveniles' last pre-adult developmental stage the syncytium attains its maximum longitudinal and radial extension, occupying a major part of the central cylinder. Its features are indicative of a very high level of metabolic activity. The hypertrophied syncytium is ensheathed by a peridermal cover in which secondary xylem and phloem elements are interspersed. When females die the syncytia degenerate. The ultrastructural and histological features of syncytia described from roots are also found in syncytia induced in aerial parts of the plant.     

The Induction of Fibre Differentiation in Peas

The problem studied in this work was that of the internal controlof the formation of strands of fibres in Pisum sativum. It isshown that fibre differentiation is dependent on stimuli originatingin young leaf primordia. Removing these primordia early enoughprevents fibre differentiation; changing the position of theleaves experimentally changes the position of the fibres aswell. It was demonstrated that some stimuli for fibre differentiationmust flow through the strands at the time they differentiate.The evidence for this flow is in experiments concerning theability of very young fibre strands to regenerate after cutsas well as in experiments concerning their pattern of joining.The stimuli which originate in the leaves and control the differentiationof fibres and xylem are shown to differ in at least one component:auxin does not cause fibre differentiation and no surgical treatments,carried out on very young tissues, caused the replacement ofpart of a strand of fibres byor xylem or vice versa.     

Unusual Network of Internal Phloem in the Pod Mesocarp of Cowpea [Vigna unguiculata (L.) Walp. (Fabaceae)]

A mycelium-like network of internal phloem was observed in theinner mesocarp of the lateral pod walls of the fruit of certaingenotypes of cowpea [Vigna unguiculata (L.) Walp.] In the cultivarVita 3, the network consists of single, or rarely double, strandsof sieve elements and associated phloem parenchyma, orientedmainly parallel with the fibres of the adjacent endocarp, andstretching marginally beyond the sheets of fibres to connectabove and below with the outermost phloem of the longitudinalstrands of the dorsal and ventral sutures of the fruit. Theinternal phloem network does not relate conformationally to,or interconnect with the conventional (xylem+phloem) vasculatureof the mid mesocarp of the pod wall. In Vita 3, sieve elementsdifferentiate in the internal phloem after those in the majorveins of the pod, but before the presumptive endocarp fibrescommence wall thickening. The pod walls of twenty-one otherspecies of legumes proved negative for internal phloem, whileof nine varied genotypes of cowpea examined, six proved positive,three negative for the trait. Presence of internal phloem incowpea is not always associated with presence of endocarp fibresor necessarily with large fruits with large seeds. Possiblefunctions suggested for the phloem network are to provide assimilatesfor fibre wall thickening or to transport solutes to or fromsites of temporary storage in the fleshy inner layers of thepod wall. Internal phloem, legume fruit, translocation, mesocarp, pod wall, Vigna unguiculata, cowpea     

Comparative Root Anatomy of Five Actinidia Species in Relation to Rootstock Effects on Kiwifruit Flowering

This study examined the root anatomy of five Actinidia specieswhich, when used as rootstocks, differ in their effects on flowerproduction of the 'Hayward' kiwifruit scion. The rootstock speciesexamined were A. hemsleyana, A. eriantha, A. rufa, A. deliciosaand A. chinensis. Roots of all the five species had similartissues and similar cell types in each tissue. The cortex andendodermis were retained and developed further during secondaryroot thickening. Each meriphyte of endodermal cells, derivedduring secondary growth, was enclosed in a suberin lamella.Crystalline idioblasts were present in the cortex and phloem.These idioblasts contained raphide crystals, often surroundedby mucilage. Roots of the five species differed significantlyin the total cross-sectional area of vessels in the xylem, inthe total cross-sectional areas of fibres and crystalline idioblastsin the phloem, and in the abundance of starch grains in parenchymaof all tissues. Roots of flower-promoting rootstock speciestended to have more and larger xylem vessels, more crystallineidioblasts, and more starch grains. These anatomical featuresmay be used as criteria for future selection of flower-promotingrootstocks in kiwifruit. The possible physiological link betweenroot anatomy and rootstock effect is discussed.Copyright 1994,1999 Academic Press Kiwifruit, Actinidia, root, anatomy, rootstock, flowering     

52Fe Translocation in Barley as Monitored by a Positron-Emitting Tracer Imaging System (PETIS): Evidence for the Direct Translocation of Fe from Roots to Young Leaves via Phloem

The real-time translocation of iron (Fe) in barley (Hordeumvulgare L. cv. Ehimehadaka no. 1) was visualized using the positron-emittingtracer ⁵²Fe and a positron-emitting tracer imaging system (PETIS).PETIS allowed us to monitor Fe translocation in barley non-destructivelyunder various conditions. In all cases, ⁵²Fe first accumulatedat the basal part of the shoot, suggesting that this regionmay play an important role in Fe distribution in graminaceousplants. Fe-deficient barley showed greater translocation of⁵²Fe from roots to shoots than did Fe-sufficient barley, demonstratingthat Fe deficiency causes enhanced ⁵²Fe uptake and translocationto shoots. In the dark, translocation of ⁵²Fe to the youngestleaf was equivalent to or higher than that under the light condition,while the translocation of ⁵²Fe to the older leaves was decreased,in both Fe-deficient and Fe-sufficient barley. This suggeststhe possibility that the mechanism and/or pathway of Fe translocationto the youngest leaf may be different from that to the olderleaves. When phloem transport in the leaf was blocked by steamtreatment, ⁵²Fe translocation from the roots to older leaveswas not affected, while ⁵²Fe translocation to the youngest leafwas reduced, indicating that Fe is translocated to the youngestleaf via phloem in addition to xylem. We propose a novel modelin which root-absorbed Fe is translocated from the basal partof the shoots and/or roots to the youngest leaf via phloem ingraminaceous plants.     

Source of Induction and Sites of Primary Phloem Fibre Differentiation in Coleus blumei

The fact that fibre induction is strictly basipetal is usedhere to study the long distance effect of young growing leaveson acropetal primary phloem fibre differentiation. Excisionexperiments are used to show that young leaves induce fibredifferentiation around a wound a few internodes below them.No fibers appeared in the younger internodes between the youngleaves and the mature internode. Young leaves yield shorterfibres than those which differentiate under mature leaves, indicatingthat more than one stimulus is involved in the induction process.Fibre differentiation in nodes is faster than in subtendinginternodes. Wounding causes rapid differentiation of phloemfibres above and beside the wound. The rapid differentiationin the node as well as around the wound can be understood asan effect of a high local concentration of inductive stimulus.It is proved that the ability of the cells to respond to inductiondetermines the pattern of their differentiation which in thiscase is counter-directional to the induction. Coleus blumei, phloem fibres, differentiation     

Identification of high levels of phytochelatins, glutathione and cadmium in the phloem sap of Brassica napus. A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC–Cd and glutathione–Cd complexes.     

Characterization of Cd translocation and identification of the Cd form in xylem sap of the Cd-hyperaccumulator Arabidopsis halleri

Arabidopsis halleri is a Cd hyperaccumulator; however, the mechanismsinvolved in the root to shoot translocation of Cd are not wellunderstood. In this study, we characterized Cd transfer fromthe root medium to xylem in this species. Arabidopsis halleriaccumulated 1,500 mg kg^–1 Cd in the shoot without growthinhibition. A time-course experiment showed that the releaseof Cd into the xylem was very rapid; by 2 h exposure to Cd,Cd concentration in the xylem sap was 5-fold higher than thatin the external solution. The concentration of Cd in the xylemsap increased linearly with increasing Cd concentration in theexternal solution. Cd transfer to the xylem was completely inhibitedby the metabolic inhibitor carbonyl cyanide 3-chlorophenylhydrazone(CCCP). Cd concentration in the xylem sap was decreased by increasingthe concentration of external Zn, but enhanced by Fe deficiencytreatment. Analysis with ¹¹³Cd-nuclear magnetic resonance (NMR)showed that the chemical shift of ¹¹³Cd in the xylem sap wasthe same as that of Cd(NO₃)₂. Metal speciation with Geochem-PCalso showed that Cd occurred mainly in the free ionic form inthe xylem sap. These results suggest that Cd transfer from theroot medium to the xylem in A. halleri is an energy-dependentprocess that is partly shared with Zn and/or Fe transport. Furthermore,Cd is translocated from roots to shoots in inorganic forms.     

Effects of Growth Hormone Application on the Secondary Growth of Roots and Stems in Picea sitchensis (Bong.) Carr

Indol-3-ylacetic acid (IAA), gibberellin A3 (GA) and 6 benzylaminopurine(BAP) were applied factorially each at 3x10^–2 M in lanolinto the roots and stems of Sitka spruce seedlings and the activityof the two secondary meristems, the vascular cambium and phellogen,and of the parenchymatous tissues between them, was examined.All the treatments, with the exception of GA produced a localizedstimulation of radial growth at the point of application andthere was a similarity in the response of the various tissuesin both the root and stem. Radial growth of the xylem was notsignificantly affected in the roots whereas in the stems BAPand IAA stimulated growth. In the phloem BAP produced significantstimulation in both roots and stems and IAA stimulated growthin the roots. Growth of the parenchyma and periderm externalto the phloem was also strongly stimulated by both BAP, andIAA in roots and stems. In roots and stems the application of BAP altered the derivativesproduced by the vascular cambium, resulting in the productionof large multiseriate rays in the xylem, and giving rise toan overall increase in the proportion of ray tissue. Picea sitchensis (Bong.) Carr, Sitka spruce, secondary growth, xylem, phloem, periderm, wood rays, Indol-3-ylacetic acid, gibberellin A3, 6 benzylaminopurine, growth hormones     

Pre-harvest retting of flax: a light microscope study of the effects of glyphosate treatment on the maturation of stem tissues

A range of glyphosate treatments was applied to flax at three stages of flowering. Spraying at a rate equivalent to 1.4 kg a. e./ha at the start of flowering gave satisfactorily uniform premature desiccation and was selected for investigation of the effects of the herbicide on stem tissues, using light microscopy. During normal maturation, increase in fibre cell wall thickness, lignification of the fibres and differentiation of the secondary xylem continued for 3–5 wk after the beginning of flowering. This differentiation was halted by the application of glyphosate. Three wk after treatment various types of tissue damage were observed. In some sections epidermal and cortical cells showed the most damage, this being consistent with herbicide uptake at the stem surface. In other sections phloem and associated parenchyma cells showed the most damage. Disintegration of phloem and cortical cells occurred in some treated stem segments and caused separation of the fibre bundles from their surrounding tissues. This resembled the release of fibre bundles which results from conventional post-harvest retting. The potential of glyphosate application to flax as a pre-harvest retting technique is discussed.     

OPT3 Is a Phloem-Specific Iron Transporter That Is Essential for Systemic Iron Signaling and Redistribution of Iron and Cadmium in Arabidopsis

Iron is essential for both plant growth and human health and nutrition. Knowledge of the signaling mechanisms that communicate iron demand from shoots to roots to regulate iron uptake as well as the transport systems mediating iron partitioning into edible plant tissues is critical for the development of crop biofortification strategies. Here, we report that OPT3, previously classified as an oligopeptide transporter, is a plasma membrane transporter capable of transporting transition ions in vitro. Studies in Arabidopsis thaliana show that OPT3 loads iron into the phloem, facilitates iron recirculation from the xylem to the phloem, and regulates both shoot-to-root iron signaling and iron redistribution from mature to developing tissues. We also uncovered an aspect of crosstalk between iron homeostasis and cadmium partitioning that is mediated by OPT3. Together, these discoveries provide promising avenues for targeted strategies directed at increasing iron while decreasing cadmium density in the edible portions of crops and improving agricultural productivity in iron deficient soils.     

The Effect of Applied Growth Hormones on Cambial Division and the Differentiation of the Cambial Derivatives

When indole-3-acetic acid (IAA) is applied to woody shoots cambialdivision is stimulated and the cambial derivatives differentiateto produce xylem tissue. When gibberellic acid (GA) is applied,cambial division occurs but the resultant derivatives on thexylem side of the cambium remain undifferentiated. The relativelevels of applied IAA and GA are important in determining whethermainly xylem or phloem tissue is produced. High IAA/low GA concentrationsfavour xylem formation, whereas low IAA/high GA concentrationsfavour phloem production. The new phloem tissue produced asa result of hormone treatment is fully differentiated, containingsieve elements and sieve plates. IAA is important in promotingthe elongation of the cambial derivatives to produce xylem vesseland fibre elements, though in the case of xylem fibres appliedGA causes further elongation. IAA is an important factor indetermining vessel diameter in the ring-porous species Robiniapseudacacia, high levels of applied IAA giving wide springwood-typevessels and low levels giving narrow ‘summerwood’vessels.     

Gibberellin signaling

A study of stem anatomy and the sclerenchyma fibre cells associated with the phloem tissues of hemp (Cannabis sativa L.) plants is of interest for both understanding the formation of secondary cell walls and for the enhancement of fibre utility as industrial fibres and textiles. Using a range of molecular probes for cell wall polysaccharides we have surveyed the presence of cell wall components in stems of hemp in conjunction with an anatomical survey of stem and phloem fibre development. The only polysaccharide detected to occur abundantly throughout the secondary cell walls of phloem fibres was cellulose. Pectic homogalacturonan epitopes were detected in the primary cell walls/intercellular matrices between the phloem fibres although these epitopes were present at a lower level than in the surrounding parenchyma cell walls. Arabinogalactan-protein glycan epitopes displayed a diversity of occurrence in relation to fibre development and the JIM14 epitope was specific to fibre cells, binding to the inner surface of secondary cell walls, throughout development. Xylan epitopes were found to be present in the fibre cells (and xylem secondary cell walls) and absent from adjacent parenchyma cell walls. Analysis of xylan occurrence in the phloem fibre cells of hemp and flax indicated that xylan epitopes were restricted to the primary cell walls of fibre cells and were not present in the secondary cell walls of these cells.     

Radial secondary growth and formation of successive cambia and their products in Ipomoea hederifolia L. (Convolvulaceae)

Ipomoea hederifolia stems increase in thickness using a combination of different types of cambial variant, such as the discontinuous concentric rings of cambia, the development of included phloem, the reverse orientation of discontinuous cambial segments, the internal phloem, the formation of secondary xylem and phloem from the internal cambium, and differentiation of cork in the pith. After primary growth, the first ring of cambium arises between the external primary phloem and primary xylem, producing secondary phloem centrifugally and secondary xylem centripetally. The stem becomes lobed, flat, undulating, or irregular in shape as a result of the formation of both discontinuous and continuous concentric rings of cambia. As the formation of secondary xylem is greater in one region than in another, this results in the formation of a grooved stem. Successive cambia formed after the first ring are of two distinct functional types: (1) functionally normal successive cambia that divide to form secondary xylem centripetally and secondary phloem centrifugally, like other dicotyledons that show successive rings, and (2) abnormal cambia with reverse orientation. The former type of successive rings originates from the parenchyma cells located outside the phloem produced by previous cambium. The latter type of cambium develops from the conjunctive tissue located at the base of the secondary xylem formed by functionally normal cambia. This cambium is functionally inverted, producing secondary xylem centrifugally and secondary phloem centripetally. In later secondary growth, xylem parenchyma situated deep inside the secondary xylem undergoes de‐differentiation, and re‐differentiates into included phloem islands in secondary xylem. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 30–40.     

Seed Coat Retention and Hypocotyl Hook Development in Mutants of Arabidopsis thaliana L.

Germination and growth of wild-type and two mutant strains (aux-1and Dwf) of Arabidopsis thaliana L. have been examined. Seedlingsof aux-1 exhibit agravitropic roots whereas Dwf display bothagravitropic roots and shoots. Wild-type seedlings retained the seed coat at the root-hypocotyltransition zone and developed hypocotyl hooks. In contrast,aux-I and Dwf seedlings did not retain their seed coats andlacked hypocotyl hooks. A positive gravitropic response of theroots was essential for the retention of the seed coat at theroot—hypocotyl transition zone by the attachment of roothairs to the seed coat. The development of the hypocotyl hookwas aided by the retention of the seed coat. The apical regionof the hypocotyl apparently remained agravitropic during formationand maintenance of the hypocotyl hook. Arabidopsis thaliana L., auxins, gravitropism, hypocotyl hook, mutants, peg formation, germination     

Root Graviresponsiveness and Cellular Differentiation in Wild-type and a Starchless Mutant of Arabidopsis thaliana

Primary roots of a starchless mutant of Arabidopsis thalianaL. are strongly graviresponsive despite lacking amyloplastsin their columella cells. The ultrastructures of calyptrogenand peripheral cells in wild-type as compared to mutant seedlingsare not significantly different. The largest difference in cellulardifferentiation in caps of mutant and wild-type roots is therelative volume of plastids in columella cells. Plastids occupy12.3% of the volume of columella cells in wild-type seedlings,but only 3.69% of columella cells in mutant seedlings. Theseresults indicate that: (1) amyloplasts and starch are not necessaryfor root graviresponsiveness; (2) the increase in relative volumeof plastids that usually accompanies differentiation of columellacells is not necessary for root graviresponsiveness; and (3)the absence of starch and amyloplasts does not affect the structureof calyptrogen (i.e. meristematic) and secretory (i.e. peripheral)cells in root caps. These results are discussed relative toproposed models for root gravitropism. Arabidopsis thaliana, gravitropism (root), plastids, root cap, stereology, ultrastructure     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. III. Long-distance transport of abscisic acid depending on nitrogen nutrition and salt stress

Seedlings of Ricinus communis L. were cultivated in quartz sandand supplied with media which contained either different concentrationsof nitrate or ammonium nitrogen and were treated with a lowsalt stress. The concentration of ABA was determined in tissuesand in xylem and phloem saps. Between 41 and 51 day after sowing,abscisic acid (ABA) flows between roots and shoots were modelled.Long-distance transport of ABA was not stimulated under conditionsof nitrate deficiency (0.2 mol m^–3). However, when ammoniumwas given as the only N source (1.0 mol m^–3), ABA transportin both xylem and phloem was increased significantly. Mild saltstress (40 mol m^–3 NaCl) increased ABA transport in nitrate-fedplants, but not in ammonium-fed plants. The leaf conductancewas lowered by salt treatment with both nitrogen sources, butit was always lower in ammonium-fed compared to nitrate-fedplants. A negative correlation of leaf conductance to ABA levelsin leaves or flow in xylem was found only in comparison of ammonium-fedto nitrate-fed plants. Key words: Abscisic acid, ammonium, Ricinus communis, phloem, xylem, transport, nitrate, nitrogen nutrition