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