Rate of Leaf Appearance and Final Number of Leaves in Wheat: Effects of Duration and Rate of Change of Photoperiod

This study was conducted to test the hypothesis that photoperiodor its rate of change significantly affects the rate of leafappearance (RLA) and final number of leaves (FNL) in wheat,as suggested from several time-of-sowing experiments. Two wheatcultivars (Condor and Thatcher) were sown in the field on 2Sep. 1992 at Melbourne (38°S). Photoperiod was extendedartificially to give five treatments up to terminal spikeletinitiation (TS) viz.: natural photoperiod (rate of change ofphotoperiod = 2 min d^-1), two faster rates of change (8·5and 13·3 min d^-1) and two constant photoperiods of 14·0and 15·5 h. After TS, the two constant photoperiods wereextended to 15·0 and 16·5 h, respectively, andtreatments were re-randomised, i.e. some plots received differentphotoperiod regimes before and after TS. The rate of leaf appearance maintained strong linear relationshipswith thermal time. It was greater for Condor [0·012-0·013(°C d)^-1] than for Thatcher [0·011-0·012 (°Cd)^-1] and did not alter during plant development or in responseto the change in photoperiod at TS. Rate of leaf appearanceon the main culm was not influenced by the rate of change ofphotoperiod nor by the average photoperiod. Cultivar and photoperiod significantly affected FNL on the mainculm. Condor produced more leaves than Thatcher under long butnot under short photoperiods. The rate of change of photoperioddid not affect FNL independently of the effect of average photoperiod.Most of the variation in FNL due to photoperiod resulted fromdifferences in duration of leaf initiation. The lack of effects of the photoperiod treatments on RLA contrastwith previous reports of its effects on the rate of phasic developmentfrom seedling emergence to double ridge. Therefore, the numberof visible leaves on the main culm (NL) at double ridge andat TS were not constant. However, NL on the main culm at doubleridge was closely correlated with FNL.Copyright 1994, 1999 AcademicPress Triticum aestivum L., wheat, leaf appearance, phyllochron, photoperiod     

Development Rate in Wheat as Affected by Duration and Rate of Change of Photoperiod

A field study was conducted to test the hypothesis that wheatdevelopment rate responds to the rate of change of photoperiod.Two wheat cultivars (Condor and Thatcher) were sown on 18 Aug.1992 at Melbourne (38° S). Photoperiod was extended artificiallyto give five treatments up to terminal spikelet initiation (TS)viz.: natural photoperiod (rate of change of photoperiod, 2·3mind d^-1), two faster rates of change (9·8 and 13·1min d^-1) and two constant photoperiods of 14·0 and 15·5h. After TS, the two constant photoperiods were extended to15·0 and 16·5 h, respectively and treatments wererandomly re-allocated, i.e. some plots received different photoperiodregimes before and after TS. There were no significant differences among treatments in thelength of the period from sowing (S) to seedling emergence (E)phase, ranging from 15 to 16·3 d. The rate of developmentfrom E to TS responded to increases in photoperiod in both cultivars,increasing with average photoperiod across all treatments butthere was no effect of rate of change of photoperiod independentof its average photoperiod. The rate of development from TSto anthesis (A) did not show any trend with average photoperiod.This lack of effect of photoperiod on the period from TS toA contrasts with other results from the literature and possiblereasons for this conflicting result are discussed. Rate of changeof photoperiod did not affect the duration of the phase fromTS to A either. Therefore, the effect of photoperiod on theduration of the S-A period was strongly and positively correlatedto that of the length of the E-TS phase.Copyright 1994, 1999Academic Press Triticum aestivum L., wheat, phasic development, photoperiod, rate of change     

The Initiation, Growth, and Emergence of Leaf Primordia in Fragaria

The leaf initiation rate in Fragaria vesca (var. ‘RoyalSovereign’) has been compared with the elongation rateof the leaf primordia at different seasons. Certain conceptionsof growth correlation within the bud are presented. Experimentson the nature of elongation and emergence of primordia are described,and the causes of emergence are discussed.     

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     

Leaf and Tiller Production of Prairie Grass (Bromus willdenowii Kunth) and Two Ryegrass (Lolium) Species

A detailed morphological study of three prairie grass cultivars(Bromus willdenowii Kunth) was conducted under ‘vegetative’and ‘reproductive’ growth conditions (short andlong photoperiods) and at different temperatures. Perennialryegrass (Lolium perenne L.) and Westerwolds ryegrass (Loliummuhiflorum Lam.) were compared during vegetative growth. Prairie grass had higher leaf appearance rates (leaves per tillerper day) and lower site filling (tillers per tiller per leafappearance interval) than the ryegrass species. Tillering rates(tillers per tiller per day) were also lower, except under vegetativeconditions at 4C. Low tiller number in prairie grass was notdue to lack of tiller sites but a result of poor filling ofthese sites. Lower site filling occurred because of increaseddelays in appearance of the youngest axillary tiller and lackof axillary tillers emerging from basal tiller buds. In prairiegrass, no tillers came from coleoptile buds while only occasionallydid prophyll buds develop tillers. Low tiller number in prairiegrass was compensated for by greater tiller weight. Prairiegrass had more live leaves per tiller, greater area per leafand a high leaf area per plant. Considerable variation between cultivars was found in prairiegrass. The cultivar ‘Bellegarde’ had high leaf appearance,large leaves and rapid reproductive development, but had lowlevels of site filling, tillering rate, final tiller numberand herbage quality during reproductive growth. ‘Primabel’tended to have the opposite levels for these parameters, while‘Grasslands Matua’ was intermediate and possiblyprovided the best balance of all plant parameters. prairie grass, Bromus willdenowii Kunth, perennial ryegrass, Lolium perenne L., Westerwolds ryegrass, Lolium multiflorum Lam., temperature, photoperiod, leaf appearance, leaf area, tillering, site filling, tillering sites, yield     

Effect of Photoperiod and Chlormequat on Apical Development and Growth in a Spring Wheat (Triticum aestivum) Cultivar

Spring wheat, cv. Timmo, was grown under three photoperiod regimes(16, 13 and 11 h) with and without treatment with the plantgrowth regulator chlormequat (applied at the glume primordiumstage of apical development) and the relationships between apicaldevelopment, primordium initiation and growth stage examined The effects of photoperiod were generally similar to those reportedfrom other studies; shorter photoperiod slowed the rate of apicaldevelopment, increased the duration of the primordium initiationphases and reduced the rate of primordium initiation. The finalnumber of spikelets was increased, but there was no effect onnumber of floret primordia per spikelet The number of tillersproduced was also higher in the shorter photoperiods. Chlormequattreatment had a similar effect to imposing short-days: floweringwas delayed and tiller production increased There were strong correlations between certain development eventsand the phasing of primordium initiation and growth stages andthese were not affected by photoperiod or chlormequat treatments.For example, the end of spikelet primordium initiation, i.e.terminal spikelet (TS) formation, coincided with the floret-stamenprimordium stage (of the most advanced spikelet) and the endof floret primordium initiation with the stigma tic branchesand hairs on ovary wall elongating stage. Similarly, rapid stemextension growth always started at TS formation while spikeextension and spike growth commenced at TS formation and thestigmatic branches stage, respectively. Tiller production alsoceased at TS formation, when rapid stem growth started Although the timing of the phases of primordium initiation andcertain growth events were linked to apical development, therate of apical development did not determine either the rateof spikelet primordium initiation or the rates of stem and eargrowth. However, there was a strong relationship between rateof development and rate of floret primordium initiation. Therewas also a strong relationship between spike length and apicaldevelopment stage Triticum aestivum, spring wheat, photoperiod, chlormequat, apical development, primordium initiation, stem and spike growth     

Physiology of Flowering in Saccharum: I. DAYLENGTH CONTROL OF FLORAL INITIATION AND DEVELOPMENT IN S. SPONTANEUM L.

Flowering in two clones of Saccharum spontaneum L. is controlledby photoperiod. The earliest stages of development, ‘induction’and ‘initiation of the inflorescence axis primordium’(IAP) were optimally promoted under intermediate days of 12h 30 min, while the subsequent stage ‘initiation of inflorescencebranch primordia’ (IBP) was inhibited by days longer than13 h. The following stage ‘initiation of spikelet primordia’(ISP) showed a quantitatively intermediate response with anoptimum photoperiod of 9 h to 11 h. The elongation of the differentiatedinflorescence was found to be only slightly sensitive to photoperiodsof 13 h or longer in one of the clones. Unfavourable photoperiodsat stages following induction resulted in the arrest or delayof inflorescence development and when these were given duringthe IAP and IBP stages, reversion to the vegetative conditioncommonly occurred.     

The Control of Flowering in Wheat and Barley: What Recent Advances in Molecular Genetics Can Reveal

The combined forces of developmental biologists, studying primordiuminitiation at the stem apex, and mathematical modellers, developingsimulations of crop growth and development, have brought aboutconsiderable advances in the understanding of the control offlowering in wheat and barley. Nevertheless, there are stillmajor gaps in this understanding including: what determinesthe basic rate of development (magnitude of the phyllochronor plastochron); how temperature and photoperiod interact tobring about the transition from vegetative to reproductive development;and how flowering occurs eventually in the absence of inductiveconditions. Although geneticists have tended to measure cerealflowering in terms of ‘days from sowing or emergence toheading’, results of studies using aneuploids and molecularmarkers are compatible with the roles for photoperiod and low-temperaturevernalization established in purely-physiological or developmentalinvestigations. They have also revealed the existence of ‘earlinessperse’loci, whose detailed roles have yet to be established.Progress towards isolating and characterizing wheat and barleyloci is hampered by the poor resolution of mapping (locationto a precision of tens of thousands of base pairs). Neitherof these broad approaches promises a rapid resolution of thefactors controlling the induction of flowering. Two expandingareas of molecular genetics now provide potential for greaterunderstanding of cereal flowering. First, the extensive homoeologyamong members of the Gramineae can be employed to establishthe existence and location of genes or quantitative trait lociin rice which correspond to controlling loci in wheat or barley.Since the rice genome is 1/30th of the size of the wheat genome,the accuracy of mapping loci can be much higher, and there isgreater potential for precise location of loci using techniquessuch as chromosome walking. With the ultimate cloning of individualgenes, and the isolation of gene products, the relative rolesof the 20 loci apparently involved in the induction of floweringof wheat could be explored. However, progress in the moleculargenetics ofArabidopsis(the second area) may provide a more rapidroute to understanding the control of flowering in cereals forseveral reasons: its small genome (1/4 that of rice); the likelihoodof extensive homoeology with cereals, in spite of differencesin codon usage between monocots and dicots; the existence ofa wide range of flowering-time mutants; and the control of floralinduction by a similar range of environmental factors includingphotoperiod and low temperature. It is likely that the MCDK(Martinez-Zapater, Coupland, Dean and Koornneef, 1994. In: MeyerowitzEM, Somerville CR.Arabidopsis.New York: Cold Spring Harbor Laboratory,403–433) model, formulated to explain the genetic andenvironmental control of flowering inArabidopsis,could be employedusefully in the formulation of experimental work on floweringin wheat and barley. This paper reviews these issues, payingparticular attention to the significance of ‘earlinessperse’ loci and the ‘constitutive floral pathway’for wheat and barley.Copyright 1998 Annals of Botany Company Wheat, barley, rice,Arabidopsis,flowering, photoperiod, vernalization, genetics, development.     

Photoperiod Sensitivity during Flower Development of Opium Poppy (Papaver somniferum L.)

Photoperiod is a major factor in flower development of the opiumpoppy (Papaver somniferum L. ‘album DC’) which isa long-day plant. Predicting time to flower in field-grown opiumpoppy requires knowledge of which stages of growth are sensitiveto photoperiod and how the rate of flower development is influencedby photoperiod. The objective of this work was to determinewhen poppy plants first become sensitive to photoperiod andhow long photoperiod continues to influence the time to firstflower under consistent temperature conditions. Plants weregrown in artificially-lit growth chambers with either a 16-hphotoperiod (highly flower inductive) or a 9-h photoperiod (non-inductive).Plants were transferred at 1 to 3-d intervals from a 16- toa 9-h photoperiod andvice versa . All chambers were maintainedat a 12-h thermoperiod of 25/20 °C. Poppy plants becamesensitive to photoperiod 4 d after emergence and required aminimum of four inductive cycles (short dark periods) beforethe plant flowered. Additional inductive cycles, up to a maximumof nine, hastened flowering. After 13 inductive cycles, floweringtime was no longer influenced by photoperiod. These resultsindicate that the interval between emergence and first flowercan be divided into four phases: (1) a photoperiod-insensitivejuvenile phase (JP); (2) a photoperiod-sensitive inductive phase(PSP); (3) a photoperiod-sensitive post-inductive phase (PSPP);and (4) a photoperiod-insensitive post-inductive phase (PIPP).The minimum durations of these phases forPapaver somniferum‘album DC’ under the conditions of our experimentwere determined as 4 d, 4 d, 9 d, and 14 d, respectively. Anthesis; days to flowering; flower bud; opium poppy; Papaver somniferum L.; photoperiod; photoperiod sensitivity; predicting time to flowering; transfer     

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     

Cell Division and Expansion in the Growth of the Leaf

Volumes and numbers of cells were determined at different stagesof development of the fifth leaf of Lupinus albus, and eachof the second pair and the tenth leaf of Helianthus annuus.In the case of the second pair of sunflower leaves the valuescover the whole life of the leaf from initiation to senescence. During both primordial development and the ensuing ‘grandperiod of growth’ division is the determinant of growth.About 10 per cent. of the cells in the fully grown leaf arelaid down before leaf-emergence; the remaining 90 per cent.are formed during unfolding. Division does not cease in thelupin leaf or the second pair of sunflower leaves until theyhave reached half their maximum area. The tenth leaf, on theother hand, is as much as three-quarters fully grown beforedivision ceases. Cell expansion commences soon after leaf initiation and continuesthroughout the life of the leaf. With lupin and the second pairof sunflower leaves there is a fourfold increase in the averagevolume of the cells before emergence from the apical region.During unfolding, there is a further tenfold increase in theaverage volume of the cells of the lupin leaf, and a twentyfoldincrease with the second pair of sunflower leaves. Expansioncontinues after the cessation of division but this further increasein volume is comparatively small. The data are discussed in relation to the ‘two phase’hypothesis of leaf development.     

Growth and Dry-weight Distribution in Callistephus chinensis as Influenced by Lighting Treatment

Plants of two cultivars of Callistephus chinensis (Queen ofthe Market and Johannistag) were grown in 8 h of daylight perday with one of the following treatments given during the 16h dark period: (a) darkness—‘uninterrupted night’,(b) I h of light in the middle of the dark period—a ‘nightbreak’, (c) I min of light in every hour of the dark period—‘cycliclighting’, (d) light throughout—‘continuouslight’. The plants receiving uninterrupted dark periods remained compactand rosetted in habit with small leaves, while leaf expansion,stem extension, and flower initiation were promoted in all threeillumination treatments (b, c, d). Although these three treatmentsproduced similar increases in leaf area, continuous light wasthe most effective for the promotion of both stem growth andflower initiation while cyclic lighting was generally more effectivethan a I-h night break. Continuous light also caused more dry matter to be divertedto stems at any given vegetative dry weight and it was shownthat the stem weight ratio of both varieties was correlatedwith stem length.     

The Influence of Genes for Vernalization Response on Development and Growth in Wheat

Studies were made of the influence of genes for vernalizationresponse on the growth and development of four near-isogeniclines of bread wheat (Triticum aestivum L.). The duration from sowing of flower initiation, terminal spikeletformation and ear emergence all increased with increasing vernalizationresponse. There was a close positive relationship between thedays from sowing to flower initiation and from sowing to earemergence, indicating that the duration of either phase of developmentis a useful measure of relative vernalization when daylengthdoes not limit the rate of development. Total spikelet number per ear and the duration of spikelet initationincreased with increasing vernalization response and there wasa correspondingly higher rate of spikelet initiation in thetwo lines with stronger vernalization response. Most of the differences in growth between the lines were associatedwith diferences in development caused by the vrn genes. Maximumtotal above-ground dry matter and total leaf area per plantincreased with increasing vernalization repsonse but relativegrowth rate and leaf area per plant were not significantly differentbetween the lines. There were no differences in net assimilationrate between the four lines until 40 d from sowing; thereafterit decreased, with the greatest decrease in the line with thestrongest vernalization response. Flower initiation, terminal spikelet formation, spikelet initiation, ear emergence, growth rate     

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     

Genetic control of duration of pre-anthesis phases in wheat (Triticum aestivum L.) and relationships to leaf appearance, tillering, and dry matter accumulation

The duration of pre-anthesis developmental phases is of interest in breeding for improved adaptation and yield potential in temperate cereals. Yet despite numerous studies on the genetic control of anthesis (flowering) time and floral initiation, little is known about the genetic control of other pre-anthesis phases. Furthermore, little is known about the effect that changes in the duration of pre-anthesis phases could have on traits related to leaf appearance and tillering, or dry matter accumulation before terminal spikelet initiation (TS). The genetic control of the leaf and spikelet initiation phase (LS; from sowing to TS), the stem elongation phase (SE; from TS to anthesis), and, within the latter, from TS to flag leaf appearance and from then to anthesis, was studied in two doubled-haploid, mapping bread wheat populations, Cranbrook × Halberd and CD87 × Katepwa, in two field experiments (ACT and NSW, Australia). The lengths of phases were estimated from measurements of both TS and the onset of stem elongation. Dry weight per plant before TS, rate of leaf appearance, tillering rate, maximum number of tillers and number of leaves, and dry weight per plant at TS were also estimated in the Cranbrook × Halberd population. More genomic regions were identified for the length of the different pre-anthesis phases than for total time to anthesis. Although overall genetic correlations between LS and SE were significant and positive, independent genetic variability between LS and SE, and several quantitative trait loci (QTLs) with different effects on both phases were found in the two populations. Several of these QTLs (which did not seem to coincide with reported major genes) could be of interest for breeding purposes since they were only significant for either LS or SE. There was no relationship between LS and the rate of leaf appearance. LS was strongly and positively correlated with dry weight at TS but only slightly negatively correlated with early vigour (dry weight before TS). Despite significant genetic correlations between LS and some tillering traits, shortening LS so as to lengthen SE without modifying total time to anthesis would not necessarily reduce tillering capacity, as QTLs for tillering traits did not coincide with those QTLs significant only for LS or SE. Therefore, the study of different pre-anthesis phases is relevant for a better understanding of genetic factors regulating developmental time and may offer new tools for fine-tuning it in breeding for both adaptability and yield potential.     

Developmental Anatomy of the Inflorescence of Bread Wheat (Triticum aestivum L.) during Normal Initiation and when Affected by 2, 4-D

In wheat, the tip of the shoot apex normally consists of a coreof irregularly arranged cells covered by two uniseriate, selfperpetuating, layers (the dermatogen and the hypodermal layer):no third, inner layer (sub-hypodermal layer) is present. Leafinitiation involves periclinals in the cells of the dermatogenand hypodermal layers, but not the core. Buds involve many periclinalsin the outer cells of the core, a few occasionally in the hypodermallayer but never any in the dermatogen. The appearance of ‘double-ridges’signals inflorescence initiation. Each double-ridge is the equivalentof an axillary bud (the future spikelet bud) and its subtendingleaf primordium. The initiation of the subtending leaf is normal:the initiation of the spikelet bud is characterized by periclinaldivisions in the outer cells of the core, though some may alsooccur in cells of the hypodermal layer immediately outside:no periclinals are observed in the neighbouring dermatogen cells.All the above events concerned with leaf and bud initiationoccur in an easily recognizable, strictly distichous, pattern.In plants affected by 2, 4-dichlorophenoxyacetic acid the cellularpattern where double-ridges would have been arising, is badlydisrupted, due mainly to increased cell divisions in the hypodermallayer and outer part of the core, though possibly includingsome in the dermatogen. The apex tip itself is unaffected, probablyexplaining why, when growth is resumed, it produces a successionof normal spikelets in the normal phyllotaxis. Triticum aestivum L, bread wheat, shoot apex, double-ridge primordia, inflorescence initiation, spikelet buds, 2, 4-dichlorophenoxyacetic acid     

Host-Parasite Relations in Loose Smut of Wheat: III. The Utilization of 14C-Labelled Assimilates

Infection of wheat by Ustilago nuda caused an increased rateof export of radioactivity from the sixth leaf of infected plantsas compared with healthy plants. The ear acted as the most important‘sink’, in both infected and healthy plants. Erythntol,mannitol, and trehalose were present in the infected ears andnodes, but were not present in the healthy plants. Radioactivitywas incorporated into these compounds in the infected ear, andthis was reflected in a lower percentage in the insoluble fractionthan in the healthy ear.     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation     

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.     

Leaf Characteristics and Net Gas Exchange of Diploid and Autotetraploid Citrus

The effect of tetraploidy on leaf characteristics and net gasexchange was studied in diploid (2x ) and autotetraploid (4x) ‘Valencia’ sweet orange (Citrus sinensis (L.)Osb.) and ‘Femminello’ lemon (Citrus limon (L.)Burm. f.) leaves. Comparisons between ploidy levels were madeunder high irradiance (I) in a growth chamber or low total Iin a glasshouse. Tetraploids of both species had thicker leaves,larger mesophyll cell volume and lower light transmittance thandiploids regardless of growth I. Mesophyll surface area perunit leaf area of 2x leaves was 5–15% greater than on4x leaves. Leaf thickness and mesophyll cell volume were greaterin high I leaves than low I leaves. In high I, average leafarea was similar for 2x and 4x leaves, whereas in low I it was30% greater in 4x than in 2x leaves. Nitrogen and chlorophyllconcentration per cell increased with ploidy level in both growthconditions. The ratio of chlorophyll a:b was 25% greater in2x than in 4x leaves. When net CO₂assimilation rate (A_CO2) wasbased on leaf area, 4x orange leaves had 24–35% lowerA_CO2than their diploids. There were no significant differencesin A_CO2between 2x and 4x orange or lemon leaves when expressedon a per cell basis. Overall, lower A_CO2per unit leaf area oftetraploids was related to increase in leaf thickness, largermesophyll cell volume, the decrease in mesophyll area exposedto internal air spaces, and the lower ratio between cell surfaceto cell volume. Such changes probably increased the resistanceto CO₂diffusion to the site of carboyxlation in the chloroplasts. Cell volume; chlorophyll; irradiance; leaf thickness; nitrogen; photosynthesis; ploidy; Citrus limon ; C. sinensis ; ‘Valencia’ sweet orange; ‘Femminello’ lemon