The Nature and Duration of Gene Action for Vernalization Response in Wheat

Four near-isogenic lines of wheat were studied to determinethe nature and duration of gene action for vernalization responseunder 2 weekly vernalization periods from 0 to 10 weeks. With time to floral initiation the Vrn 1 Vrn 2 and Vrn 1 vrn2 genotypes showed a cumulative response whereby days to floralinitiation decreased as the period of vernalization increased.The vrn 1 Vrn 2 and the vrn 1 vrn 2 genotypes also showed acumulative response for periods of vernalization less than 6weeks for the former and 8 weeks for the latter. Days to earemergence was closely related to days to floral initiation dueto the constancy of the period from floral initiation to earemergence across all lines and treatments and, consequently,they gave similar measures of the relative strength of vernalizationresponse. It appears that genes for vernalization response ceaseto act after floral initiation. The implications of these findings to breeding for increasedadaptability and yield in wheat are discussed. Triticum aestivum, wheat isogenic lines, vernalization, floral initiation, ear emergence, gene action     

Expression of Vernalization Requirement and Spikelet Number in Synthetic Hexaploid Wheats and their Triticum tauschii and Tetraploid Wheat Parents

Vernalization requirement, as measured by days from sowing toear emergence (plants grown under an 18-h photoperiod), andspikelet number per ear were recorded for 17 synthetic hexaploidwheats and the six tetraploid (Triticum durum) and the ninediploid T. tauschii parents used to synthesize them. The tetraploid parents and the synthetic hexaploids had springphenotypes (little or no vernalization requirement) whereasthe T. tauschii parents were all winter types (strong vernalizationrequirement). The tetraploid wheats and the synthetic hexaploidsreached ear emergence 50·3 to 63·8 d and 58·2to 75·3 d after sowing, respectively, while the T. tauschiilines reached ear emergence 114·3 to 179·5 d aftersowing. The spring habit of the synthetic hexaploids demonstrates theepistasis of spring over winter habit. It is considered thatwith a presumed single vrn locus in the diploid species T. tauschiithe range of ear emergence in these lines is consistent withthe action of multiple alleles at that locus. Although there was no general epistasis for spikelet number,the tetraploid parents appear to be exerting more influenceover spikelet number in the synthetic hexaploids than T. tauschii.The well established association between the duration from sowingto ear emergence and spikelet number was not evident eitherwithin each ploidy group or when the 32 lines were consideredtogether. Triticum tauschii, Triticum durum, hexaploid wheat, spikelet number, vernalization requirement     

Duration and Rate of Development Phases in Wheat in Two Environments

Six cultivars of bread wheat (Triticum aestivum ssp. vulgareL. amend. Thell.) of diverse climatic origin and different developmentalpatterns were studied under two environments, (a growth roomand an outdoors sowing) for the duration and rate of completionof their developmental phases. The need for vernalization in the cultivar Cappelle Desprezsubstantially increased the length of the vegetative phase,particularly in the growth room. Large differences in the durationof reproductive initiation and stem elongation phases betweenCappelle Desprez and the other cultivars in the growth roomsowing suggests an influence of veralization beyond the vegetativephase. Differences between the two environments influenced the durationof all phases of development, giving pronounced between-cultivarvariation in both the stem elongation and ripening phases. Rates of reproductive initiation and stem elongation for thecultivars, within and between the two environments appearedto be largely independent. The rate of spikelet initiation wassignificantly decreased in the growth room compared with theoutdoor sowing. The duration of the phases of development withineach cultivar appeared to be independent of each other, indicatingthe possibility for adjusting the rate, or duration, of a phaseof development comparatively free of a compensatory change inthe rate, or duration, of other phases. Triticum aestivum ssp, vulgare, wheat, vegetative phase, flower initiation, vernalization, photoperiodism     

Expression of Vernalization Genes in Near-isogenic Wheat Lines: Effects of Night Temperature

Four near-isogenic lines of wheat (Triticum aestivum L.em Thell)were used to compare selected night temperatures for their effectivenessas vernalizing temperatures. All treatments (conducted withina phytotron) had a common day temperature of 20 °C for 12h and night temperatures were 4, 7, 10, 13 and 20 °C. Interpretationof results for reproductive development was confounded by threeinteracting factors, their relative importance varying withgenotype. Firstly, development rate was generally slower atlower night temperatures. Secondly, in contrast, there was atendency for lower night temperatures to hasten developmentrate if vernalization requirements were satisfied. Thirdly,the lower night temperatures provided a more favourable environmentfor leaf production such that for some genotypes, vernalizedplants had higher final leaf numbers than unvernalized plants.Only for the genotype with the strongest vernalization response(vrn1 vrn2) did hastening of development due to vernalizationoverride any delaying effects. For this genotype, 4, 7 and 10°C were vernalizing temperatures. For the other three genotypes,any hastening of development due to vernalization was outweighedby delaying effects of lower night temperatures. Spikelet numberand days to anthesis were positively correlated in three ofthe four genotypes. It appeared that differences in spikeletnumber were a direct result of night temperature influencingthe duration of the spikelet phase and/or rate of spikelet initiation.Plant size at flowering was determined by the differential effectsof night temperature on growth and development rates. Triticum aestivum L., wheat, vernalization, night temperature, isogenic lines     

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     

Effect of Defoliation on Rate of Development and Spikelet Number per Ear in Six Wheat Varieties

The effect of leaf removal either before or at floral initiationon times from sowing to floral initiation and from floral initiationto terminal spikelet initiation, spikelet number per ear, andrate of spikelet initiation investigated in six spring wheatsunder controlled environment conditions. Defoliation delayed the times to floral and terminal spikeletinitiations significently in all varieties. However, defoliationincreased the number of spikelets per ear in the varieties 8–23,8–27 and Selkirk (up to 43 per cent in 8–27 comparedwith the control) but decreased the number in Triple Dirk, Sunsetand Kogat (up to 28 per cent in Triple Dirk). Although the rateof spikelet initiation apparently decreased in all varieties,dissection at regular intervals of Sunset and 8–23 plantsthowed that the rate dropped sharply just after defoliationin both varieties, but later the rate in 8–23 (but notin Sunset) surpassed that of the control plants. The possiblemechanism of control of spikelet number in wheat is discussed.     

Influence of Vernalization and Photoperiod on Supernumerary Spikelet Expression in Wheat

Two tetraploid (Triticum turgidum L.emend gr. turgidum and gr.durum) and five hexaploid wheats (Triticum x aestivum L. emendgr. aestivum) with reported tendencies for ‘branched heads’(supernurnerary spikelets) exhibited variation in its expressionunder different vernalization photoperiod and temperature regimes. Two main types of supernumerary spikelets were identified, multiplesessile spikelets (MSS) with two or more complete spikeletsat a rachis node and indeterminate rachilla spikelets (IRS)with two to 13 spikelets on an extended rachilla. The degree of supernumerary spikelet expression in wheats withvernalization response differed from those without. Short photoperiods(9–14 h) both outdoors and in a glasshouse environment,were more conducive to supernumerary spikelet expression than24 h photoperiod in both environments. The 24 h photoperiodglasshouse environment (higher mean temperatures) was leastconducive to its expression except in lines with a strong vernalizationresponse. The high stability of supernumerary spikelet expression in certaingenotypes in the different environments indicated the feasibilityof incorporating this character in breeding and selecting commercialwheats to increase grain number per head. Triticum, wheat, ear-branching, supernumerary spikelets, vernalization, photoperiod, temperature     

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

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

Responses of Leaf and Tiller Emergence and Primordium Initiation in Wheat and Barley to Interchanged Photoperiod

The influence of constant (9, 13 and 19 h) and reciprocally-interchangedphotoperiods [at terminal spikelet (TS) or triple mound (TM)]on leaf, tiller and primordium development were examined usingphotoperiod-responsive cultivars of spring wheat, ‘UQ189’ and spring barley, ‘Arapiles’. In bothspecies, constant longer photoperiod reduced the duration fromsowing (S) to double ridge (DR), as expected. However, photoperiodsensitivity was not restricted to this mainly vegetative phase.There was also a marked increase or reduction in the durationof reproductive phases between TS/TM and heading (H) when plantswere transferred to shorter or longer photoperiods respectively,compared with controls. These responses were largely independentof the photoperiod during previous phases although minor effectsof the previous photoperiod were observed. For both species,the time course of leaf emergence was linear, or bi-linear,depending on the final leaf number on the main stem. The rateof leaf emergence was faster for the first six to eight leavesthan for the leaves appearing subsequently. The rate of emergenceof early-formed leaves was independent of photoperiod whereasthe rate of emergence of later leaves varied with photoperiod.Photoperiod also affected the dynamics of tillering. The rateof leaf primordium initiation was little affected by variationin photoperiod, but the rate of spikelet initiation increasedwith increases in photoperiod. The rates of leaf and spikeletprimordium initiation were both substantially higher in barleythan in wheat. The fact that the reproductive phase from TS/TMto H was largely independent of the duration of the previousphase provides evidence that this phase might be geneticallymanipulated to increase the time for floret development andhence grain number. Copyright 2000 Annals of Botany Company Photoperiod, rate of development and leaf emergence, tillering, wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.)     

Leaf Growth and Apex Development of Perennial Ryegrass During Winter and Spring

The relationships between rates of leaf extension, leaf appearance,and primordia production on the apex were studied in the fieldduring winter and spring in established swards of four contrastingperennial ryegrass lines (Aurora, Melle, a hybrid selectionfrom a cross between Aurora and Melle and S.24). All four linesshowed an increase in leaf extension rates which commenced whenspikelet primordia were first initiated at the apex. This wassome time after vernalization requirements had been satisfied.In early-flowering lines the stimulus to leaf growth rates occurredearlier than in late-flowering lines. Maximum leaf growth ratesoccurred about the time of double ridge formation, in the middleof the period of spikelet primordia production. The rate andduration of the period of spikelet primordia production variedbetween lines. By the time of flowering, leaf growth rates declinedto values recorded for vegetative plants in the winter. Leafappearance rates followed a similar pattern to leaf growth rates,although the increase in leaf appearance rate was less thanin primordia production or leaf growth rates. Hybrids from across between early- and late-flowering lines showed early enhancementof leaf extension rates due to early initiation of spikeletprimordia production. These high rates of leaf growth were maintainedfor longer, compared with the early-flowering line, as the durationof spikelet production was longer. This illustrates a mechanismfor combining early spring growth with lateness of floweringin ryegrass breeding programmes Leaf growth, apex development, vernalization, initiation, spikelet primordia production, flowering, thermal time, Lolium perenne, perennial ryegrass, spring growth     

Growth of Near-isogenic Wheat Lines Differing in Development--Plants in a Simulated Canopy

An understanding of the principal factors regulating the growthof temperate cereals will identify opportunities to manipulatecrop growth. In an accompanying paper (Gomez-Macpherson, Richardsand Masle,Annals of Botany82: 315–322, 1998), growth aroundthe start of floral initiation was studied in isogenic wheat(Triticum aestivumL.) lines grown as spaced plants. In thispaper, two of the same near-isogenic wheat lines were grownas mini-canopies in a growth chamber. The objective was to determinewhether results obtained using spaced plants also apply to plantsgrown in a simulated canopy as a first step to emulate fieldconditions. Biomass of plant organs, leaf area and leaf andtiller appearance were determined from sowing to ear emergenceof the early developing line. Contrary to results obtained usingspaced plants, lines differed in their above-ground biomassaccumulation, although total plant biomass accumulation wassimilar. After the early line reached terminal spikelet stage(TS), biomass partitioning to the roots and leaves decreased,whereas partitioning to the stem and ear increased. This resultedin a lower root:shoot ratio in the early flowering line thanin the late line which remained vegetative. Tiller senescencealso began after TS in the early line whereas no tiller senescencewas observed in the late line during the experiment. Furthermore,after TS, net assimilation rate was greater and leaf area ratiowas lower in the early line. It is suggested that, after reachingTS, plants grown in a canopy become source limited comparedto widely spaced plants, or compared to plants that have notreached TS, and this results in less root growth.Copyright 1998Annals of Botany Company Development, growth, partitioning,Triticum aestivumL., wheat.     

Effect of photoperiod on development and number of spikelets of a temperate and some low-latitude wheats

Four low-latitude cultivars (two Mexican and two Rhodesian) and a Canadian cultivar were grown in controlled environments in four photo-periods, 16, 13, 10 and 7 h, respectively. Plants of two of the low-latitude cultivars were also transferred between long and short photoperiods during ear differentiation. The intervals from sowing to successive stages of ear development up to the formation of the terminal spikelet, and to ear emergence, and the number of leaves, all increased as photoperiod decreased. The Canadian cultivar responded most and one of the Rhodesian cultivars least to changes of photoperiod in these respects. However, with all the low-latitude cultivars, the interval between formation of the terminal spikelet and emergence of the ear responded similarly and relatively little to decreasing photoperiod except when photoperiod was reduced from 10 to 7 h. The mean rate of spikelet production decreased as the duration of the period of spikelet production (DSP) increased, i.e. as ear development slowed down with decrease in photoperiod. Accordingly number of spikelets per ear increased curvilinearly as DSP increased, suggesting a maximal number of spikelets of about thirty. Rate of spikelet production apparently differed between cultivars. Development of the ear slowed down when plants were moved to a shorter photoperiod and accelerated when they were moved to a longer photoperiod, both at the time at which the shoot apex began to elongate and at the double ridge stage. Final number of spikelets per ear increased when ear development was slowed down and decreased when it was accelerated.     

Pre-flowering Growth and Development of the Inflorescences of Maize: I. PRIMORDIA PRODUCTION AND APICAL DOME VOLUME

The growth and development of the main stem and the two uppermostaxillary apices of maize were studied during the period fromemergence until flowering. Plants were grown in the field undervarying levels of applied nitrogen fertilizer at two times ofsowing. The effects of daylength were isolated from those oftemperature by making comparisons of growth and developmenton a thermal time basis. The growth and development of the terminal (male) apex and thetwo uppermost axillary (female) apices followed the same patterns,with apex volumes increasing curvilinearly with increase innumber of leaf or husk primordia. The RGR(relative growth rateof volume) of the terminal apex was, however, only approximatelyone-tenth of the axillary apices. There was no difference ingrowth and development between the first and second axillaryapices before flowering: other factors, such as accumulationof dry weight, rather than primordia production, must be responsiblefor an axillary apex's potential to bear grain. Applied N, andto a lesser extent short days, increased the rates of growthand development of all the apices. For example, applied N increasedthe RGR (volume) of the apical domes, and the rate of productionof spikelet primordia, by about 25%. All axillary apices and treatments showed a single relationshipbetween number of spikelets and surface area of the ear: a favourableenvironment (e.g. high N) simply accelerated the progressionof spikelet production and area expansion along this singlepath. We conclude that this path is probably determined geneticallyand that N and time of sowing influence potential yield of maizethrough effects on the surface area of the ear but not on thedensity of spikelets formed. Key words: Maize, inflorescence, N application, daylength, temperature, apex volume     

The Effects of Vernalization on the Growth of the Wheat Shoot Apex

he effect of vernalization on the growth of the wheat shootapex was examined by comparing three genetic lines of ChineseSpring (CS) wheat having strong [CS (Hope 5D)], medium (CS Euploid),or no [CS (Hope 5A)] vernalization requirement. The mean volumeof the apical dome increased gradually in all lines, and thenthe apical dome enlarged rapidly as its relative growth rate(RGR) increased prior to double ridge formation. Phytomer volumeat initiation remained constant, so that the ratio of phytomerto apical dome at primordium initiation decreased in successiveplastochrons. In CS Euploid and in unvernalized CS (Hope 5D),the RGR of the apical dome tended to decrease at least untilinitiation of the collar primordium. The rate of primordiuminitiation at double ridge formation increased in proportionto the RGR of the apex at that time; i.e. it increased greatlyin CS (Hope 5A) and vernalized CS (Hope 5D), less so in CS Euploid,but no increase was observed in unvernalized CS (Hope 5D). Thetime of formation of double ridges seemed to be independentof the growth rate or size of the apical dome. The number oftillers present at ear emergence was inversely proportionalto vernalization requirement and was reduced by vernalization.Vernalization resulted in a decrease in the RGR of the newly-initiatedleaf primordia in relation to the RGR of the apical dome andthe axial part of the phytomer. Transfer of plants from longto short days at various times during growth showed that vernalizationincreased the number of labile primordia which could developinto either leaf, collar or spikelet. Vernalization thereforeseems to alter the ability of the apex to respond to subsequentphotoperiod rather than to affect its growth directly. Triticum aeslivum, wheat, chromosome substitution lines, shoot apex growth, vernalization     

Influence of Photoperiod on Culm Elongation and Apical Development in Semi-dwarf and Standard-height Wheats

Flower initiation (FI) coincided with the commencement of culmelongation under both long (18 h) and normal (104–144h) photoperiod in eight spring wheats, including both gibberellicacid-sensitive and -insensitive types, which differed widelyin photoperiod sensitivity At FI the apex was significantly (P = 005) higher (above ground)in three of the wheats under long, compared with normal photoperiod;with no difference between the remaining five. Differences inresponse were not related to photoperiod response or gibberellicacid-sensitivity/insensitivity differences between the wheats. Long photoperiod prolonged the phase from terminal spikeletinitiation (TSI) to anthesis (A) in all the wheats, except Sunset(with the greatest photoperiod insensitivity), with no cleardifferences in response between semi-dwarf and standard-heightwheats. Respective rates of culm elongation from FI to TSI were lowerunder normal, compared with long, photoperiod in all varieties.That from TSI to A was unaffected by photoperiod, except inSunset when it was significantly (P = 001) slower under long,compared with normal photoperiod. Rate of culm elongation from FI to TSI across cultivars andphotoperiods was inversely related to spikelet number per head(r = –053, P = 005) but not to rate of spikelet initiation(r = –014 n.s.). Gibberellin-sensitivity, spikelet number, flower initiation, terminal spikelet initiation     

Nitrogen Deficiency Slows Leaf Development and Delays Flowering in Narrow-leafed Lupin

Effects of nitrogen (N) supply on leaf and flower developmentinLupinus angustifolius L. cv Merrit were examined in a temperature-controlledglasshouse. Low N supply (0.05 or 0.4 mM N) had little effecton leaf initiation but slowed leaf emergence on the main stemcompared with plants receiving high N supply (6.0 or 6.4 mMN), or with symbiotic N₂-fixation. Plants experiencing transientN deficiency had slower leaf emergence than plants with a continuoussupply of 6.4 mM N. Nitrogen supply did not affect the timeof floral initiation, which occurred within 4 weeks of sowing,by which time nine to ten leaves had emerged. However, the floweringof low-N plants was delayed by 68 to 220 °C d (i.e. 4–14d) even though they had fewer leaves. The effect of N deficiencyon flowering time was largely a result of slower leaf emergence. Lupinus angustifolius L.; nitrogen; leaf; flower initiation; thermal time; plastochron; phyllochron     

Environmental Influences on Panicle Differentiation and Spikelet Number of Pennisetum americanum

Pearl millet (Pennisetum americanum (L.) Leeke) has a juvenilephase after which the time to panicle initiation is reducedby short daylengths. To understand more fully the mechanismunderlying temperature ? daylength interactions on panicle initiationand differentiation, plants were grown (a) at a range of constanttemperatures under a short daylength from sowing until afterpanicle differentiation and (b) at one temperature until 20d after emergence and then at a range of temperatures duringa 10 d exposure to short daylength. Temperature prior to panicle initiation determined the numberof leaves initiated on the main stem and the size of the apicaldome at the start of panicle initiation. The number of leaves,in turn, influenced the duration of the phase from panicle initiationto anthesis: this phase required a constant thermal time whenexpressed as day degrees per leaf. At anthesis, panicle lengthwas positively correlated with the number of leaves on the mainstem (and temperature) prior to panicle initiation. Changingthe temperature only during exposure to inductive daylengthsaffected the rate of growth of the apical dome so that panicledifferentiation began within 10 d at high temperature (30?C)whereas differentiation did not commence in 10 dat 21?C. Paniclesdeveloped normally if differentiation had commenced under inductivedaylengths whereas panicles were abnormal when plants were returnedto long daylengths after panicle initiation but before visibledifferentiation. Relative extension rates of the panicle during differentiationwere correlated positively with temperature. The results areconsistent with the hypothesis that panicle initiation dependson the apex attaining a critical size and that temperature,by determining the number of leaves initiated on the main stem,affects the size of the apical dome and thus the onset of panicleinitiation, the duration of paniclc differentiation and thenumber of spikelets differentiated. Key words: Pennisetum americanum, panicle differentiation, spikelet number     

Photoperiod and Vernalization Responses in Triticum turgidumxT. tauschii Synthetic Hexaploid Wheats

Studies of synthetic hexaploid wheat developed from Triticumturgidum(AABB genomes) and T. tauschii(DD genome) can provideinformation on potentially useful characters in T. tauschiiand/or T. turgidum for genetic improvement of hexaploid wheat(T. aestivum). Synthetic hexaploid wheats and the T. turgidumand T. tauschii parents were assessed for their developmentalresponses to photoperiod and vernalization for days to ear emergence,final leaf number and the number of spikelets per spike. Theresponses to photoperiod and vernalization of the synthetichexaploids were generally intermediate between those of theparents but in some instances the levels of expression exhibitedby the T. tauschii or T. turgidum parents were epistatic inthe synthetic hexaploids. The relatively strong photoperiodresponse of the T. tauschii accessions was not expressed inthe synthetic hexaploids, but rather the synthetic hexaploidsreflected the photoperiod response of the respective T. turgidumparents. The synthetic hexaploids had vernalization responsesstronger than those of the T. turgidum and bread wheats usedin the study. The expression of ear emergence in response tovernalization of these synthetic hexaploids appeared to be modifiedby the T. turgidum parent. Copyright 2001 Annals of Botany Company Photoperiod, synthetic hexaploids, Triticum aestivum, Triticum tauschii, Triticum turgidum, vernalization     

Expression of Vernalization Genes in Near-lsogenic Wheat Lines: Duration of Vernalization Period

Vernalization periods ranging from 0 to 11 weeks at 4 °Cwere used to study the reproductive development of four near-isogeniclines of wheat (Triticum aestivum L. em. Thell). From the resultsfor days to anthesis two types of gene action were identified,a threshold (all-or-nothing) response (vrn3 and/or vrn4) anda cumulative (graded) response (vrn1). The action of anothergene (vrn2) intensified these two responses. Based on the actionof genes, a model relating days to anthesis to genotype wasderived. Final leaf number and days to anthesis were shown tobe closely related after adjusting for differences due to theduration of vernalization treatment. No relationship betweendays to anthesis and spikelet number was observed. This studyemphasises the need to understand vernalization at the levelof the gene in terms of responses and interactions. Such knowledgeshould enable the plant breeder to predict and more preciselycontrol reproductive development. Triticum aestivum L., wheat, vernalization, gene action, isogenic lines     

Development in wheat as affected by timing and length of exposure to long photoperiod

Seeds of a spring wheat (Triticum aestivum L. cv. ‘Condor’)were vernalized and then grown at 19C in two naturally–litenvironments, one with a moderate (12 h) and the other withlong (18 h) photoperiod. Treatments consisted of transfers ofplants from the moderate to the long photoperiod chamber ondifferent occasions, or for periods of different durations.The main objectives were to determine whether wheat developmentresponds to current and previous photoperiodic environmentsand whether there is a juvenile phase when the plants are insensitiveto photoperiod. Plants under constant 18 h photoperiod had fewerleaves which appeared faster than those under constant 12 hphotoperiod (i.e. phyllochron was increased from 4.4 to 5.1d leaf^–1). Plants transferred from 12 h to 18 h photoperiodat terminal spikelet appearance (TSA) reached anthesis 4 d earlierthan plants retained at 12 h, while plants under continuouslong photoperiod (18 h) completed this phase most rapidly. Thus,there was some evidence for a historic effect of photoperiodon development. Exposure to long photoperiod during the first 5 d after plantemergence accelerated the rate of development towards anthesis,suggesting that there was no juvenile period of photoperiodicinsensitivity. There were, however, changes during ontogenyin the degree of sensitivity to long photoperiod, increasingfrom seedling emergence to a maximum c. 15 d later, and thendecreasing again. Although all treatments were imposed beforeTSA, the response was not limited to the pre-TSA phase, suggestingthat well before the terminal spikelet appeared, the plant wasalready committed to the initiation of this spikelet. Spikeletnumber decreased with delayed transfer to long photoperiod witha minimum for plants transferred to long days from 16-20 d afterseedling emergence. Additionally, there was a trend for an increasein the rate of leaf appearance (decrease in phyllochron) whenthe plants were exposed to long days between 10 and 35 d afterseedling emergence. Although the differences were small, whenconsidered in conjunction with the effects on final leaf numberthey become important in explaining differences in time to anthesis. Key words: Development, flowering, leaf number, photoperiod, phyllochron, Triticum aestivum