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.     

Inhibition of growth and synchronised cell division in the shoot apex in relation to flowering in Silene

When plants of Silene coeli-rosa (L.) Godron were induced by seven long days, then exposed to darkness for 48 h before being returned to short days, they went on to initiate flowers with a delay of about 2 d. The synchronisation of cell division which normally occurs before flower initiation was suppressed, showing that it is not essential for flowering. Periods of darkness of up to 240 h inhibited apical growth and leaf initiation but did not prevent eventual flowering in short days. The commitment of the apex to flower was therefore maintained while apical growth was inhibited.Abbreviations SD short day(s) - LD long day(s)     

Apical Growth and Modification of the Development of Primordia During Re-flowering of Reverted Plants of Impatiens balsamina L

Impatiens balsamina L. was induced to flower by exposure to5 short days and then made to revert to vegetative growth byreturn to long days. After 9 long days reverted plants wereinduced to re-flower by returning them to short days. Petalinitiation began immediately and seven primordia already presentdeveloped into petals instead of into predominantly leaf-likeorgans. However, the arrangement of primordia at the shoot apex,their rate of initiation and size at initiation remained unchangedfrom the reverted apex, as did apical growth rate and the lengthof stem frusta at initiation. The more rapid flowering of thereverted plants than of plants when first induced, and the lackof change in apical growth pattern, imply that the revertedapices remain partially evoked, and that the apical growth patternand phyllotaxis typical of the flower, and already present inthe reverted plants, facilitate the transition to flower formation. Impatiens balsamina, flower reversion, partial evocation, shoot meristem, determination, leaf development     

The facultative photoperiodic response in the reproductive development of Amaranthus retroflexus L.: Changes in the dose response during ontogeny

The first inductive (short-day; SD) cycle advanced the initiation of reproductive development, while additional SD cycles progressively reduced the lag phase between the start of induction and initiation. The sensitivity to SD increased during ontogeny in long-days (LD) until even the requirement for the first SD cycle disappeared at the onset of autonomous flowering. In photo-induced plants, the postinitiation rate of elongation of the apex was accelerated as the SD dose was increased, but was progressively slower as the start of induction was delayed closer to autonomous flowering, approaching asymptotically the rate of non-induced controls. The inflorescences were branched in plants growing continuously in LD and unbranched in those growing continously in SD. The subsequent branching of the inflorescence could be repressed by SD at any time prior to autonomous flowering, and the degree of repression increased with the induction dose. After the initial SD cycle, 1–2 additional SD could induce the loss of apical dominance, causing excessive elongation and leaf production in the subjacent branches. Further increase in the SD dose inhibited this elongation by accelerating the transformation of the apices of these branches to the reproductive state.Abbreviations LD long day(s) - SD short day(s) - ContSD continuous short day(s) - RGR relative growth rate     

Jasmonates Inhibit Flowering in Short-Day Plant Pharbitis nil

The role of jasmonates in the photoperiodic flower induction of short-day plant Pharbitis nil was investigated. The plants were grown in a special cycle: 72 h of darkness, 24 h of white light with lowered intensity, 24-h long inductive night, 14 days of continuous light. At 4 h of inductive night the cotyledons of non-induced plants contained about two times the amount of endogenous jasmonates (JA/JA-Me) compared to those induced. A 15-min long pulse of far red light (FR) applied at the end of a 24-h long white light phase inhibited flowering of P. nil. The concentration of jasmonates at 2 and 4 h of inductive night in the cotyledons of the plants treated with FR was similar. Red light (R) could reverse the effect of FR. R light applied after FR light decreased the content of jasmonates by about 50%. Methyl jasmonate (JA-Me) applied to cotyledons, shoot apices and cotyledon petioles of P. nil inhibited the formation of flower buds during the first half of a 24-h long inductive or 14-h long subinductive night. Application of JA-Me to the cotyledons was the most effective. None of the plants treated with JA-Me on the cotyledons in the middle of the inductive night formed terminal flower buds. The aspirin, ibuprofen and phenidone, jasmonates biosynthesis inhibitors partially reversed the effect of FR, stimulating the formation of axillary and terminal flower buds. Thus, the results obtained suggests that phytochrome system control both the photoperiodic flower induction and jasmonates metabolism. Jasmonates inhibit flowering in P. nil.     

Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.     

Changes in adenine-nucleotide content in the apical bud of Sinapis alba L. during floral transition

The total adenylate pool of the apical buds of vegetative plants of Sinapis alba L. continuously grown in short days fluctuates over a 24-h cycle with the minimum occurring at the end of the dark period. In the buds of plants induced to flower by a single long-day treatment, total adenylate pool increases above the control level 16 h after the start of the long day, resulting mainly from a rise in ATP and ADP contents. This occurs 6 h after the increase in the soluble carbohydrate content previously shown to occur in the apical buds of plants induced to flower (Bodson 1977, Planta 135, 19–23). A transient rise of the energy charge occurs 22 h after the start of the inductive long day.Abbreviations LD long day - SD short day     

Effects of Indol-3yl acetic Acid on Floral Induction and Apical Differentiation in Chenopodium rubrum L.

Indol-3yl acetic acid (10^–4M) was applied to the plumulesof Chenopodium rubrum. Effects on the anatomical structure andthe growth pattern in the apical meristem, as well as DNA synthesisand nucleolus size were investigated. When auxin is applied before or during photoperiodic inductionit inhibits DNA synthesis and meristematic activity. The axillarymeristem (i.e. a group of cells in the axils of the leaf primordia)is most affected. A similar inhibition of the axillary meristemwas also observed in non-induced control plants grown in continuouslight. Auxin applied simultaneously with photoperiodic inductioncounteracts the reduction of apical dominance in the apex andthus inhibits the onset of floral differentiation. Auxin appliedfollowing induction inhibits the previously-formed buds andmakes possible a more complete development of the apical flower. The dual effect of IAA on flowering, inhibitory and stimulatory,manifests itself as a growth response at different stages ofthe changing shoot apex.     

Changes in Apical Growth and Phyllotaxis on Flowering and Reversion in Impatiens balsamina L.

When plants of Impatiens balsamina L were subjected to 5 shortdays and then re-placed in long days, they began to form a terminalflower and then reverted to vegetative growth at this terminalshoot apex The onset of flowering was accompanied by an increasein the rate of initiation of primordia, an increase in the growthrate of the apex, a change in primordium arrangement from spiralto whorled or pseudo-whorled, a lack of internodes, and a reductionm the size at initiation of the primordia and also of the stemfrusta which give rise to nodal and internodal tissues On reversion,parts intermediate between petals and leaves were formed, followedby leaves, although in reverted apices the size at initiationand the arrangement of primordia remained the same as in thefloweing apex The apical growth rate and the rate of primordiuminitiation were less in the reverted apices than in floral apicesbut remained higher than in the original vegetative apex Sincethe changes in apical growth which occur on the transition toflowering are not reversed on reversion, the development oforgans as leaves or petals is not directly related to the growthrate of the apex, or the arrangement, rate of initiation orsize at initiation of primordia Impatiens balsamina L, flower reversion, evocation, phyllotaxis, shoot meristem     

Changes in the content of endogenous auxins in apical buds ofchenopodium rubrum L. Induced with respect to the endogenous rhythm in capacity to flower

The content of endogenous auxins was examined in apical buds ofChenopodium rubrum plants induced by a photoperiodic cycle of 16h darkness and 8h light followed by a dark period of various duration so as to correspond with either maximal or minimal flowering response in the endogenous rhythm in capacity to flower initiated by the photoperiodic treatment. Apical buds of potentially generative plants contained less auxins than apical buds of plants which remained in the vegetative state. Apical buds from plants treated with kinetin (1. 10^-3 M) and therefore remaining in the vegetative state showed an auxin level comparable to that of untreated plants exhibiting minimal flowering response irrespective of the duration of the second dark period. Plants cultivated on a sucrose solution (0.6 M) during the second dark period became generative even at the normal minimum of flowering. The auxin content of the apical buds was low, similarly as in untreated plants induced for a period leading to maximal flowering response. On the other hand, apical buds from plants grown on sucrose solution during a dark period leading to the manifestation of maximal flowering response showed a relatively high auxin content comparable to that found in untreated plants which had obtained a more extended induction by three photoperiodic cycles. The results are discussed with respect to the possible role of endogenous auxins in the regulation of the changes in growth correlations occurring in the shoot apex during photoperiodic induction and in the expression of the competence to flower.     

Effects of exogenous cytokinins on flowering of the short-day plantChenopodium rubrum L.

Kinetin at a concentration from 3.10^-6 M to 1.10^-3 M was applied to the plumule ofChenopodium rubrum plants during photoperiodic induction. Different levels of induction were compared (one and three short days). The higher concentrations of kinetin applied to induced plants inhibited flower formation. The rate of leaf initiation was increased under these treatments. Lower concentrations of kinetin (from 3.10^-6 M to 1.10^-5 M) usually promoted lateral bud formation and flowering. The step-wise application of kinetin revealed that the inhibitory effect on flowering had been restricted to the inductive period. The effects of kinetin, benzyladenine and trans-zeatin were compared in plants partially induced by two short days. High concentrations always inhibited flowering. Benzyladenine was the most effective in this respect. Root removal diminished the inhibitory effects of cytokinins on flowering as was stated with benzyladenine. It is assumed that endogenous cytokinins play a role in the regulation of organogenetic activity of the stem apical meristem. Depending on the photoperiodic conditions, they presumably exert their activity by maintaining the vegetative functions of the apex.     

Localization of starch in shoot apices of vegetative and photoperiodically induced plants ofChenopodium rubrutn

Starch was determined by means of IKI reaction in shoot apices ofChenopodium rubrum plants induced to flowering by two short days and in non-induced plants. Small starch grains were already observed in the meristematic cells at an age of four days after sowing. Larger grains were found in the subapical region of the apex. Heterogeneity increases during further growth of the plants in induced, as well as in non-induced vegetative plants. Starch disappears from the cells potentially giving rise to axillary buds, while the number and size of starch grains increase in cells from which leaf primordia will be formed. This metabolic specifity of leaf and bud primordia is preserved during morphological differentiation and applies to vegetative, as well as to prefloral apices of photoperiodically induced plants. The amount of starch in the different regions of the apex is linked rather with organogenesis than with the quantitative growth in the apex.     

Effect of Photoperiod and Chlorogenic Acid on Morphogenesis in Leaf Discs of Streptocarpus Nobilis

Leaf discs from vegetative plants greatly increase their phenolic content when cultivated in vitro. Under long days the values remained constant, and were higher when compared with short days cultures. Under short days total phenolics decreased after 10 d, corresponding to the induction and expression of in vitro flowering. The effect of photoperiod and chlorogenic acid (0.01 mM) on leaf discs cultured from induced and non-induced plants, were analyzed regarding the neo-formation of roots, as well as vegetative and flower buds. Chlorogenic acid enhances the regeneration of roots in all treatments tested, with the highest stimulation on induced leaf discs cultivated in short days. The flowering was not affected by chlorogenic acid, but an inhibitory effect was observed on the neo-formation of vegetative buds in non-induced explants maintained in short days. Vegetative buds were reduced by 50% in flower-induced leaf discs cultivated under short days.     

Growth correlations and rna synthesis in different parts of the shoot apical meristem ofChenopodium rubrunt L. induced to flowering

Uridine-³H incorporation and RNA concentration were investigated in different parts of the shoot apical meristem ofChenopodium rubrum using autoradiography and cytophotometry. A single inductive cycle was sufficient to bring about postinductive first events in the shoot apex but not for complete flower differentiation. The initial activation of RNA synthesis manifested itself in all zones of the apex. The first increase was more conspicuous in the peripheral than in the central zone. The indications of the first events in the apices after a single inductive cycle disappear prior to morphological reversal to the vegetative state. Induction by three short days led to rapid flower differentiation. The increase in RNA synthesis and concentration was most conspicuous in the central zone in this case. The ratio of RNA synthesis and content between bud and leaf primordia (B/L) also change in relation to photoperiodic induction. In vegetative plants the B/L ratio was low while after induction it increased. The shifts in activity of RNA synthesis observed in the shoot apical meristem are related to the changes in growth activity of the different parts of the apex. The growth ratios in the apices bear the character of growth correlations. The change in the growth correlations following photoperiodic induction together with the total activation of RNA synthesis are considered to represent one of the first events of the transition to the reproductive state.     

Floral and growth responses in Chenopodium rubrum L. to an extract from flowering Nicotiana tabacum L.

Flowering of Chenopodium rubrum seedling plants was obtained in continuous light after application of fractions of a partially purified extract from leaves of flowering Maryland Mammoth tobacco (Nicotiana tabacum). The stage of flowal differentiation was dependent on the age of the Chenopodium plants used for the bioassay. Apices of plants treated with the extract at the age of four or seven days showed an advanced branching of the meristem or the beginning of formation of a terminal flower; treatment with the extract of plants 12 d old resulted in rapid formation of flower buds in all assay plants. Non-treated control plants kept in continuous light remained fully vegetative. The effects of the extract on flowering were associated with pronounced growth effects. Floral differentiation was preceeded by elongation of the shoot apex. Extension of all axial organs occurred, while growth of leaves, including leaf primordia, was inhibited. The pattern of growth after application of the flower-inducing substance(s) did not resemble the effects of the known phytohormones, but showed some similarities to growth changes resulting from photoperiodic induction of flowering.     

Cell division and morphological changes in the shoot apex of Arabidopsis thaliana during floral transition

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.     

Effects of Abscisic Acid on the Growth Pattern of the Shoot Apical Meristem and on Flowering in Chenopodium rubrum L.

Abscisic acid (ABA) at 1 x 10^–4 M or 3 x 10^–4 Mwas applied to the apical buds of Chenopodium rubrum plantsexposed to different photoperiodic treatments and showing differentpatterns of floral differentiation. Stimulation of growth inwidth of the apical meristem of the shoot and/or inhibitionof growth in length was obtained under all photoperiodic treatments.This change of growth pattern was followed by different effectson flowering. In non-induced plants grown under continuous light ABA stimulatedpericlinal divisions in the peripheral zone and the initiationof leaves as well as the growth in width of bud primordia. Inplants induced by two short days reduced growth of the meristemcoincided with ABA application. Longitudinal growth of the meristemwas inhibited in this case and only a temporary stimulationof inflorescence formation took place. In plants induced ata very early stage, ABA exerted a strong inhibitory effect onflowering. A permanent and reproducible stimulatory effect onflowering was obtained in plants induced by three sub-criticalphotoperiodic cycles if ABA was applied to apices released fromapical dominance. In this case formation of lateral organs andinternodes was promoted by ABA and was followed by stimulatedinflorescence formation. Gibberellic acid (GA2) at 1x 10^–4M or 3 x 10^–4 M brought about a similar effect on floweringas ABA, although the primary growth effect was different, i.e.GA₂ stimulated longitudinal growth. The effects of ABA and GA₂ on floral differentiation have beencompared with earlier results obtained from auxin and kinetinapplications. These growth hormones are believed to regulateflowering by changing cellular growth within the shoot apex.Depending on the actual state of the meristem identical growthresponses may result in different patterns of organogenesisand even in opposite effects on flowering. Shoot apex, flowering, photoperiodic induction, abscisic acid, gibberellic acid, Chenopodium rubrum L.     

Effects of Kinetin on Growth of the Apical Meristem and Floral Differentiation in Chenopodium rubrum L.

Kinetin (1•10^–4 M and 1•10^–3 M) was appliedto the plumules of 6-day-old Chenopodium rubrum plants. Effectson growth, anatomical structure and organogenesis in the apicalmeristem were followed. Floral differentiation as affected bykinetin was also investigated in plants induced to flower byshort-day treatment. Kinetin increased mitotic activity in the apical meristems inboth induced and non-induced plants. The effect was most pronouncedin the peripheral and subcentral zone. An increase in nucleolussize and a higher degree of pyroninophilia in the peripheralzone was also observed, indicating a localized promotion ofRNA synthesis. A higher rate of leaf initiation and a stimulationof leaf and stem growth was subse quentiy recorded. The growthof axillary meristems and of bud primordia was promoted onlyat the lower concentration of kinetin (1•^10–4 M),in both photoperiodically-induced and non-induced plants. However,the pattern of lateral bud growth differed from that found innormal floral differentiation. In kinetintreated plants, thebud primordia are isolated from the summit of the shoot apexby a succession of rapidly growing leaves. The enhancement ofleaf growth leads to correlative inhibition of axillary budpriniordia and results, finally, in a suppression of floraldifferentiation. The inhibitory effect of kinetin on floweringwas compared with that of auxin. Inhibition of flowering occurredin both cases but is achieved in two different ways.     

Gibberellins and the floral transition in Sinapis alba

The putative role of gibberellins in the transition to flowering was investigated in Sinapis alba , a caulescent long-day (LD) plant. It was observed that: (1) physiological doses of exogenous gibberellins (GA₁, GA₃, GA₉) do not cause the floral shift of the meristem when applied to plants grown in short days but have some positive effect on the flowering response to a suboptimal LD; no inhibition was observed in any case; (2) GA-biosynthesis inhibitors (prohexadione-Ca and paclobutrazol) considerably inhibit stem growth but have some negative effect on flowering only when a suboptimal LD is given; and (3) the floral transition induced by one 22-h LD does not correlate with any detectable change in GA content of the apical bud, of the leaves, and of the phloem exudate reaching the apex. Taken together, these results suggest that GAs do not act as a major signal for photoperiodic flower induction in Sinapis .     

Promotive effect of abscisic acid in flowering ofChenopodium rubrum as the result of decreasing apical dominance

Abscisic acid (ABA) was applied in a concentration of 1. 10^?3 M and 1. 10^?4 M to the quantitative SD plantChenopodium rubrum under various light regimes. ABA did not influence flowering in plants under continuous illumination, enhanced flowering in plants subjected to long days and inhibited it in plants induced by short days. It was concluded that ABA can not substitute for inductive treatment but its action may be additive to initial stages of reproductive morphogenesis (enhanced growth rate and branching of the apical meristem) as evoked by long days.