Wheat. Peter R. Shewry

1.2 The Wheat Plant

      As already said, wheat, like other cereals, is a grass plant grown primarily for its edible seed. The lineages of wheat, rice, and maize may have diverged from a common ancestor about 40 MYA (Huang et al. 2002; Gill et al. 2004). The wheats (Triticum spp.) form part of the Triticeae tribe, which also includes closely related cereals, notably barley ( Hordeum vulgare ) and rye ( Secale cereale ), as well as many wild grasses, such as the weed common couch grass ( Elymus repens ). The lineages of wheat and barley appear to have separated 10–14 MYA, and of wheat and rye 7 MYA.

Table 1.2 Köppen climate classifications used in Figure 1.6.

      Source: adapted from Peel et al. (2007).

Grouping	Water distribution	Temperature description	Composite classification
B Desert/semi‐arid
	S Semi‐arida
		k coldb	BSk
C Temperate/mesothermalc
	s Dry summerd (Mediterranean)
		a Hot summere	Csa
	w Dry winterf (e.g. subtropical)
		a Hot summere	Cwa
	f No dry season (e.g. oceanic)
		b Warm summerg	Cfb
D Continental/microthermalh
	w Dry winterf
		a Hot summere	Dwa
	f No dry season
		a Hot summere	Dfa
		b Warm summerg	Dfb

      Definitions:

      ^a ‘Semi‐arid’ is where the mean annual precipitation is > 5× but < 10 × a threshold precipitation. When precipitation is relatively evenly distributed, the threshold precipitation is 2 × mean annual temperature (°C) +14.

      ^b ‘Cold’ is where mean annual temperature is < 18 °C.

      ^c ‘Temperate’ is where the mean temperature of the hottest month is > 10 °C and the coldest month is between 0 and 18 °C.

      ^d A ‘dry summer’ is where the precipitation in the driest summer month is < 40 mm and also less than a third of the wettest month in winter.

      ^e A ‘hot summer’ is where the mean temperature of the hottest month is ≥ 22 °C.

      ^f A ‘dry winter’ is where the wettest winter month has less than a tenth of the precipitation than the wettest summer month.

      ^g A ‘warm summer’ is where the hottest month is < 22 °C but there are at least four months where the mean temperature is > 10 °C.

      ^h A continental climate is where the hottest month is > 10 °C but the coldest month is ≤ 0 °C.

The Triticeae have common structures and patterns of development typical of many grasses. It is normal to define phases of development in terms of growth stage scores (Table 1.4; Figure 1.7). Growth stage scores are used extensively to interpret the effects of the environment on crop growth and yield, and to optimize the timings of agronomic treatments including irrigation and the application of fertilizers, growth regulators, and crop protection agrochemicals (Barber et al. 2015). They can also be used to define safe grazing periods for dual‐purpose crops (Gooding et al. 1998). The most widely used growth stage scoring method for wheat, and that used throughout this book, is the decimal growth stage (DGS) system of Zadoks et al. (1974). Equivalents between the Feekes and Zadoks scales are given in Table 1.4. Equivalents with lesser‐used scoring systems are provided by Landes and Porter (1989) and Harrell et al. (1993, 1998).

Table 1.3 Mega‐Environments (ME) for wheat.

      Source: Adapted from Rajaram et al. (1993) and Gbegbelegbe et al. (2017).

ME	Wheat type	Agro‐climatic conditions
1	Spring	Irrigated. Temperate in winter to late heat stress.
2	Spring	High rainfall (> 500 mm during cropping season). Temperate.
3	Spring	High rainfall. Temperate. Acid soils.
4A	Spring	Low rainfall. Winter rain followed by drought. Temperate.
4B	Spring	Low rainfall. Early drought preceding summer rain. Temperate.
4C	Spring	Low