Procrop Wheat Growth And Development

p4 P R OC R OP WH E AT GR OW TH & DE V ELO PM ENTZadoks is a decimal growth scaleproposed for cereal plants by J Zadok,T Chang and C Konzak in 1974. The scaleis based on 10 primary growth stages(0–9). Each primary growth stage isdivided into 10 secondary growth stagesthat indicate the number of plant partson the main stem or secondary stage ofdevelopment, extending the scale from00–99. So each point on the scale has twodigits, the first indicating the growth stageand the second the number of plant partsor secondary stages of development. Forexample, Z15 means growth stage 1 with5 leaves on the main stem. Z24 meansgrowth stage 2 with 4 tillers. Several of thegrowth stages occur together, so a plantmay have more than one decimal codeapplied at the same time. For example,a plant may be producing leaves andtillering at the same time and so couldhave a code of Z15, 22, meaning it has 5leaves on the main stem and 2 tillers.Where appropriate, throughout Wheatgrowth and development a Zadoksreference is provided for the developmentstage under discussion. Zadoks decimalgrowth scale is provided in Table i.The wheat grainThe wheat grain is the reproductive unitof the wheat plant as well as the end-useproduct. A wheat grain can be broadlydivided into three components (Figure iv): seed coat and aleurone layer (orbran) endosperm embryo (germ).In most varieties, the proportion of eachcomponent of the grain is 14% seed coat,83% endosperm and 3% embryo.Seed coat : the outer protective covering ofthe seed.Aleurone: a layer of protein surroundingthe endosperm that secretes enzymesto break down starch reserves in theendosperm.Endosperm: Tissue that surroundsthe embryo and provides energy forgermination. The germinating seed relieson these reserves until it has developed aroot system. It makes up the bulk of thegrain and stores the starch and proteinthat are milled for the production of whiteflour.scutellumseed coatplumulealeuroneendospermupper glumelower glumerachisbrushFigure iv: The wheat grain.Source: wheatbp.net.radicle

I N T R O D U C T I O N p5Embyro: contains the main plantstructures, so it holds all the elements ofthe growing plant. It is made up of thescutellum, plumule (shoot) and radicle(primary root). It is found at the pointwhere the grain is attached to the spikelet. Scutellum: a shield–shapedstructure that absorbs thesoluble sugars from thebreakdown of starch in theendosperm. It also secretessome of the enzymes involvedin germination. Plumule: the growing pointof the seed that develops intothe shoot bearing the first trueleaves. At the growing pointis the coleoptile, three leafprimordia and the shoot apex. Radicle: develops into theprimary root and is the firststructure to emerge aftergermination.Once filled, the wheat grain is 70%carbohydrate, and 97% of thiscarbohydrate is starch (Figure v). Theprotein content is between 8% and 15%,depending on final grain weight; thisequates to between 4 and 10 mg.The wheat plantThe main structures of the plant are thecoleoptile, leaves, tillers, stem, roots andhead.ColeoptileThe coleoptile is a protective sheath thatencases the first leaf. It pushes through thesoil to the surface.LeavesThe leaf consists of a sheath, which wrapsaround the newly emerging leaf, and a leafblade. The leaf sheath contributes to stemstrength. The leaf collar is the point wherethe leaf sheath joins the leaf blade. Theleaf collar has two features, the ligule andthe auricles. The ligule is a thin colourlessmembrane around the base of the collar.The auricles are small hairy projectionsthat extend from the side of the leaf collar.These features can be used to help identifygrass species (Figure vi).Leaves are produced in a set order, onalternate sides of the stem. The final leafto grow before head emergence is the flagleaf.Leaf collar showing the ligule andauricles.Photo: J White.leaf bladebladecrude fibre 2%vitamins & minerals 2%lipids 2%water12%carbohydrate70%bladesheathsplitleaf sheathsheathprotein12%nodeinternodeFigure v: The basic ingredients of the wheat grain.Source: Based on P Stone & M Nicolas, 1996.Figure vi: Structure of the leaf.Source: Based on W Anderson & J Garlinge, 2000.liguleauricles

p6 P R OC R OP WH E AT GR OW TH & DE V ELO PM ENTTillersRootsTillers are lateral branches or shoots thatarise from buds in the axil of the leaves atthe base of the main stem. Primary tillersare produced from the leaves of the mainstem and can form their own, secondarytillers.Wheat has primary and secondary rootsystems (Figure vii). The first roots toappear are the primary roots (also calledseminal or seedling roots). Atgermination, the radicle breaks throughthe seed coat, followed by four or fivelateral roots. These form the primary rootsystem that supports the plant until thesecondary root system develops. Thesecondary roots (also called adventious,nodal or crown roots) initiate from nodeswithin the crown after germination.StemThe stem is made up of nodes andinternodes. Nodes are where structuressuch as leaves, roots, tillers and spikeletsjoin the stem. The internode is the tissuebetween adjacent nodes that elongates asthe stem grows. The stem is wrapped inthe sheaths of the surrounding leaves.This structure of stem and leaves givesstrength to the shoot, helping to keep theplant upright.As the stem grows it changes functionfrom providing support for leaves tostoring carbohydrates and nutrients forgrain filling.Roots of a wheat plantPhoto: L Turton.Ground levelCrownSecondary RootsSub-crown internodePrimary rootsFigure vii: The wheat plant, showing the primary and secondary root systems.Source: Based on W Anderson & J Garlinge, 2000.

I N T R O D U C T I O N p7HeadThe head of the wheat plant has a rachis(stem) made up of nodes and short,flattened internodes. At the nodes are thefloral structures, called spikelets, that holdup to 10 florets containing the flower ofthe wheat plant, where grain is formed.Figure viii shows the structure of thewheat head, spikelet and floret.Each floret is enclosed within twoprotective bracts called the lemma andpalea. These structures wrap around thecarpel. The carpel contains the ovarywith the feathery stigmas, three stamensholding the anthers (pollen sacs), and theovule. Once fertilised, the ovule forms thegrain.Figure ix shows the structure of the floret,and the location of florets on the spikelet.antherpaleaovarystigmastamenlemmalower glumerachisFigure ix: A spikelet showing the structures of the floret.Source: Based on E Kirby & M Appleyard , 1984.Wheat head.Photo: M sFigure viii: Structures of the head, showing the spikelet and floret.Source: Based on W Anderson & J Garlinge, 2000.

p8 P R OC R OP WH E AT GR OW TH & DE V ELO PM ENTReferencesAnderson, WK & Garlinge, JR (eds) 2000,The wheat book: principles and practice,2nd edn, Agriculture Western Australia.Kirby, EJM & Appleyard, M 1984, Cerealdevelopment guide, 2nd edn, Coventry,UK: Arable Unit, National AgricultureCentre.Scott, F 2006, NSW Grains ReportSummary, NSW Department ofPrimary Industries, Orange.Stapper, M 2007, ‘Crop monitoring andZadoks growth stages for wheat’, GrainsResearch and Development Corporation(see GRDC), Research Update.Stone, PJ & Nicolas, ME 1996, ‘Effect oftiming of heat stress during grain–fillingon two wheat varieties differing inheat tolerance. II. Fractional proteinaccumulation’, Australian Journal ofPlant Physiology, vol. 23, pp. 739–749.Zadoks, JC, Chang, TT & Konzak, CF 1974,‘A decimal code for the growth stagesof cereals’, Weeds Research, vol. 14, pp.415–42.www.wheatbp.net, ‘Wheat: the big picture’,the Biotechnology and BiologicalSciences Research Council (BBSRC),the Home-grown Cereals Authority(HGCA) & University of Bristol.Accessed on 5 June 2007.

C H A P T E R 1: G E R M I NAT I O N & EMERGENCE p91. Germination & emergenceGermination& Emergenceby Phillip Bowden & Nathan FergusonChapter SnapshotGermination – 10Phase 1 Water absorption, Phase 2 Activation,Phase 3 Visible germinationFactors affecting germination andemergence – 11Coleoptile formationDormancy, Moisture, Temperature, Oxygen,Seed quality, Coleoptile length, Nutrition, Seedtreatments, Sowing, Plant population,Seed storageEstablishment – 11References and bibliography – 22Emergence – 10In the paddock – 23IntroductionUnder the right conditions, a viable wheat seed germinates. Chapter 1 is aboutthe processes that see the first shoot emerge from the ground and the beginning ofroot growth. The phases covered in this chapter are germination, emergence andestablishment.Learning OutcomesAt the end of this chapter, you will be able to: describe the germination process and the role of moisture, temperatureand oxygen explain plant emergence and establishment and the role of moisture andtemperature understand the factors that influence coleoptile length recognise the qualities to look for when selecting seed conduct a germination and 1000-grain weight test calculate a sowing rate to target a plant population.

p10 PR OC R OP WH EAT GR OW TH & DE V ELO PM ENTGermination–Z 00Radiclepart of the seed embryo thatgrows into the primary root.Germination begins when the seedabsorbs water and ends with theappearance of the radicle. Germinationhas three phases: water absorption (imbibition) activation visible germination(see Figure 1–1).Phase 1 Water absorption (Z01)Phase 1 starts when the seed begins toabsorb moisture. As a general rule, awheat seed needs to reach a moisturecontent of around 35% to 45% of itsdry weight to begin germination. Watervapour can begin the germination processas rapidly as liquid can. Wheat seeds beginto germinate at a relative humidity of97.7%. Soil so dry that roots can’t extractwater still has a relative humidity of 99%,much higher than that of a dry seed.So even in dry conditions, there can beenough moisture for the seed to absorband begin Phase 1, but it takes longer.Phase 3 Visible germination(Z05–Z09)In Phase 3 the embryo starts to visiblygrow. The radicle emerges followed soonafter by other primary roots and thecoleoptile. The enzymes produced inPhase 2 mobilise sugars and amino acidsstored in the seed and enable their transferto the growing embryo.Emergence–Z 07As the first primary roots appear thecoleoptile bursts through the seed coatand begins pushing towards the surface.Emergence is when the coleoptile or thefirst leaf becomes visible above the soilsurface. (See Chapter 2: Vegetative growthfor detail on root growth).Coleoptile formationOnce the embryo has swollen it produceshormones that stimulate enzyme activity.The enzymes break down starch andprotein stored in the seed to sugarsThe coleoptile is well developed in theembryo, forming a thimble-shapedstructure covering the seedling tubeleaf and the shoot. Once the coleoptileemerges from the seed it increases inlength until it breaks through the soilsurface.Phase 2Phase 3 water is taken uprapidly in Phase 1and Phase 3imbibitionPhase 2 continues until the rupture ofthe seed coat, the first visible sign ofgermination.Phase 2 Activation (Z03)Water content of seedPhase 1and amino acids, providing energy tothe growing embryo. If the seed driesout before the embryo starts to grow itremains viable.activationvisiblegerminationTimeFigure 1–1: Pattern of water uptake by wheat seeds in a moist environment.Source: Based on J. Passioura, 2005.The fully elongated coleoptile is a tubularstructure about 50 mm long and 2 mmin diameter. It is white, except for twostrands of tissue that contain chlorophyll.The end of the coleoptile is bullet shapedand is closed except for a small pore,0.25 mm long, a short distance behind thetip.When the coleoptile senses light it stopsgrowing and the first true leaf pushesthrough the pore at the tip. Up to thispoint the plant is living on reserves withinthe seed.

C H A P T E R 1 : G E R M I N AT I O N & E M E R G E N C E p11Establishment–Z 10The plant is established once it has rootsand a shoot. It is no longer relying onreserves in the seed as it is producing itsown energy.Factors affecting germinationand emergenceDormancyIn a wheat seed, germination beginsafter a very short period of dormancy.Australian wheats have a low level ofdormancy that it is easily broken down,allowing germination to begin. Bycontrast, European and North Americanred wheat varieties have a dormancyderived from their seed coat that lasts 3 to7 months. This dormancy is linked to theenzymes called anthocyanins that give theseed coat the red colour.radiclecoleoptileradicleIn Australian white wheats at least twogenes influence the level of dormancy.One gene is in the embryo of the seed andneeds to be present for any level of seeddormancy to develop. This gene makesthe grain sensitive to the plant hormoneabscisic acid, which prevents germinationat the time of crop maturity.The second gene is in the seed coatand, in combination with the embryogene, produces a more robust and stabledormancy. This level of dormancyis essential in varieties targeted forQueensland and northern New SouthWales because of summer rainfall, andis highly desirable in southern Australia.(See also Chapter 4: Grain development,Pre-harvest sprouting).MoistureSoil moisture influences the speed ofgermination. Germination is rapid if thesoil is moist. When the soil dries to nearthe permanent wilting point, the speed ofgermination slows. Instead of 5 days atcoleoptileprimary rootsFigure 1–2: Germinating wheat seeds with the radicle andcoleoptile emerging. Photo: L Turton.7oC when there is adequate moisture,germination will take 10 days at 7oC whensoil reaches the permanent wilting point.The germination process in a seed maystop and start in response to availablemoisture. So seeds that have taken upwater and entered Phase 2, but notreached Phase 3, remain viable if the soildries out. This can happen when drysowing is followed by a small fall of rainthat keeps the soil moist for a few daysPermanentwilting point(or crop lower limit) is thepoint at which roots can’textract water.

p12 PR OC R OP WH EAT GR OW TH & DE V ELO PM ENTbefore drying out. When the next fall ofrain comes, the seed resumes germinating,taking up water and moving quicklythrough Phase 2 so that germination israpid.This ability to start and stop thegermination process (in responseto conditions) before the roots andcoleoptile have emerged is an importantconsideration when dry sowing. If theseed bed dries out before the coleoptilehas emerged, the crop needs to bemonitored to determine if it will emergeso that the critical decision to re-sow canbe made.Soil moisture also affects emergence.Sowing into hard setting or crusting soilsthat dry out after sowing may result inpoor emergence. The hard soil makes itdifficult for the coleoptile to push throughto the surface, particularly in varietieswith short coleoptiles.In some crusting soils, gypsum and/orlime may improve soil structure and assistseedling emergence.Stubble reduces the impact of raindropson the soil surface and helps prevent soilcrusts from forming. Stubble retentionalso encourages biological activity andincreases the amount of organic matter,which improves the stability of the soil bybinding the soil particles together.Table 1–1: Degree-days required for germination andemergence.NO. OF DEGREE-DAYS REQUIREDRoot just visibleColeoptile visibleEmergence (40 mm)Each leaf2735130100Source: J Passioura, 2005.Table 1–2: Examples of how different temperatures affectgermination.TEMPERATURENO. OF DAYS TOGERMINATION3.5 C5 C7 C10 C10753.5Source: J Passioura, 2005.it takes 5 days before visible germination.At 10 C it takes 3.5 days. Other examplesare presented in Table 1–2. (See alsoChapter 2: Vegetative growth, Thermal time).EmergenceTemperatureExtension of the coleoptile is directlyrelated to soil temperature. Soils that aretoo cold or too hot shorten the coleoptilelength. Research shows that coleoptilesare longest when soil temperatures arebetween 10 and 15 C. This is one reasonwhy there is variation in emergence andestablishment in the different wheatgrowing areas.GerminationEstablishmentGermination is dependent ontemperature. The ideal temperature forwheat germination is between 12 and25 C, but germination will occur between4 and 37 C.The speed of germination is driven byaccumulated temperature or degree–days.Degree-days are the sum of the averagedaily maximum and minimumtemperatures over consecutive days.Wheat requires 35 degree-days for visiblegermination to occur (see Table 1–1). Forexample, at an average temperature of 7 CPlant emergence and establishmentare the starting points of crop growth.High temperatures during establishmentcause seedling mortality, reducing thenumber of plants that establish. In hotenvironments, the maximum temperaturein the top few centimetres of soil can be10 to 15 C higher than the maximum airtemperature, especially with a dry, baresoil surface and high radiation intensity.In these conditions, soil temperature canreach 40 to 45 C, seriously affectingseedling emergence. Brief exposure toextreme soil temperatures can also restrictroot growth and tiller initiation.

C H A P T E R 1 : G E R M I N AT I O N & E M E R G E N C E p13Table 1–3 shows the average number ofplants that established with increasing soiltemperatures. The equivalent to 100 kgseed/ha was planted at a depth of 30 to40 mm. The soil temperature wasmeasured in the field at a depth of 50 mm.Table 1–3: Number of wheat plants established at varioussoil temperatures.MEAN MAX SOIL TEMP( C)20.2 C33.2 C42.2 CPLANTS ESTABLISHED(plants/m2)315.3256.789.8Note: the difference between 20.2 C and 33.2 C is statisticallysignificant.Source: E Acevedo, P Silva & H Silva, 2002.OxygenOxygen is essential to the germinationprocess. Seeds absorb oxygen rapidlyduring germination, and without enoughoxygen they die. Germination is slowedwhen the soil oxygen concentration isbelow 20%. During germination, watersoftens the

Wheat growth and development . is to link plant physiology and crop management. It will help agronomists and farmers to understand the life cycle of the wheat plant, and the factors that influence growth and development, and to identify the growth stages of the wheat plant using Zadoks decimal growth scale. This knowledge can then be applied to .