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     

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 Effects of Photoperiod and Mineral Nutrient Supply on Growth and Primordia Production at the Stem Apex of Barley Seedlings

Growth and development of the shoot apex in seedlings of threebarley cultivars was examined in two daylengths (8, 16 h) andat two mineral nutrient levels (x 1, x 0.1). Production of primordiawas greater at the higher nutrient level and in the longer days.The rate of production varied with cultivar but in all casesthe plastochron shortened with transition to spike formation.Early flowering (cv. Clipper) was associated with a high rateof primordial production and early transition to spike formation,late flowering (cv. Proctor) with a low rate of production anda longer vegetative phase. The cultivar Akka showed intermediatecharacteristics. The volume of the apical dome increased linearlywith increasing numbers of primordia, the rate of increase varyingwith cultivar and treatment. Enlargement of the dome was duemainly to increase in cell number. The transition of the apexto produce spikelet primordia occurred with widely differingvolumes of the apical dome, thus invalidating the hypothesisthat transition is dependent upon attainment of a critical domesize. Although both the rate of production of primordia andenlargement of the dome were markedly affected by photoperiod,both were unaffected when the photoperiodic treatment was givendirectly to the shoot apex. It is considered that the fate of a primordium once initiatedis determined by competition for available metabolites betweenit, other primordia and the apical dome. Hordeum vulgare L, barley, apical dome, primordia, plastochron, cell division     

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.     

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.     

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     

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     

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     

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 quantitative ultrastructure of the pea shoot apex in relation to leaf initiation

Summary The ultrastructure of the pea shoot apical meristem was examined quantitatively in longitudinal sections. Photographs were taken at eleven defined positions in the apex, at six developmental stages within a single plastochron. The only change in ultrastructure during the period of a single plastochron was the increase in the proportion of plastids with starch in the central regions of the apex and in the young leaf axils. This increase occurred midway in time between the emergence of successive leaves, at precisely the time that the orientation of growth changes in the region where a new leaf is to emerge. There were quantitative changes in ultrastructure associated with cell differentiation. In the sequence of cell development from the summit of the apex (central zone) to the incipient pith, cell enlargement was accompanied by an increase in the volume of endoplasmic reticulum, dictyosomes, microbodies and vacuoles per cell, an increase in the number of mitochondria, microbodies and vacuoles per cell, and an increase in the volume, but not the number, of plastids per cell. In the sequence of axillary development (before the axillary bud begins to grow) the number of mitochondria per cell decreased as cell volume decreased but the number of plastids per cell remained constant. The number of plastids per cell increased only in the developmental sequence leading to leaf development, in which the number of mitochondria and dictyosomes per cell also increased. There appeared to be no features of ultrastructure, qualitative or quantitative, which could be correlated with the different rates of cell division in different regions of the meristem. The differences in ultrastructure throughout the apex were mainly quantitative and seemed to be associated with cellular differentiation rather than with the plastochronic functioning of the apex during leaf initiation.     

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.     

The rate of cell division in the shoot apical meristem during photoperiodic induction and transition to flowering

Cell division contributing to longitudinal growth of the shoot apex was investigated inChenopodium rubrum in segments marked by the axils of leaf primordia. Plants treated with two short days (16h of darkness and 8h of light) were compared with two non-induced controls (cultivated in continuous light or treated by alternations of 8 h of darkness and 4 h of light for two days). During the short-day treatments the rate of cell division contributing to the longitudinal growth decreases in all segments of the shoot apex irrespective of whether the darkness was given in inductive or non-inductive photoperiods. The rate of cell division contributing to longitudinal growth increases in the upper internodes of the shoot apex after the termination of the photoperiodic treatment and transfer of the plants to continuous light. However, cell division remains inhibited in the lowest segment of the shoot apex. This inhibition in the differentiating parts of the shoot apical meristem is a direct consequence of photoperiodic induction. It is supposed that this inhibition is related to evocation similarly as the well-known phenomenon of stimulation of cell division in the apical dome.     

A QUANTITATIVE STUDY OF CELLULAR EVENTS IN THE SHOOT APICAL MERISTEM OF BRASSICA CAMPESTRIS (CRUCIFERAE) DURING TRANSITION FROM VEGETATIVE TO REPRODUCTIVE CONDITION

Vegetative plants were induced to flower by 16-hr-long days. Apical buds were collected at intervals during several developmental phases up to 63 hr. A stereologic analysis and mitotic index study was conducted on median longitudinal sections of shoot apical meristems. A rise in the mitotic index occurred between 12 and 24 hr within central, peripheral and pithrib meristem zones. Preceding the floral stage a second increase in the mitotic index was observed in peripheral and central zones, but not in the pith-rib meristem zone. A significant rise in apical volume, cell number, height, and width began in the transitional stage and continued to the floral stage. Significant correlation coefficients were observed between these apical parameters. Relative volume and cell population of each zone remained constant from the vegetative to the reproductive stage. Volume fraction occupied by the nucleus and nucleolus remained constant within each zone during the same time period. In each zone the volume of the nucleus was significantly correlated to volume of the nucleolus. It appears a pre-inflorescence apex, while larger, is structurally similar to a vegetative apex.     

Planes of Cell Division and Growth in the Shoot Apex of Pisum

The planes of cell division and growth were examined in thecourse of a single plastochron in the shoot apical meristemby observing the orientations of mitotic spindles. In the I₁region of the apical dome, cell divisions were at first anticlinalbut 30 h before a leaf primordium emerged at this site 20 percent of the cell divisions became periclinal. These periclinaldivisions were found only in the corpus. Periclinal divisionsin the tunica were coincident with the appearance of the primordiumas a bulge. The change in the direction of growth in I₁ at thesite of the incipient leaf primordium occurred without any changein the rate of growth in this region of the meristem.     

Structure and Cytogenesis of the Shoot Apex of Syringa oblata var. affinis Lingelsh

The structure, growth and mitotic activity of 211 shoot apices of developing sprouts of Syringa oblata var. affinis Lingelsh. in longitudinal sections and 67 in transverse sections have been studied with the view to understanding the nature of zonation patterns and cytogenesis of the apical meristems during a double plastochron. The external morphology and the anatomical structure of the apices in 4 plastochronic stage-early, middle, late Ⅰ and late Ⅱ stages are described. In the shoot apices examined, especially those at late plastochronic stage, the following zones may be delimited: Zone of tunica initials, zone of corpus initials, peripheral zone and zone of rib meristem. The location and orientation of mitotic figures observed in longisections of the apices in 4 plastochronic stages are plotted in diagrams and the mitotic frequency has been calculated. Information obtained from these investigations reveals that the tunica and corpus inititals constitute an active region of the apex, but their mitotic activity changes periodically within the double plastochron. In the middle plastochronic stage when the apex is at its minimal area and the cells of peripheral zone and rib meristem zone have been completely transformed into constituent parts of foliar primordia and the subjacent tissues of the stem and the pith mother cells respectively, the mitotic frequency of the initials is at its maximium and its intensity of mitotic activity is not much lower than that of any other meristematic zone at any stage. When the apical dome is reformed by the activity of these initials in late plastochronic stages, the mitotic frequency of the initials gradually drops and the region of high mitotic frequency shifts to the flank of the apex, the peripheral zone. Anticlinal divisions are predominant in this zone. On the other hand, those cells directly left behind by the corpus initials, which constitute the rib meristem, are vacuolated and marked by the pre- dominance of transverse divisions. Thus the entire zonation pattern reappears. In the next early plastochronic stage, the mitotic frequency of the tunica and corpus initials drops to its mimimium, but other regions of the apex still maintain a high mitotic frequency. It may be concluded that the tunica and corpus initials form a cytogenerative center of the shoot, and the cytohistological zonation is actually a result of the fact that different regions of apical meristems are different in mitotic activety, different in state of cell differentiation and different in their function in morphogenesis.     

Changes in the Pattern of Cell Division in the Shoot and Root Meristems of Lolium perenne during the Transition from Vegetative to Floral Growth

For Lolium perenne cv. Cropper, a system which resulted in 100%flowering comprised 90 short days (SD) at 4 ?C (vernalization)and 30 SD at 18 ?C followed by 8 long days (LD). The mitoticindex and G1 and G2 percentages were measured in the shoot androot apices of plants following 2, 5 or 8 LD and in SD controlssampled at the beginning and end of induction. Identical measurementswere made in plants given 48 SD at 18 ?C followed by 2, 5 or8 LD; plants remained vegetative in response to this treatmentlacking vernalization. Significant increases in both mitoticindex and meristem size occurred in the shoot apex in LD followingthe vernalizing, but not the non-vernalizing, treatment. A clusterof mitoses in the apical dome of the shoot apex was unique tothe vernalized plants given 5 or 8 LD. However, an increasein root meristem size occurred regardless of vernalization,but a significant increase in the mitotic index was limitedto vernalized plants given 5 or 8 LD. Whilst the vernalization-LDtreatment resulted in an increase in the G2 percentage in theshoot apex following 2, 5 or 8 LD, no such alteration was observedin the root meristem. Thus, the changes to the cell cycle whichcorrelated with flowering were increased mitotic indices andG2 percentages in the shoot apex at each sampling time and increasedmitotic indices in the root apex following 5 and 8 LD. Key words: Cell division, flowering, Lolium perenne L.     

CELL DIVISION STUDIES OF THE SHOOT APEX OF DATURA STRAMONIUM DURING TRANSITION TO FLOWERING

Seedlings of Datura stramonium L., although not photoperiodically sensitive, are useful for floral transition studies when raised in a growth chamber at a constant temperature of 25 C with a photoperiod of 8 hr of light (1,600-2,000 ft-c) and 16 hr of darkness. A terminal flower is formed after the seventh or eighth leaf primordium is produced. A constant rate of leaf initiation up to the time of flowering enables specific apical stages to be obtained and studied. Changes in the mitotic index, substantiated with calculated rates of cell division (measured by the accumulation of metaphases following treatment with colchicine) were studied in shoot apical zones during transition to flowering. Fluctuations in the mitotic index of each zone in the vegetative and transition apex with respect to apical stage as well as time of day were not statistically significant. The mitotic index of the summit zone of the vegetative apex was significantly lower than in the other zones whose mitotic indices were not significantly different from one another. During floral transition the mitotic index of the summit zone as well as the central zone (just below the summit zone) significantly increased while no significant changes were detected in the flank zones. It was shown that the mitotic index could be considered representative of the rates of cell division in Datura.     

In vitro Growth of Vegetative Tomato Shoot Apices

Explants from the shoot apex of the tomato, comprising the apicaldome and youngest primordium together with small amounts ofsub-apical tissue were cultured for periods of 1 to 4 plastochrons.By the use of a simple parameter, the axillary distance, thegrowth-rate could be accurately monitored throughout each plastochron. Gibberellic acid, coconut milk, and kinetin, in addition tosucrose and inorganic salts, all promoted growth of the apex;a combination of gibberellic acid and coconut milk gave thefastest growth. Temperature had a large effect on the growth-ratewith an in vitro Q₁₀ of 2.1 contrasted with an in vivo Q₁₀ of1.2 over the range of 15 to 25 ?C. On gibberellic acid and coconutmilk at 15 ?C two-thirds of the in vivo growth--rate was sustainedin culture for two plastochrons after which the growth-rategradually declined; at 20 and 25 ?C growth-rates slightly higherthan in vivo rates were sustained for 1 plastochron before amore rapid decline. The anatomy of these in vitro apices wasnormal for 1? plastochrons after which there were small increasesin cell volume in the developing primordium. Reducing the amount of sub-apical tissue drastically reducedthe growth rate but had little effect on the responses to gibberellicacid and coconut milk. Explants are considered to be useful material for studying thechanges that take place in the apex during the course of 1 or2 plastochrons, but inadequate on the media tested for experimentsinvolving longer periods of growth. Explants also provide asensitive assay system for the effects of growth factors onthe rate of shoot apical growth.     

ON THE MECHANISM OF DECUSSATE PHYLLOTAXIS: BIOPHYSICAL STUDIES ON THE TUNICA LAYER OF VINCA MAJOR

Phyllotaxis theory typically assumes that an acropetal influence from recently formed leaves acts on the apical dome to initiate new leaves. Biophysical theory postulates that established plant organs elongate because their primary walls, particularly those in the organ surface layer, are transversely reinforced by cellulose to give the organ overall hoop reinforcement. These two postulates are combined here in a biophysical theory for phyllotaxis. The essential acropetal influence from young leaves is proposed to be the stretching of the adjacent dome tissue by the growth of leaf bases. Cytoskeletal responses on the dome produce reinforcement patterns which initiate new hoop reinforced leaves. Growth of these leaves remodels the dome for the next round of organs. Data pertinent to this theory are presented here for Vinca major. The surface (tunica) layer of the apical dome was isolated by paradermal cuts. Using polarized light, the cellulose alignment in this surface layer was determined, cell by cell, for various stages of the plastochron. The growing dome is typically elliptical, with the major axis shifting by 90° during each plastochron. The periphery of the dome always has cellulose oriented parallel to its margin; the central region, when the major axis is pronounced, has reinforcement normal to this axis. During the plastochron this reinforcement pattern is modified, by plausible biophysical mechanisms, to account for the three major activities of the dome: 1) production of a hoop-reinforced leaf at each end of the ellipse, 2) formation of a hoop-reinforced stem segment, 3) revision of dome structure to produce the same initial reinforcement pattern as at the start of the plastochron, but at 90°.     

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.