Cell Division and Expansion in the Growth of the Shoot Apex

The number and the total volume of cells produced in small portionsof the apical region of the shoot have been measured at differentstages of development. In non-vernalized seedlings of winter rye, grown in July, thecells in the portions examined divided about once every 1.8days during the first week of growth, and there was about a2.2-fold increase in the volume of each cell generation. Fourweeks later the apices were still vegetative, but the rate ofdivision in the portions then examined had fallen to once every5.8 days and there was slightly less than a 2.2-fold increasein the volume of each generation. The average cell volume decreasedas the apices developed. In vernalized seedlings more cells were produced in unit timeduring the first 4 days of growth than in non-vernalized seedlings,but, as in the latter, the rate of production fell during vegetativedevelopment. The rate of cell expansion in the vernalized seedlingswas probably slightly lower than in the non-vernalized seedlingsduring the first 4 days of growth and tended to increase duringvegetative development. At the time of transition to reproductivegrowth the cells were dividing about once every 2.0 days andthere was about a 2.2-fold increase in the volume of each succeedinggeneration. After transition the rates increased. From 4 to8 days after transition the cells divided once every 1.4 daysand there was then a 2.4-fold increase in the volume of eachsucceeding generation. Subsequently, the rate of division changedlittle but the rate of expansion decreased. Higher rates of division and expansion were found in the apicesof lupin seedlings grown in July than in November. In the portionsof the meristem examined during the first 5 days of growth thecells divided about once every 1.3 days in July, but only onceevery 1.8 days in November. The increase in volume of each generationin the same time was slightly higher in July than in November.Both the rates of division and expansion decreased until transitionto reproductive growth occurred, and then increased. About 5days after transition the cells divided once every 1.4 daysin July and once every 2.0 days in November. In both experimentsthere was a 2.4-fold increase in the volume of each generation.Both the rates of division and expansion decreased subsequently. The data are discussed in relation to the changes in size whichaccompany the development of the shoot apex.     

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     

Cell Cycle Patterns in the Shoot Apex of Lolium temulentum L. cv. Ceres During the Transition to Flowering Following a Single Long Day

Six-week-old Lolium temulentum cv. Ceres plants were inducedto flower by a single long day (day 1). The ‘double ridge’stage was reached on days 4/5. A detailed analysis of apicesevery 4 h on days 3 to 5 demonstrated synchronized cell divisionin the apex. However, this synchronized cell division occurredonly in the apical summit and axillary bud sites, i.e. onlyin those regions of the apex which give rise to the spikelets.This indicates a specific activation of the cells in these regions,rather than a general activation of the whole apex. Key words: Cell cycle, flowering, Lolium, shoot apex, spikelet     

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.     

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.     

Effects of Developing Leaves on Stelar Pattern Development in the Shoot Apex of Matteuccia struthiopteris

A developmental study of the normal shoot apex of Matteucciastruthiopteris suggested that patterned stelar differentiationis initiated immediately beneath the single layer of promeristemand occurs prior to the initiation of the youngest leaf primordium.A developmental study in which all leaf primordia were suppressed,with or without lateral isolation of the terminal meristem byvertical incisions, has confirmed this interpretation of stelardifferentiation. Experimentally-induced changes in the tissueimmediately below the promeristem were reflected in the resultingmature structure of the stele. Failure of leaf gap initialsto differentiate, if all leaf primordia were suppressed at theincipient stage, resulted in a mature stele without leaf gaps.Similarly the disappearance of pith mother cells after severalweeks of leaf removal was associated with the formation of astele without pith. Leaf influence was further assessed by allowingone primordium to develop while all others were suppressed.The developing leaf had a small promoting effect on caulinevascular tissue differentiation but its major impact on theexpansion of the parenchymatous tissues of the stele. Characteristicprotoxylem and protophloem failed to differentiate when allleaves were suppressed and, when leaf was allowed to develop,formed only in relation to the leaf.Copyright 1995, 1999 AcademicPress Leaf influence, vascular pattern formation, experimental surgery, shoot apex development, protoxylem, protophloem, Matteuccia struthiopteris     

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.     

Cell Cycle Changes in the Shoot Apex of Silene coeli-rosa During the Second and Third Days of Floral Induction

The cell cycle in Silene coeli-rosa shoot apices was measuredto test whether or not early components of the floral stimulus,produced during the 2nd and 3rd long days (LD) of an inductiveLD treatment, resulted in an increase in the duration of G₂phase in constant 20–24 h cell cycles. Plants were grownat 20°C in short days (SD) of 8 h light and 16 h darknessfor 28 d (day 0). Starting on day 0, plants were given SD or3 LD each comprising an identical 8 h day and 16 h photo-extension,or 3 dark-interrupted (d.i.) non-inductive LD, interrupted at1700 h of each day with 1 h of darkness. The cell cycle (percentagelabelled mitoses method) and changes in cell number were determinedin the shoot apical meristem. During days 1–2 of the SDtreatment, the cell cycle and mean cell generation time (MCGT)was 18 and 32 h, respectively, giving a growth fraction of 56%.During days 2–3, the cell cycle and MCGT shortened to15 and 23 h, respectively (growth fraction = 65%). During days1–2 of the LD and d.i. LD treatments, cell cycles andMCGTs were 9–10 and 27–29 h, respectively, resultingin smaller growth fractions (about 33%). Thus, shortened cellcycles and altered growth fractions occurred regardless of whetheror not the treatment was inductive. The LD treatment resultedin a marked shortening of G₁ and, to a lesser extent, S-phase,whilst G₂ remained constant. These changes were consistent withincreases in the proportion of cells in G₂ during the photoextensionof each LD which were suppressed during the comparable periodsof the d.i. LD treatment. The latter treatment resulted in eachphase occupying virtually identical proportions of the cellcycle as in the SD treatment. Thus, the unique cell cycle responsesto the initial part of the inductive LD treatment were increasesin the proportion of cells in G₂ coupled with G₁ and G₂ beingof similar duration. Cell cycle, mean cell generation time, shoot apex, Silene coeli-rosa     

Growth and Differentiation of Plant Tissue Cultures: II. Synchronous Cell Divisions in Developing Callus Cultures

Explants isolated from Jerusalem Artichoke tubers are stimulatedto divide when placed in contact with a nutrient medium containingsucrose, mineral salts, coconut milk, and 2: 4-dichlorophenoxyaceticacid. The first two or three cell divisions, which only occurin the outer layers of the explant, do not occur uniformly withtime but are, at least, partially synchronous. This synchrony,which decreases with successive divisions, is inherent. DNAsynthesis, which is an essential prerequisite for division inthese cells, is also partially synchronous. These conclusions,while being of some significance in relation to the interpretationof the early development of the callus, are also of some interestin relation to the possible exploitation of this system forthe study of cell division.     

Shoot Growth and Mortality Patterns of Urtica dioica, a Clonal Forb

The growth and mortality patterns of the clonal forb Urticadioica were investigated at the level of the individual shootin two growing seasons, 1991 and 1992, in a natural stand. Shootheight and diameter at ground level of each shoot tagged inspring were measured repeatedly five times during the growingseason. Dry weights of these repeatedly measured shoots wereestimated using an allometric relationship between dry weight,height and diameter of harvested shoots. A large decrease inshoot density occurred with stand development from the beginningof the growing season in both the years: (1) shoot survivalrate was about 30% at the end of the growing season; (2) shootmortality rate per 10 x 10 cm subplot between censuses was positivelydependent on shoot density per subplot; (3) the mortality rateof individual shoots was negatively dependent on shoot size(height, diameter and weight) at each growing stage, suggestingone-sided competition between living and dying shoots; (4) shootsize (height, diameter and weight) variability in terms of thecoefficient of variation and skewness decreased in accordancewith shoot mortality. Symmetric competition between living shootswas detected by regression analysis based on a model for individualshoot growth considering the degree of competitive asymmetry.However, the competitive effect on individual shoot growth wasvery small (nearly absent). The mortality pattern of Urticadioica indicates that shoot self-thinning occurred from theearly growing stage as in non-clonal crowded monospecific stands,and contrasts with many clonal plants where shoot self-thinningrarely occurs or, if any, is confined only to a short periodof the later growing stage. The pattern of growth and competitionbetween living shoots of Urtica dioica contrasts with non-clonalcrowded plants undergoing intense competition (usually asymmetric)between individuals, but is a common feature of many clonalplants where shoot competition is supposed to be reduced by'physiological integration' between shoots. These form a newpattern not reported yet for clonal plants. It is pointed outthat clonal plants show a wider spectrum of the growth, competitionand mortality patterns of shoots than non-clonals. Some possiblemechanisms for the pattern of Urtica dioica are discussed.Copyright1995, 1999 Academic Press Shoot competition, diffusion model, individual shoot growth, shoot self-thinning, shoot size variability, Urtica dioica L     

Concentration and Delivery of Abscisic Acid in Xylem Sap are Greater at the Shoot Base than at a Target Leaf Nearer to the Shoot Apex

Abstract: Samples of xylem sap from 5-week-old Ricinus corn-munis L. were obtained after severing a lamina, or shoot, from plants pressurized at the roots with air to raise hydrostatic xylem water potentials to atmospheric. In situ sap flow gauges, and mass flow measurements, showed that removing the lamina approximately doubted sap flow rate through the petiole stub that remained attached to the plant. This was a consequence of flow out of the roots being diverted along this low-resistance pathway and away from leaves higher in the canopy. Leaf and whole shoot excision temporarily released extra solutes in to sap as it discharged from the cut petiole or from the hypo-cotyl stump. This contamination prevented the use of sap extracted from detached lamina by overpressurizing in a Scholan-der bomb. To minimise distortions to sap flow and wound-induced contamination, estimates of in planta concentration and delivery (concentration × sap flow rate) of ABA and osmolality in xylem sap were made using sap flow rates measured before excision and concentrations in flowing sap collected approximately 30 mm after excision. At this time, effects of excision on solute contamination had subsided. The approach revealed that withholding water from upper roots increased ABA delivery from roots into the shoot base 3-fold. However, approximately half this ABA was lost en route to the youngest fully open leaf. This loss of ABA may explain the slow stomatal response to drying of upper roots shown by R. communis .     

Effects of Culture Conditions on Cell Division and Composition of Regenerated Cell Walls in Vinca rosea Protoplasts

Protoplasts prepared from suspension-cultured Vinca rosea cellswere grown in a liquid medium, and the effects of osmolarityand growth regulators on cell division and on the compositionof regenerated cell walls were investigated. The concentrationof mannitol optimal for cell division was 0.3–0.4 M. Thepresence of 2,4-D was essential for cell division, and 6-benzylaminopurine(BAP) enhanced cell division at concentrations of 0.03–0.1ppm. However, the composition of regenerated cell walls wasabnormal under suspension culture; the predominant sugar wasglucose, indicating that the regenerated cell walls consistedmostly of glucans, and that the other cell-wall components werereleased into the medium. Mannitol, 2,4-D, and BAP at variousconcentrations did not significantly affect the sugar compositionof the regenerated cell walls. Compared with liquid culture,cell division was stimulated when protoplasts were culturedon agar, and their regenerated cell walls had a compositionsimilar to that of the original culture. The importance of thephysical environment for the deposition of polysaccharide componentsin cell walls and the interrelationship between cell divisionand the regeneration of cell walls by protoplasts are discussed. ¹ Present address: Division of Environmental Biology, NationalInstitute for Environmental Studies, Yatabe, Tsukuba, Ibaraki305, Japan. (Received June 10, 1981; Accepted December 12, 1981)     

Comparison of Combined and Sequential Surgery for Proliferative Diabetic Retinopathy: A Single Surgeon Study

Purpose

To compare the results of combined and consecutive surgeries to treat proliferative diabetic retinopathy and cataract.

Methods

Retrospective comparative study. Forty-one patients with proliferative diabetic retinopathy (PDR) were enrolled. Twenty-nine eyes for the combined surgery group and twelve eyes for the sequential group were included. All surgeries were performed by one surgeon. Phacoemulsification was performed using a clear cornea incision. The vitrectomy was performed using a 20-gauge vitreous cutter.

Results

The best corrected visual acuity (BCVA) and intra- and post-operative complications were the main outcome measures. In the combined surgery group, the BCVA increased in 18 (62.1%) eyes, while eight (27.6%) eyes remained stable and three (10.3%) eyes decreased. Postoperative complications included fibrinous exudation in nine eyes, macular edema in three eyes and vitreous hemorrhage in three eyes. In the sequential surgery group, the BCVA increased in seven (58.3%) eyes, remained the same in four (33.3%) eyes and was reduced in one (8.3%) eye. Postoperative complications included macular edema in two eyes, neovascular glaucoma in two eyes and vitreous hemorrhage in one eye.

Conclusions

Both combined and sequential surgeries are safe and effective for treating PDR and cataracts. The combined surgery had a higher incidence of fibrinous exudation.     

Studies on the Fragmented Shoot Apex of Grapevine: IV. SEPARATION OF PHENOTYPES IN A PERICLINAL CHIMERA IN VITRO

Adventitious shoots were induced on apical fragments of thegrapevine, cv. Meunier, a presumptive periclinal chimera. Thiscultivar resembles Pinot noir in essential characteristics,but is phenotypically distinguishable by the dense white matof hairs on the apex and expanding leaves (tomentose phenotype).Plants derived from apical culture were of three types: (i)phenotypically resembling Meunier, (ii) bearing tomentose leaveswith comparatively hairless sectors, and (iii) with completelynon-tomentose shoots phenotypically similar to Pinot noir. These results establish that adventitious buds of grapevineproduced by this method are multicellular in origin and arenot derived from single cells. Furthermore, it is concludedthat fragmented shoot apex culture of grapevine periclinal chimerasdisturbs the integrity of apical tissue allowing separationof component genotypes. Fragmented apex culture of such chimerasis not recommended where trueness-to-type is of prime importance.However, circumstances under which this technique may be usefullyapplied to separate chimeral genotypes are discussed. Key words: Vitis vinifera, Grapevine, In vitro, Chimera separation     

植物细胞壁研究进展

植物细胞壁是一种复杂的网状结构,其成分包含纤维素、半纤维素、果胶和少量的结构蛋白等。在植物细胞生长过程中,细胞能产生伸展素蛋白,打断纤维素和半纤维素之间的氢键,引起细胞膨压驱动的细胞壁扩张。成熟细胞壁扩张性的丧失是由于细胞壁硬化作用而对扩张性蛋白的作用不敏感造成的,细胞壁成熟过程中很多不同的连接会同时发生,当细胞壁基质多聚体分子之间的连接增加到一定的程度。细胞壁的伸长就会被完全抑制。     

Effects of the Photoperiod Genes Ppd1 and Ppd2 on Growth and Development of the Shoot Apex in Wheat

Two major genes influencing the photoperiod response in wheat,Ppd1 and Ppd2, have been identified on the group 2 chromosomes.Substitution lines, which had been characterized on the basisof time of ear emergence as carrying either the insensitiveor sensitive alleles of the two Ppd genes, were used to investigatethe effect of these genes on development. They were grown undershort photoperiods, and growth and development of the shootapex was measured. The primary influence of the Ppd genes was on ear growth. Inthe plants carrying the insensitive alleles, Ppd1 and Ppd2,the relative growth rate of the floral apex was faster thanthat of plants with the sensitive alleles, ppd1 and ppd2. Therewere no differences in the rate of spikelet initiation, butthe spikelets of the ppd lines grew and developed more slowly. The Ppd2 material segregated for another gene located on chromosome2B affecting duration of the life cycle. This gene also affectedthe relative growth rate of the ear. It was considered that the major effect of the Ppd1, Ppd2 andthe second genetic factor on chromosome 2B is on floral growthrate. Differences in apex morphology, stem growth and ear emergenceare thought to be due to the differences in floral apex growthand size. Wheat, photoperiod genes, shoot apex development, shoot apex growth     

Studies on the Fragmented Shoot Apex of Grapevine: III. A SCANNING ELECTRON MICROSCOPE STUDY OF ADVENTITIOUS BUD FORMATION IN VITRO

Scanning electron microscopy (SEM) was used to provide directevidence that shoots produced in vitro from fragmented shootapices of grapevine were adventitious in origin. The effectof temperature on the formation of the adventitious buds wasalso examined using SEM. At 27°C, shoot buds were initiatedby 31 d following fragmentation of the apex, while at 35°Cshoot initiation and multiplication were already well-advancedat only 18 d after the start of culture. At 38°C, apicalfragments quickly browned and died. After 25 d at 35°C,structures resembling inflorescence primordia were also visible.These did not occur in cultures at 27°C. The primordia laterdeveloped into multiple-branched tendrils, structures whichappear to be intermediate between tendrils and inflorescencesand have not been previously described.