Relationships between Bulb Size, Apex Size and Flowering in Bulbous Iris cv. Ideal

Large, retarded bulbs of the Iris cv. Ideal flower readily after exposure to appropriate preplanting conditions, whereas smaller bulbs flower with decreased frequency. Flower initiation occurs when apices from large, retarded bulbs are cultured on either Murashige-Skoog medium supplemented with gibberellic acid, or on the same medium without gibberellic acid on which scales from large bulbs have been incubated. Since floral initiation seldom occurs in explants from small bulbs, it is likely that reduced flowering of small bulbs relates in part to characteristics of the apical meristem. Apical dome diameter is one characteristic of retarded (and freshly dug) bulbs that is positively correlated with bulb size. However, whereas prolonged storage at retarding temperature increases the frequency of flowering of smaller bulbs, there is not a concomitant increase in apical dome diameter. Moreover, the ratio of apical dome diameter to bulb size in freshly dug bulbs does not increase measurably with later digging date indicating that apical dome size is not correlated with bulb maturity.     

Cytopathology of Shoot Apical Meristem of Hop Plants Infected with Hop Stunt Viroid11)

Shoot apical meristems of hop plants infected with hop stunt viroid (HSV) were examined for cytopathic changes. No measurable changes in ultrastructure were found in shoot tip 0.2 mm long bearing apical dome and two pairs of the primordia, whereas in the 3rd leaf primordium cell walls were undulate and/or frequently were irregular in thickness. These cell wall abnormalities were not observed in comparable shoot tips of the uninoculated hop plants or HSV-free hop plants obtained by meristem tip culture.     

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     

A study by cryo-scanning electron microscopy of the initiation and early development of an inflorescence in glasshouse celery (Apium graveolens L. var. dulce (Miller) Pers.) grown under inductive conditions

The morphology of flower initiation and early development in glasshouse celery (Apium graveolens L. var. dulce (Miller) Pers.) cv. Celebrity was studied by means of apical dissections and cryo-scanning electron microscopy. Easily recognisable morphological features were used to define seven stages in the early development of the inflorescence. A highly significant linear regression was established between the logarithm of the apical diameter (measured diametrically across the apical dome between the two most recently initiated leaf or inflorescence primordia) and these discrete floral stages. There was no strong evidence that either the origin or the slope of the regression varied with different combinations of temperature (viz. 10°C or 14°C) and daylength (viz. natural, short or long) which were conducive for the initiation and development of an inflorescence. It is suggested that both apical diameter and floral stage may be used as parameters for assessing the influence of environmental factors such as temperature and daylength on the floral development of glasshouse celery.     

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     

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     

Floral morphogenesis in Anagallis: Scanning-electron-micrograph sequences from individual growing meristems before,during, and after the transition to flowering

Non-destructive scanning electron microscopy allows one to visualize changing patterns of individual cells during epidermal development in single meristems. Cell growth and division can be followed in parallel with morphogenesis. The method is applied here to the shoot apex of Anagallis arvensis L. before, during, and after floral transition. Phyllotaxis is decussate; photoperiodic induction of the plant leads to the production of a flower in the axil of each leaf. As seen from above, the recently formed oval vegetative dome is bounded on its slightly longer sides by creases of adjacent leaf bases. The rounded ends of the dome are bounded by connecting tissue, horizontal bands of node cells between the opposed leaf bases. The major growth axis runs parallel to the leaf bases. While slow-growing at the dome center, this axis extends at its periphery to form a new leaf above each band of connecting tissue. Connecting tissue then forms between the new leaves and a new dome is defined at 90° to the former. The growth axis then changes by 90°. This is the vegetative cycle. The first observed departure from vegetative growth is that the connecting tissue becomes longer relative to the leaf creases. Presumably because of this, the major growth axis does not change in the usual way. Extension on the dome continues between the older leaves until the axis typically buckles a second time, on each side, to form a second crease parallel to the new leaf-base crease. The tissue between these two creases becomes the flower primordium. The second crease also delimits the side of a new apical dome with the major axis and growth direction altered by 90°. During this inflorescence cycle the connecting tissue is relatively longer than before. Much activity is common to both cycles. It is concluded that the complex geometrical features of the inflorescence cycle may result from a change in a biophysical boundary condition involving dome geometry, rather than a comprehensive revision of apical morphogenesis.Abbreviation SEM scanning electron microscopy, micrograph Use of the SEM facility of Professor G. Goffinet, Institute of Zoology, University of Liège, is greatly appreciated. We thank Dr. R. Jacques, C.N.R.S., Le Phytotron, Gif-sur-Yvette, France, for providing the experimental material, and Mr. Philippe Ongena for expert photography. Support was from grants from the U.S. Department of Agriculture and National Science Foundation as well as from the Fonds National de la Recherche Scientifique, Fonds de la Recherche Fondamentale et Collective, and the Action de Recherche Concertée of Belgium.     

A theory for inflorescence development and flower formation based on morphological and biophysical analysis in Echeveria

Floral development is generally viewed as involving interactions between recently made organs and generative activity on the apical dome; one set of floral organs is thought to induce the next. To investigate such interactions, flowering in Echeveria derenbergii (J. Purpus) was studied at two levels of structure. At the larger, morphological, level the inflorescence apex is shown to have simple cyclic development. Seen from above, it elongates horizontally, then forms a transverse cleft to demarcate a flower primordium in one of two rows. The meristem then elongates at 90° to its previous axis, also horizontally, and demarcates a flower in the other row. Activity on the apical surface correlates well with the nature and activity of adjacent sub-apical organs. For example, the 90° shifts in elongation of the meristem correlate with that tissue's being attached, laterally, to successive large growing bracts whose bases lie at 90°. Also, on the flower primordium, the five sepals arise in a spiral sequence which correlates with one of increasing age, since formation by the cleft, of the edges of the primordium.The second level of study was to test whether the developmental correlations could have a biophysical explanation. By biophysical theory, organs arise where the dome surface is structurally predisposed to bulge. This is a function of the cellulose reinforcement pattern in the surface. Successive patterns of cellulose reinforcement in isolated surface layers from floral organs were determined using polarized light. This was done for the cyclic activity of the inflorescence meristem and the development of the flower. The results indicate that patterns of cellulose reinforcement on the apical dome surface could lead to the production of organs, through local promotion of bulging of the tunica. Subsequent growth of the base of each organ stretches the adjacent dome tissue in a directional fashion. Cytoskeletal responses of these stretched cells lead to new cellulose alignments on the dome which generate the reinforcement pattern for the next round of organs.Abbreviations F floral meristem tissue which will directly produce a flower, starting with sepals - I inflorescence meristem tissue, generally oval in top view and bounded by two bracts, that produces both floral tissue (F) and additional I meristem tissue - I-max the maximal size of I tissue before it bifurcates into F tissue and I tissue (I-min) - I-min the minimal size of I tissue just after it has bifurcated to produce F tissue and I tissue     

Rates of Growth and Cell Division in the Shoot Apex of Silene During the Transition to Flowering

The growth rates of the shoot apex during and after floral inductionwere measured in Silene, a long-day plant. Plants were inducedto flower with 4 or more long days (LD) but not with 3 longdays or with short days (SD). The rate of increase of cell numberin the apical dome, above the youngest leaf pair, was exponentialand in plants given 3 LD remained the same as in plants in SD.In plants induced to flower with 7 LD, until the end of theinductive period the rate of increase of cell number in theapical dome remained the same as in plants in SD. Only whenthe apex began to enlarge as the first stage in the formationof the flower did the growth rate of the apical dome increase.The rates of increase of cell numbers in the apex correspondedto mean cell generation times of 20 to 33 h for plants in SD,for plants given 3 LD, and during the 7 days of induction forplants given 7 LD, and 6 to 8 h for induced plants when flowerformation was beginning. The distribution of cell division in the apex was examined bytreating plants with colchicine and noting in sections the positionsof the resulting metaphases. In vegetative apices and also inapices undergoing transition to flowering the whole of the apicaldome appeared to consist of cells dividing at a similar rate. The rate of leaf initiation during induction was the same asin vegetative, non-induced plants.     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. III. SOME AGE-RELATED ASPECTS OF MORPHOGENESIS

We have extended our previous analyses of growth in shoot apices of Picea abies seedlings. Quantification of apical dome volume changes, more detailed analysis of the subapical caulis profile, and of the vertical distance from the base of the dome to the nth primordium, all as functions of age, revealed the dynamics of various growth variables. As seedlings age from 10 to 136 days, apical dome volume increases about 30-fold, plastochron duration declines from 31 h to about 5 h, height of primordial internodes declines from near 10 μm, to only 3.5 μm, and the caulis assumes a distinct neck-and-shoulder profile. Relative volume growth rates for the apical dome as a whole are about twice the base-of-dome values and decline from 36% to 21% per day as age increases to 136 days. Relative growth rates (radial, vertical, and volume) in the caulis change in a complex manner with both plant age and internode number. We also computed the total volume of tissue generated by an apical dome per day including that part invested in increased dome volume. The investment (ϕ) ratio is greater than 20% in the 10-day dome, but declines rapidly to become negative after 136 days. The ϕ ratio controls apical dome volume and hence augurs future growth yield.     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. IV. PROTOXYLEM INITIATION AND AGE OF INTERNODES

In Picea abies seedlings the distance below the base of the shoot apical dome to the first protoxylem (px) to be differentiated in the internodes beneath is a linear function of apical dome basal diameter. By using mathematical relations derived in earlier papers of this series, we computed the morphogenic age in plastochrons and the chronometric age in days of the internode in which px is first differentiated (n_px). As the seedlings age from 30 to 140 days, the distance from the base of the apical dome to px increases from 186 to 295 μm, the n of n_px increases from 16 to 53, but the chronometric age of n_px remains within the range of 10 to 12 days. Protoxylem differentiation in young internodes is, therefore, more closely related to chronometrie age than it is to morphogenic age or to distance from the base of the shoot apical dome.     

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     

A Modification of Flowering and Phyllotaxis in Silene

Modified proliferous flowers arose spontaneously in a smallproportion of plants of Silene coeli-rosa growing in gardenplots. The modified flowers consisted of leaves, arranged spirallywith a mean divergence angle of 138.4° instead of the pentamerousarrangement of the normal flower, and sometimes also carpelswhich ranged from open structures with exposed ovules to follicle-likestructures, free or fused, to fully fused carpels with free-centralplacentation. In the modified flowers petals and stamens werenot formed. The primordia at initiation were intermediate insize (relative to the apical dome) between normal leaf and normalsepal primordia but were the same absolute size as the latter.The structure of these anomalous flowers is discussed in relationto the normal flowering process. Silene coeli-rosa, flowering, phyllotaxis     

Development of Floral Apex After Floral Induction in Stylo (Stylosanthes guianensis (Aubl.) Sw. var. guianensis)

The apex morphology of stylo (Stylosanthes guianensis var. guianensis)is described in four developmental phases (vegetative, transitional,initiated and floral) further subdivided into a total of tenstages. The apical dome broadens and flattens as induction proceedsuntil the initiation phase when apical diameter within 0.05mm of the dome apex is 55 per cent greater than in the vegetativeapex. Changes in vegetative morphology during induction aredescribed. Stylosanthes guianensis var. guianensis, stylo, flowering, reproductive anatomy, developmental stages     

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     

APICAL SIZE IN THE DETERMINATION OF COROLLA LOBE NUMBER IN LINANTHUS ANDROSAECUS SSP. ANDROSAECUS

A hypothesis in the literature suggests that apical size is one of the critical determinants of the number of organs a flower will form. Genetically selected and control lines of Linanthus androsaecus ssp. androsaecus (Polemoniaceae) were used to test this hypothesis. Plants in the selection up (SU) line of this species produce flowers with greater than the normal number of corolla lobes, and plants in the selection down (SD) line produce less than the normal number of corolla lobes. Apices from lateral branches collected from plants just prior to flowering differed in size among these two lines and controls. Plants in the SU line showed an increase in apical diameter in median section while plants in the SD line showed a decrease in apical diameter in comparison to controls. SU plants may be flowering later, and SD plants earlier, than controls. SU and control apices from the main stem, collected from plants in a late stage of vegetative growth, also differed in apical diameter. Increase in the number of apical cells is associated with increased size in SU apices in comparison to controls. Apical size differences associated with petal number appear to become established during vegetative growth by the time the first two pairs of foliage leaves are formed. The selected and control lines of L. androsaecus offer support for the hypothesis that apical size is an important factor in the determination of petal number.     

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     

Phase change-related variations of dome shape in Eucalyptus urophylla × Eucalyptus grandis shoot apical meristems

Shoot apical meristem (SAM) domes derived from five different outdoor and in vitro sources of juvenile and mature Eucalyptus urophylla × Eucalyptus grandis akin genotypes were compared. Overall measurements of SAM dome height H and diameter D ranged from 2 to 35 μm and 20 to 80 μm, with significant differences according to the various physiological origins of plant material investigated. SAM domes from the mature trees “Mat” were taller than those from the rejuvenated ministock plants “Rej”; from the in vitro microcuttings “IVM” of the same clone and also from the in vitro juvenile seedlings “IVJ”, whereas outdoor seedlings “Juv” exhibited intermediate SAM dome height. SAM domes from the rejuvenated material “Rej”, from the in vitro mature “IVM” and juvenile “IVJ” origins were also narrower than those from the outdoor seedlings “Juv” and to lesser extent than those from the mature trees “Mat”. Overall, the mature source “Mat” displayed bigger and somehow sharper hemispherical domes than those from “Rej” and “Juv”, physiologically more juvenile, or those from the in vitro origins “IVM” and “IVJ” which looked flatter and smaller. SAM dome height, diameter D and H/D values varied also significantly according to the plastochron. More specifically, H, D, and H/D SAM differences between the five origins were not significant during the early plastochron phase corresponding to leaf initiation, to become more salient as leaf structures started to elongate and to differentiate. This was particularly obvious for mature tree “Mat” SAM dome shapes which showed at this stage much higher H/D values than the other SAM sources. A shape index S used for characterizing more accurately dome shape confirmed these trends. These observations provide additional arguments to the view that juvenility in trees becomes more and more time- and shoot-tip restricted as ageing increases in the course of time during the ontogenetical process and could be ultimately confined to the most organogenic phase of SAM, from which shoot characteristics derive.     

THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. I. ANATOMY,APICAL DOME DIAMETER,AND PLASTOCHRON DURATION

Developing embryos in immature Picea abies seeds already have well-delineated shoot apical meristems with clearly evident cytohistological zonation. During early seedling development the zonation characteristic of gymnospermous apical meristems is attained. Seedling development is also accompanied by an approximately threefold increase in apical dome diameter. The latter approaches a steady state about 140 days after germination. Seedlings display a spiral phyllotaxis consisting of a contact parastichy system, usually of the primary Fibonacci series. As the seedlings age and apical domes enlarge, higher Fibonacci number-pairs characterize their phyllotaxis. Mathematical analysis of the relation between cumulative leaf number and age revealed that the length of the plastochronic time interval declines from about 18.5 hr to 5.7 hr as seedling age increases from 20 to 140 days.     

Cell Division and Expansion and Resultant Tissue 3

By following the movement of carbon particle markers on theexposed surface of a cultured tomato apex it has been shownthat a leaf primordium is formed by growth on the flank of theapex raising the tissue upwards and outwards to form the leafbuttress. The whole of the apical surface is in an active stateof cell division and expansion except in the axillary regionabove the primordium. The data provide direct estimates of therates of division in the outer layer of cells. The distribution of blocked metaphase figures following treatmentwith colchicine, shows that in the early stages of primordiumformation cell divisions are concentrated in what appears tobo a ‘growth centre’ in the corpus to one side ofthe apical dome. As the bulge of the primordium develops, thegrowth centre spreads out and splits into two parts continuingthe growth of the dome (proximal side) and the primordium (distalside). Between these two regions of active division there arisesa small pocket of cells in the axil, whose rate of divisionrapidly declines. Cuts made in the apical surface in the early stages of primordiumformation immediately gape widely, apparently as a result ofpressure exerted on the outer layers from within by divisionsin the corpus. Once the upper surface of the primordium becomesraised above the dome, the axillary cells seem to become compressedbetween the two zones of active division. In the axil at thisstage (a) cuts do not gape but close up after exuding cell sapand (b) the carbon particle markers move slightly together.