initiated primordia,instead of becoming spikelet primordia,develop into glumeand floretprimordia (Figure 2.5g). The terminal spikelet stage is regarded as a key stage in wheatphenology.Attheterminal spikeletstage,theshootapex isafullyformedembryoearandfurtherdevelopmentisdescribedinsection"Earandearformation"DomeSite of spikelet ridgeAxillarysplkelot ridgeLower leaf ridgeLeafLower loaf ridgeprimordiaLeafprimordiumCbFloret meristemLemmaGlumeSpikeletmeristemGlumeprimordium-Lemma, Floret 1] ofterminalLowerglumeJspikeletSpikelet meristemSpikeletmeristemFloretLemma, Floret 3LemmaStamenFloret1GlumeLemma9FIGURE2.5Successivestagesofshootapexdevelopmentfromavegetativeapex(a)toterminalspikelet stage (g)9
9 a f d e g initiated primordia, instead of becoming spikelet primordia, develop into glume and floret primordia (Figure 2.5g). The terminal spikelet stage is regarded as a key stage in wheat phenology. At the terminal spikelet stage, the shoot apex is a fully formed embryo ear and further development is described in section "Ear and ear formation". FIGURE 2.5 Successive stages of shoot apex development from a vegetative apex (a) to terminal spikelet stage (g) b c
Studies of the anatomyof the shoot have revealedthat the leaf and the florets originate in different tissues(Williams,1975).Leafprimordiaarisefromthesuperficiallayer of cells (dermatogen, tunica),while the spikeletprimordia are initiated in the deeper layers of the apex,thecorpus or core (Figure 2.6)LEAVESTheleafFIGURE2.6Longitudinalsectionof shoot apex at the double ridgestageThe leaf is divided attheliguleinto a cylindricalsheathandtheflatbladeorlamina.Thesheath istubularatthebase,butnearertotheblade it is split and the margins overlap. The lamina has a fairly well-marked midrib, alongwhichrunsthemajorvascularbundleof theleaf.It dividestheblade intotwo subequalparts,eachof whichhasa numberofparallellateral ribsorveins.Eachveinmarksthepositionofavascularbundle,andthetissue overthebundle israisedproducingaridge sothattheadaxialsurfaceofthebladeiscorrugated.TheabaxialsurfaceismoreorlessflatThe midrib extends down into the sheath for a short distance as a pronounced ridge.Theleaf blade naturally assumes a twist,and just belowthetip,usually about two-thirds alongthe leaf, there is frequently a constriction. This constriction is produced by the constraintupon growth produced by the closely investing ligule of the subtending leaf duringdevelopment.Theliguleisathincolourlessflapoftissueabout1to2mminlength,whichencirclestheleafortheculmaboveitbeyondwherethebladediverges.Associatedwiththeligule are the auricles, two small earlike projections fringed with unicellular hairs.At the base of the leaf sheathof the culm leaves,there is athicker zone calledvariously the joint knot or pulvinus. It is considerably thicker and generally lighter greenthanthesheathaboveorthestembelow.Thenodeofthestemisbelowthejointanditspositionismarkedbyaslightconstrictionofthestem.Thejointhasan importantfunctionlifting the ear of a lodged stem off the ground and restoring it to a more or less verticalposition.Leafshapeandsizechangewithleafposition.Thelowermostleafonthemainshoothasparallelsidestowithin1cmorsoofthetipsothatthetipitselfischaracteristicallybluntTheleavesabovethefirst havemoreorlessparallel sidesforabout two-thirds their lengthabovewhichtheytapertoasharppoint.Thelastleafproducedupontheculm,theflagleaf,tapersfromaboutthelowerthird,givingtheleafanelongatedovateshapeIn spring wheat, lamina length increases with increasing leaf number from the base,reachingamaximumoneortwoleavesbeforetheflagleafafterwhichthelengthdeclinessotheflagleafissomewhatshorterthanthelongestleaf.Laminawidthincreaseswithleafpositionso thattheflagleaf is thewidestleaf.Sheathlengthalsoincreaseswithleafposition, markedly so for the culm leaves. Winter wheat shows comparable size changes,butassociatedwiththelongvegetativeperiod,thefirst-producedleaves show litlechange.Heteroblastic development isalso seen in someanatomicalfeatures ofthe leaf.10
10 FIGURE 2.6 Longitudinal section of shoot apex at the double ridge stage Studies of the anatomy of the shoot have revealed that the leaf and the florets originate in different tissues (Williams, 1975). Leaf primordia arise from the superficial layer of cells (dermatogen, tunica), while the spikelet primordia are initiated in the deeper layers of the apex, the corpus or core (Figure 2.6). LEAVES The leaf The leaf is divided at the ligule into a cylindrical sheath and the flat blade or lamina. The sheath is tubular at the base, but nearer to the blade it is split and the margins overlap. The lamina has a fairly well-marked midrib, along which runs the major vascular bundle of the leaf. It divides the blade into two subequal parts, each of which has a number of parallel lateral ribs or veins. Each vein marks the position of a vascular bundle, and the tissue over the bundle is raised producing a ridge so that the adaxial surface of the blade is corrugated. The abaxial surface is more or less flat. The midrib extends down into the sheath for a short distance as a pronounced ridge. The leaf blade naturally assumes a twist, and just below the tip, usually about two-thirds along the leaf, there is frequently a constriction. This constriction is produced by the constraint upon growth produced by the closely investing ligule of the subtending leaf during development. The ligule is a thin colourless flap of tissue about 1 to 2 mm in length, which encircles the leaf or the culm above it beyond where the blade diverges. Associated with the ligule are the auricles, two small earlike projections fringed with unicellular hairs. At the base of the leaf sheath of the culm leaves, there is a thicker zone called variously the joint knot or pulvinus. It is considerably thicker and generally lighter green than the sheath above or the stem below. The node of the stem is below the joint and its position is marked by a slight constriction of the stem. The joint has an important function, lifting the ear of a lodged stem off the ground and restoring it to a more or less vertical position. Leaf shape and size change with leaf position. The lowermost leaf on the main shoot has parallel sides to within 1 cm or so of the tip so that the tip itself is characteristically blunt. The leaves above the first have more or less parallel sides for about two-thirds their length above which they taper to a sharp point. The last leaf produced upon the culm, the flag leaf, tapers from about the lower third, giving the leaf an elongated ovate shape. In spring wheat, lamina length increases with increasing leaf number from the base, reaching a maximum one or two leaves before the flag leaf after which the length declines so the flag leaf is somewhat shorter than the longest leaf. Lamina width increases with leaf position so that the flag leaf is the widest leaf. Sheath length also increases with leaf position, markedly so for the culm leaves. Winter wheat shows comparable size changes, but associated with the long vegetative period, the first-produced leaves show little change. Heteroblastic development is also seen in some anatomical features of the leaf
LeafanatomyThere are three main features of the anatomy of the leaf.The ad- and abaxialepidermis of the mature leaf enclose the mesophyll, which is traversed at intervals by thevasculartissue(Esau,1953).Thevasculartissueandmesophyllareorganizedinalternatestripsoftissuerunningparallelwitheachotheralongthelongaxisoftheleaf.Thevasculartissue lies beneath the ridges of the lamina and the associated thickening capping thevascular bundle of the midrib,and themajor veins extend from the ad-to the abaxialepidermis.EpidermisTheadaxialepidermisisacomplextissuewithseveralcelltypes(Esau,1953).Thebulliform(bubble-shaped)cellsarethelargestcellslyingbetweentheveinsatthebottomofthefurrows.When seen in optical section,they are roughly coffin-shaped with the longaxisof the cell running parallel to the long axis of the leaf. Flanking the bulliform cells are longcylindrical cells with a smaller diameter than the buliform cells, alternating in a regularmanner with stomata.There is usually a single row of stomata between each rank ofbulliformcellsandthevasculartissue.Eachstomaismadeupoftwocharacteristicshapedguardcellsandhastwoassociatedaccessorycells.Thestomalengthvariesfrom42to51μm.Thefrequencyofstomatavariesfromabout63/mm2to109/mmTherearemoreontheadaxial surface and aremore denselydistributed towards the tip.On the other flank of therow of stomata, over the veins there are long cylindrical cells characterized by thickenedwavy walls. The long cells are interspersed in a regular manner by short cells of two types,corkcellsandsilicacells.Short,unicellularhairsoccurmainlyovertheveinsandoneitherside of the row ofstomata.The abaxial epidermis has fewer celltypes, mainly the long cylindrical cells with wavywalls interspersed by short cells. Stomata occur in the same position relative to the veinsasintheadaxialepidermisand,althoughhairsoccur,theyarelessfrequentthanontheadaxial epidermis.The epidermis on both surfaces of the leaf has a cuticle with strongly developedepicuticularwax.Theformofthewaxdependsuponthesurfaceoftheleafandthepositionof the leaf on the stem. This occurs as lobed plates, simple plates, flat ribbons and tubes,the amount and form of the wax depending on the position and surface of the leaf.MesophyllThe mesophyll cells are of a complex lobed shape, resembling armed palisade cells(Esau,1953).Whenviewedintransversesection,thesubepidermalcellsofthemesophyllare elongated similar to palisade cells. The cells in the middle layers of the leaf are not soelongated.Viewedinlongitudinalsection,thelobednatureofthesecellsisapparent.Intheleavesatthebaseof theplant,thedegreeof lobingislowandthedimensionsofthelobesare large. With ascending leaf position up the stem, the degree of lobing increases and thediameterofthelobesdecreases.Theeffectofthesechangesistoincreasethecell surfaceareaperunitareaofleafwithascendingleafpositionupthestem.11
11 Leaf anatomy There are three main features of the anatomy of the leaf. The ad- and abaxial epidermis of the mature leaf enclose the mesophyll, which is traversed at intervals by the vascular tissue (Esau, 1953). The vascular tissue and mesophyll are organized in alternate strips of tissue running parallel with each other along the long axis of the leaf. The vascular tissue lies beneath the ridges of the lamina and the associated thickening capping the vascular bundle of the midrib, and the major veins extend from the ad- to the abaxial epidermis. Epidermis The adaxial epidermis is a complex tissue with several cell types (Esau, 1953). The bulliform (bubble-shaped) cells are the largest cells lying between the veins at the bottom of the furrows. When seen in optical section, they are roughly coffin-shaped with the long axis of the cell running parallel to the long axis of the leaf. Flanking the bulliform cells are long cylindrical cells with a smaller diameter than the bulliform cells, alternating in a regular manner with stomata. There is usually a single row of stomata between each rank of bulliform cells and the vascular tissue. Each stoma is made up of two characteristic shaped guard cells and has two associated accessory cells. The stoma length varies from 42 to 51 µm. The frequency of stomata varies from about 63/mm2 to 109/mmThere are more on the adaxial surface and are more densely distributed towards the tip. On the other flank of the row of stomata, over the veins there are long cylindrical cells characterized by thickened wavy walls. The long cells are interspersed in a regular manner by short cells of two types, cork cells and silica cells. Short, unicellular hairs occur mainly over the veins and on either side of the row of stomata. The abaxial epidermis has fewer cell types, mainly the long cylindrical cells with wavy walls interspersed by short cells. Stomata occur in the same position relative to the veins as in the adaxial epidermis and, although hairs occur, they are less frequent than on the adaxial epidermis. The epidermis on both surfaces of the leaf has a cuticle with strongly developed epicuticular wax. The form of the wax depends upon the surface of the leaf and the position of the leaf on the stem. This occurs as lobed plates, simple plates, flat ribbons and tubes, the amount and form of the wax depending on the position and surface of the leaf. Mesophyll The mesophyll cells are of a complex lobed shape, resembling armed palisade cells (Esau, 1953). When viewed in transverse section, the subepidermal cells of the mesophyll are elongated similar to palisade cells. The cells in the middle layers of the leaf are not so elongated. Viewed in longitudinal section, the lobed nature of these cells is apparent. In the leaves at the base of the plant, the degree of lobing is low and the dimensions of the lobes are large. With ascending leaf position up the stem, the degree of lobing increases and the diameter of the lobes decreases. The effect of these changes is to increase the cell surface area per unit area of leaf with ascending leaf position up the stem
Thereisvariationinthecompactnessandarrangementofthemesophyll cells.Somecultivars have a relatively loose arrangement of cells, while in others the cell arrangementis more compact and files of cells radiate in a regularmannerfrom thevascular bundlesProminent sub-stomatal cavitiesoccur,particularly beneath the stomata of the abaxialsurfaceof the leaf.VasculartissueThe vascular bundle has the structure typical of a C3 plant. The phloem is abaxial tothe xylem and inthelarger bundles consists of regularlyarranged sieve tubesandcompanioncells.Thexylemhastwolarge,prominentxylemvesselsbetweenwhicharesmallermetaxylemvessels andfibres.Adaxialtothemetaxylem,there isanarea ofdisrupted protoxylem.The conducting elements are surrounded byan inner (mestome)sheath and an outer (parenchyma)sheath,though these are not as clearly defined as insomeotherC3plants.Thecellsofthemestomesheatharesmall andthick-walledandarewithout chloroplasts. Those of the outer bundle sheath are large and thin-walled andcontain chloroplasts.Inlongitudinal section,the cells of the bundle sheathsareelongatedwithblunt ends.Thewallsofthemestomesheatharelignified,andsometimesthewalladjacenttotheconductingelementsisthickerthantheotherwallsofthecell.Thecomplexfinestructureofthemestomesheathis important inregulatingthetransportofwaterandsolutes(O'BrienandZee,1971)Thesmallveins that interconnect themain longitudinal veinsconsist only of a singlesievetubeandxylemvesselandtwofilesof parenchymacells.Theypassthroughthemestome and parenchyma sheaths and connect directlywith the metaxylem andmetaphloemofthemainbundles.Theydonothavebundlesheaths,andthevesselwallshave a complex fine structure depending on the adjacent mesophyll walls.VascularsystemThemajorbundlesrunparallelwitheachotherthewholelengthoftheleaf.Thesmalltransverseveins,whichconstituteabout7percentofthetotal length,occurevery2.5to3mm(Figure2.7).Towardsthetipoftheleaf,thesmallerlongitudinal bundlesterminateinaY-shape, the forks of the Y comprising small transverse veins that link to the longitudinalveins at either side.At the pointed tip of the leaf,the veins converge and connect with eachother.Thedistancebetweenthelongitudinalveinsvariesfromabout0.3mminthefirstleaftoabout0.15mminaculmleaf(Black-man,1971)LeafsheathbaseIn the mature, erect stem, there is a ring of vascular bundles in the joint nearer to theinner wall of the cylindrical leaf sheath.Unlikethe sheath above the joint where eachbundlehas prominentsclerenchymagirder,thereisno sclerenchyma and lignificationisminimal.Associatedwitheachvascularbundleandperipheraltoit,thereisamassivezoneof collenchyma.Theparenchyma cellsofthegroundtissueofthisregionarearranged inregularfilesandhaveveryshortverticalaxes.12
12 There is variation in the compactness and arrangement of the mesophyll cells. Some cultivars have a relatively loose arrangement of cells, while in others the cell arrangement is more compact and files of cells radiate in a regular manner from the vascular bundles. Prominent sub-stomatal cavities occur, particularly beneath the stomata of the abaxial surface of the leaf. Vascular tissue The vascular bundle has the structure typical of a C3 plant. The phloem is abaxial to the xylem and in the larger bundles consists of regularly arranged sieve tubes and companion cells. The xylem has two large, prominent xylem vessels between which are smaller metaxylem vessels and fibres. Adaxial to the metaxylem, there is an area of disrupted protoxylem. The conducting elements are surrounded by an inner (mestome) sheath and an outer (parenchyma) sheath, though these are not as clearly defined as in some other C3 plants. The cells of the mestome sheath are small and thick-walled and are without chloroplasts. Those of the outer bundle sheath are large and thin-walled and contain chloroplasts. In longitudinal section, the cells of the bundle sheaths are elongated with blunt ends. The walls of the mestome sheath are lignified, and sometimes the wall adjacent to the conducting elements is thicker than the other walls of the cell. The complex fine structure of the mestome sheath is important in regulating the transport of water and solutes (O’Brien and Zee, 1971). The small veins that interconnect the main longitudinal veins consist only of a single sieve tube and xylem vessel and two files of parenchyma cells. They pass through the mestome and parenchyma sheaths and connect directly with the metaxylem and metaphloem of the main bundles. They do not have bundle sheaths, and the vessel walls have a complex fine structure depending on the adjacent mesophyll walls. Vascular system The major bundles run parallel with each other the whole length of the leaf. The small transverse veins, which constitute about 7 percent of the total length, occur every 2.5 to 3 mm (Figure 2.7). Towards the tip of the leaf, the smaller longitudinal bundles terminate in a Y-shape, the forks of the Y comprising small transverse veins that link to the longitudinal veins at either side. At the pointed tip of the leaf, the veins converge and connect with each other. The distance between the longitudinal veins varies from about 0.3 mm in the first leaf to about 0.15 mm in a culm leaf (Black-man, 1971). Leaf sheath base In the mature, erect stem, there is a ring of vascular bundles in the joint nearer to the inner wall of the cylindrical leaf sheath. Unlike the sheath above the joint where each bundle has prominent sclerenchyma girder, there is no sclerenchyma and lignification is minimal. Associated with each vascular bundle and peripheral to it, there is a massive zone of collenchyma. The parenchyma cells of the ground tissue of this region are arranged in regular files and have very short vertical axes
The tissue of the jointremains capable of further elongation long aftermeristematicactivityhas ceased inthebaseof theleaf sheathandthe internode.Whenthestem isbentfrom the vertical, as forinstance when the crop is lodged,thelower side of the joint growsby cell elongation and raises the peduncle and ear to a vertical position.LeafdevelopmentEach leaf is initiated at the shoot apex. It appears first as a bump on the flank of theapex,whichbycontinuedlateralandacropetalgrowthfomsacrescent-shapedridgeandlater, as it overtops the shoot apex, becomes cowl-shaped (Figure 2.5a). Further growthgives rise to a split cylindrical structure with the margins overlapping. When the leaf isabout 20 mm long, the ligule is initiated. The leaf eventually grows up through thesub-tending leaf sheaths,the lamina expands and lastlythegrowthof the sheath stops.Cytologically,theeventsthatleadtotheformationofaleafsheathbeginwithperclinaldivisions intheoutermostcellayer(tunica)oftheapex,whichoccurfirstinthemid-positionoftheleaf andextend round theflanks of theapex togiverisetothecrescent-shapebump.Growththenbecomes apicalandmarginal.Aftertheleafovertopstheapex,apicalgrowthceases, and increase in length is by cell division throughout the primordium. Later growth iscentred atthebasal part oftheyoungleaf in anintercalary meristem.At thisthereisnostage,distinction between sheathand lamina, but when theleaf is about 20 mm long,theliguledevelopsfromtheadaxial protoderm and afurtherintercalarymeristemproduces the cell division,FIGURE2.7Drawingofthevenationof leaf1 (upperrank)andleaf6(lowerrank),aculmleaf,showingfromlefttorightthetip,themid-partofwhichgivesrisetothethe laminaandthemid-partofthesheath;notetheblunttipofleaf1sheath. The concentrationof growth at an early stage in the basal intercalary meristems means that the tip of the leafmaturesbeforethebaseandthatthelaminaexpandsandstopsgrowthbeforethesheath.Procambium,whichlaterdifferentiatestoformthevasculartissue,firstappearsinthemedian position about two plastochrons after primordium initiation.It is not continuous withthemainvascularsystemoftheshoot.Itextendsbothbasipetallyandacropetally,andsixplastochronsafterinitiation.itconnectswiththeyascularsysteminthenodalcomplexoftheleaf.Thefirst lateralprocambiumis seen aboutfourplastochronsafterinitiation,andfurther strands are initiated in intermediate positions as the leaf becomes more mature(Sharman and Hitch, 1967).The procambium of the small transverse vascular bundles does not initiate until theleaf is almost completelydifferentiated.Cells in thefiles ofmesophyl cells donot elongate,and arow of disc-shaped unelongatedcells extendsbetweenthelongitudinalvascularstrands. These rows of disc-shaped cells occur at intervals of 10 to 15 cells in themesophyll cell files. Each disc cell then divides tangentially, and the daughter cells also13
13 The tissue of the joint remains capable of further elongation long after meristematic activity has ceased in the base of the leaf sheath and the internode. When the stem is bent from the vertical, as for instance when the crop is lodged, the lower side of the joint grows by cell elongation and raises the peduncle and ear to a vertical position. Leaf development Each leaf is initiated at the shoot apex. It appears first as a bump on the flank of the apex, which by continued lateral and acropetal growth forms a crescent-shaped ridge and later, as it overtops the shoot apex, becomes cowl-shaped (Figure 2.5a). Further growth gives rise to a split cylindrical structure with the margins overlapping. When the leaf is about 20 mm long, the ligule is initiated. The leaf eventually grows up through the sub-tending leaf sheaths, the lamina expands and lastly the growth of the sheath stops. Cytologically, the events that lead to the formation of a leaf sheath begin with perclinal divisions in the outermost cell layer (tunica) of the apex, which occur first in the mid-position of the leaf and extend round the flanks of the apex to give rise to the crescent-shape bump. Growth then becomes apical and marginal. After the leaf overtops the apex, apical growth ceases, and increase in length is by cell division throughout the primordium. Later growth is centred at the basal part of the young leaf in an intercalary meristem. At this stage, there is no distinction between sheath and lamina, but when the leaf is about 20 mm long, the ligule develops from the adaxial protoderm and a further intercalary meristem produces the cell division, which gives rise to the sheath. The concentration of growth at an early stage in the basal intercalary meristems means that the tip of the leaf matures before the base and that the lamina expands and stops growth before the sheath. Procambium, which later differentiates to form the vascular tissue, first appears in the median position about two plastochrons after primordium initiation. It is not continuous with the main vascular system of the shoot. It extends both basipetally and acropetally, and six plastochrons after initiation, it connects with the vascular system in the nodal complex of the leaf. The first lateral procambium is seen about four plastochrons after initiation, and further strands are initiated in intermediate positions as the leaf becomes more mature (Sharman and Hitch, 1967). The procambium of the small transverse vascular bundles does not initiate until the leaf is almost completely differentiated. Cells in the files of mesophyll cells do not elongate, and a row of disc-shaped unelongated cells extends between the longitudinal vascular strands. These rows of disc-shaped cells occur at intervals of 10 to 15 cells in the mesophyll cell files. Each disc cell then divides tangentially, and the daughter cells also FIGURE 2.7 Drawing of the venation of leaf 1 (upper rank) and leaf 6 (lower rank), a culm leaf, showing from left to right the tip, the mid-part of the lamina and the mid-part of the sheath; note the blunt tip of leaf 1