Interaction of vernalization, photoperiod and high temperature in flowering of Arabidopsis thaliana (L.) HEYNH.

Flowering of Arabidopsis thaliana (L.) HEYNH., var. "Stockholm",plants, raised from vernalized seeds, may be modified by thephotoperiodic conditions or a short (1 week) exposure to hightemperature (32°C) following vernalization, depending onthe duration of the cold treatment. When vernalization is partial(1 to 4 weeks at 4°C), both short days (8hr light) and hightemperature have a devernalizing effect, but when the cold requirementhas been fully satisfied, after 5 to 6 weeks at 4°C, devernalizationis no longer possible. There is no interaction between photoperiodand high temperature. Fully vernalized plant flower in bothlong and short days, although flowering is delayed in shortdays. This delay is not a photoperiodic effect, however, butmay be ascribed to the decreased radiant energy available inan 8-hr photoperiod. Thus, fully vernalized Arabidopsis plantsare day-neutral. (Received November 5, 1969; )     

Factors controlling Flowering in the Chrysanthemum: V. DE-VERNALIZATION IN RELATION TO HIGH TEMPERATURE AND LOW LIGHT INTENSITY TREATMENTS

Experiments are described which indicate that the annual vernalizationrequirement of the basal shoots of the Chrysanthemum is dueto annual devernalization of these shoots as the main axis growsup and flowers. Plants sprayed with varying concentrations ofmaleic hydrazide were arrested in their growth for considerableperiods, but this enforced ‘dormancy’ did not affecttheir vernalization status. This makes it appear unlikely thatmere suppression of growth through apical dominance of the mainshoot is the cause of this de vernalization of basal shoots.Fully or partly vernalized plants heated to 40° C. for upto 30 hours did not become dc-vernalized. Heat treatment at35° C. for as long as 30 days also failed to achieve completedc-vernalization, but here flowering was delayed by periodsequivalent to the time spent at high temperature. However, atthe end of the heat treatment progress towards flowering wasresumed at the normal rate. Complete dc-vernalization can bebrought about by prolonged exposure to low intensity illumination.This treatment appears to be effective right up to the stagewhen the first morphological changes leading to inflorescenceformation take place. These results are discussed in relationto similar experiments on the de-vernalization of rye and Hyoscyamusniger.     

Effects of Temperature, Photoperiod and Seed Vernalization on Flowering in Faba Bean Vicia faba

Factorial combinations of three photoperiods (10, 13 and 16h), two day temperatures (18 and 28 °C) and two night temperatures(5 and 13 °C) were imposed on nodulated plants of six diversegenotypes of faba bean (Vicia faba L.). Plants were grown inpots in growth cabinets from both vernalized (1.5±0.5°C for 30 d) and non-vernalized seeds. The times from sowingto the appearance of first open flowers (f) were recorded. Seedvernalization decreased the subsequent time taken to flowerin almost all genotype x growing environment combinations (theexceptions were plants of the cv. Maris Bead grown in threecooler, short-day regimes). The influence of temperature andphotoperiod on the rate of flowering was quantified, using amodel applied previously to other long-day species of grainlegume in which positive linear relations between both temperatureand photoperiod and the rate of progress towards flowering areassumed to apply. A significant positive linear response ofrate of progress towards flowering to limited ranges of meandiurnal temperature was detected in all six genotypes, but inthree genotypes (Syrian Local Large, Aquadulce and Maris Bead)the 28 °C day temperature reduced the rate of progress towardsflowering - suggesting that the optimum temperature for floweringin these genotypes is below 28 °C. In four genotypes (MarisBead, Giza-4, Aquadulce and BPL 1722) a significant positiveresponse to photoperiod, typical of quantitative long-day plants,was observed only in plants grown from vernalized seeds. Incontrast, plants of the genotype Zeidab Local grown from bothnon-vernalized and vernalized seeds showed the same positiveresponse to photoperiod, whereas plants of the land-race SyrianLocal Large were consistently unresponsive to photoperiod. Theimplications of this range of responses amongst diverse genotypesare discussed in relation to screening germplasm. Vicia faba, faba bean, flowering, photoperiod, temperature, seed vernalization, germplasm screening     

Effects of Interactions Between Low and High Temperature Treatments on Flowering of Spring Rape (Brassica napusvar.annua)

Exposure to high temperature (30 °C) before or after exposureto low temperature (0, 4 or 8 weeks at 4 °C) consistentlyincreased the number of leaf nodes at flowering and delayedflowering in a range of genotypes of spring rape(Brassica napusvar.annuaL.).Four days of prior exposure to high temperature had more effectthan 2 d, and the effect of subsequent exposure to high temperaturewas maximized when exposure commenced 1 week after the end ofthe low-temperature treatment. In genotypes that showed a vernalizationresponse (i.e. in which the number of leaf nodes at floweringwas reduced or flowering was advanced by low temperature), thisresponse was reduced or eliminated by either prior high-temperaturetreatment (antivernalization) or subsequent high-temperaturetreatment (devernalization). A biochemical model to accountfor these effects is proposed.Copyright 1998 Annals of BotanyCompany Brassica napusvar.annua, spring rape, antivernalization, devernalization, vernalization     

The Mechanism of Action of Vernalization in Lathyrus odoratus L.

In the sweet pea (Lathyrus odoratus L.) genes Dn^l andDr^h controlthe production of a graft-transmissible substance which delaysflowering and promotes outgrowth of basal laterals. Seed vernalizationpromotes flowering and reduces lateral outgrowth in intact plantsand grafted scions of genotype DnⁱDn^l, suggesting that vernalizationreduces output of the Dnⁱ system, possibly by disrupting therelationship between chronological and plastochronic age. Whenlateral outgrowth and floral abortion are used as indicatorsof inhibitor levels, it can be shown that vernalized Dnⁱ plantspossess more inhibitor but initiate flower buds at a lower nodethan unvernalized dn plants. This supports the suggestion thatin regard to floral initiation vernalization also alters thesensitivity of the shoot apex to the flowering hormone(s). InLathyrus odoratus an hormonally based vernalization responseof considerable magnitude can be shown for day-neutral (dndn)lines, supporting the suggestion that vernalization also influencesthe level of a flower promotor. Lathyrus odoratus L., sweet pea, vernalization, flowering, branching, genotype, grafting     

Temperature Response of Vernalization in Wheat: A Developmental Analysis

The vernalization response of wheat ( Triticum aestivum L.)was reinterpreted from a developmental perspective, using currentconcepts of the developmental regulation of wheat morphologyand phenology. At temperatures above 0 °C, the effects ofthe process of vernalization per se in wheat are confoundedby the effects of concurrent vegetative development. These effectsare manifested by differences in the number of leaves initiatedby the shoot apex prior to floral initiation, which in turnaffects the subsequent rate of development to ear emergenceand anthesis. Leaf primordia development during vernalizationand total leaf number at flowering were used to develop criteriato define both the progress and the point of saturation of thevernalization response. These criteria were then used to reinterpretthe results of Chujo ( Proceedings of the Crop Science Societyof Japan 35 : 177–186, 1966), and derive the temperatureresponse of vernalization per se for plants grown under saturatinglong day conditions. The rate of vernalization increased linearlywith temperature between 1 and 11 °C, such that the timetaken to saturate the vernalization response decreased from70 d at 1 °C to 40 d at 11 °C. The rate declined againat temperatures above 11 °C, and 18 °C was apparentlyineffective for vernalization. Total leaf number at saturation,however, increased consistently with temperature, as a resultof the balance between the concurrent processes of leaf primordiuminitiation and vernalization. Total leaf number at saturationincreased from 6 at 1 °C to 13.3 at 15 °C, which inturn influenced the time taken to reach ear emergence. The advantagesof using this developmental interpretation of vernalizationas the basis for a mechanistic model of the vernalization responsein wheat are discussed. Triticum aestivum L.; wheat; vernalization; rate; temperature; primordia; leaf number; flowering     

Changes in Dormancy and Sensitivity to Vernalization in Axillary Buds of Satsuma Mandarin Examined in vitro During the Annual Cycle

Effect of season and the presence of fruit on bud-endodormancyand the flowering response to low temperature treatments weredetermined using bud cultures of Owari satsuma mandarin (Citrusunshiu Marc). Bud dormancy was deeper in fruiting as comparedto defruited trees. In fruiting trees, the intensity of buddormancy was highest in spring, decreased to a low value byearly Jul. and then increased until early winter. This increasein dormancy during summer and early autumn did not occur innon-fruiting trees. No flowers formed in buds cultured betweenMay and Sep. Both in fruiting and defruited trees, buds becamecompetent to show a vernalization response to chilling by theend of Oct., at the time they also became capable of sproutingin vitro at low temperature (15/10 °C). There was a directeffect of fruit on the buds which persisted long after fruitremoval and resulted in a reduction of the flowering responseto chilling.Copyright 1995, 1999 Academic Press Citrus flowering, Citrus unshiu Marc., dormancy, flower induction, flowering, in vitro flowering, satsuma mandarin, vernalization     

Action du CCC et du B 995 sur la mise à fleur d'une plante bisannuelle,l'Œnothera biennis

Summary Oenothera biennis L. is a typical biennial plant with an absolute cold requirement for subsequent floral initiation under long-day conditions. Flowering of vernalized Oe. biennis is associated with transition from a rosette habit to the formation of a long flower-bearing stem. Vernalization in Oe. biennis consists of two consecutive stages: (1) preparation for flowering; (2) preparation of stem elongation. At the end of the second stage, the level of endogenous gibberellin-like substances is sufficient to allow the stem elongation which is requisite for subsequent floral development.Applications of (2-chloroethyl)trimethylammonium chloride (CCC) via the roots, when made after the cold treatment, have no effect on internodal elongation and subsequent flowering of vernalized Oe. biennis. Relatively small amounts of CCC applied onto the apical bud of the rosette promote stem growth. However, 750 g of CCC act as a growth inhibitor and delay the beginning of stem elongation but this retardant has no effect on the number of flower buds.In contrast, N-dimethylaminosuccinamic acid (B 995), when applied after a cold treatment, delays the beginning of stem growth, this delay increasing with greater amounts of applied B 995. However, with relatively small amounts of B 995, the rate of later stem growth increases rapidly so that treated plants flower at the same time as controls.If gibberellic acid (GA₃) is applied on the apical bud just at the same time as B 995, it reverses the effects of the growth retardant, even when the amount of B 995, applied by itself, is sufficient to inhibit entirely stem elongation and flowering.B 995 is particularly effective if the preceding cold treatment was just sufficient for effective vernalization. If the cold treatment was extended beyond this time, a greater amount of retardant was required to obtain the same degree of stem growth inhibition.B 995 probably acts by interference with the metabolism of endogenous gibberellin-like substances. The delay of floral initiation in vernalized Oe. biennis by B 995 is a consequence of the retarded internode elongation; if this retardation exceeds a certain limit, the plant is devernalized and exhibits a cold requirement identical with the original one.     

Gibberellin-like Substances Obtained from Chilled Plants of Dianthus barbatus L. by an Agar Diffusion Technique

Plants of Dianthus barbatus with a cold requirement for floweringwere subjected to chilling treatments at 5 °C. An agar diffusiontechnique was used to collect gibberellin-like substances fromshoot tips excised from these plants and from plants that hadbeen kept in the glasshouse at a minimum temperature of 14 °C.Shoot tips from chilled plants gave markedly higher yields ofgibberellin-like substances. The effect of chilling was no longerso apparent if plants were returned to the higher temperaturesof the glasshouse for 1 week before the shoot tips were excised. The proportion of plants that flowered as a result of the differentchilling treatments varied widely but this variation was notassociated with any obvious differences in the yields of gibberellin-likesubstances. Application of gibberellins to plants grown at aminimum temperature of 14 °C did not promote flowering.     

Physiological Analysis of Flower Formation in Wedgwood Iris

A series of experiments is described in which buds and scalesof iris bulbs were implanted in nutrient media and subjectedto varying temperature treatments, in an attempt to analysethe effect of different endogenous growth factors and of gibberellicacid on flower formation. In isolated scales incubated at 13°C for one to three weeks much of a heat-stable flower-inducingcompound and a small quantity of a second growth factor is produced.At 25.5° C no such substances are formed. It is shown thatat different times during a prolonged lowtemperature treatmentof iris bulbs, different growth factors are formed which activatesuccessive stages in the differentiation of the flower primordium.At 25.5° C the production and activity of the factor responsiblefor the transition to the reproductive stage are inhibited.Probably not inhibited at this temperature are differentiationprocesses in the reproductive primordium controlled by otherfactors with gibberellin-like characters.     

The Influence of Donor Plant Genotype and Environment on Production of Multicellular Microspores in Cultured Anthers of Brassica napus ssp. oleifera

Studies of the effects of genotype and pre-flowering environmentalconditions on the production of multicellular microspores wereundertaken th four highly inbred lines of Brassica napus sap.oleifera. These lines were first grown in shaded and unshadedenvironments at 20/15°C arid unshaded at 30/25°C ina daylight phytotron. Buds were harvested from half the plantswhen first visible in the rosette and later from the remainingplants at the time when the first flower opened. The frequencyof microspores at a specific stage of development varied widelywithin a relatively narrow range of bud lengths. Uninucleatemicrospores were not detected in anthers from buds less than1·5 m or greater than 3·0mm long, but were generallypresent in frequencies of greater than 50 per cent in anthersfrom buds which were between 2·0 and 2·5 mm inlength. However, the bud length at which the highest frequencyof uninucleate microspores was detected varied significantlybetween genotypes and between the environments in which theywere grown. Examination of the remaining anthers from each budafter a period in culture revealed that the proportion of microsporesdeveloping into multicellular units varied greatly with budlength, an increase in frequency of multicellular microsporesbeing associated with an increase in the frequency of uninucleatemicrospores in the uncultured anther. Genotypes differed, however,in respect of the relationship between uninucleate microsporefrequency and production of multicellular units. Although thefrequency of multicellular units was as high as 57 percent,further development was limited and the number of embryoldsformed was low in all cases (<10 per cent). The frequency of multicellular units in pollen samples frombuds of a length in which uninucleate microspore frequency washigh varied significantly with genotype, temperature and lightconditions under which donor plants were grown, and the stageof inflorescence development at which buds were removed. Underconditions most conducive to multicellular unit formation (20/15°C,unshaded), the maximum frequency of multicellular units foreach genotype in buds from young inflorescences ranged from11·5 to 56·5 per cent. Shading or exposure tothe higher temperature was associated with a marked reductionin production of multicellular units. Higher frequencies ofmulticellular units were generally detected in microspore samplesfrom younger inflorescences irrespective of genotype or environment. Two of the four inbred lines were selected for a second experimentin which responses to vernalization and photoperiod durationwere monitored. There was a significant reduction in the numberof leaf nodes formed prior to floral initiation in both genotypesfollowing exposure to vernalization and/or a longer photoperiod,the response to photoperiod being more pronounced. Exposureto 4 weeks vernalization was accompanied by a significant increasein the frequency of multicellular units in both genotypes, thefrequency being double that in unvernalized plants under thelonger photoperiod. By contrast, genotypes differed sharplyin their response to photoperiod. In TB 20, the frequency ofmulticellular units was unaffected by an increase in day lengthirrespective of whether seed had been vernalized or not. Onthe other hand, in TB 42 the frequency of multicellular unitswas substantially greater in the 24 h day than in the 12 h day,being 27·3 per cent vs 13·0 per cent in the caseof unvernalized plants and 66·7 per cent vs 18·2per cent in the case of vernalized plants. Brassica napus, anther culture, pollen embryogenesis, genotype-environment interaction     

Photothermal Time for Flowering in Faba Bean (Vicia faba) and the Analysis of Potential Vernalization Responses

One cultivar and one land-race of faba bean were subjected to18 potentially vernalizing pre-treatments (constant temperaturesof 1, 5 or 9 °C factorially combined with photoperiods of8 or 16 h d^–1 for 10, 30 or 60 d), and then transferredinto four different growing regimes (‘day’/‘night’temperatures of 18/5 °C or 24/13 °C factorially combinedwith photoperiods of 11 or 16 h d–1). Control plants weregrown entirely in the latter four regimes. The times from sowingto appearance of first open flowers were recorded for all plants.Control plants of the land-race Zeidab Local flowered soonerin long days and in the warmer regime. Pre-treatment reducedthe subsequent time to flower in the four growing-on regimesbut most of the variation in the total time to first flowerfor the pre-treated plants was accounted for by differencesin the combined photothermal time accumulated in the two successiveenvironments - which was predicted by a simple photothermalmodel. Thus, there was neither a specific low-temperature nora short-day vernalization response in this accession. Similarly,no true low-temperature or short-day vernalization responsewas detected in the cv. Maris Bead. However, this UK cultivarflowered later than predicted in the 24/13 °C regime, indicatingthat the 24 °C ‘day’ temperature was supraoptimal.Delays to flowering at 24/13 °C were, however, less evidentwhen plants were grown in long days or following prolonged (30–60d) pre-treatments at cool temperatures. Viciafaba faba, bean, flowering, photoperiodism, vernalization, photothermal time, screening germplasm.     

Pattern of growth regulating substances in the leaves of vernalized sugar-beet during flowering period

The changes in the levels of growth regulating substances using the wheat coleoptile straight growth test were determined in the leaves of vernalized (flowering) and non-vernalized (non-flowering) plants of sugar-beet, cv. Poly-AG-Poland at two stages; the end of vernalization treatment (210 days from planting) and full-flowering stage. IAA was detected only in the extracts of the leaves of non-vernalized plants after210 days from planting. No inhibitory activity was detected, except in the case of the concentrated extract of the leaves of non-vernalized plants. This growth promoting zone was found at R_f 0.5–0.8 in the leaves of flowering plants after cold treatment and at flowering time. This zone of growth promoting action was suggested to have a major role in the flowering of sugar-beet.     

Effects of Irradiance and Temperature on Flowering of Chrysanthemum morifolium Ramat. in Continuous Light

The short-day plant Chrysanthemum morifolium cv. Polaris initiatedflower buds in all irradiances of continuous light from 7.5to 120 W m^–2. As the irradiance increased, the transitionto reproductive development began earlier and the number ofleaves initiated before the flower bud was reduced. The autumn-floweringcultivars Polaris and Bright Golden Anne, and the summer-floweringGolden Stardust were also grown in continuous light at differenttemperatures; all initiated flower buds at temperatures from10 to 28 °C but only the buds of Golden Stardust developedto anthesis and then only at 10 and 16°C. Flower initiationbegan earliest at 16–22 °C, and the number of leavesformed before the flower bud was increased at 28°C. GoldenStardust was exceptional in that the number of leaves formedwas also increased at 10 °C. Axillary meristems adjacentto the terminal meristem initiated flower buds rapidly at 10°C but not at 28 °C in all three cultivars. These resultsare discussed in relation to the autonomous induction of flowerinitiation and the effects of the natural environment on floweringof chrysanthemum. Chrysanthemum morifolium Ramat, flowering, irradiance, temperature     

Environmental Factors Controlling Flower Opening and Closing in aPortulacaHybrid

To examine flower opening and closing of aPortulacahybrid, flowerbuds were placed in darkness for 12 h (2030–0830 h) at20 °C and then exposed to various light-temperature conditions.Flower buds exposed to light at 25, 30 or 35 °C opened within1 h, and wilted 10–14 h later. Flower buds exposed tolight at 20 °C started to open after 4 h but opened slowlyand not completely. Flower buds subjected to 25, 30 or 35 °Cin darkness also opened rapidly, but did not reach full opening.Flowers opened at 30 °C in light, and partially closed andopened repeatedly in response to cycles of a 2-h exposure to20 °C and a 2-h exposure to 30 °C at any time between1000 to 1600 h. Similar phenomena were observed when the flowersopened at 30 °C in light and then were subjected to darknessand light alternately at 30 °C, although the effect of lightwas less obvious than that of alternating temperature. Floweropening and closing were not affected by relative humidity.These results indicate that a rise in temperature is requiredfor rapid flower opening in the buds kept at 20 °C, andthat light intensifies the effect of high temperature. Exposureto light without a temperature change delayed and slowed floweropening which was never complete. The involvement of an endogenousrhythm in flower opening byPortulacais indicated. Portulacahybrid, flower opening, flower closing, temperature shift, endogenous rhythm.     

Endogenous gibberellins and inhibitors in relation to flower induction and inflorescence development in the olive

Comparative analyses of reproductive and vegetative tissues of the olive (Olea europaea L. cv. Manzanillo) for endogenous hormones, particularly inhibitors and gibberellin like substances, were made to study the relation between such hormones and thermoinduction of flowering. Qualitative and quantitative changes in gibberellin-like subtance(s) were observed in lateral buds (potential flower buds) but not in leaves or terminal buds (potential vegetative buds) sampled from orchard trees at intervals during the winter and spring. At least two types of gibberellin-like substances were found in extracts of lateral buds; their levels increased progressively during the low temperature induction period, reaching a maximum shortly before floral initiation. Two types of inhibitors were extracted from buds and leaves. A nonacidic type did not change during the induction stage but decreased considerably during the initiation period. An acidic inhibitor, which was identified as an abscisic acid-like substance, was present at a relatively lower level in lateral (flower) buds than in terminal (vegetative) buds during the induction period.     

Photothermal Time for Flowering in Lentils (Lens culinaris) and the Analysis of Potential Vernalization Responses

Two cultivars of lentils, Laird and Precoz, were subjected to18 potentially vernalizing treatments, comprising constant temperaturesof 1, 5 or 9 °C in factorial combination with photoperiodsof 8 or 16 h for 10, 30 or 60 d. These seeds or seedlings, togetherwith non-vernalized seeds (as controls), were then transferredto four different growing regimes (‘day’/‘night’temperatures of 18/5 °C or 24/13 °C, factorially combinedwith photoperiods of 11 or 16 h). Variation in the number ofdays from sowing to first flower (f) in the growing regimesfor the controls conformed to the equation I/f = a+b+cP, whereis mean temperature (°C), P is photoperiod (h) and a, band c are genotype-specific constants. Accordingly, when theenvironment varies during development, the photothermal timerequired to flower in day-degrees (°C d) is given by 1/babove a base temperature defined as —(a+cP)/b. Most variationin time to flower could be accounted for by the photothermaltime accumulated in the two successive environments. Therefore,there was no evidence of a specific low-temperature vernalizationresponse in either cultivar. Neither was there evidence of ‘short-day’vernalization, i.e. advancement of flowering resulting frompreliminary short-day treatments. A potential error inherentin the predictive model described arises because it ignoresthe presence of a pre-inductive, photoperiod-insensitive phase;but agro-ecological considerations suggest that this error maynot be important in practice. Lens culinaris, lentil, flowering, photoperiodism, vernalization, photothermal time, screening germplasm     

Flowering Responses of Contrasting Ecotypes of Poa annua and their Putative Ancestors Poa infirma andPoa supina

Flowering responses of two Australian and six Norwegian populationsof Poa annua and their putative ancestors P. infirma and P.supina were studied in controlled environments. The two Australianpopulations originating from suburban parks in Canberra hadopposite daylength flowering responses across the range of temperaturestested (9–21 °C), one being a quantitative short-day(SD) plant with no response to vernalization, the other a quantitativelong-day (LD) plant with a quantitative vernalization requirement(winter annual type). Variation in earliness of flowering withinthe former population was shown to be genetically determined,and testing of selfed progenies indicated that the populationis an aggregate of several largely homozygous lines with divergentflowering responses. Two lowland populations from southern Norwaywere both quantitative LD plants with no vernalization response,while two alpine snowbed populations from southern Norway andtwo high-latitude, subarctic populations from northern Norwaywere quantitative SD plants with an obligatory plant vernalizationor SD requirement for flowering. Two populations of P. supinaexhibited the same flowering responses as the alpine and high-latitudepopulations of P. annua with an obligatory plant vernalizationor SD requirement for flowering. A combination of SD and lowtemperature (9–12 °C) for 8–10 weeks was optimalfor induction and inflorescence initiation. On the other hand,P. infirma was found to be an early-flowering quantitative SDplant which flowered freely across the range of temperatures(9–21 °C) as a typical summer annual. The experimentsdemonstrate that virtually any kind of photoperiodic and vernalizationresponses can be found among populations of P. annua. Theseversatile flowering responses reflect the contrasting floweringresponses of P. supina and P. infirma, and add strong supportto the hypothesis that P. annua has originated from these species.Copyright 2001 Annals of Botany Company Adaptation, evolution, flowering, Poa annua, P. infirma, P. supina, photoperiod, vernalization     

The Role of Leaves in the Perception of Vernalizing Temperatures in Sugar Beet

Annual and biennial sugar beet varieties require long days toinduce flowering but the biennial genotypes additionally requirevernalization. Previous research has suggested that the inabilityof non-vernalized biennial plants to flower can be explainedby a lack of competence of the leaves to respond to long days.In this study defoliation experiments were used to investigatewhich leaves could perceive long daylengths and, in particular,whether leaves initiated from a non-vernalized shoot apicalmeristem could perceive vernalizing temperatures and producea floral stimulus in long days. Annual and vernalized biennialplants flowered if young leaves (i.e. those formed during orafter vernalization) were kept on the plants, but they did notflower if only older expanded leaves (including those expandedprior to vernalization) were present. No evidence was obtainedto indicate that the older leaves contained inhibitors of floweringand it seems most likely that there is a decline in responsivenessto daylength with increasing leaf age. Exposure to vernalizingtemperatures accelerated flowering of the annual and was essentialfor flowering of the biennial. The presence of a single leafinitiated, but not expanded, prior to the transfer of biennialplants to vernalizing temperatures was sufficient to induceflowering. This indicates that expanding leaves do not needto be initiated from a vernalized apical meristem to becomecompetent to produce a floral stimulus in long days. Key words: Beta vulgaris L., sugar beet, vernalization, flowering     

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

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