Growth changes in the shoot apex of Sinapis alba during transition to flowering

The aim of the work was to report morphological changes whichoccur in the shoot apex during the morphogenetic switch to floweringin the model long day (LD) plant, Sinapis alba. During the floraltransition induced by 1 LD the growth rate of all componentsof the shoot apex is modified profoundly. The earliest changes,detected at 24 h after start of LD, include a decrease in plastochronduration and an increase of growth of leaf primordia. One daylater, the meristem dome starts to increase in volume, apicalinternodes have an increased height and there is a precociousoutgrowth of axillary meristems. All these changes precede initiationof flower primordia, which starts at about 60 h after the startof LD. Later changes include meristem doming, a decrease inthe plastochron ratio and a shift to a more complex phyllotaxis.All the changes, except the decreased plastochron ratio, arecharacteristics of an apex with an increased tempo of growth.The stimulation of longitudinal growth (height of apical intemodes)is more marked and occurs earlier than the reduction of radialgrowth (plastochron ratio). Key words: Axillary meristem, internode growth, leaf growth, plastochron ratio, plastochron duration     

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     

Growth of the Shoot Apex of Agropyron repens (L.) Beauv. During Successive Plastochrons

Two kinds of size change occur in the apical dome of Agropyronrepens during development of the shoot. A cyclic increase anddecrease in size results from the production of a new stem segmentand associated leaf primordium during each plastochron. A progressiveincrease and then decrease in size, which occur over a periodof several plastochrons, is attributable to discrepancies betweenthe size increment during each plastochron and the size of thestem segment formed at the end of the plastochron. The volumedoubling time of the dome remains constant at approximatelyone plastochron. Fluctuations in mean cell generation time correlatewith changes in mean cell volume and do not contribute to thesize changes of the dome. Agropyron repens (L.), Beauv, couch grass, shoot apex, cell growth, cell divisions     

RELATIONSHIPS BETWEEN SHOOT GROWTH AND CHANGING PHYLLOTAXY OF RANUNCULUS

Growth of Ranunculus shoots through ontogeny is quantified by techniques utilizing scanning electron microscopy and studies on living plant material. The order of the contact parastichy phyllotaxy in the apical system is related to the relative plastochron rates of growth of the shoot. There is a change in the (2, 3) contact parastichy pattern of vegetative phyllotaxy to a transitional (3, 5) contact pattern which is maintained through sepal production. Formation of a 5(1, 1) whorl of petal primordia establishes a (5, 8) contact pattern with the sepal primordia. Subsequent initiation of stamen primordia, in spiral sequence, results in (5, 8, 13) triple contacts between petal and stamen primordia. The stamen primordia and carpel primordia arrangement is characterized by a (8, 13) contact parastichy pattern of phyllotaxy. Through ontogeny the volume of the shoot apex progressively increases but the shape of the apex, described by a second degree polynomial, remains constant. The plastochron and the relative plastochron rates of radial and vertical displacement of primordia progressively decrease during transition but there is no alteration of the chronological rate of apical expansion. The change in contact parastichy phyllotaxy through ontogeny is interpreted as a change in the relative positions of primordia insertion on the apex resulting from an increase in apical volume and an increased rate of primordia initiation.     

Changes in Volume and Cell Number in the Different Regions of the Shoot Apex of Pisum during a Single Plastochron

The length of the ninth plastochron in shoot apices of Pisumsativum was measured and found to be almost 46 h. This singleplastochron was divided into 11 morphologically recognizablestages and the time taken to reach each stage was measured.The cell number and cell volume of five regions of the apexwas measured at each stage of the plastochron. Although theapex as a whole grew exponentially, growth during the first30 h of the plastochron was predominantly in the primordiumand the adjacent tissues, whereas in the last 16 h growth wasmainly in the apical dome. Since the mean cell volume remainedconstant, different rates of growth were due to different ratesof cell division. The data suggested that the apex probablygrows by the formation of growth centres on alternate sidesof the apex, the beginning of each new growth centre being apparentas an increased rate of growth in the apical dome 16 h beforethe beginning of the next plastochron. The inception of a newprimordium may therefore precede its appearance as a bump byabout 16 h, and precede the first periclinal division in thetunica by 26 h. A central zone of larger cells with lightly-stainingnuclei was found at the extreme apex. This central zone becamereduced in size or disappeared at the time at which a new primordiumwas about to become visible.     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

Size of the Chrysanthemum Shoot Apex in Relation to Inflorescence Initiation and Development

The size of the apical dome of Chrysanthemum morifolium Ramat.at the transition to inflorescence initiation in continuouslight (long days) was not systematically influenced by eitherthe temperature or the irradiance under which the plants weregrown. It was generally 0.26 mm in diameter and c. 3.6 x 10^–3mm³ in volume when the first bract was initiated. The dimensionsof the apical dome of plants in short days were only slightlysmaller at this stage. Similarly, each step in the further developmentof the chrysanthemum inflorescence was associated with a narrowrange of apex sizes, indicating that inflorescence initiationand development are closely related to apex size. Chrysanthemum morifolium Ramat, shoot apex, inflorescence initiation     

THE GROWTH OF THE STEM TIP OF KALANCHOËU CV. ‘BRILLIANT STAR’

Stein , Diana B. and o . L. Stein . (Montana State U., Missoula.) The growth of the stem tip of Kalanchoë cv. ‘Brilliant Star.‘ Amer. Jour. Bot. 47 (2) : 132—140. Illus. I960.–The purposes of this investigation were (1) to define as clearly as possible the events in the shoot apex and its immediate derivatives during the ontogeny of the shoot; and (2) to determine the changes which occur during the transition from a vegetative to a reproductive meristem. Rate of leaf production in Kalanchoë is basically constant. The rate of leaf growth subsequent to the early primordial state is, however, dependent on the age of the plant and on the environment in which the plant is grown. By keeping these factors constant a correlation can be demonstrated between the size of the youngest visible leaf and the microscopic primordia. Throughout its ontogeny the general architecture of the shoot apex remains essentially the same. Two tunica layers cover the corpus in the vegetative shoot apex, and even in the flowering meristem these 2 layers can be detected. The apex is essentially flat and blends into the adjacent leaf primordia early in the plastochron. About 10 days after flower induction has been started the apex changes its form to a dome, primarily by increased cell division. At the same time the rate of elongation of the youngest internodes increases thus placing the flowering stem tip atop an elongated stem. Axillary development is ultimately responsible for the development of a dichasium.     

Apical Growth and Modification of the Development of Primordia During Re-flowering of Reverted Plants of Impatiens balsamina L

Impatiens balsamina L. was induced to flower by exposure to5 short days and then made to revert to vegetative growth byreturn to long days. After 9 long days reverted plants wereinduced to re-flower by returning them to short days. Petalinitiation began immediately and seven primordia already presentdeveloped into petals instead of into predominantly leaf-likeorgans. However, the arrangement of primordia at the shoot apex,their rate of initiation and size at initiation remained unchangedfrom the reverted apex, as did apical growth rate and the lengthof stem frusta at initiation. The more rapid flowering of thereverted plants than of plants when first induced, and the lackof change in apical growth pattern, imply that the revertedapices remain partially evoked, and that the apical growth patternand phyllotaxis typical of the flower, and already present inthe reverted plants, facilitate the transition to flower formation. Impatiens balsamina, flower reversion, partial evocation, shoot meristem, determination, leaf development     

A Model of Flowering in Chrysanthemum

A mathematical model of flowering in Chrysanthemum morifoliumRamat. is described which may be used to predict quantitiessuch as the number of primordia initiated by the apex, plastochronduration and apical dome mass before, during and after the transformationof the apical meristem from vegetative to reproductive development.The model assumes that primordial initiation is regulated byan inhibitor present in the apical dome. Within each plastochronthe apical dome grows exponentially, and the inhibitor concentrationdeclines through chemical decay and dilution. When the inhibitorconcentration falls to a critical level a new primordium isinitiated. There is instantaneous production of inhibitor, anda decrease in dome mass corresponding to the mass of the newprimordium. The process continues until the apical dome attainsa particular mass when the first bract primordium is produced.Subsequent primordia compete with the apical dome for substrates,and the specific growth rate of the dome declines with successiveplastochrons. Eventually, the net mass of the dome starts todecline until it is entirely consumed in the production of floralprimordia. Chrysanthemum morifoliumRamat, flowering, primordial initiation     

The Growth of the Shoot Apex and the Apical Dome of Barley During Ear Initiation

The growth of the floral main shoot apex of spring barley wasstudied during the period of ear initiation (that is, from initiationof the collar primordium until maximum primordium number wasattained). While floral primordia were being initiated the relativelength growth rate of the shoot apex was low. After maximumprimordium number there was about a twofold increase in relativelength growth rate. Estimates of the volume, fresh and dry weightof the floral apex indicated that the relative weight growthrate was also low at first and increased after maximum primordiumnumber. The rates of growth and the size at initiation of thefloral primordia was affected by their position on the floralshoot apex. The relative volume growth rate increased acropetallyfrom the first initiated (collar) primordium. The collar wasthe smallest and each subsequently-initiated primordium increasedin length. The diameter of the newly-initiated primordium alsoincreased until more than half the primordia had been initiatedand then it declined. The apical dome increased in both lengthand diameter and both were at a maximum at the time of the double-ridgestage and then both measurements declined. Length and diameterwere at a minimum at maximum primordium number. Subsequentlythere was an increase in the length of the dome, after whichboth the dome and some of the last formed, distal primordiadied. The period of spikelet initiation therefore is a stage duringwhich the relative growth rate of the floral shoot apex is low,there are changes in the size of the dome and the primordiashow a progression of increasing relative growth rates acropetallyalong the shoot apex. These changes produce the embryo ear inwhich the most advanced spikelets are in the lower mid-partof the ear. Changes in size of the apical dome prior to maximumprimordium number may be related to the subsequent death ofspikelet primordia and therefore also to grain number in themature ear.     

CHANGE IN EPILOBIUM PHYLLOTAXY DURING REPRODUCTIVE TRANSITION

The ontogeny of Epilobium hirsutum grown under natural summer photoperiod in a glasshouse was divided into vegetative, early transitional, transitional, and floral stages. Bijugate phyllotaxy, common to both the vegetative and early transitional stages, is transformed into spiral phyllotaxy during the transitional stage by an initial change in the divergence angle of a single primordium inserted at a unique level on the shoot. Leaf primordia subsequently are inserted in a spiral arrangement in the indeterminate floral shoot apex. The early transitional shoot apical meristem is about 1.5 times the volume of the vegetative meristem but expands at about two-thirds the relative plastochron rate of volume increment of the vegetative meristem. There are progressive decreases in the plastochron and relative plastochron rates of radial and vertical shoot growth through ontogeny. Relative chronological rates of shoot growth, however, are not altered during ontogeny. Spiral transformation results from changes in the relative points of insertion of leaf primordia on the shoot meristem. These changes are accompanied by an increased rate of primordia initiation on a more circular shoot meristem. The change in phyllotaxy during ontogeny is similar to that which was artificially induced by chemical modification of auxin concentration gradients in the shoot apex, with the additional feature that there is an initial increase in the volume of the shoot meristem prior to the natural spiral transformation. Size of the shoot apical meristem, however, appears to have little influence on Epilobium phyllotaxy; but the geometric shape of the meristem is well correlated with bijugate to spiral transformations. This suggests that geometric parameters of the shoot meristem should be considered in theoretical models of phyllotaxy.     

Rates of Growth and Primordial Initiation During Flower Development in Silene at Different Temperatures

The growth of the flower and its constituent parts was measuredin Silene coeli-rosa plants, induced at 13, 20 and 27 °C,in order to try and identify those processes which consistentlyoccurred and would therefore be more likely to be essentialfor flower formation. The increased growth rate of the apical dome just before orabout the time of sepal initiation was not maintained in theflower, the growth rate of which was comparable to that of avegetative apex until all the carpels had been initiated, whenit decreased further. The primordia of the same whorl all hadsimilar growth rates so that the relative sizes of the primordiareflected their relative ages since their initiation. The relativegrowth rate of the stamens was the same (13 and 20 °C) orless (27 °C) than that of the sepals, but the relative growthrate of the petals was lower than either. The growth rate ofthe flower axis was least at the sepal node and increased bothdistally and proximally from this region. The plastochron during sepal initiation was shorter than forleaf initiation and tended to be shorter still during initiationof stamens and petals. Increasing temperature increased therate of primordial initiation but at 27 °C the growth ratesof the primordia were lowest although the rates of primordiainitiation were highest. The form of the flower, as exemplifiedby the relative sizes of the primordia at the moment when allcarpels had been initiated, was constant despite the differinggrowth rates and sizes of the primordia on initiation in differenttemperatures. It is concluded that neither the initiation ofthe primordia in the flower nor the form of the flower is determinedprimarily by the relative growth rates of its component parts. Silene coeli-rosa, flower development, primordia initiation, growth     

Structure of the vegetative shoot apex ofCassiope lycopodioides

The structure of the vegetative shoot apex ofCassiope lycopodioides D. Don, which has a decussate leaf arrangement, was analyzed using trans- and longisections to generate a three-dimensional viewpoint. The apical dome of this species is relatively high from the middle to the maximal area phase of a plastochron. Therefore, the initial protrusion of a pair of leaf primordia occurs laterally on an apical dome conspicuously in contrast to the cases ofDaphne pseudo-mezereum andClethra barbinervis whose apices are nearly flat or slightly convex. The structure of the apex ofCassiope, however, may be understood with the concept of “apical sectors” on the same basis asDaphne andClethra (Hara, 1961, 1962, 1971a, b, c).     

Analysis of Shoot Apical Growth of Picea sitchensis Seedlings

During the first 100 days after sowing (March-June) the followingchanges took place at the terminal shoot apices of Picea sitchensisseedlings: plastochrones (T) decreased from over 24 h to 4 h;apical domes enlarged from less than 0·20 mm to 0·45mm diameter (D); the ‘projected’ area of tissuesproduced by the apical domes (i.e. viewed from above) increasedin amount from less than 0·012 to 0·024 mm² day^-1;about 15 per cent of this tissue was re-invested in the apicaldomes, the rest was used to produce primordia; and the volume-doublingtimes of the apical dome tissues decreased from over 150 h to50 h. After 100 days there was no further re-investment in theapical domes, but the domes did not decrease abruptly in size.Less tissue was produced per day, but the primordia were smallerso that the rate of primordia formation did not fall precipitously.Plastochrone ratios were inversely related to D, but the relationshipbetween T and D depended on whether T was decreasing or increasing.Progenies which were known to be fast growing tended to build-uptheir apical domes rapidly (i.e. have large ‘re-investmentratios’) and to be capable of producing small primordia.These attributes can evidently be evaluated on seedlings andcould help to lessen the cost of tree breeding progeny-testprogrammes. meristem, Picea sitchensis, Sitka spruce, growth, shoot apex     

Changes in Apical Growth and Phyllotaxis on Flowering and Reversion in Impatiens balsamina L.

When plants of Impatiens balsamina L were subjected to 5 shortdays and then re-placed in long days, they began to form a terminalflower and then reverted to vegetative growth at this terminalshoot apex The onset of flowering was accompanied by an increasein the rate of initiation of primordia, an increase in the growthrate of the apex, a change in primordium arrangement from spiralto whorled or pseudo-whorled, a lack of internodes, and a reductionm the size at initiation of the primordia and also of the stemfrusta which give rise to nodal and internodal tissues On reversion,parts intermediate between petals and leaves were formed, followedby leaves, although in reverted apices the size at initiationand the arrangement of primordia remained the same as in thefloweing apex The apical growth rate and the rate of primordiuminitiation were less in the reverted apices than in floral apicesbut remained higher than in the original vegetative apex Sincethe changes in apical growth which occur on the transition toflowering are not reversed on reversion, the development oforgans as leaves or petals is not directly related to the growthrate of the apex, or the arrangement, rate of initiation orsize at initiation of primordia Impatiens balsamina L, flower reversion, evocation, phyllotaxis, shoot meristem     

The Effects of Vernalization on the Growth of the Wheat Shoot Apex

he effect of vernalization on the growth of the wheat shootapex was examined by comparing three genetic lines of ChineseSpring (CS) wheat having strong [CS (Hope 5D)], medium (CS Euploid),or no [CS (Hope 5A)] vernalization requirement. The mean volumeof the apical dome increased gradually in all lines, and thenthe apical dome enlarged rapidly as its relative growth rate(RGR) increased prior to double ridge formation. Phytomer volumeat initiation remained constant, so that the ratio of phytomerto apical dome at primordium initiation decreased in successiveplastochrons. In CS Euploid and in unvernalized CS (Hope 5D),the RGR of the apical dome tended to decrease at least untilinitiation of the collar primordium. The rate of primordiuminitiation at double ridge formation increased in proportionto the RGR of the apex at that time; i.e. it increased greatlyin CS (Hope 5A) and vernalized CS (Hope 5D), less so in CS Euploid,but no increase was observed in unvernalized CS (Hope 5D). Thetime of formation of double ridges seemed to be independentof the growth rate or size of the apical dome. The number oftillers present at ear emergence was inversely proportionalto vernalization requirement and was reduced by vernalization.Vernalization resulted in a decrease in the RGR of the newly-initiatedleaf primordia in relation to the RGR of the apical dome andthe axial part of the phytomer. Transfer of plants from longto short days at various times during growth showed that vernalizationincreased the number of labile primordia which could developinto either leaf, collar or spikelet. Vernalization thereforeseems to alter the ability of the apex to respond to subsequentphotoperiod rather than to affect its growth directly. Triticum aeslivum, wheat, chromosome substitution lines, shoot apex growth, vernalization     

Growth of the Stem of Agropyron repens (L.) Beauv

The growth rate of the stem of Agropyron repens (L.) Beauv.begins to decline when the sixth foliage leaf has expanded butthe relative growth rate declines throughout the period betweenthe production of one and ten mature leaves. On an absolutetime scale there is a progressive decline in growth rate ofsuccessively formed stem (node-internode) units. On a plastochronscale the relative growth rate of successive stem units declineswithin the apical region but increases behind the apex. Thedecline in the apical region is related to a decrease in therate of cell division and in the later formed stem units thereis no significant increase in cell number from the time of theirformation by the apex until the internode is initiated duringtheir fourth plastochron. These changes are related to concurrentchanges in the size of the shoot apex and in rates of leaf growth.     

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     

Dual effects of miR156-targeted SPL genes and CYP78A5/KLUH on plastochron length and organ size in Arabidopsis thaliana

Leaves of flowering plants are produced from the shoot apical meristem at regular intervals, with the time that elapses between the formation of two successive leaf primordia defining the plastochron. We have identified two genetic axes affecting plastochron length in Arabidopsis thaliana. One involves microRNA156 (miR156), which targets a series of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes. In situ hybridization studies and misexpression experiments demonstrate that miR156 is a quantitative, rather than spatial, modulator of SPL expression in leaf primordia and that SPL activity nonautonomously inhibits initiation of new leaves at the shoot apical meristem. The second axis is exemplified by a redundantly acting pair of cytochrome P450 genes, CYP78A5/KLUH and CYP78A7, which are likely orthologs of PLASTOCHRON1 of rice (Oryza sativa). Inactivation of CYP78A5, which is expressed at the periphery of the shoot apical meristem, accelerates the leaf initiation rate, whereas cyp78a5 cyp78a7 double mutants often die as embryos with supernumerary cotyledon primordia. The effects of both miR156-targeted SPL genes and CYP78A5 on organ size are correlated with changes in plastochron length, suggesting a potential compensatory mechanism that links the rate at which leaves are produced to final leaf size.