THE EFFECT OF LIGHT ON THE CELLULAR COMPONENTS OF POLARIZED GROWTH IN BEAN INTERNODES

First internodes of light-grown bean seedlings exposed to supplementary red and far-red light and those of dark-grown seedlings were sectioned and studied to determine the effects of irradiation on the cellular components of polarized growth. Cell counts and measurements of epidermis, cortex, and pith are given. Increased length of internodes of far-red-treated plants was caused by both increased rate and increased duration of cell elongation. The effect of far-red light is interpreted as a reversal of the accelerating effect of light upon cell maturation. It is suggested that investigations of the mechanism of the red, far-red response of stems be concerned with the processes involved in cell elongation. In darkness, rate and duration of cell division as well as rate and duration of cell elongation were greater than in any of the irradiated plants, indicating that only part of the photocontrol of stem elongation is mediated through the red, far-red system.     

Thigmomorphogenesis: Changes in Cell Division and Elongation in the Internodes of Mechanically-perturbed or Ethrel-treated Bean Plants

Examination of first internodes of young Phaseolus vulgarisL. plants which have been subjected to mechanical perturbationshows decreased elongation and increased radial growth. Thedecreased elongation can be attributed to both reduced cellelongation of epidermal and cortical cells and a reduced numberof cells in the vascular and pith tissues. The increased radialenlargement is due to increased cortical cell expansion andincreased secondary xylem production resulting from increasedcambial activity. All of these responses are observable withina few hours of a single mechanical perturbation. Treatment ofplants with ethrel mimics all of these effects of mechanicalperturbation. Phaseolus vulgarisL, Kidney bean, thigmomorphogenesis, mechanical perturbation, ethrel, (2-chloroethyl phosphonic acid), cell division, internode elongation     

SHOOT GROWTH AND HISTOGENESIS OF TREES POSSESSING DIVERSE PATTERNS OF SHOOT DEVELOPMENT

Shoot growth and histogenesis were followed in five unrelated tree taxa possessing inherently diverse patterns of shoot development. Following the resumption of growth in spring, each species differs quantitatively in the number of internodes elongating contemporaneously, in rates and duration of internodal elongation and seasonal periodicity of shoot growth. The basic pattern of internode elongation and histogenesis is qualitatively similar in each of the dicotyledonous species observed irrespective of growth habit or final form of the shoot produced. During the intial phase of internode development, growth is essentially uniform throughout young internodes, corresponding to an active period of cell division during which time pith cells increase in size to about one-third their final length. Subsequently, the pattern of cell division shifts progressively upward concomitant with increased elongation and maturation of pith cells in the basal portion of developing internodes. Thereafter, a wave of cell division accompanied by cell elongation continues to proceed acropetally until growth finally ceases in the distal portion of each internode. As long as internode elongation continues, frequently at distances 15–20 cm below the shoot apex, cell divisions still occur in the distal growing portion. As successive portions of each internode mature acropetally, final length of pith cells becomes relatively uniform throughout the internode. During the process of internode growth and development, cell lengths increase only two- to threefold, whereas cell numbers increase ten- to 30-fold, indicating the dominant role of cell division and increases in cell number to final internode length. Morphological patterns of shoot expression associated with differences in internode lengths along the axis of either preformed or neoformed shoots, as well as sylleptic branches, are due to differences in cell number rather than final cell length. Significant variations in final internode lengths along the axis of episodic shoots, caused by either endogenous or exogenous factors, are also attributed to differences in cell number.     

Factors responsible for deep-sowing tolerance in wheat seedlings: varietal differences in cell proliferation and the co-ordinated synchronization of epidermal cell expansion and cortical cell division for the gibberellin-mediated elongation of first internodes

MethodsThe lengths and numbers of epidermal and cortical cells of the first internodes in three wheat cultivars were measured. These parameters were compared in wheat seedlings treated with gibberellin A₃ (GA₃) or an inhibitor of GA biosynthesis, uniconazole.ConclusionsThe deep-sowing-tolerant cultivar ‘Hong Mang Mai’ is able to elongate the first internode to a greater degree due to enhanced cell division and a heightened response to GA. In addition, cell expansion in the epidermis and cell division in the cortex are synchronized for the elongation of the first internodes. In response to GA, this well-co-ordinated synchronization yields the rapid elongation of the first internodes in wheat seedlings.     

The Role of the Epidermis in the Control of Elongation Growth in Stems and Coleoptiles

The peripheral cell wall(s) of stems and coleoptiles are 6 to 20 times thicker than the walls of the inner tissues. In coleoptiles, the outer wall of the outer epidermis shows a multilayered, helicoidal cellulose architecture, whereas the walls of the parenchyma and the outer wall of the inner epidermis are unilayered. In hypocotyls and epicotyls both the epidermal and some subepidermal walls are multilayered, helicoidal structures. The walls of the internal tissues (inner cortex, pith) are unilayered, with cellulose microfibrils oriented primarily transversely. Peeled inner tissues rapidly extend in water, whereas the outer cell layer(s) contract on isolation. This indicates that the peripheral walls limit elongation of the intact organ. Experiments with the pressure microprobe indicate that the entire organ can be viewed as a giant, turgid cell: the extensible inner tissues exert a pressure (turgor) on the peripheral wall(s), which bear the longitudinal wall stress of the epidermal and internal cells. Numerous studies have shown that auxin induces elongation of isolated, intact sections by loosening of the growth-limiting peripheral cell wall(s). Likewise, the effect of light on reduction of stem elongation and cell wall extensibility in etiolated seedlings is restricted to the peripheral cell layers of the organ. The extensible inner tissues provide the driving force (turgor pressure), whereas the rigid peripheral wall(s) limit, and hence control, the rate of organ elongation.     

Buckling of inner cell wall layers after manipulations to reduce tensile stress: observations and interpretations for stress transmission

The inner layer of the cell wall in tissues that are under tensile stress in situ, e.g. epidermis and collenchyma of etiolated sunflower hypocotyls, shows a pattern of transverse folds when the tissues are detached and plasmolysed. This can be observed by Nomarski imaging of inner surfaces of the outer cell walls and electron microscopy of longitudinal sections after peeling the epidermis and bathing it in plasmolysing solutions. The folds are apparently caused by buckling of the inner layer due to the longitudinal compressive force exerted on this layer by the outer wall layer, when it shrinks after the removal of the longitudinal tensile stresses. In these stresses, two components can be distinguished: the tissue stress, disappearing on peeling, and that caused directly by turgor pressure, disappearing in hyperosmotic solution. Investigation of the buckling indicates that the outer layer of the cell wall transmits in situ most of the longitudinal tensile stress in the wall. The common concept that the inner layer of the wall is the region bearing most stress and therefore regulating growth can still be valid with respect to the transverse stress component.     

The Distribution of Growth and Cell Division in the Fruit of Cox's Orange Pippin

An apple fruit may be treated as approximately spherical, butallometric analysis shows that its growth is far from uniformlydistributed throughout its tissues. Along the fruit axis, longitudinal(i.e. lengthwise) growth is most rapid at the eye end and slowestat the stalk end, and in each region is slower than transversegrowth in the equatorial zone (i.e. growth in diameter). Onthe surface of the fruit cheek longitudinal growth is most rapidin the equatorial region and slower at the stalk and eye ends;growth at the eye end is slightly slower than at the stalk end.Near the equator, longitudinal growth of the cheek is also fasterthan latitudinal growth, despite the slower growth in over-allfruit length. In the transverse plane at the equator, growthis initially much faster in the cortex (the outer tissues) thanin the pith (the inner tissues). Later, about one month afterblossom, there is practically no difference in the growth-ratesof the cortex and pith. The change in the relative transverse growth-rates of the cortexand pith occurs at about the same time as cell division in thecortex stops, and at this time also there is an increase inthe rate of expansion of the air-spaces, relative to growthin fruit diameter. Other aspects of growth, such as growth inover-all length or weight, both relative to fruit diameter,do not appear to change in any way when cell division stops. Cell division in the epidermis continues for a longer time thanin the cortex, and accounts for a greater proportion of tissuegrowth. Allometric analysis of growth in cell size shows thatcell division in the epidermis stops when the fruit is about45 mm in diameter, or about 65–70 days after blossom.The same data show that in the stalk and eye cavities, longitudinalgrowth of the cheek is substantially faster than latitudinalgrowth.     

The influence of intermittent soil drying on growth, cell number, and final cell length in the pith of mature internodes in Helianthus annuus L. and Liquidambar styraciflua L.

The sensitivity of cell division in developing internodes to plant water deficits has not been previously documented. In this study two diverse taxa, Helianthus annuus L. and Liquidambar styraciflua L. were chosen because cell divisions in the pith and cortex continue to occur acropetally throughout the period of internode elongation. Potted plants were given 6-d cycles of soil drying between waterings to observe the effects of moderate, intermittent water deficits on final cell pattern in developing internodes. Under this regime, internode and leaf growth were inhibited although leaf and shoot turgidity were restored daily by nocturnal rehydration. The percent inhibition of final internode lengths was similar in both taxa, increasing from 23–58% in contemporaneously developing internodes. Of this inhibition, 9–48% in H. annuus compared with 97–100% in L. styraciflua was attributable to decreases in cell number in mature internodes. While cell divisions were severely inhibited in both taxa, differences in sensitivity appear related to differences in patterns of histogenesis associated with pronounced inherent differences in final cell lengths. Final cell lengths in H. annuus exceed those of L. styraciflua by 7–8-fold and can play a more dominant role in final internode lengths than total cell number. Conversely, in L. styraciflua total cell number, rather than final cell length, accounts for most of the variation in final internode length. These studies demonstrate species differences in sensitivity of cell division in developing internodes to intermittent water deficits.     

Tissue stresses in growing plant organs

Rapidly growing plant organs (e.g. coleopties, hypocotyls, or internodes) are composed of tissues that differ with respect to the thickness, structure, and extensibility of their cell walls. The thick, relatively inextensible outer wall of the epidermal cells contains both transverse and longitudinally oriented cellulose-microfibrils. The orientation of microfibrils of the thin, extensible walls of the parenchyma cells seems to be predominantly transverse. In many growing organs (i.e. leafstalks), the outer epidermal wall is supported by a thickened inner epidermal wall and by thick-walled subepidermal collenchyma tissue. Owing to the turgor pressure of the cells the peripheral walls are under tension, while the extensible inner tissue is under compression. As a corollary, the longitudinal tensile stress of the rigid peripheral wall is high whereas that of the internal walls is lowered. The physical stress between the tissues has been described by Sachs in 1865 as 'tissue tension'. The term 'tissue stress'. however, seems to be more appropriate since it comprises both tension and compression. Hitherto no method has been developed to measure tissue stresses directly as force per unit cross-sectional area. One can demonstrate the existence of tissue stresses by separation of the tissues (splitting, peeling) and determining the resulting strain of the isolated organ fragments. Based on such experiments it has been shown that rapid growth is always accompanied by the existence of longitudinal tissue stresses.     

Effects of gibberellin on the cellular dynamics of dwarf pea internode development

This study analysed the dynamics of cell production and extension, and how these were affected by applied gibberellic acid (GA₃), during internode development in dwarf peas (Pisum sativum L. cv. Meteor). Image analysis was used to obtain cell number and length data for entire cell columns along the epidermis, the two outermost cortical layers, and the pith, from internode 7, over a time period covering the whole of the internode's growth phase. For a few days following the inception of an internode at the shoot apex, little further growth occurred, and there was no significant effect of GA₃ on cell division or cell extension. The subsequent growth of the internode was stimulated more than fourfold by GA₃ as a result of the production of more than twice the number of cells, which were twice as long. At least 96.5% of the cells of the mature internode were actually formed within the internode itself during this period of growth, demonstrating that the internode cells themselves represent the morphogenetic site of response to GA₃. Mitoses and cell extension occurred along the full length of the internode throughout its development. The daily changes in cell numbers were modelled by the Richards function, and manipulations of the fitted functions to reveal time trends of absolute and specific cell production rates were performed for each stem tissue. The increase in cell numbers in the +GA₃ plants was brought about by an increase in the rate of cell production, over a shorter time interval; specific cell production rates declined continuously from initial rapid rates in the +GA₃ epidermis and pith, but declined more slowly in the cortex. The control (−GA₃) epidermis and cortex cells exhibited a constant specific cell production rate (i.e. purely exponential) for several days. Cell extension rates were calculated so as to compensate for the size-reduction effects of concurrent cell division. These calculations confirmed that `real' cell extension rates were higher in the +GA₃ internodes. Models of the cellular controls of internode growth, based on the estimated dynamics of cell division and extension, are discussed. Received: 1 July 1997 / Accepted: 30 July 1997     

Modification by ethylene of the cell growth pattern in different tissues of etiolated lupine hypocotyls

The influence of ethylene on growth in etiolated lupine (Lupinus albus L.) hypocotyls was studied in ethephon-treated plants. Ethephon reduced the length and increased the diameter of hypocotyls. At the end of the hypocotyl growth period (14 days), the fresh weight was reduced by 53%, and the dry weight was reduced by 16%. Thus, ethylene reduced water uptake in the tissues to a greater extent than the incorporation of new materials. Light microscopic measurements showed that the thickness of tissues was stimulated by ethylene, the vascular cylinder and cortex exhibiting greater increases (55 and 45%, respectively) than pith (26%) or epidermis (12%). Ethephon modified the cell growth pattern, stimulating lateral cell expansion and cell wall thickness, while reducing cell elongation. The response to ethylene varied in the different tissues and was higher in cortex and pith cells than in the epidermis cells. The ethylene-induced cell expansion in the cortex varied according to the localization of cells in the tissue: the central and subepidermal layers showed little change, whereas the innermost layers exhibited the greatest increase. Electron microscopy revealed that ethylene increased both the rough endoplasmic reticulum and dictyosomes, suggesting that ethylene stimulated the secretion of cell wall materials. In untreated seedlings, the pattern of cell growth was similar in cells from the epidermis, cortex, and pith. The final cell size varied along the hypocotyl, the cells becoming shorter and broader the closer to the basal zones of the organ.     

Microtubules spatial alterations in root cells of Brassica rapa under clinorotation

Organization of tubulin cytoskeleton in epidermis and cortex cells in different root growth zones in Brassica rapa L. 6-day-old seedlings under clinorotation has been investigated. It was shown that changes in cortical microtubules orientation occur only in the distal elongation zone. In control, cortical microtubule arrays oriented transversely to the root long axis. Whereas under clinorotation an appearance of shorter randomly organized cortical microtubules was observed. Simultaneously, a significant decrease in a cell length in the central elongation zone under clinorotation was revealed. It is suggested that the decline of anisotropic growth, typical for central elongation zone cells, is connected with cortical microtubules disorientation under clinorotation.     

Acid-induced elongation of Reynoutria stems requires tissue stresses

Measurements of the elongation rate of strips of outer tissue composed of epidermis and collenchyma, peeled from the elongating internodes of Reynoutria japonica Houtt. (Japanese knotweed) and stretched in buffer, showed that the rate depended on pH and the stretching force. At pH 6.8, elongation was barely perceptible in the force range studied, but the rate of elongation increased rapidly with the force at pH 5.0 (acid-induced elongation). The yield threshold stress at pH 5.0 amounted to 1.8×10⁶ N m^?2. It was three times higher than the osmotic pressure in the outer tissue, but was lower than the average tensile tissue stress in this tissue in intact stems. We infer that tensile tissue stress in the outer tissue is required for the manifestation of acid-induced elongation of this tissue in situ.     

双穗雀稗(Paspalum distichum)通透性生理和茎解剖结构补充研究

双穗雀稗根外皮层,茎角质层和茎节中的质外体屏障结构阻挡黄连素示踪液透过植物体。茎中机械组织包括周缘厚壁机械组织层,厚壁组织层和维管系统,髓部和皮层的蜂窝状厚角组织。茎中通气组织包括茎节间髓部和皮层的蜂窝状通气组织,茎节内的通气组织。双穗雀稗茎节间具有外侧、内侧和维管系统的质外体屏障,以及茎节周围质外体屏障的封闭结构。因此,该植物体完善的机械组织、通气组织、质外体屏障结构及其离子不通透性是其适应湿地环境的重要结构。     

Studies on the Structure and Elongating Growth of the Gynophore of Peanut plant

The epigeal portion of the gynophore has a typical herbaceous stem structure. It is found that the young gynophore is composed of epidermis, cortex and vascular cylinder. Periderm, lenticelle and secondary vascular tissue in the gynophore are produced due to its secondary growth. The region of cell division in the gynophore is located at 0.9–1.9 mm from the peg apex, and that of cell elongation at 2.0–4.5 mm. These two regions are found to overlap at 2.0–2.5 mm from peg apex. The results of the experiment exhibit clearly that the growing region of the gynophore grows toward the gravitional direction of the earth when the gynophore is placed either in a vertical position or in a horizontal position, thus the elongating gynophore shows a positive geotropic response. The distributed position of starch grains in the pith parenchyma sedimented to the side of cell wall near the earth surface. All this shows that there is a close relation between the positive geotropic growth of the peanut gynophore and the distribution of starch statoliths in the pith of the gynophore under the influence of gravity.     

The slender phenotype of pea: stem growth, peroxidase levels and ethylene responses

Stem elongation growth in a slender line of Pisum sativum L. was much greater than in its two dwarf parental lines due to a greater rate of production of longer internodes. The cellular basis of the greater length of slender internodes was tissue specific. In the epidermis, greater cell length primarily accounted for the longer internodes of the slender plants, but in the outer cortex, greater cell number was the more important factor. The soluble and salt-extractable peroxidase (EC 1.11.1.7) activities of expanding internodes were much lower in slender plants than in dwarf plants. The proportional effects of ethylene treatment on epicotyl length, diameter and orientation were similar in etiolated slender and dwarf seedlings.     

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

Analysis of Cell Division and Elongation Underlying the Developmental Acceleration of Root Growth in Arabidopsis thaliana

To investigate the relation between cell division and expansion in the regulation of organ growth rate, we used Arabidopsis thaliana primary roots grown vertically at 20°C with an elongation rate that increased steadily during the first 14 d after germination. We measured spatial profiles of longitudinal velocity and cell length and calculated parameters of cell expansion and division, including rates of local cell production (cells mm⁻¹ h⁻¹) and cell division (cells cell⁻¹ h⁻¹). Data were obtained for the root cortex and also for the two types of epidermal cell, trichoblasts and atrichoblasts. Accelerating root elongation was caused by an increasingly longer growth zone, while maximal strain rates remained unchanged. The enlargement of the growth zone and, hence, the accelerating root elongation rate, were accompanied by a nearly proportionally increased cell production. This increased production was caused by increasingly numerous dividing cells, whereas their rates of division remained approximately constant. Additionally, the spatial profile of cell division rate was essentially constant. The meristem was longer than generally assumed, extending well into the region where cells elongated rapidly. In the two epidermal cell types, meristem length and cell division rate were both very similar to that of cortical cells, and differences in cell length between the two epidermal cell types originated at the apex of the meristem. These results highlight the importance of controlling the number of dividing cells, both to generate tissues with different cell lengths and to regulate the rate of organ enlargement.