2 Botanical Aspects of Cereals Gras Cereals are cultivated grasses that grow througl out the temperate and tropical regions of the orld. As members of the gramineae (or grass amily) they share the following characteristics but these are developed to different degrees in the various members Vegetative features of grasses 1. Conspicuous nodes in the stem Branching 2. A single leaf at each node. 3. Leaves in two opposite ranks. 4. Leaves consist of sheath and blade 5. Tendency to form branches at nodes and adventitious roots at the bases of node 6. Lower branches may take root and develop into stems as tillers Variation in vegetative features among species may be illustrated by reference to maize and wheat In wheat branches occur only at the base of the main stem or culm, to produce tillers(fig FIG 2. 1 The pattern of branching 2.1)(Percival, 1921). While all tillers have the in the wheat plant ttern of capacity to bear ears, the later formed ones may branching not actually do so; this habit is characteristic of most cereal In maize branches occur higher on the main among cereals in that, on the branches, only the tems and they are much shorter as the internodes female organs develop in the forets and on the do not extend(Fig. 2.2) main culm only male organs develop. The advent Leaf bases are very close together and the leaves itious roots that develop at the base of the main consist almost entirely of blades which surround stem provide support for the aerial parts of the the inflorescence, and the shortness of its stalk plant (Ennos, 1991); in maize they are called leads to branches that are almost entirely infore prop-roots and they are particularly well develope cence.At the tip of the main culm there is also as is appropriate to the large and heavy nature of an inflorescence as in wheat, but maize is unique the aerial parts
2 Botanical Aspects of Cereals Grasses Cereals are cultivated grasses that grow throughout the temperate and tropical regions of the world. As members of the Gramineae (or grass family) they share the following characteristics, but these are developed to different degrees in the various members: Vegetative features of grasses 1. Conspicuous nodes in the stem. 2. A single leaf at each node. 3. Leaves in two opposite ranks. 4. Leaves consist of sheath and blade. 5. Tendency to form branches at nodes and adventitious roots at the bases of nodes. 6. Lower branches may take root and develop into stems as tillers. Variation in vegetative features among species may be illustrated by reference to maize and wheat. In wheat branches occur only at the base of the main stem or culm, to produce tillers (Fig. 2.1) (Percival, 1921). While all tillers have the capacity to bear ears, the later formed ones may not actually do so; this habit is characteristic of most cereals. In maize branches occur higher on the main stems and they are much shorter as the internodes do not extend (Fig. 2.2). Leaf bases are very close together and the leaves consist almost entirely of blades which surround the inflorescence, and the shortness of its stalk leads to branches that are almost entirely inflorescence. At the tip of the main culm there is also an inflorescence as in wheat, but maize is unique FIG 2.1 The pattern of branching in the wheat plant. FIG 2.2 The pattern of branching in the maize plant among cereals in that, on the branches, only the female organs develop in the florets and on the main culm only male organs develop. The adventitious roots that develop at the base of the main stem provide support for the aerial parts of the plant (Ennos, 1991); in maize they are called prop-roots and they are particularly well developed as is appropriate to the large and heavy nature of the aerial parts. 29
TECHNOLOGY OF CEREALS Reproductive features of grasses 1. All stems and branches normally form terminal inforescences 2. Flowers are produced in spikelets 3. Each flower is enclosed between two bracts the lemma and palea (pales or flowering 4. At the base of each spikelet are two glumes (empty or sterile glumes) All cereal inflorescences are branched structures but the type of branching varies. The loose spreading structure found in oats is known as a panicle(Fig. 2.3) The main axis of the panicle, the peduncle bears several extended branches on which the spikelets are attached through short stalks or edicels. Within the spikelet forets alternate (Fig. 2. 4); the two closest to the base are similar in size but forets become progressively smaller towards the Each floret(Fig. 2.5)contains the female organs, a carpel containing a single ovule, with its stigma; and the male parts three stamens each consisting of filament and anther pollen released from the anthers, which split when ripe, FlG 2.3 The oat panicle Reproduced from Poehlman(1987) another plant. The elaborate feathery style has an extensive sticky surface well suited to intercept ing wind-borne pollen. Before the anthers mature udimentary tertiary floret the time of fowering or'anthesis'the pales are foret g corday pened primary floret forced open by the expansion of organs called Palea Lemmo lodicules at their base (lodicules swell as a result of an influx of water). The filaments of the stamens rapidly extend, projecting the opening others outside the pales, allowing the pollen to be shed onto the wind lets contain only one floret. Glumes are mostly insignificant small scales. Rice florets are unlike those of other cereals in having six stamens. Fig 26) In sorghum the situation is complex: infore- scences are panicles but they may be compact or Glume open(Hulse et aL., 1980). Spikelets occur in pairs one is sessile and the other borne on a short FIG 2.4 Spikelet of oat Reproduced from Poehlman(1987) pedicel. The sessile spikelet contains two florets, by courtesy of Avi Publishers, New York
30 TECHNOLOGY OF CEREALS Reproductive features of grasses 1. All stems and branches normally form terminal 2. Flowers are produced in spikelets. 3. Each flower is enclosed between two bracts, the lemma and palea (pales or flowering glumes). 4. At the base of each spikelet are two glumes (empty or sterile glumes). All cereal inflorescences are branched structures but the type of branching varies. The loose spreading structure found in oats is known as a panicle (Fig. 2.3). The main axis of the panicle, the peduncle, bears several extended branches on which the spikelets are attached through short stalks or pedicels. Within the spikelet florets alternate (Fig. 2.4); the two closest to the base are similar in size but florets become progressively smaller towards the tip. Each floret (Fig. 2.5) contains the female organs, a carpel containing a single ovule, with its stigma; and the male parts, three stamens, each consisting of filament and anther. Pollen released from the anthers, which split when ripe, is transferred by wind to the receptive stigma on another plant. The elaborate feathery style has an extensive sticky surface well suited to intercepting wind-borne pollen. Before the anthers mature, the time of flowering or ‘anthesis’ the pales are forced open by the expansion of organs called lodicules at their base (lodicules swell as a result of an influx of water). The filaments of the stamens rapidly extend, projecting the opening anthers outside the pales, allowing the pollen to be shed onto the wind. Rice inflorescences are also panicles but spikelets contain only one floret. Glumes are mostly insignificant small scales. Rice florets are unlike those of other cereals in having six stamens. (Fig. 2.6) In sorghum the situation is complex: inflorescences are panicles but they may be compact or open (Hulse et al., 1980). Spikelets occur in pairs, One is sessi1e and the Other borne On a short pedicel. The sessile spikelet contains two florets, inflorescences. FIG 2.3 The oat panicle. Reproduced from Poehlman (1987) by courtesy of Avi publishers, New yo&. Rudimentary tertiary floret they remain enclosed between the pales but at ned primary floret f‘oret FIG 2.4 Spikelet of oat. Reproduced from Poehlman (1987) by courtesy of Avi Publishers, New York
BOTANICAL ASPECTS OF CEREALS Carpel FIG 2.5 Reproductive organs in an oat floret. Repro Poehlman(1987)by courtesy of Avi Publishers Fertile spikelet Poehlman (1987) by courtesy of Avi Publishers, New York FIG 2.6 Reproducti of a rice floret. Note the six of Avi Publishers d from Pochlman(1987)courtesy one perfect and fertile and the other sterile. The pedicelled spikelet is either sterile or develops male organs only(Fig. 2.7) In barley the type of inflorescence is a spike (Fig.28) It is more compact than a panicle, the spikelets being attached to the main axis or rachis by much shorter rachilla. The rachis is fattened and adopts a zigzag form, spikelets occur in groups of three alternately on the rachis. In six-rowed types all spikelets develop to maturity and each bears a grain in its single foret. In two-rowed pes only the central spikelet in each three develops in this way; the others are sterile. The glumes are very small but the pales fully surround the grain and remain closely adherent to it even fter threshing. The lemma tapers to a long awn which does break off during threshing. Many variants of the barley spike are illustrated by Briggs (1978) FIG 2.8 Spikes of barley, showing: a. the le two- rowed and b The inforescences of wheat, rye and triticale the six-rowed form
BOTANICAL ASPECTS OF CEREALS 31 Poehlman (1987) by courtesy of Avi Publishers, New York. Sterile spikelet Fertile spikelet FIG 2.7 A pair of spikelets of sorghum. Reproduced from Poehlman (1987) by courtesy of Avi Publishers, New York. FIG 2.6 Reproductive organs of a rice floret. Note the six stamens present. Reproduced from Poehlman (1987) courtesy of Avi Publishers, New York. one perfect and fertile and the other sterile. The pedicelled spikelet is either sterile or develops male organs only (Fig. 2.7). In barley the type of inflorescence is a spike (Fig. 2.8). It is more compact than a panicle, the spikelets being attached to the main axis or rachis by much shorter rachillas. The rachis is flattened and adopts a zigzag form, spikelets occur in groups of three alternately on the rachis. In six-rowed types all spikelets develop to maturity and each bears a grain in its single floret. In two-rowed types only the central spikelet in each three develops in this way; the others are sterile. The glumes are very small but the pales fully surround the grain and remain closely adherent to it even after threshing. The lemma tapers to a long awn which does break off during threshing. Many variants of the barley spike are illustrated by Briggs (1978). The inflorescences of wheat, rye and triticale FIG 2.8 Spikes of barley, showing: a. the two-rowed and b. the six-rowed forms
TECHNOLOGY OF CEREALS FIG 2.9 Spikes of A. wheat and B rye. Wheat may be awned(bearded)(ii)or awnless (i) are also spikes with spikelets alternating on a are many different species belonging to several rachis, each spikelet however contains up to six different tribes(see Fig. 2. 25) and no generaliza forets(Fig. 2. 9). tions about their inforescences are possible( details It is unusual for all six forets in a spikelet to of most are given in hulse et al., 1980 ) Pearl be fertile and those at the extremes of the millet has a spike which may be anything between inflorescence may bear only one or even no fertile a few centimeters to a metre long. It is densely forets. Variants of wheat spikes are illustrated packed with groups of 2-5 spikelets, surrounded by Peterson(1965). As in oats the grains in the by 30-40 bristles(Fig. 2. 11). Florets may be wo basal forets are the largest. Those in the bisexual or male only centre of the spike are larger than those at In maize the male spikes occurring at the top the extremes(Bremner and rawson, 1978). The of the culm bear spikelets in pairs, one being variation in size occurs as a function of the ability sessile, and the other pedicellate. Both types of each grain to compete for nutrients but also contain two forets, each with three anthers. The of the period of development, the earliest to entire male inflorescence is known as the tassel flower being those in the basal forets of the (Fig. 2. 2). On the female inflorescences the central spikelets(Fig. 2.10) spikelets again carry two florets but only one is Millets are an extremely diverse group there fertile, the upper functions while the lower one
32 TECHNOLOGY OF CEREALS FIG 2.9 Spikes of A. wheat and B. rye. Wheat may be awned (bearded) (ii) or awnless (i). are also spikes with spikelets alternating on a are many different species belonging to several rachis, each spikelet however contains up to six different tribes (see Fig. 2.25) and no generalizaflorets (Fig. 2.9). tions about their inflorescences are possible (details It is unusual for all six florets in a spikelet to of most are given in Hulse et al., 1980.) Pearl be fertile and those at the extremes of the millet has a spike which may be anything between inflorescence may bear only one or even no fertile a few centimeters to a metre long. It is densely florets. Variants of wheat spikes are illustrated packed with groups of 2-5 spikelets, surrounded by Peterson (1965). As in oats the grains in the by 30-40 bristles (Fig. 2.11). Florets may be two basal florets are the largest. Those in the bisexual or male only. centre of the spike are larger than those at In maize the male spikes occurring at the top the extremes (Bremner and Rawson, 1978). The of the culm bear spikelets in pairs, one being variation in size occurs as a function of the ability sessile, and the other pedicellate. Both types of each grain to compete for nutrients but also contain two florets, each with three anthers. The of the period of development, the earliest to entire male inflorescence is known as the tassel flower being those in the basal florets of the (Fig. 2.2). On the female inflorescences the central spikelets (Fig. 2.10). spikelets again carry two florets but only one is Millets are an extremely diverse group, there fertile, the upper functions while the lower one
BOTANICAL ASPECTS OF CEREALS FIG 2 10 Typical profiles of wheat grain weights within a towards the tip of the spikelet, and represented by o, A,t O.( Bremner and Rawson, 1978) aborts(Fig. 2. 12). Each fertile floret contains a single ovary; its style is not of the feathery type typical of most cereals but a long threadlike structure covered in fine hairs which entrap wind-borne pollen. A single ear may contain 800 fertile florets so the same number of stigmas or silks'is present The ear or cob is wrapped by modified leaf sheaths forming husks or shucks and the silks emerge together from the distal open end of the Perhaps the protection afforded by the husk spikelet. earl millet: a spike, b. part of spike enlarged, c protective husk(Fig. 2. 13) FIG 2.11 obviates the need for enclosure of the reproduc tive structures by bracts and pales. these are outbreeding habit fits it admirably for Fl hybrid insignificant in maize and as a result grains are production, whereby yields have been increased not separated one from another on the cob In dramatically through the heterosis or hybrid some cases their mutual pressure imposes an vigour which results angular form on them. While most cereals are dependent to some degree on cultivation for their Life cycle of cereals survival, maize has the ultimate dependence on man since there is no mechanism for dispersal of Although other parts of cereal plants have ts seeds remaining The concentration of the value, particularly in providing feed and bedding sexual organs on separate spikes encourages cross- for livestock, the ripe fruit or grains are economic pollination, which is the norm for maize. Its ally by far the most valuable parts of the plant
c j_ II tl8- g 9- * 2 1 w a 10 12 11 0-l 14 15 13- 16 17 ie 19 20 I- % ** \* (b) - \e ‘It \.4 - 7) *i; 7- A\A \* \+ A \‘* - +/:,a - - f J1 */ I I 30 40 50 60
TECHNOLOGY OF CEREALS FIG 2. 12 Radial section of a maize cob, showing(i) a perfect (fertile)and (ii a rudimentary(empty) floret. Based Nevertheless the quality of the grain is dependent e condition of the plant; diseases that affect the leaves, roots and stem can reduce the photosynthetic area, the ability of the plant to take up water and nutrients from the soil, and the ability of the plant to stand. Agricultural scientists have through experimentation, determined the optimal imes for field treatments such as fertilizer, herbi- cide and pesticide applications. For communicating this information it has been found convenient to FIG 2. 13 Cob of maize, showing the protective husk define stages of plant growth and several scales have been devized. Those that exist for wheat, the same foret. Once on the stigma, pollen grains barley and oats have been compared by Landes have a mechanism whereby a pollen tube is and Porter(1989). The decimal scale of Zadoks produced. The tube progresses towards the micro- et al.(1974)is illustrated here(Fig. 2. 14). pyle and having effected access by this route, it Scales usually start at the time of seed germina- allows nuclei from the pollen grain to pass into tion but a life cycle is, by definition, a continuously the ovule and fuse with nuclei present there. The repeating sequence of events and, as such, has no primary fusion is of the sperm nucleus with the absolute beginning or end egg nucleus. The product is a cell, the e successive The beginning of each new generation occurs divisions of which produce the embryo. A further when pollination is effected. As in all plants this set of fusions, however, produces the first endo- results when pollen produced in the anther con- sperm nucleus. Three, not two, nuclei are involved tacts the stigma on the carpel of another, or even one from the pollen and two polar nuclei from the
34 TECHNOLOGY OF CEREALS FIG 2.12 Radial section of a maize cob, showing (i) a perfect (fertile) and (ii) a rudimentary (empty) floret. Based on A. L. and K. B. Winton (1932). Nevertheless the quality of the grain is dependent upon the condition of the plant; diseases that affect the leaves, roots and stem can reduce the photosynthetic area, the ability of the plant to take up water and nutrients from the soil, and the ability of the plant to stand. Agricultural scientists have, through experimentation, determined the optimal times for field treatments such as fertilizer, herbicide and pesticide applications. For communicating this information it has been found convenient to define stages of plant growth and several scales have been devized. Those that exist for wheat, barley and oats have been compared by Landes and Porter (1989). The decimal scale of Zadoks et al. (1974) is illustrated here (Fig. 2.14). Scales usually start at the time of seed germination but a life cycle is, by definition, a continuously repeating sequence of events and, as such, has no absolute beginning or end. The beginning of each new generation occurs when pollination is effected. As in all plants this results when pollen produced in the anther contacts the stigma on the carpel of another, or even FIG 2.13 Cob of maize, showing the protective husk. the same floret. Once on the stigma, pollen grains have a mechanism whereby a pollen tube is produced. The tube progresses towards the micropyle and, having effected access by this route, it allows nuclei from the pollen grain to pass into the ovule and fuse with nuclei present there. The primary fusion is of the sperm nucleus with the egg nucleus. The product is a cell, the successive divisions of which produce the embryo. A further set of fusions, however, produces the first endosperm nucleus. Three, not two, nuclei are involved, one from the pollen and two polar nuclei from the
BOTANICAL ASPECTS OF CEREALS ovule. All endosperm cells are ultimately derived An important phase in the life cycle, from the from this first endosperm cell and each inherits point of view of grain quality, is germination chromosomes from three nuclei rather than the This occurs when ripe grain is subjected to more usual two Endosperm cells thus have one and damping to an adequate moisture content at an a half times as many chromosomes as cells else- appropriate temperature. In primitive grains the where in the plant. The details of the develop appropriate conditions are those to be expected ment of endosperm, embryo and other grain in the natural habitat of the species at the issues of different cereals are described in relevant beginning of the growing season, but breeding texts(Kiesselbach, 1980; Bushuk, 1976; Wat and cultivation over many years have diminishe 1987; Hulse et al., 1980; Percival, 1921; Evers this relationship Bechtel, 1988; Palmer, 1989; Hoshikawa, 1967) The processing requirements in respect of Stem extension FIG 2. 14 Stages of plant growth corresponding to the phases defined in Zadoks scale. Major phases are represented by higher order numbers thus: 0. germination, 10 seedling growth, 20. tillering, 30 stem elongation, 40. booting, 50. inflorescence emergence, 60. anthesis, 70. milk development, 80 Within major phases additional lower order numbers indicate events of lesser importance The descriptions corresponding to the numbers on the horizontal axis are: ll, first leaf unfolded ves unfolded; 21, main shoot and I tille 23, main shoot and 3 tillers; 24,ma 4 tillers: 25, main shoot and 5 37, Aag(last)leaf just visible; 39, fag leaf ligule/collar just visible; 41, flag leaf sheath g the term boot refers to the swollen sheath of the last leaf, when the inflorescence within causes it to expand. The inflorescence is said to be in boot
TECHNOLOGY OF CEREALS germination vary with uses. In grains destined caryopsis), which is a type of achene. all achenes for malting the requirement is for ready and are dry (rather than fleshy like many common vigorous germination as soon after harvest as fruits). All fruits, whether dry or fleshy typically possible but this must be combined with resistance contain one or more seeds. In the case of caryopses to sprouting, or premature germination prior to the number of fruits contained is one, and the able in all cereals, irrespective of their intended fruit when mature. It comprises accompanied by the production of hydrolytic 1. Embryonic axis enzymes which render stored nutrients in the 2. Scutellum endosperm soluble, thus reducing the amount of 3. Endosperm starch and protein harvested. Additionally the 4. Nucellus presence of high germination enzyme levels 5. Testa or seedcoat in cereals intended for flour production gives rise to excessive hydrolysis during processing Bread-making fours are particularly sensitive to such high enzyme levels as processing con- Aleurone ditions are well suited to enzyme-cataly Nucellus hydrolysis Germination is a complex syndrome, the detail of which are not fully understood. The important Pericarp events are shown in the flow diagram( Fig 2.15) beloy Grain anatom Scutellum The basic structural form of cereal caryopses is surprisingly consistent, to the extent that a FIG 2. 16 Generalized cereal grain, showing the relationships ' generalized cereal grain can be described(Fig. among the tissues. The proportions that they contribute, in individual cereals, are shown in Table 2.1 Although frequently referred to as seeds, cereal grains are in botanical terms fruits. The fruits Embryo of grasses are classified as caryopses(singula The embryonic axis and the scutellum together constitute the embryo. The embryonic axis is the plant of the next generation. It consists of pri mondial roots and shoot with leaf initials. It is Resting grair connected to and couched in the shield-like scutel If dormant lum, which lies between it and the endosperm change Hormones There is some confusion about the terminology of the embryo as the term is also used by Enzyme synthesis cereal chemists to describe part or all of the in endosperm embryo. If the botanical description is adopted Growth of roots proteins ati stof cet wot s, as above and germ'reserved for the embryo-rich then the FIG 2. 15 The main events involved in germination of a be no confusion The scutellum behaves as a ry and
36 TECHNOLOGY OF CEREALS germination vary with uses. In grains destined for malting the requirement is for ready and vigorous germination as soon after harvest as possible but this must be combined with resistance to sprouting, Or Premature germination Prior to harvest. Resistance to sprouting is, in fact, desirable in all cereals, irrespective of their intended use, because the growth of the embryonic axis is accompanied by the production of hydrolytic enzymes which render stored nutrients in the endosperm soluble, thus reducing the amount of starch and protein harvested. Additionally the presence of high germination enzyme levels 5. Testa or seedcoat. in cereals intended for flour production gives rise to excessive hydrolysis during processing. Bread-making flours are particularly sensitive to such high enzyme levels as processing conditions are well suited to enzyme-catalyzed hydrolysis. Germination is a complex syndrome, the details of which are not fully understood. The important events are shown in the flow diagram (Fig. 2.15) below. caryopsis), which is a type of achene. All achenes are dry (rather than fleshy like many common fruits). All fruits, whether dry or fleshy, typically contain one or more seeds. In the case of caryopses the number of fruits contained is one, and the seed accounts for the greater part of the entire fruit when mature. It comprises: 1. Embryonic axis; 2. Scutellum; 3. Endosperm; 4. Nucellus; Starchy endosperm Grain anatomy The basic structural form of cereal caryopses is surprisingly consistent, to the extent that a ‘generalized’ cereal grain can be described (Fig. FIG 2.16 Generalized cereal grain, showing the relationships among the tissues. The proportions that they contribute, in individual cereals, are shown in Table 2.1. 2.16). Although frequently referred to as seeds, cereal grains are in botanical terms fruits. The fruits of grasses are classified as caryopses (singular: Embryo The embryonic axis and the scutellum together constitute the embryo. The embryonic axis is the plant of the next generation. It consists of primordial roots and shoot with leaf initials. It is connected to and couched in the shield-like scutellum, which lies between it and the endosperm. There is some confusion about the terminology of the embryo as the term ‘germ’ is also used by cereal chemists to describe part or all of the embryo. If the botanical description is adopted as above and ‘germ’ reserved for the embryo-rich fraction produced during milling, then there can be no confusion. The scutellum behaves as a secretory and Wote. Yxygen Resting grain If dormant no change iw ErnbryJ Horrnones 7 1 t L K%j~~r!hes’s Growth of roots and shoots Solublllzatlon of cel wals, proteins and starch FIG 2.15 The main events involved in germination of a seed
BOTANICAL ASPECTS OF CEREALS SA IG 2. 17 Part of a transverse section of a grain of Hard Red Winter wheat, 14.4% protein content, showing concentration of protein in subaleurone endos Protein concentration diminshes towards the central parts of the grain in all cereals P, pericarp curone layer; SA, subaleurone endosper n(Reproduced from N. L. Kent Chem. 1966, 43: 585, by courtesy of the absorptive organ, serving the requirements of Although the fusions of nuclei, occurring dur- the embryonic axis when germination occurs. ing sexual fertilization, and leading to the forma- It consists mainly of parenchymatous cells, each tion of endosperm and embryo respectively, take containing nucleus, dense cytoplasm and oil place approximately at the same time, the develop. bodies or spherosomes. The layer of cells adja- ment of the embryo tissue, by cell division, is cent to the starchy endosperm consists of an relatively delayed When the embryo does enlarge as a palisade. Cells are joined only near their tissue giving rise to a few layers of crushed, empty cells the contents of which have either been Exchange of water and solutes between scutel- resorbed or have failed to develop The crushed lum and starchy endosperm is extremely rapid cells are described variously as the cementing Secretion of hormones and enzymes and absorp- depleted or fibrous layer tion of solubilized nutrients occurs across this bundary during germination. The embryonic axis is well supplied with conducting tissues of a End simple type and some conducting tissues are also The endosperm is the largest tissue of the grain present in the scutellum(Swift and O'Brien, 1970). It comprises two components that are clearl
BOTANICAL ASPECTS OF CEREALS 37 p A SA FIG 2.17 Part of a transverse section of a grain of Hard Red Winter wheat, 14.4% protein content, showing concentration of protein in subaleurone endosperm. Protein concentration diminshes towards the central parts of the grain in all cereals. P, pericarp; A, aleurone layer; SA, subaleurone endosperm; I, inner endosperm. (Reproduced from N. L. Kent, Cereal Chem. 1966,43: 5'85, by courtesy of the Editor.) Although the fusions of nuclei, occurring during sexual fertilization, and leading to the formation of endosperm and embryo respectively, take place approximately at the same time, the development of the embryo tissue, by cell division, is relatively delayed. When the embryo does enlarge, it compresses the adjacent starchy endosperm tissue giving rise to a few layers of crushed, empty cells the contents of which have either been resorbed or have failed to develop. The crushed cells are described variously as the cementing, depleted or fibrous layer . absorptive organ, serving the requirements of the embryonic axis when germination occurs. It consists mainly of parenchymatous cells, each containing nucleus, dense cytoplasm and oil bodies or spherosomes. The layer of cells adjacent to the starchy endosperm consists of an epithelium of elongated columnar cells arranged as a pallisade. Cells are joined only near their bases. Exchange of water and solutes between scutellum and starchy endosperm is extremely rapid. Secretion of hormones and enzymes and absorption of solubilized nutrients occurs across this boundary during germination. The embryonic axis is well supplied with conducting tissues of a simple type and some conducting tissues are also present in the scutellum (Swift and O'Brien, 1970). Endosperm The endosperm is the largest tissue of the grain . It comprises two components that are clearly
TECHNOLOGY OF CEREALS distinguished. The majority, a central mass des- At maturity the starchy endosperm dies but cribed as starchy endosperm, consists of cells aleurone cells continue to respire, albeit at a packed with nutrients that can be mobilized to very slow rate, for long periods. The aleurone support growth of the embryonic axis at the tissue covers the outer surface of the embryo but onset of germination. Nutrients are stored in its cells in this area may become separated and insoluble form, the major component being the degenerate. Their ability to respire and to pro- carbohydrate starch. Next in order of abundance duce enzymes on germination is in some doubt is protein. In all cereals there is an inverse Briggs, 1978) gradient involving these two components, the protein percentage per unit mass of endosperm tissue increasing towards the periphery (Fig. Seed coats 2.17 Surrounding the endosperm and embryo lie the Cell size also diminishes towards the outside remains of the nucellus, the body within the ovule nd this is accompanied by increasing cell wall in which the cavity known as the embryo sac thickness. The walls of the starchy endosperm of develops. Following fertilization the embryo and wheat are composed mainly of arabinoxylans endosperm expand at the expense of the nucellus while in barley and oats(1-3)and(1-4)B-D which is broken down except for a few remnants glucans predominate. Cellulose contributes little of tissue and a single layer of squashed empty to cereal endosperm walls except in the case of cells from the nucellar epidermis. epidermal cells rice(see p. 64). in many higher plants secrete a cuticle and a Surrounding the starchy endosperm is the cuticle is present on the outer surface of the other endosperm tissue, the aleurone, consisting nucellar epidermis of many cereals of one to three layers of thick-walled cells with The outermost tissue of the seed is the testa or dense contents and prominent nuclei seed coat(the nucellar epidermis is also regarded The number of layers present is characteristic as a seed coat but its origin is different from that of the cereal species, wheat, rye, oats, maize and of the testa which develops from the integuments) sorghum having one and barley and rice having The testa may consist of one or two cellular layers three. Unlike the tissue they surround, aleurone In some varieties of sorghum a testa is absent cells contain no starch but they have a high altogether. Where two layers are present the long protein content and they are rich in lipid They axes of their elongated cells lie at approximately are extremely important in both grain deve- 90 to each other. Frequently the testa accumulates lopment, during which they divide to produce corky substances in its cells during grain ripening starchy endosperm cells, and germination, when and this may confer colour on the grain and in most species they are a site of synthesis of certainly reduces the permeability of the testa. a hydrolytic enzymes responsible for solubilizing cuticle, thicker than that of the nucellar epidermis the reserves is typical, and this also plays a role in regulating The balance between aleurone and scutellum water and gaseous exchange in the latter role varies among species. both Both testa and nucellus are tissues which once tissues synthesize the enzymes in response to formed part of the ovule of the mother plant hormones including giberellic acid, transmitted They are thus of an earlier generation than the from the embryonic axis(via the scutellum in the endosperm and embryo which they surround and case of the aleurone). A further function of to which they closely adhere aleurone cells in some millets and sorghum is the transfer of metabolites into the starchy endo- Pericarp sperm during grain maturation. This activity is deduced from the knobbly, irregular thickenings The pericarp (or fruitcoat) is a multilayered on their walls often associated with such transfers structure consisting of several complete and in- Rost and Lersten, 1970 complete layers. In all cereal grains the pericarp
38 TECHNOLOGY OF CEREALS distinguished. The majority, a central mass described as starchy endosperm, consists of cells packed with nutrients that can be mobilized to support growth of the embryonic axis at the onset of germination. Nutrients are stored in insoluble form, the major component being the carbohydrate starch. Next in order of abundance is protein. In all cereals there is an inverse gradient involving these two components, the protein percentage per unit mass of endosperm tissue increasing towards the periphery (Fig. 2.17). Cell size also diminishes towards the outside and this is accompanied by increasing cell wall thickness. The walls of the starchy endosperm of wheat are composed mainly of arabinoxylans, while in barley and oats (1-3) and (1-4) p-D glucans predominate. Cellulose contributes little to cereal endosperm walls except in the case of rice (see p. 64). Surrounding the starchy endosperm is the other endosperm tissue, the aleurone, consisting of one to three layers of thick-walled cells with dense contents and prominent nuclei. The number of layers present is characteristic of the cereal species, wheat, rye, oats, maize and sorghum having one and barley and rice having three. Unlike the tissue they surround, aleurone cells contain no starch but they have a high protein content and they are rich in lipid. They are extremely important in both grain development, during which they divide to produce starchy endosperm cells, and germination, when in most species they are a site of synthesis of hydrolytic enzymes responsible for solubilizing the reserves. The balance between aleurone and scutellum in the latter role varies among species. Both tissues synthesize the enzymes in response to hormones including giberellic acid, transmitted from the embryonic axis (via the scutellum in the case of the aleurone). A further function of aleurone cells in some millets and sorghum is the transfer of metabolites into the starchy endosperm during grain maturation. This activity is deduced from the knobbly, irregular thickenings on their walls often associated with such transfers (Rost and Lersten, 1970). At maturity the starchy endosperm dies but aleurone cells continue to respire, albeit at a very slow rate, for long periods. The aleurone tissue covers the outer surface of the embryo but its cells in this area may become separated and degenerate. Their ability to respire and to produce enzymes on germination is in some doubt (Briggs, 1978). Seed coats Surrounding the endosperm and embryo lie the remains of the nucellus, the body within the ovule in which the cavity known as the embryo sac develops. Following fertilization the embryo and endosperm expand at the expense of the nucellus, which is broken down except for a few remnants of tissue and a single layer of squashed empty cells from the nucellar epidermis. Epidermal cells in many higher plants secrete a cuticle and a cuticle is present on the outer surface of the nucellar epidermis of many cereals. The outermost tissue of the seed is the testa or seed coat (the nucellar epidermis is also regarded as a seed coat but its origin is different from that of the testa which develops from the integuments). The testa may consist of one or two cellular layers. In some varieties of sorghum a testa is absent altogether. Where two layers are present the long axes of their elongated cells lie at approximately 90" to each other. Frequently the testa accumulates corky substances in its cells during grain ripening and this may confer colour on the grain and certainly reduces the permeability of the testa. A cuticle, thicker than that of the nucellar epidermis, is typical, and this also plays a role in regulating water and gaseous exchange. Both testa and nucellus are tissues which once formed part of the ovule of the mother plant. They are thus of an earlier generation than the endosperm and embryo which they surround and to which they closely adhere. Pericarp The pericarp (or fruitcoat) is a multilayered structure consisting of several complete and incomplete layers. In all cereal grains the pericarp