Surgical Experiments on the Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carota L.)

Surgical techniques were applied to the shoot apex of carrot(Daucus carota L.) to test the interpretation that provasculartissue is the initial stage of vascular differentiation andto localize the sources of the influences that control its differentiation.If the apex is isolated laterally by vertical incisions leavingit at the summit of a plug of pith tissue, vascular differentiationproceeds normally and an independent vascular system is formedin the pith plug. If all leaf primordia are systematically suppressed,provascular tissue continues to differentiate as an acropetalextension of the pre-existing vascular system but no furtherdifferentiation occurs. When the apex is isolated laterallyand all leaf primordia are suppressed, provascular tissue continuesto be formed acropetally and is extended basipetally into thepith plug by redifferentiation of pith cells, but no furtherdifferentiation occurs. This tissue reacts positively to histochemicaltests for esterase indicating its vascular nature. If only oneleaf primordium is allowed to develop on an isolated shoot apex,its vascular system develops normally and extends basipetallyinto the pith plug, but there is no extension of provasculartissue into the pith plug. These results support the interpretationthat the initial stage of vascular differentiation is controlledby the apical meristem but that further maturation of vasculartissue depends upon influences from developing leaf primordia.Copyright 2000 Annals of Botany Company Provascular tissue, differentiation, carrot (Daucus carota L.), shoot apex, surgical techniques, leaf primordia     

Auxin Effects on Vascular Differentiation in Ostrich Fern

Surgical experiments have confirmed the capacity of a fern shootapex to give rise to a mature vascular system in the absenceof leaves When the shoot apex of Matteuccia struthiopteris Todwas isolated by vertical incisions and incipient leaf pnmordiawere suppressed systematically for more than 5 weeks, a maturesiphonostele with reduced diameter and uninterrupted by leafgaps replaced the normal dictyostele If one or two leaf primordiawere allowed to develop, a transitional stele with one or twogaps was formed To test the possible role of the leaf primordiaas an auxin source, IAA in anion exchange beads was used toreplace the suppressed leaf primordia in experimental apicesThis treatment resulted in the formation of parenchymatous regionssimulating leaf gaps with an increase in stelar diameter Thisresponse contrasts with the well known promotion of vasculardifferentiation by auxin in seed plants Auxin, vascular differentiation, stele, shoot apex, Matteuccia struthiopteris     

THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. I. THE APICAL REGION

Thomson , B. F., and P. M. Miller . (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum. I. The apical region. Amer. Jour. Bot. 49(3): 303–310. Illus. 1962.—Seedlings of Pisum sativum grown under constant conditions and kept in total darkness or exposed daily to red or white light were harvested at the same plastochron age and examined histologically to determine what specific aspects of histogenesis and differentiation are affected by light. The tissue organization of the shoot apex is the same in all light conditions to a point below the 2 youngest leaf primordia. The first detectable difference is a slight thickening of the internode in light due to more and larger cells. The first effect on longitudinal growth appears below the fourth youngest primordium and consists of an increase of internode length in light-grown plants. This is associated with a greater distance between the apex and the first mature protoxylem. The distance from apex to the first pith, provascular strands, and protophloem and the distances between the 4 youngest leaf primordia are not affected by light.     

LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.     

Leaf Ontogeny in Digitaria eriantha

The shoot apex consists of two layers, the dermatogen and thehypodermis. The leaf primordia arise through periclinal divisionswithin these two layers on the side of the apex. Further divisionsof the dermatogen push the little protuberance upward and togetherwith divisions the hypodermis add internal tissues of the youngleaf. When the median and lateral bundles of the primordia arisein Digitaria eriantha they are isolated from the vascular supplyof the rest of the plant. The median strand, the first to form,and the first order laterals form at the disc of insertion ofthe primordium. The other laterals form higher up in the primordium.These strands extend both acropetally and basipetally to linkwith the vascular supply of the rest of the plant. Digitaria eriantha, apical meristem, leaf primordium, vascular bundle, orange G, tannic acid, iron alum     

Morpho-Histological Investigation of Plantlet Formation In Vitro in Taiwania flousiana Gaussen

Histological events during adventitious shoot formation in cultured shoot apex of 10–12-day-old seedlings and adventitious root formation in the elongated shoot of Taiwania floudana Gaussen were examined. Ceils of the peripheral subsurface layers of the shoot apex responded to cytokinin and divided into meristematic cells from which the shoot primordia were proliferated. A few bud primordia also originated from the epidermis and hypodermis of the adaxial surface of the cotyledon. The parenchyma of leaf gap of the shoots cultured in rooting medium dedifferentiated to regain the capacity of division and form adventitious root. Besides, cells that had relatively low potential of differentiation, such as the cortex parenchyma, pith ray, phloem parenchyma and cambium zone, albeit initiated to divide, but seldom formed root primordium. The origin of the adventitious roots in the leaf gap facilitated the establishment of the vascular connection between the shoot and root.     

Leaf Determination in the Fern Osmunda cinnamomea--A Reinvestigation

Earlier studies of leaf determination in the fern Osmunda cinnamomeaL., using the technique of excision and sterile culture, indicatedthat the apices of incompletely determined leaf primordia areconverted into shoot apices after isolation. Subsequently otherworkers questioned this interpretation, suggesting that a primordiumis determined as a leaf shortly after its inception and thatany shoot which arises from it is an adaxial adventitious bud.Because of the importance of this distinction in the interpretationof determination, the phenomenon has been reinvestigated byscanning electron microscopy and histological methods. Sincefixation for electron microscopy was inconsistence, the workwas performed on the primordia using resin models formed indental plastic moulds. Our data, on explanted P3, P4, and P5leaf primodia, provide strong confirmation for the interpretationthat the leaf apex is indeed converted to a shoot apex in incompletelydetermined primodia after explantation. This new evidence formsthe basis for interpreting leaf determination in this experimentalsystem.Copyright 1993, 1999 Academic Press Leaf determination, in vitro, surface replica, scanning electron microscopy, Osmunda cinnamomea L., cinnamon fern     

Vascular Differentiation in the Shoot Apex of Matteuccia struthiopteris

Initial vascular differentiation is generally considered tooccur in procambium. In ferns, however, a provascular tissueimmediately subjacent to the promeristem has been suggestedas an initial stage within which the procambium is subsequentlyformed. In contrast to this interpretation, a zonation conceptapplied in ferns recognizes a promeristem consisting of severallayers of cells in which no differentiation takes place. Thisstudy demonstrates that the shoot apex of Matteuccia struthiopterishas one cell layer of promeristem. Immediately subjacent tothe promeristem is the provascular tissue surrounding a centralgroup of pith mother cells. The developmental continuity betweenthe provascular tissue and the mature vascular tissue, and betweenthe pith mother cells and the pith, through transitional stages,indicates that the initial differentiation of vascular tissueand pith takes place in this prestelar tissue. The continuityof vascular differentiation in the area confronting young leavesor incipient leaf positions is interrupted by the formationof leaf gap initials. Developing leaves thus begin to exertinfluence on the vascular system at the prestelar stage. Smallprotoxylem elements with helical cell wall thickening, and distinctiveprotophloem elements are present in the leaf traces, but endabruptly near the junction regions of leaf traces to the meristele.Copyright1994, 1999 Academic Press Initial vascular differentiation, provascular tissue, pith mother cells, shoot apex, Matteuccia struthiopteris     

The prediction of leaf canopy expansion in the leek from a simple model dependent on primordial development

In an analysis of leaf development of leek plants grown in the field in 1988, successive leaves initiated, appeared (tip and ligule) and senesced at equal intervals of accumulated temperature/thermal time. These intervals corresponded to a plastochron of 92°C days and phyllochrons of 135 (tip) and 233 (ligule) °C days. The rate of appearance of ligules was exactly equal to the rate of leaf senescence, with the result that the number of fully-expanded leaves per plant remained constant at 1.4. These data, which were compatible with results from previous seasons, were used to develop a model of the interrelationships between primordium initiation at the shoot apex and subsequent events in the development of individual leaves. Primordium initiation is considered to be the primary controlling event in the life of a leaf, and the processes of tip appearance, ligule appearance and death can be predicted from knowledge of the number of primordia which have been initiated, without reference to the environment. A model of canopy expansion, based on the central role of the shoot apex, was developed using the temperature relations of primordium initiation and additional data on leaf expansion and leaf dimensions. Leaf area indices computed in this way provided a satisfactory simulation of the thermal-time course of leaf area index observed in a previous season, 1985.     

The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

Periclinal cell divisions in vegetative shoot apices of Pisumand Silene were recorded from serial thin sections by mappingall the periclinal cell walls formed less than one cell cyclepreviously. The distribution of periclinal divisions in theapical domes corresponded to the distributions subsequentlyoccurring in the apices when the young leaf primordia were forming.In Pisum, periclinal divisions were almost entirely absent fromthe I₁ region of the apical dome for half a plastochron justafter the formation of a leaf primordium and appeared, simultaneouslyover the whole of the next potential leaf site, about half aplastochron before the primordium formed. In Silene periclinaldivisions seemed to always present in the apical dome at thepotential leaf sites and also round the sides of the dome wherethe ensheathing leaf bases were to form. Periclinal divisionstherefore anticipated the formation of leaf primordia by occuring,in Pisum about one cell cycle and in Silene two or more cellcycles, before the change in the direction of growth or deformationof the surface associated with primordial initiation. Pisum, Silene, planes of cell division, orientation of cell walls, leaf primordia, shoot apical meristem, plastochron     

Initial Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carotaL.)

Although an initial stage of vascular differentiation precedingprocambium has been demonstrated in ferns, its presence in seedplants has not been accepted generally. In the shoot apex ofcarrot, a short cylinder of provascular tissue is recognizedas the initial stage of vascular differentiation. This firstbecomes apparent through the enlargement and vacuolation ofpith and cortical tissue rather than as a result of specificchanges in the provascular tissue itself. Procambium in discretestrands differentiates acropetally in the provascular tissuein relation to developing leaf primordia. Provascular tissueis not recognized above the axil of the youngest leaf primordiumbut it is distinct at or above the level at which the traceof the youngest primordium diverges. Support for the recognitionof provascular tissue is provided by a positive reaction tohistochemical tests for carboxylesterases in this tissue aswell as in procambium and later stages of vascular differentiation.Copyright1999 Annals of Botany Company. Provascular tissue, carboxylesterase, shoot apex, vascular differentiation, carrot (Daucus carotaL.).     

FOLIAR ONTOGENY AND HISTOGENESIS IN MAGNOLIA GRANDIFLORA L. I. APICAL ORGANIZATION AND EARLY DEVELOPMENT

Foliar ontogeny of Magnolia grandiflora was studied to elucidate possible unique features of evergreen leaves and their development. The apex of Magnolia grandiflora is composed of a biseriate or triseriate tunica overlying a central initial zone, a peripheral zone and a pith rib meristem. Leaf primordia are initiated by periclinal divisions on the apical flank of the tunica in its second layer. This initiation and expansion is seasonal just as in related deciduous magnolias. Following leaf initiation, a foliar buttress is formed and the leaf base gradually extends around the apex. As growth continues, separation of the leaf blade primordium from the stipule proceeds by intensified anticlinal divisions in the surface and subsurface layers near the base. Marginal growth begins in the blade primordium when it reaches approximately 200 μm in height and results in the formation of two wing-like extensions, the lamina. This young blade remains in a conduplicately folded position next to the stipule until bud break.     

Temperature Response of Vernalization in Wheat: A Developmental Analysis

The vernalization response of wheat ( Triticum aestivum L.)was reinterpreted from a developmental perspective, using currentconcepts of the developmental regulation of wheat morphologyand phenology. At temperatures above 0 °C, the effects ofthe process of vernalization per se in wheat are confoundedby the effects of concurrent vegetative development. These effectsare manifested by differences in the number of leaves initiatedby the shoot apex prior to floral initiation, which in turnaffects the subsequent rate of development to ear emergenceand anthesis. Leaf primordia development during vernalizationand total leaf number at flowering were used to develop criteriato define both the progress and the point of saturation of thevernalization response. These criteria were then used to reinterpretthe results of Chujo ( Proceedings of the Crop Science Societyof Japan 35 : 177–186, 1966), and derive the temperatureresponse of vernalization per se for plants grown under saturatinglong day conditions. The rate of vernalization increased linearlywith temperature between 1 and 11 °C, such that the timetaken to saturate the vernalization response decreased from70 d at 1 °C to 40 d at 11 °C. The rate declined againat temperatures above 11 °C, and 18 °C was apparentlyineffective for vernalization. Total leaf number at saturation,however, increased consistently with temperature, as a resultof the balance between the concurrent processes of leaf primordiuminitiation and vernalization. Total leaf number at saturationincreased from 6 at 1 °C to 13.3 at 15 °C, which inturn influenced the time taken to reach ear emergence. The advantagesof using this developmental interpretation of vernalizationas the basis for a mechanistic model of the vernalization responsein wheat are discussed. Triticum aestivum L.; wheat; vernalization; rate; temperature; primordia; leaf number; flowering     

Symplastic communication between primary and developing lateral roots of Arabidopsis thaliana

The functional symplastic connections between primary and developinglateral roots of Arabidopsis were studied non-invasively usingconfocal laser scanning microscopy (CLSM), following ester-loadingof the phloem with carboxyfluorescein (CF). Prior to the formationof lateral primordia in the pericycle, the phloem of the primaryroot behaved as an isolated conducting domain. However, thedifferentiation of phloem connector elements within the dividingpericycle allowed the rapid establishment of intercellular communicationbetween the phloem and the cells of the lateral primordium.This communication was often established prior to the completeemergence of the lateral root from the parent root. Shortlyafter its emergence, functional conducting phloem became differentiatedwithin the developing lateral root. A progressive isolationbetween the phloem and surrounding cells at the base of thelateral root was observed as the lateral continued to grow;the new phloem conducting CF to the elongation zone where itwas unloaded symplastically from the protophloem into surroundingcells of the cortex and stele, a feature mirroring the patternfound near the apex of growing primary roots. Anomalous patternsof intercellular communication were found which indicated thatpreviously functional symplastic pathways may have become sealedoff following the emergence of some of the lateral roots. Key words: Arabidopsis, carboxyfluorescein, confocal laser scanning microscopy (CLSM), intercellular transport, lateral roots, phloem (unloading), symplast     

A Sequential Study of Cell Divisions and Expansion Patterns on a Single Developing Shoot Apex of Vinca major

During the growth of a single developing vegetative apex ofVinca major, both the orientation and frequency of cell divisions,and the pattern of cell expansion, were observed using a non-destructivereplica technique. Micrographs taken at daily intervals illustratethat the central region of the apical dome remains relativelyinactive, except for a phase of cell division which occurs after2 d of growth. The majority of growth takes place at the proximalregions of the dome from which develop the successive pairsof leaves. The developing leaf primordia are initiated by aseries of divisions which occur at the periphery of the centraldome and are oriented parallel to the axis of the subsequentleaves. The cells which develop into the outer leaf surfaceof the new leaves undergo expansion and these cells divide allowingfor the formation of the new leaf. This paper describes thefirst high-resolution sequential study of cell patterns in asingle developing plant apex. Sequential development, cell division, expansion patterns, SEM, Vinca major, apical dome, leaf primordium, leaf initiation     

Changes in Poparity of Growth During Leaf Initiation in the Pea, Pisum sativum L.

In the apical dome of the pea shoot apex the axis of growthof the epidermal cells becomes predominantly longitudinal inthe I₁ region one plastochron before a leaf is initiated, andthis orientation persists into the young primordium. In contrast,in the underlying, non-epidermal cells the growth axis in theI₁ region becomes randomized half a plastochron before leafinitiation, but in the young primordium it becomes the sameas in the epidermis. The initiation of a leaf primordium thereforetakes place without any major change in the orientation of growthaxes in the epidermis. A controlling role for the epidermisis therefore suggested. No marked reorientation of the growthaxis occurs on the sides of the newly initiated primordium.The shape of the young primordium can be related to the differentialrates of growth and division within it rather than to changesin growth orientation. Pisum sativum, pea, shoot apex, meristem, growth, epidermis, polarity     

Biophysical mechanisms for morphogenetic progressions at the shoot apex.

Leaf primordia, first visible as small bumps, are produced in a cyclical pattern at the edges of the shoot apex, a smooth region at the top of the stem. Their formation is a biomechanical process. This review first considers hypothetical construction mechanisms and then summarizes research that provides information about how and where the primordia are made. Studies of growth at the primordium site indicate the importance of growth parallel to the surface in generating the forces for primordium emergence. The symmetry of the pattern of reinforcement by cellulose microfibrils correlates with the subsequent pattern of primordium production. Finite element models of the apex reveal that lateral bulging of the apex results in a gradient of shear stress, with high shear at the future primordium site. In contrast, tension parallel to the surface is lowest at the primordium site. Response of apical surface tissue to punctures indicates that an existing primordium can exert a pulling force tangential to its base and a compressive force perpendicular to its base. These observations lead to identification of a continuous biophysical cycle for apex morphogenesis, in which most of the steps are direct physical consequences of the previous step. Biophysical processes, subject to input from genetic, hormonal, and environmental sources, are thus involved in the construction and patterning of leaf primordia.     

DEVELOPMENT AND ORGANIZATION OF THE PRIMARY VASCULAR SYSTEM IN POPULUS DELTOIDES ACCORDING TO PHYLLOTAXY

An actively growing cottonwood bud was embedded in epon-araldite and serially sectioned at 2 μm. The sections were analyzed microscopically with the optical shuttle system of Zimmermann and Tomlinson, and all data were quantitatively recorded relative to the apex and to leaf plastochron index (LPI). Analysis of the sections revealed an acropetally developing procambial system organized according to a precise phyllotaxy. Six procambial strands could be recognized and followed long before the leaf primordia that they would enter were evident at the apex. Origin of these strands coincided with developmental events both in the parent trace and its primordium and in the antecedent leaf on the same orthostichy. Once a primordium and its trace attained a certain stage of development, trace bundles began to develop basipetally from the primordium base. These trace bundles appeared to be the earliest progenitors of wood formation in cottonwood. It was concluded that the concept of residual meristem and its corollary, the hypothesis that acropetally developing procambial strands determine the inception sties of new primordia, apply to the cottonwood apex.     

Effect of Assimilate Supply on Development and Growth Potential of Axillary Buds in Roses

The effect of assimilate supply on axillary bud developmentand subsequent shoot growth was investigated in roses. Differencesin assimilate supply were imposed by differential defoliation.Fresh and dry mass of axillary buds increased with increasedassimilate supply. The growth potential of buds was studiedeither by pruning the parent shoot above the bud, by graftingthe bud or by culturing the bud in vitro. Time until bud breakwas not clearly affected by assimilate supply during bud development,Increase in assimilate supply slightly increased the numberof leaves and leaf primordia in the bud; the number of leavespreceding the flower on the shoot grown from the axillary budsubstantially increased. No difference was found in the numberof leaves preceding the flower on shoots grown from buds attachedto the parent shoot and those from buds grafted on a cutting,indicating that at the moment of release from inhibition thebud meristem became determined to produce a specific numberof leaves and to develop into a flower. Assimilate supply duringaxillary bud development increased the number of pith cells,but the final size of the pith in the subsequent shoot was largelydetermined by cell enlargement, which was dependent on assimilatesupply during shoot growth. Shoot growth after release frominhibition was affected by assimilate supply during axillarybud development only when buds sprouted attached to the parentshoot, indicating that shoot growth is, to a major extent, dependenton the assimilate supply available while growth is taking place.Copyright1994, 1999 Academic Press Assimilate supply, axillary bud, cell number, cell size, defoliation, development, growth potential, meristem programming, pith, Rosa hybrida, rose, shoot growth     

INITIAL VASCULARIZATION OF THE VEGETATIVE SHOOT OF ABIES CONCOLOR

Parke , Robert V. (Colorado State U., Fort Collins.) Initial vascularization of the vegetative shoot, of Abies concolor. Amer. Jour. Bot. 50(5): 464–469. Illus. 1963.—In the dormant winter bud, the future vascular system of the shoot exists as a rather ill-defined system of procambial strands, which extends acropetally from the scale traces through a plate of thick-walled, deeply staining cells, the crown, and into the axis and the numerous foliar primordia making up the telescoped shoot. Each foliar primordium receives a single procambial strand or leaf trace. The procambial strands differentiate acropetally. No differentiated vascular tissue was observed in the dormant shoot. As the shoot elongates in the spring, vascular differentiation progresses at a rapid rate. In the leaf traces, protophloem differentiates acropetally. The protoxylem, which appears first in the axial region of the trace, differentiates acropetally into the foliar primordium and basipetally into the stem. The first-formed phloem elements are short-lived. They are nucleate and without sieve areas. In the protoxylem, the first-formed tracheids are mostly of the annular or spiral-thickened type.