Wheat. Peter R. Shewry
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phase of wheat. Boxed numeral is the decimal growth stage score from Table 1.4.
1.2.2.1 Stem Extension
The first signs of the transition from the vegetative to the reproductive phase are that the stem apex elongates to 0.3 mm with the appearance of primordia as single ridges. At this stage, the stem apex is still close to ground level. The buds in the axils of the apex ridges are spikelet primordia and, with their leaf initials form double ridges as the developing spike elongates to between 0.8 and 1 mm (Figure 1.7). In broad terms, double ridges occur as the plants take on a more upright appearance as leaf sheaths elongate and pass through the pseudostem erect growth stage (Figure 1.11) (Barber et al. 2015).
The structures of the ear develop as it is simultaneously elevated through the leaf sheaths of the canopy by the lengthening stem. Stem elongation is evidenced by the detection of nodes behind the rising apex (Figure 1.11). As the spike continues to elongate, the central spikelets swell while additional double ridges are formed acropetally (upwards) until the terminal spikelet is formed at the apex. By this time, the developing ear is usually 10–30 mm above the base (or crown) of the plant, and between one or two nodes are detectable (DGS31–32; Figure 1.7; Barber et al. 2015). The spikelets continue to differentiate and develop scaly tissues (glume, lemma, palea) and potential sites for grain formation (florets) (Figure 1.12). Floret initiation continues until about the time that the last leaf (the flag leaf) emerges and unfolds (DGS37–39; Figure 1.11). Between 7 and 11 florets may be initiated in each spikelet but, as with tillers, many more florets are initiated than develop to maturation with a grain.
Figure 1.11 Stem extension, booting, ear emergence, and anthesis of wheat. Boxed numerals are the decimal growth stage scores from Table 1.4.
1.2.2.2 Booting and Ear Emergence
Once the flag leaf has emerged, the swelling of the developing ears within the flag leaf sheath becomes visible as a boot (DGS 40–49; Figure 1.11). It is approximately during this booting phase that cell division by meiosis occurs to produce the male (pollen) and female (ovule) gametes of the flowers. Booting is soon followed by the emergence of the ear above the flag leaf (DGS 50–59; Figure 1.11). At ear emergence, the structure and colour of the ears can be used to distinguish wheat cultivars. For example, the length of the awns extending from the lemmas (Figure 1.12) can either be short (awnless cultivars) or long (awned cultivars), as compared in Figure 1.13. Awns are a wild‐type adaptation that would have aided seed dispersal. However, there appears to be a small yield advantage for awned cultivars in warmer areas, and particularly for preventing excessive grain shrivelling in dry conditions (Rebetzke et al. 2016).
1.2.2.3 Anthesis
Anthesis (flowering; DGS 60–69; Figure 1.11) usually occurs between three and eight days after ear emergence, starting in the lower florets of the central spikelets. Most flowering throughout the spike is often completed within two to four days, although late flowering in distal spikelets and florets can extend this period to seven days (Percival 1921). Wheat is mostly self‐pollinated, but some outcrossing is possible, particularly in genotypes with more open florets and when plants are grown close together. However, cross‐pollination is limited because wheat pollen is heavy and short lived.
Figure 1.12 Structures of a single mature spikelet. Gl = Glume; L = Lemma; P = Palea; Gr = Site of grain, obscured by lemma and palea; a = awn; IF = Infertile floret.
Figure 1.13 Ears (spikes) of wheats and their relatives.
1.2.2.4 Grain Growth
After fertilization, the developing caryopses (grain) expand rapidly with net ingress of water. When squeezed, the immature grains first exude a clear watery droplet, i.e. signifying the watery‐ripe growth stage (DGS = 71). Later, the liquid becomes milky as starch is accumulated (DGS = 73). A wet mass, surrounded by milky liquid, denotes medium milk (DGS = 75), whereas late milk occurs when the grain contents are wet and sticky (Tottman 1987). The growth stage scores move from milk development to dough development when no liquid droplet can be squeezed from the grain. At early dough (DGS = 83) the grain contents are soft and cheesy. At soft dough the grain contents cannot be easily squeezed out, but a fingernail impression into the dorsal side quickly disappears (DGS = 85). This stage is broadly coincident with the senescence of the flag leaf and with the maximum accumulation of dry matter in the grain (physiological maturity; Barber et al. 2015). Grain moisture content (w/w) is often between 50 and 40% at this stage. Later stages describe the net loss of water from the grain as harvest is approached (harvest maturity). At the hard dough stage (DGS = 87) a fingernail impression is retained, whereas by DGS = 92 the caryopsis is hard and cannot be dented. Further details of grain development, particularly as they pertain to grain quality, are given in Chapter 7.
Harvest can proceed once grain moisture content drops below 18–20% but at this level further drying before or during storage will be necessary. In the UK, the target moisture content for safe temporary storage at ambient conditions going into autumn after harvest is 14.5% or below (AHDB 2011). The equivalent for Australia, with its warmer conditions, is 12%, and grain loads with higher than 12.5% moisture are prevented from entering the marketing system (Newman 2008). In Canada, wheat is graded as Straight, Tough, and Damp for moisture contents of < 14.6, 14.6–17.0, and > 17% respectively (CGC 2020). In the US moisture does not affect wheat grading, but the price of wheat will depend on contract specifications: various discounts can be applied as moisture contents rise above 12% (Wilson and Dahl 2002).
1.3 Wheat Evolution and Migration
1.3.1 Origin in the Fertile Crescent
Wheat was first used, domesticated, and farmed in and around what has become known as the Fertile Crescent. This term was first coined by J.H. Breasted (1916), when he described an area covering a narrow arc from the southeast corner of the Mediterranean Sea to the northern tip of the Persian Gulf (Figure