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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 rainfall. Soil water stored from monsoon. Temperate.
5A	Spring	High rainfall. Hot (coolest month > 17.5 °C).
5B	Spring	Irrigated. Hot.
6	Spring	Moderate rainfall. Winter would kill wheat. Coolest month < –10 °C.
7	Facultative	Irrigated. Cool.
8A	Facultative, daylength sensitive	High rainfall. Cool.
8B	Facultative, daylength insensitive	High rainfall. Cool.
9	Facultative	Low rainfall (< 500 mm available to crop). Cool.
10A	Winter	Irrigated. Cold winter.
10B	Winter	Rainfall + irrigation. Cold winter.
11A	Winter, daylength sensitive	High rainfall. Cold winter.
11B	Winter, daylength insensitive	High rainfall. Cold winter.
12	Winter	Low rainfall. Cold winter, hot summer.

The growth of wheat can be broadly divided into two phases: the vegetative phase and the reproductive phase. The distinction between the two phases depends on events in the bud tip, or stem apex (i.e. the apical meristem if referring to the main stem) of the plant. The vegetative phase encompasses germination, and then a period during which growth from the stem apex gives rise only to vegetative tissues: leaves, tillers (i.e. the branches or side stems of grass plants), and adventitious roots.

Table 1.4 Growth stage scores of wheat plants.

	Decimal growth stages (DGS) according to Zadoks et al. (1974)	The scale of Feekes (1941)
Major growth phases	Major division	Minor divisions described by n	Score	Description	Equivalent DGS score
Germination	0n	n indicates development before leaf emergence i.e. from dry seed (n = 0) to first leaf at coleoptile tip (n = 9)
Leaf production	1n	n is the number of unfolded leaves on the main stem to a max. n = 9	1	Seedling emergence to three leaf stage	10–13
Tiller production	2n	n is the number of tillers with their own unfolded leaf to a max. n = 9	2	Start of tillering	21–25
3	End of tillering	26–29
Stem extension	3n	n is the number of nodes to a max. of 6, then n = 7 for flag leaf just visible, and n = 9 flag leaf unfolded	4	Leaf sheaths lengthen	30
5	Leaf sheaths strongly erect	30
6	One node detectable	31
7	Two nodes detectable	32
8	Flag leaf just visible	37
9	Flag leaf emerged	39
Booting	4n	n indicates degree of swelling of the boot	10	Booting	45
Ear emergence	5n	n indicates proportion of ear or ears emerged	10.5	Full ear emergence	59
Anthesis	6n	n indicates degree of completion	10.51	Start of anthesis	60
Grain expansion	7n	i.e. droplet exuded when caryopsis squeezed changes from watery (n = 1) to milky (n = 7)	10.54	Caryopsis water ripe	71
11.1	Caryopsis milky ripe	75
Dough development	8n	i.e. no droplet exuded from squeezed caryopsis, thumbnail imprint not retained (n = 3) to thumbnail imprint retained (n = 5+)	11.2	Soft dough	85
Ripening	9n	Grain dry enough to harvest (n = 2) to seed with no primary dormancy (n = 7)	11.4	Harvest ripe	92

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.