Article

The growth of the shoot apical meristem during flower initiation

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Abstract

The vegetative apex grows in an indeterminate, iterative mode. When flowers are formed it changes to determinate, sequential growth. The essential change in growth at the transition to flowering seems to be the decrease in primordium size at initiation, relative to the size of the apex. In the formation of the flower itself there may be a further requirement, that primordia undergo a decrease in absolute size at initiation. Where measured, this decrease is parallelled by a decrease in the size of the stem frustum at initiation. While in the case of the stem frustum this decrease in size has a predictable consequence, the reduced size of internodes in the mature flower, it is not clear how the reduction in size of the primordium is related to the subsequent development of flower parts. Reduction in primordium size seems to be fundamental to the changes in primordium arrangement and divergence angle that typically occur on flower formation, but not sufficient to cause the subsequent differentiation of floral parts. This presumably occurs as a consequence of the interactions between the developing primordia and substances present in the flowering apex. Recent models of flowering show that a realistic simulation of apical growth on the transition to flowering can be made using the equations of catastrophe theory. To account for the growth and development of a flower meristem models must also account for changes in divergence angle and the new developmental pathways followed by the floral organs. The data on the role of growth substances in the transition to flowering at the apex are at present insufficient. What is needed particularly is information on the identity of the putative inhibitor of primordium initiation that may operate within the apex, and its role in the growth changes that occur during and after the floral transition.

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... Both types of SAM geometry changes may accompany the VR transition. These changes take place at or just before the beginning of flower initiation (Lyndon and Battey, 1985; Bernier, 1988 Bernier, , 1997). Their type and extent depend on the species, in particular, on the habitus of the flowering plant (whether the terminal flower or inflorescence is formed, and what is the inflorescence type). ...
... Presumably due to the typical increase in meristem size, shape, and growth, as well as the switch in identity of the lateral organs produced, changes at the VR transition are also in some parameters characterizing phyllotaxis (Lyndon and Battey, 1985), i.e. the pattern of lateral organ arrangement. In particular, the rate of primordium initiation increases and the plastochron duration decreases, also resulting in a decrease in the plastochron ratio (the ratio of distances from the meristem centre to the centres of two successively initiated primordia, older over the younger; Richards, 1948). ...
... Such changes in relative sizes of primordia and the SAM are known to accompany quantitative or even qualitative changes in the phyllotactic pattern (Meicenheimer and Zagórska-Marek, 1989; Meicenheimer, 1998; Kwiatkowska and FlorekMarwitz, 1999), and explain a phenotype of certain mutants (Golz and Hudson, 2002). It has therefore been suggested that it is the change in relative size of primordia that is important for VR transition rather than the absolute change in the SAM size (Lyndon and Battey, 1985). In other species, such as Anagallis, the relationships between changes in phyllotaxis and the VR transition are not straightforward (Kwiatkowska, 1995 ). ...
Article
The shoot apical meristem generates stem, leaves, and lateral shoot meristems during the entire shoot ontogeny. Vegetative leaves are generated by the meristem in the vegetative developmental phase, while in the reproductive phase either bracts subtending lateral flower primordia (or paraclades), or perianth and strictly reproductive organs are formed. Meristem growth is fully characterized by the principal growth rates, directions, volumetric, and areal growth rates. Growth modelling or sequential in vivo methods of meristem observation complemented by growth quantification allow the above growth variables to be estimated. Indirectly, growth is assessed by cell division rates and other cell cycle parameters. Temporal and spatial changes of growth and geometry take place at the meristem during the transition from the vegetative to the reproductive phase. During the vegetative phase, meristem growth is generally indeterminate. In the reproductive phase it is almost always determinate, but the extent of determinacy depends on the inflorescence architecture. In the vegetative phase the central meristem zone is the slowest growing region. The transition from the vegetative to the reproductive phase is accompanied by an increase in mitotic activity in this zone. The more determinate is the meristem growth, the stronger is this mitotic activation. However, regardless of the extent of the activation, in angiosperms the tunica/corpus structure of the meristem is preserved and therefore the mitotic activity of germ line cells remains relatively low. In the case of the thoroughly studied model angiosperm plant Arabidopsis thaliana, it is important to recognize that the flower primordium develops in the axil of a rudimentary bract. Another important feature of growth of the inflorescence shoot apical meristem is the heterogeneity of the peripheral zone. Finally, the role of mechanical factors in growth and functioning of the meristem needs further investigation.
... In further experiments, the pattern of terminal flower development was assessed in treatments involving variable timings of the start of the inductive SD treatment and transferring from SD to long day (LD) those plants of a nonreverting purple-flowered line. This purpleflowered line is known to be able to continue flowering when transferred from SD to LD (Battey, 1985; Tooke et al., 1998), in contrast to the red-flowered line, which reverts, halting flowering and returning to leaf production, when so treated. ...
... The organ number increases in Impatiens, described here, appear to be unique among these cases. Although meristem size was not measured in these experiments, careful observations of dissected meristems provided no evidence for such a change; indeed, the Impatiens meristem is known not to change in size on flowering, reversion, or reflowering ( Lyndon, 1984, 1986; Lyndon and Battey, 1985). There was also no obvious variation in organ patterning/positioning associated with the increases in organ numbers observed. ...
Article
The completion of flower development in Impatiens balsamina requires continuous inductive (short-day) conditions. We have previously shown that a leaf-derived signal has a role in floral maintenance. The research described here analyzes the role of the leaf in flower development. Leaf removal treatments, in which plants were restricted to a specified number of leaves, resulted in flowers with increased petal number, up to double that of the undefoliated control. Similar petal number increases (as well as changes in bract number or morphology) were recorded when plants began their inductive treatment at a late developmental age or when plants of a nonreverting line (capable of floral maintenance in the absence of continuous short days) were transferred from short days to long days. Our data imply that the increased petal number was neither a response to stress effects associated with leaf removal nor a result of alterations in primordium initiation rates or substitutions of petals for stamens. Rather, the petal initiation phase was prolonged when the amounts of a leaf-derived signal were limiting. We conclude that a leaf-derived signal has a continuous and quantitative role in flower development and propose a temporal model for the action of organ identity genes in Impatiens. This work adds a new dimension to the prevailing ABC model of flower development and may provide an explanation for the wide variety and instabilities of floral form seen among certain species in nature.
... Another process accompanying the transition from the vegetative to reproductive phase is structural reorganization of the SAM (Lyndon and Battey, 1985;Bernier, 1988;Kwiatkowska, 2008) that affects the SAM zonation, shape, and mitotic activity. In the majority of vegetative SAMs the cytohistological zones are distinguished by differences in their mitotic activity (Romberger et al., 1993). ...
... The central zone is the least active zone while the rib meristem and the peripheral zone are characterized by higher mitotic activity. In the reproductive (inflorescence or terminal flower) SAM the activity of the central zone often strongly increases and the zonation described above is replaced by the mitotically active meristematic mantle overlaying the less active core (Lyndon and Battey, 1985;Bernier, 1988). However, in some plants, the central zone of lower mitotic activity can still be recognized in the inflorescence SAM, as in Arabidopsis, though the observed zonation is less profound than in the vegetative phase (Vaughan, 1955;Laufs et al., 1998;reviewed in Kwiatkowska, 2008). ...
Article
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Quantitative analysis of geometry and surface growth based on the sequential replica method is used to compare morphogenesis at the shoot apex of Anagallis arvensis in the reproductive and vegetative phases of development. Formation of three types of lateral organs takes place at the Anagallis shoot apical meristem (SAM): vegetative leaf primordia are formed during the vegetative phase and leaf-like bracts and flower primordia during the reproductive phase. Although the shapes of all the three types of primordia are very similar during their early developmental stages, areal growth rates and anisotropy of apex surface growth accompanying formation of leaf or bract primordia are profoundly different from those during formation of flower primordia. This provides an example of different modes of de novo formation of a given shape. Moreover, growth accompanying the formation of the boundary between the SAM and flower primordium is entirely different from growth at the adaxial leaf or bract primordium boundary. In the latter, areal growth rates at the future boundary are the lowest of all the apex surface, while in the former they are relatively very high. The direction of maximal growth rate is latitudinal (along the future boundary) in the case of leaf or bract primordium but meridional (across the boundary) in the case of flower. The replica method does not enable direct analysis of growth in the direction perpendicular to the apex surface (anticlinal direction). Nevertheless, the reconstructed surfaces of consecutive replicas taken from an individual apex allow general directions of SAM surface bulging accompanying primordium formation to be recognized. Precise alignment of consecutive reconstructions shows that the direction of initial bulging during the leaf or bract formation is nearly parallel to the shoot axis (upward bulging), while in the case of flower it is perpendicular to the axis (lateral bulging). In future, such 3D reconstructions can be used to assess displacement velocity fields so that growth in the anticlinal direction can be assessed. In terms of self-perpetuation, the inflorescence SAM of Anagallis differs from the SAM in the vegetative phase in that the centrally located region of slow growth is less distinct in the inflorescence SAM. Moreover, the position of this slowly growing zone with respect to cells is not stable in the course of the meristem ontogeny.
... All these changes in apical growth have been already described individually in one or another species where the vegetative apical meristem is converted into an inflorescence meristem (see literature reviews by Bernier et al., 1981;Lyndon and Battey, 1985;Bernier, 1988), but only in very rare cases has a complete spatial and temporal description been performed, as is the case here in S. alba. ...
Article
The aim of the work was to report morphological changes which occur in the shoot apex during the morphogenetic switch to flowering in the model long day (LD) plant, Sinapis alba. During the floral transition induced by 1 LD the growth rate of all components of the shoot apex is modified profoundly. The earliest changes, detected at 24 h after start of LD, include a decrease in plastochron duration and an increase of growth of leaf primordia. One day later, the meristem dome starts to increase in volume, apical internodes have an increased height and there is a precocious outgrowth of axillary meristems. All these changes precede initiation of flower primordia, which starts at about 60 h after the start of LD. Later changes include meristem doming, a decrease in the plastochron ratio and a shift to a more complex phyllotaxis. All the changes, except the decreased plastochron ratio, are characteristics 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 radial growth (plastochron ratio).
... SAM growth is indeterminate and changes of SAM shape and size are cyclical, with plastochron (time lapse between successive lateral organ initiation) being the main cycle (Lyndon, 1998). Shape and size changes in the course of leaf or flower formation are directional and growth of these organs is regarded as determinate, at least in the case of flowers and simple leaves (Lyndon and Battey, 1985). This means that early in lateral organ formation, the SAM surface has to be divided into two portions of different fate and growth (determinate versus indeterminate), which is the process of SAM surface partitioning (Lyndon, 1998). ...
Article
Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.
... Stimulation of cell division and changes in size and shape in various parts of the shoot apex are known to be common features of the¯oral transition in manyÐif not allÐplant species (Lyndon and Francis, 1984;Lyndon and Battey, 1985;Bernier, 1988;Francis, 1992;Francis and Herbert, 1993). As a rule, acceleration of the cell division cycle in the shoot apical meristem (SAM) is a very early event of thē oral transition which precedes, by many hours, sometimes by days, the earliest growth changes affecting various apex parts as well as the initiation of the ®rst reproductive structures. ...
Article
Eight-week-old vegetative plants of Arabidopsis thaliana, ecotype Columbia, were induced to flower by a single long day (LD). In this experimental system, it is known that the last component of the floral stimulus moves from the leaves to the apex 24-36 h after the start of the LD, and the first floral meristem is initiated by the shoot apical meristem (SAM) at 44-56 h (Corbesier et al., 1996, The Plant Journal 9: 947-952). Here we show that the rate of cell division is increased at floral transition in all SAM parts but not in the sub-apical pith cells. Mitotic activity starts to increase 24 h after the start of the LD and is two- to three-fold higher at peak times than that in non-induced plants. This activation is followed by the start of SAM enlargement at 44 h, SAM doming at 48 h, and the elongation of apical internodes (bolting) at 52 h.
... The maize SAM has three stages of development during vegetative plant growth The morphology and growth rate of the maize SAM are not static. The most evident examples of dynamic growth are the changes that take place in the SAM before or at reproductive transition, among them the enlargement of the apex as the growth rate increases, a change in phyllotaxy and suppression of internodes in the flower, and an increased rate of initiation of primordia accompanied by decreased size of primordia at initiation (Lyndon and Battey 1985). In Sinapis alba these changes have been detected as early as 24 hr after long day floral induction; flower primordia initiated around 60 hr after (Bernier 1997). ...
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... In contrast with the stable vegetative SAM size, the transition to flowering is usually associated with dramatic SAM changes; overall enlargement and altered dimensions, shifting from a flat top to a domed shape, are common to many plants (Bernier, 1988). SAM doming during the floral transition has been reported for both monocot and dicot species (Metcalf et al., 1975;Lyndon and Battey, 1985) and for plants with diverse growth habits. For most plants, the first visible change toward the transition to flowering is an increase in height relative to width (i.e., doming), followed by broadening of the meristem (Bernier et al., 1981). ...
Article
Full-text available
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... Further investigations of carbohydrates and hormones availability and local specialization of the FI signal within the meristematic tissues themselves could provide further comprehension of FI. Indeed, the central zone where FI is triggered is tightly regulated and maintained in very isolated conditions (Lyndon and Battey 1985). ...
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The flower production of Antirrhinum majus L. ("snapdragon") is commonly carried out in greenhouse conditions using traditional cultivars. In the present paper we shown that it is possible the production of A. majus L. in the field conditions of the east of the State of Mexico municipality of Texcoco with commercial quality. In order to know that, it was designed an experiment in which: the 1) epoch or date of cultivation: autumn–winter 2006 and spring–summer 2007 and 2) four cultivars of A. majus L.: 'Plumblossom' and 'Orange' Potomac series and 'Bronze' and 'Red' Rocket series; were the two factors of an experimental design of randomized blocks. The population density was 80 plants·m–2. The cultivars of the Potomac series in the cycle autumn–winter need less accumulation of development day degrees (1,254 °C·día–1) to reach the commercial maturity than the series Rocket (1,727 °C·día–1). Therefore, they were more efficient in the use of the radiation than the series Rocket (0.61g biomasa·MJ–1 and 0.36 g biomasa·MJ–1 respectively). The cultivars of the Potomac series obtained greater fresh biomass (137 g), length of stem (100 cm) and quality index (1.35 g·cm–1). In addition, 70% of the stems of its cultivars 'Plumblossom' and 'Orange' were of "special" quality, the highest one for this species. The behavior of the cultivars of both series was similar in the cycle spring–summer. Although in autumn–winter the dry biomass accumulated, the leaf number and the days to crop were smaller. It is concluded that the cultivars of the series Potomac reached better development in both cycles of cultivation, and produced inflorescences with greater commercial quality that the cultivars of the series Rocket. Therefore, they would be cultivated in similar field conditions to present study.
Article
The ontogeny of Epilobium hirsutum grown under natural summer photoperiod in a glasshouse was divided into vegetative, early transitional, transitional, and floral stages. Bijugate phyllotaxy, common to both the vegetative and early transitional stages, is transformed into spiral phyllotaxy during the transitional stage by an initial change in the divergence angle of a single primordium inserted at a unique level on the shoot. Leaf primordia subsequently are inserted in a spiral arrangement in the indeterminate floral shoot apex. The early transitional shoot apical meristem is about 1.5 times the volume of the vegetative meristem but expands at about two-thirds the relative plastochron rate of volume increment of the vegetative meristem. There are progressive decreases in the plastochron and relative plastochron rates of radial and vertical shoot growth through ontogeny. Relative chronological rates of shoot growth, however, are not altered during ontogeny. Spiral transformation results from changes in the relative points of insertion of leaf primordia on the shoot meristem. These changes are accompanied by an increased rate of primordia initiation on a more circular shoot meristem. The change in phyllotaxy during ontogeny is similar to that which was artificially induced by chemical modification of auxin concentration gradients in the shoot apex, with the additional feature that there is an initial increase in the volume of the shoot meristem prior to the natural spiral transformation. Size of the shoot apical meristem, however, appears to have little influence on Epilobium phyllotaxy; but the geometric shape of the meristem is well correlated with bijugate to spiral transformations. This suggests that geometric parameters of the shoot meristem should be considered in theoretical models of phyllotaxy.
Article
One application of gibberellic acid (GA3) to Xanthium shoots resulted in an initial large stimulation, followed by inhibition, of internode elongation. After presumed translocation of the hormone from the locus of its application to the stem apex several morphological changes were observed. There was a significant increase in number of mitotic figures in the apical meristem and a twofold increase in volume of the apical dome. With time, the rate of leaf production was accelerated about 1.8 times. The phyllotaxis of leaf primordia initiated under the influence of GA:, changed from a (2, 3) contact parastichy pattern in control shoot to a (3, 5) pattern. Final petiole length was smaller than the control, and the absolute rate of lamina expansion decreased under prolonged treatment. Gibberellic acid had a pronounced effect on leaf morphology. GAa induced the development of lanceolate leaves instead of typical deltoid leaves. The reduction in leaf area coincided with a 32% reduction in the average area of epidermal cells. Plastochron changes were correlated with anatomical and morphological changes during the course of leaf development.
Article
Whereas a medium containing kinetin alone enabled a few Dianthus caryophyllus L. apical meristem dome explants to develop into rooted plants, the highest frequency of plants was obtained in one containing supplements of both IAA and kinetin. In an unsupplemented medium, continued development required that explants have 2 pairs of primordial and a pair of expanding leaves. Kinetin alone caused production of many new leaves, but the development was significantly less than when it was furnished in combination with IAA. IAA given alone caused meristem explants to develop primarily callus, roots, and a few leaves. Gibberellin and abscisic acid were without promotive effects on leaf and shoot formation. A balance of hormonal substances, synthesized in young leaf structures and relocated to the meristem, is proposed as the fundamental mechanism that regulates new leaf initiation in the shoot apex.
Article
Preliminary studies establishing relationships between leaf plastochron index and Epilobium hirsutum L. shoot growth provide a method for rigorous selection of plants utilized in experiments designed to test the working hypothesis that endogenous auxin gradient interactions are factors of phyllotactic control in this species. Application of N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor, to one of the youngest bijugate primordia on the shoot meristem results in increased growth of the treated primordium. Fasciation between the treated primordium and one of the next primordia to be initiated alters relative vertical spacing of primordia. Angular shifts between subsequent primordia result in spiral transformation of Epilobium bijugate phyllotaxy. Application of α-4-chlorophenoxyisobutyric acid (CPIB), an auxin antagonist, to one of the youngest bijugate primordia on the shoot meristem results in decreased growth of the treated primordium that alters both radial and vertical spacing of primordia. This is followed by angular shifts between subsequent primordia resulting in spiral transformation of the bijugate phyllotaxy. Changes in the growth parameters of NPA- and CPIB-treated shoots are similar. Relative plastochron rates of radial and vertical shoot growth of induced spiral shoots are about half those of lanolin paste control shoots, as are the plastochrons and relative plastochron rates of leaf elongation. Treated shoot meristems have eccentricities of 0.5 as compared to bijugate control meristem eccentricities of 0.7. No significant difference is apparent between basal transverse areas of treated and control shoot meristems. The relative chronological rates of growth of treated shoots are not significantly different from those rates of control shoots. Spiral transformation results from changes in relative positions of leaf primordia insertion on the shoot meristem, not from changes in growth of treated shoots. These changes are accompanied by an increased rate of leaf initiation on a more circular shoot meristem. Existing theoretical models of phyllotaxy are discussed in relation to these chemically induced changes of Epilobium leaf arrangement.
Article
Growth of Ranunculus shoots through ontogeny is quantified by techniques utilizing scanning electron microscopy and studies on living plant material. The order of the contact parastichy phyllotaxy in the apical system is related to the relative plastochron rates of growth of the shoot. There is a change in the (2, 3) contact parastichy pattern of vegetative phyllotaxy to a transitional (3, 5) contact pattern which is maintained through sepal production. Formation of a 5(1, 1) whorl of petal primordia establishes a (5, 8) contact pattern with the sepal primordia. Subsequent initiation of stamen primordia, in spiral sequence, results in (5, 8, 13) triple contacts between petal and stamen primordia. The stamen primordia and carpel primordia arrangement is characterized by a (8, 13) contact parastichy pattern of phyllotaxy. Through ontogeny the volume of the shoot apex progressively increases but the shape of the apex, described by a second degree polynomial, remains constant. The plastochron and the relative plastochron rates of radial and vertical displacement of primordia progressively decrease during transition but there is no alteration of the chronological rate of apical expansion. The change in contact parastichy phyllotaxy through ontogeny is interpreted as a change in the relative positions of primordia insertion on the apex resulting from an increase in apical volume and an increased rate of primordia initiation.
Article
Application of 1.5 μg indoleacetic acid (IAA) in a lanolin droplet to the exposed apical meristem of Lupinus albus seedlings caused: (1) axillary buds to form closer to the apex than normal, (2) displacement of primordia formed during the first two plastochrons following treatment, and (3) significant increases in concentration of RNA, protein, and unsaturated lipids in the meristems. Primordial displacement tended to be random relative to the site of the treatment, which may be a feature common to dicotyledonous plants exhibiting spiral phyllotaxis. That IAA conferred initiation site capabilities to all of the peripheral zone for a short time was indicated by (2) and (3) above, and by decreases in concentrations of the observed compounds toward control levels after the second plastochron following treatment. Effects of IAA on RNA suggest that nucleic acid metabolism, and possibly gene action, was involved in the response. Kinetin or gibberellic acid had no apparent morphogenetic effect on Lupinus meristems.
Article
Photoperiodic floral induction in Xanthium, achieved by subjecting the plants to two long nights, is accompanied by a transient change of the phyllotaxis from the (2, 3) contact parastichy pattern of vegetative plants, to a (3, 5) pattern during the transition. To specify the phyllotaxis, two parameters were estimated from transverse sections of apical buds of control and treated plants: the divergence angle, α, and the plastochron ratio, a. The plastochron ratio decreased progressively during transition from the vegetative to the reproductive state of growth, from a = 1.48 initially to a = 1.15 six days after the beginning of induction. The divergence angle was not altered during the transition. This change in phyllotaxis is interpreted as a change in the relative positioning of leaf primordia on the transitional apex. This transient change appears to be identical with the previously described long-term change of the phyllotaxis of Xanthium brought about by treatment of plants with gibberellic acid.
Article
Gibberellic acid (GA) treatment of vegetative shoots of Xanthium leads to a change in phyllotaxis as diagnosed in transverse sections of apical buds. A method of analysis is proposed for estimating the phyllotactic parameters, the plastochron ratio, a, and the divergence angle, α, from measurements of the angular and radial positions of leaf primordia in sections. GA treatment significantly decreases the plastochron ratio, a, from 1.35 in controls, to 1.19 in GA-treated plants, as shown by an analysis of variance, but has no significant effect on the divergence angle. The estimates of a and α are compared with the parameters of theoretical phyllotaxis models, leading to the designation (2, 3) for controls, and (3, 5) for GA-treated plants, where the integers 2, 3, and 5 designate sets of contact parastichies. The change in a is interpreted as indicating a change in the relative position at which leaf primordia are initiated in the apical meristem, and this effect is discussed in relation to theories of leaf initiation.
Article
The diameter and shape of the apical dome of hop shoots were measured in the perennial hop during four growing seasons in the field as well as under controlled conditions. After 10 to 12 weeks' growth with almost constant apical diameter, there is a marked decrease in diameter in June. Since this takes place after the attainment of ‘ripeness to flower’ it may be interpreted as the first sign of flower initiation. Holding plants in non-inductive daylengths prevents this decrease. There is also a progressive change in the ratio of diameter to height of the apical dome which occurs throughout the entire growth period and this appears to be independent of flower induction. The period between the change in apical diameter and the microscopic appearance of floral organs is analogous to the prefloral stage described for other species.
Article
A mathematical model is constructed to describe the morphopnetic switch that occurs when a vegetative plant apex becomes reproductive. The cusp equation from catastrophe theory is modified, and is used to relate primordial size at initiation to apex size. The resulting equation may be viewed as an equation of state defining the allowed organizational modes of the shoot apex. The model simulates the growth of the apex from the vetative stage to early reproductive growth, and makes reasonable predictions about apex size and growth rate, primordial sizes, and the lengths of the plastochron.
Article
The measured divergence angles between successive primordia in the developing flower were compared with angles expected on several hypotheses concerning primordial initiation. The results lead to the conclusion that the position and sequence of initiation of the younger sepals is determined by the older ones but that the influence of an older primordium lasts for only two plastochrons. The petals and carpels are apparently positioned by the sepals. The positions of the stamens are consistent with their king determined by the sepals (antesepalous stamens) or petals (antepctalous stamens), but their sequence of initiation is consistent with its being determined, like the sepals, by the two youngest primordia. The data indicate that there are two sets of factors governing the initiation of the primordia subsequent to the sepals: one governing the positioning of the primordia and resembling the factors governing the positions of axillary buds, and the other governing the sequence of primordia and resembling the factors which determine the initiation of leaves. Measurements of the plastochron ratios were used to calculate the sizes of the sepal, petal and stamen primordia at initiation. At the moment of initiation the sepal primordia were about one third, and the petal and stamen primordia about one sixth, of the size of the leaf primordia. In its early development the Silene flower therefore resembles a condensed leafy shoot with precocious axillary buds but with primordia which are small compared to leaf primordia.
Article
When plants of Impatiens balsamina L were subjected to 5 short days and then re-placed in long days, they began to form a terminal flower and then reverted to vegetative growth at this terminal shoot apex The onset of flowering was accompanied by an increase in the rate of initiation of primordia, an increase in the growth rate of the apex, a change in primordium arrangement from spiral to whorled or pseudo-whorled, a lack of internodes, and a reduction m the size at initiation of the primordia and also of the stem frusta which give rise to nodal and internodal tissues On reversion, parts intermediate between petals and leaves were formed, followed by leaves, although in reverted apices the size at initiation and the arrangement of primordia remained the same as in the floweing apex The apical growth rate and the rate of primordium initiation were less in the reverted apices than in floral apices but remained higher than in the original vegetative apex Since the changes in apical growth which occur on the transition to flowering are not reversed on reversion, the development of organs as leaves or petals is not directly related to the growth rate of the apex, or the arrangement, rate of initiation or size at initiation of primordia
Article
Pith tissue of Nicotiana tabacum L. cv. "Havana 425" exhibits a gradient in its tendency to habituate for cytokinin on an auxin-containing medium at 35° C, about 10° C above the standard culture temperature. Explants of pith from below the 8th to 11th internode, counting from the bottom of the plant, rarely habituate for cytokinin; explants from above this threshold habituate rapidly. The explants must also be above a critical size, about 20-30 mg, to habituate. There was a pronounced interaction between size and position effects; the threshold position for cytokinin habituation shifted upward with decreasing explant size.
Article
Current methods of classifying the phyllotaxis patterns found at stem apices are discussed, and it is concluded that the numerical measures employed do not characterize solely the arrangement or positioning of the primordia, but are partially determined also by primordial shape. A method of phyllotaxis assessment is proposed that is free from this ambiguity. For a complete description three parameters are necessary, namely, the angle of the cone tangential to the apex in the region under consideration, the divergence angle, and the plastochrone ratio, i.e. the ratio of the radial distances of two successive primordia from the central axis. For assessment of the transverse component of the system the two last parameters are alone required. Of these three characteristics the plastochrone ratio is the most useful, and from it a 'phyllotaxis index' may be calculated that immediately conveys the most essential information relating to the primordial arrangement. Since this index is a continuously varying function, phyllotaxis assessment is no longer confined to a few discrete recognized systems, and it becomes possible to compare quantitatively with one another phyllotaxis arrangements of very diverse kinds. The relationship between the index and primordial pattern is presented for all important divergence angles, and also for those systems wherein more than one leaf appears at each node. The phyllotaxis index is rigidly related to the ratio of the transverse components of two areas, that of the central apex and that of the newly initiated primordium; a simple extension enables a similar relationship to be determined on the actual apical surface, opening the way for a practical adaptation of the older theoretical concept of 'bulk ratio'.
Article
Growth of Ranunculus shoots through ontogeny is quantified by techniques utilizing scanning electron microscopy and studies on living plant material. The order of the contact parastichy phyllotaxy in the apical system is related to the relative plastochron rates of growth of the shoot. There is a change in the (2, 3) contact parastichy pattern of vegetative phyllotaxy to a transitional (3, 5) contact pattern which is maintained through sepal production. Formation of a 5(1, 1) whorl of petal primordia establishes a (5, 8) contact pattern with the sepal primordia. Subsequent initiation of stamen primordia, in spiral sequence, results in (5, 8, 13) triple contacts between petal and stamen primordia. The stamen primordia and carpel primordia arrangement is characterized by a (8, 13) contact parastichy pattern of phyllotaxy. Through ontogeny the volume of the shoot apex progressively increases but the shape of the apex, described by a second degree polynomial, remains constant. The plastochron and the relative plastochron rates of radial and vertical displacement of primordia progressively decrease during transition but there is no alteration of the chronological rate of apical expansion. The change in contact parastichy phyllotaxy through ontogeny is interpreted as a change in the relative positions of primordia insertion on the apex resulting from an increase in apical volume and an increased rate of primordia initiation.
Article
One application of gibberellic acid (GA3) to Xanthium shoots resulted in an initial large stimulation, followed by inhibition, of internode elongation. After presumed translocation of the hormone from the locus of its application to the stem apex several morphological changes were observed. There was a significant increase in number of mitotic figures in the apical meristem and a twofold increase in volume of the apical dome. With time, the rate of leaf production was accelerated about 1.8 times. The phyllotaxis of leaf primordia initiated under the influence of GA3 changed from a (2, 3) contact parastichy pattern in control shoot to a (3, 5) pattern. Final petiole length was smaller than the control, and the absolute rate of lamina expansion decreased under prolonged treatment. Gibberellic acid had a pronounced effect on leaf morphology. GA3 induced the development of lanceolate leaves instead of typical deltoid leaves. The reduction in leaf area coincided with a 32 % reduction in the average area of epidermal cells. Plastochron changes were correlated with anatomical and morphological changes during the course of leaf development. MAKSYMOWYCH AND MAKSYMOWYCH (1973) presented a quantitative description of vegetative development of Xanthium pennsylvanicum as influenced by gibberellic acid (GA3). GA3 caused stimulation of stem elongation, altered leaf morphology, and changed the rate of leaf initiation. The area and length of leaves on treated plants were significantly reduced. Each response could be regulated by increasing or decreasing the concentration of gibberellic acid. In this paper we present additional information concerning the long-range effects of gibberellic acid upon Xanthium development. Increased rates of leaf initiation are associated with an enlargement of the apical dome and an increase in the number of mitotic figures seen in the dome. To our knowledge this is the first report that prolonged treatment with GA3 causes marked changes in phyllotaxis. Morphological changes in leaves are correlated with inhibition of cell enlargement in the lamina and petiole elongation.
Article
Gibberellic acid (GA) treatment of vegetative shoots of Xanthium leads to a change in phyllotaxis as diagnosed in transverse sections of apical buds. A method of analysis is proposed for estimating the phyllotactic parameters, the plastochron ratio, a, and the divergence angle, a, from measurements of the angular and radial positions of leaf primordia in sections. GA treatment significantly decreases the plastochron ratio, a, from 1.35 in controls, to 1.19 in GA-treated plants, as shown by an analysis of variance, but has no significant effect on the divergence angle. The estimates of a and a are compared with the parameters of theoretical phyllotaxis models, leading to the designation (2, 3) for controls, and (3, 5) for GA-treated plants, where the integers 2, 3, and 5 designate sets of contact parastichies. The change in a is interpreted as indicating a change in the relative position at which leaf primordia are initiated in the apical meristem, and this effect is discussed in relation to theories of leaf initiation.
Article
The photoperiodic sensitivity of two “early” oilseed sunflower cultivars, (Helianthus annuus L.), Sunfola 68-2 and HA-124, was assessed by determining the time for flower initiation to progress through a sequence of 10 floral stages, defined by reference to scanning electron microscope photographs of fixed stem apices. It was found for plants maintained in controlled environments with daylengths of 11 or 18 h that there was no significant difference in the time of flower initiation in either cultivar, although post-initiation development of the primordial inflorescence was faster in the 11 h short day leading to an earlier anthesis. It is concluded that post-initiation sensitivity to daylength may account for some of the variation in the time of anthesis and the “head visible” stage, reported for Sunfola 68-2 and other cultivars and hybrids.
Article
Previous experiments have shown that in seedlings of Lupinus albus, as probably in other species, a certain minimum free area must be available on the surface of the apical cone far enough below the summit or growing-point before a leaf primordium can be determined. If now the growing-point is weakened by being pricked with a fine glass needle, the apical cone below it grows on, and new leaf rudiments arise from much narrower bases than normal. This indicates that normally the growing-point inhibits the determination of a leaf rudiment below it until the growth of the apical cone makes available a free area that can overcome the inhibition from the growing-point, being far enough below it and large enough to possess the necessary vigour. In this way it is now possible to interpret physiologically the need for a minimum free area for leaf determination, and also certain other observations and results.
Article
Previously we found (1952, 1955) that in Lupinus albus when the summit of the stem apex was weakened with a prick, the leaves arose subsequently from smaller areas; and we concluded that normally the summit of the apex exerts some inhibiting influence on the leaf-forming zone, which needs to be overcome before a new leaf can be determined. Here it is shown that if several of the leaves of the bud are cut down repeatedly rather low, the youngest leaf and the new leaves are found subsequently to cover larger arcs of the circumference, so that normally the leaves of the bud must somehow decrease the arcs of the youngest and of the new leaves. In order, therefore, to compare more closely the effects of the leaves with those of the summit of the apex, we repeated the experiment by which we previously estimated the primary arc over which a leaf is determined, splitting with a vertical cut the presumptive area of I1 or I2, the next two leaves due to arise, and later measuring the angular distance from the wound of the morphological centre of a new leaf determined at one side of the wound and abutting on it. But this time we combined the operation with cutting down the leaves of the bud either very lightly or more severely. No difference was found to be made to the primary arc of the new leaf by the severity with which the leaves of the bud were cut down, although the variation was very small. So it follows that normally the leaves of the bud decrease the secondary lateral extension of the bases of the new leaves, whereas the summit of the apex increases the area of the apical surface which is needed for their determination. The primary leaf arc estimated from the present experiments is 90 degrees, which is much less than the previous estimate of 124 degrees or 122 degrees (1933, 1952). This is apparently because a quite different race of Lupinus was used, and other evidence confirms that in this race the primary leaf arc is much smaller.
Article
Removal of a leaf of Coleus before or early in the differentiation of leaf traces severely reduced vascular differentiation in the subjacent internode. Auxin (IAA) in anion exchange beads (applied on 2 out of every 3 d for 2 to 4 weeks) restored normal structure with respect to arrangement of leaf traces and their composition, size, and rate of development. That IAA is the likely leaf-limiting factor for vascular differentiation in situ is suggested by the formation of normal quantities of procambium, phloem and xylem under the influence of IAA after leaf excision. However, some peculiarities were induced as well. Plants treated with IAA showed earlier xylem maturation in side bundles, earlier and more frequent differentiation of xylem opposite putative phloem-only bundles, and corner-trace xylem connections at the node of IAA application. These can be accounted for by a lack of well-defined channelling of IAA through petiolar tissues when IAA was applied to the cut surface after leaf excision. Phloem-only bundles are regarded as an early auxin-deficient stage in collateral bundle formation, which depends on auxin availability to the bundle for fulfillment.
Article
On explants composed of 3–6 layers of epidermal and sub-epidermal cells of Nicotiana tabacum L. from the floral branches, it is possible to obtain mitoses followed very rapidly by meioses and the direct formation of anthers and pistil without any intermediate callus.
Article
A mathematical model of flowering in Chrysanthemum morifolium Ramat. is described which may be used to predict quantities such as the number of primordia initiated by the apex, plastochron duration and apical dome mass before, during and after the transformation of the apical meristem from vegetative to reproductive development. The model assumes that primordial initiation is regulated by an inhibitor present in the apical dome. Within each plastochron the apical dome grows exponentially, and the inhibitor concentration declines through chemical decay and dilution. When the inhibitor concentration falls to a critical level a new primordium is initiated. There is instantaneous production of inhibitor, and a decrease in dome mass corresponding to the mass of the new primordium. The process continues until the apical dome attains a 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 successive plastochrons. Eventually, the net mass of the dome starts to decline until it is entirely consumed in the production of floral primordia.
Article
The size of the apical dome of Chrysanthemum morifolium Ramat. at the transition to inflorescence initiation in continuous light (long days) was not systematically influenced by either the temperature or the irradiance under which the plants were grown. It was generally 0.26 mm in diameter and c . 3.6 × 10−3 mm3 in volume when the first bract was initiated. The dimensions of the apical dome of plants in short days were only slightly smaller at this stage. Similarly, each step in the further development of the chrysanthemum inflorescence was associated with a narrow range of apex sizes, indicating that inflorescence initiation and development are closely related to apex size.
Article
Rates of cell division in the central and the peripheral zones of vegetative and evoked meristems of Sinapis alba have been measured by accumulation of metaphases after colchicine treatment. The cells of the central zone had a longer cycle than the cells on the flanks of both kinds of meristems. The duration of the cell cycle was shortened in both zones of the meristem during transition to flowering. It was shown that the mitotic indices of the two regions of the meristem were closely comparable to their rates of cell division and therefore could be considered representative of the rates of cell division.
Article
A quantitative two-dimensional model for phyllotaxis is described. The model is based on the production, diffusion and degradation of a morphogen, and it is assumed that primordial initiation can only take place in competent tissue when the morphogen concentration drops to a certain critical level. The model predicts the angles between successive primordia; under appropriate conditions a steady state is achieved where the divergence angle is constant; and for limiting values of the parameters, the predicted steady-state divergence angle approaches the Fibonacci angle. To account for non-spiral phyllotaxis, an additional hypothesis, called spatial competence , is introduced. As well as a morphogen level below the threshold level, and for competence in the usual sense, this assumes that particular spatial demands must be satisfied before primordial initiation can proceed.
In shoot apices of Lupinus albus the greater part of the presumptive area of I2, the leaf due to arise next but one, was confined between vertical radial cuts as nearly as possible 100 degrees apart. No leaf was formed between the cuts except in apices near to flowering, in which the leaves are always much smaller. But when, in addition, the growing-point of the apex was destroyed by a shallow split, then I2 always arose between the confining cuts. In Euphorbia lathyris the greater part of the presumptive area of one of the I2's was similarly confined between cuts intended to be 70 degrees apart. In nine apices in which the arcs between the cuts were found afterwards to range from 58 to 88 degrees, no leaf was formed in the confined region. In one apex in which the arc was 99 degrees, an I2 was formed between the cuts. In eleven apices of Lupinus albus the central part of P1, the youngest visible leaf, was removed by two sloping cuts or destroyed by a prick. The angles P2-I1 and I1-I2 were not changed at all by the operation. The angle I2-I3 diminished by 3 degrees.4, indicating a slight shift of I3 towards the wound which is interpreted as due to an increase of available space. It is concluded, in agreement with previous conclusions, that in Lupinus, Euphorbia, and probably in most dicotyledons, a leaf is determined only when there is available a space on the apex far enough below the growing-point and covering some minimum arc. This arc is estimated at 122 degrees in non-flowering lupin apices with intact growing-points. It is also concluded that in Lupinus the central parts of young leaves do not repel the centres of leaves about to arise. Leaf determination in these plants is further discussed in comparison with plants of other groups.
Article
Formation of indoleacetic acid synthetases in tobacco pith explants was determined by following the growth of tissue cultures under conditions of indole-3-acetic acid (IAA) deprivation and by measuring the enzymatic conversion of tryptophan to IAA in the cultures. The pith explants obtained from the parent plant (Nicotiana glauca) and from basal regions of the tumor-prone hybrid (N. glauca x N. langsdorffii) both show a requirement for exogenous IAA for growth initiation in culture. The parent pith requires the constant presence of added IAA for continued growth, but hybrid pith, after initial treatment with IAA, will grow without further additions. IAA synthetases are detected in the cell homogenates of hybrid pith explants cultured with either continuous or initial IAA addition. These observations indicate that IAA may induce its own production. In contrast, IAA synthetases are not found in the parent pith under comparable culture conditions. Besides IAA, nonhormonal compounds such as indole and tryptophan are also capable of stimulating growth of hybrid pith, possibly through the induction of IAA synthetases needed for IAA formation. Indole and tryptophan are, however, inactive in growth promotion of the parent pith. These results suggest that the genomic expression of IAA synthetase formation is more stringently controlled in N. glauca than in the tumorprone hybrid.
Article
The mechanism for leaf position determination by the diffusion of an inhibitor has been studied in relation to the geometry of leaf positions. A computer model has been constructed for the inhibitor-diffusion process on a cellular cylindrical surface. The behavior of the model has been analyzed mathematically. The main results are: (a) that our model generates most of the phyllotactic patterns observed in nature; and (b) that restraints have been found for permissible values of diffusion rates and decay rates of the hypothetical inhibitor.
Article
Pith tissue from Nicotiana tabacum L. cv ;Maryland Mammoth' or ;Wisconsin 38' was isolated, free of vascular tissue, and cultured on a medium containing auxin but no cytokinin. Explants from the apical 1 cm of stem, within the pith rib meristem, initiated callus growth with 100% efficiency. Macroscopically visible callus was evident 5 days after the tissue was isolated, and the cultures grew persistently in the absence of cytokinin. Heat treatment, sometimes used to initiate cytokinin habituation, was not required. Explants from tissue basipetal to the pith rib meristem declined in the frequency of habituation with increasing distance from the shoot apex. Although pith tissue which was growing, in vivo, was more prone than mature tissue to establish cytokinin-habituated callus, the basipetal decline in habituation frequency extended well beyond the zone of cell expansion. Explants from mature pith 40 centimeters or more from the shoot apex grew in the absence of cytokinin with 18% frequency, although the response required at least 2 weeks of culture. Further analysis demonstrated that tissue near the periphery of mature pith was more prone to cytokinin-habituation than tissue from the pith center.
The response of the shoot apex to light-generated signals from the leaves Photoperiodic sensitivity of inflorescence initiation and development in sunflower Long-term developmental changes in Xanthium induced by gibberellie acid
  • R F Ly~don
  • F Lyndoi% R
  • D F2a~-Cls
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The nature of flower induction The Induction of Flowering: Some Case Histories
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