Wheat. Peter R. Shewry

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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

Schematic illustration of stem extension, booting, ear emergence, and anthesis of wheat.

      1.2.2.2 Booting and Ear Emergence

      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.

Schematic illustration of structures of a single mature spikelet. Schematic illustration of the Fertile Crescent as originally described by J.

      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).

      1.3.1 Origin in the Fertile Crescent


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