Wheat. Peter R. Shewry

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СКАЧАТЬ although national production levels of wheat in the UK and Kazakhstan are similar, the two countries contrast in how these levels are achieved (Table 1.1). In Kazakhstan, low yields per hectare are produced over a large area where wheat growth is mostly limited by low water availability (Chapter 3) and high temperatures (Chapter 4). Much of the wheat is planted as late as May, giving a short growing season, but still exposed to abiotic stresses. By contrast, in the UK high land‐use efficiency is achieved on a much smaller area. Growth proceeds in clement conditions where yield is, therefore, mostly limited by light interception (Chapter 5). Most of the wheat grown in the UK is planted early in the autumn and proceeds to harvest over a period of 10–11 months. If, as in the UK, abiotic stresses are less frequently severe, wheat crops are particularly responsive to inputs, such as fertilizer and agrochemical applications. Together, these inputs serve to maximise light capture and therefore photosynthesis and yield (Chapter 6).

      The environment clearly has an impact on wheat supply and, conversely, wheat influences the environment. If we are to address the sustainability of wheat production, both factors need to be understood. Our current reliance on cereal grain, and on wheat in particular, has significant implications for human health and well‐being. The relationships between wheat production, the environment, and human diet and health are therefore the overarching themes of this book.

      For the remainder of this chapter, we will introduce three of the main factors that account for the global success of the wheat crop. Firstly, the wheat plant is described in terms of its growth and development as a grass. Secondly, we introduce the evolution of wheat and explain its wide adaptability. Finally, we introduce the unique processing properties of wheat and the diversity of wheat grain types and wheat‐based products.

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 СКАЧАТЬ