Wheat. Peter R. Shewry
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Figure 1.7 Wheat growth and development. Only main stem shown from stem erect growth stage. Horizontal bars indicate when yield components are principally determined on the main stem. Numerals in brackets are decimal growth stage scores (Table 1.4).
Source: Reproduced from Gooding (2009) with permission.
1.2.1 Vegetative Phase
1.2.1.1 Germination
Starting with the germination stages, the dried grain (DGS 00) is markedly asymmetric (Figure 1.8) with the ventral side divided by a crease parallel to the long axis. The embryo is present at the germ end but typically 80% of the grain weight is allocated to the nutrient store of the seed, i.e. the starchy endosperm. The outer parts of the grain consist of compressed cell layers, notably the true seed coat (the testa) and the fruit coat (the pericarp), which, together with the outermost layer of endosperm cells (the aleurone), collectively form the bran on milling (Evers et al. 2006). Germination (DGS = 0n) starts in conditions that break or circumvent dormancy, and with adequate moisture, temperature, and aeration (Chapters 3 and 4). Water is imbibed and the aleurone cells are stimulated to produce hydrolytic enzymes that break down the endosperm to release simple sugars and amino acids for the growth of the seedling. Externally, the radicle (root) and coleoptile emerge (Figure 1.8). The coleoptile is a pale tube‐like structure that protects and facilitates the emergence of the first true leaf. The coleoptile elongates to the soil surface. At least three seedling (seminal) roots will usually have been produced before the plants emerge above the soil surface. The germination phase is complete when the first true leaf reaches the tip of the coleoptile which encases it. The apical meristem is raised towards the soil surface by expansion of an underground stem.
Figure 1.8 The germination phases of wheat. Boxed numerals are the decimal growth stage scores from Table 1.4.
1.2.1.2 Leaves
The stem apex usually remains just below ground level during the further vegetative phases, where it is partially protected from herbivory and extremes of temperature. The apex itself may only be 0.1 mm in length (Figure 1.7) and produces primordia, from which leaves expand and are pushed upwards. The leaf production growth stages (DGS = 1n) start with the emergence of the first leaf through the coleoptile (Figure 1.9). A leaf is counted as being fully emerged or unfolded based on the emergence of the collar, which is the junction between the leaf lamina (or blade) and the leaf sheath. The leaf sheaths are usually hairy (non‐glabrous). The leaf laminas are narrow with about 12 veins. Small additional structures on the collar become visible, particularly on the larger leaves; these are the auricles and the ligule (Figure 1.9). The auricles are useful for distinguishing wheat from other cereals before the ears emerge. Whereas the wheat auricles are commonly 1–3 mm long and are usually covered in hairs, barley auricles are hairless, oats have no auricles, and rye auricles are much shorter. The ligule is blunt and between 2 and 4 mm long. The sheath and lamina are contiguous. The sheath connects the leaf to its node on the true stem. While the plant is in the vegetative stage, the nodes remain compressed on a short stem (< 5 mm) behind the stem apex at the base of the plant. New leaves become externally visible as they emerge from within the leaf sheath of the previous leaf. The DGS progresses as each leaf becomes fully unfolded (Figure 1.9).
Figure 1.9 The leaf production phase of wheat. Boxed numerals are the decimal growth stage scores from Table 1.4.
1.2.1.3 Tillers
The production of tillers (DGS = 2n) allows the plant to spread and increase its canopy size more than would be possible through just increasing the number of leaves on the main stem. Tillers originate in the axils of the leaf nodes and rise between the leaf sheaths. Tillers are counted as such when they have their own fully unfolded leaf (Figure 1.10). Each tiller has the potential to produce an ear, but the plant will often produce more tillers than survive to maturity. Tillering is particularly important when inter‐plant competition is low. This can arise, for example, when seedling establishment has been poor due to difficult seed bed conditions and/or when seed of low vigour has been used. Additionally, tillering can help compensate when neighbouring stems or plants have been lost through frost or hail damage, or through herbivory by insects and mammals.
1.2.1.4 Roots
The seminal roots continue to develop during the vegetative phase, but further adventitious (or nodal) roots start to develop about one to two months after germination. The adventitious roots derive from the coleoptile node or lower nodes of the main stem and tillers. They generally overtake the seminal roots and can occupy over 90–95% of the total root volume of a fully grown crop.
1.2.2 Reproductive Phase
The reproductive stages commence when the stem apex or growing point starts producing the structures of the ear (or spike). The timing of this transition is strongly dependent on wheat genotype and the environment. Of particular relevance are the interactions between genetic factors, temperature, and day length (or photoperiod), i.e. cultivars vary greatly in their need to pass through a cold period (i.e. to become vernalised), and for days to lengthen. These interactions are detailed in Chapter 4, but for now it suffices to recognise that those cultivars that have a significant requirement to pass through a cold period for timely transition to the reproductive phase and hence to maturation are classed as winter wheats (as in Figure 1.6, Table 1.3). Those that have little such requirement are classed as spring wheats. Facultative wheats are intermediate between winter and spring wheats in their requirement for vernalization (Braun and Sãulescu 2002). Those cultivars that do or do not have a strong requirement for days to lengthen for timely development are respectively classed as day length sensitive and day length insensitive cultivars (Table 1.3).