Wheat. Peter R. Shewry
Читать онлайн книгу.
Other countries
Figure 1.4 Growth in wheat production since 1961 compared with the wheat area, the wheat yield per unit area, and human population growth.
Source: Data from FAOSTAT (FAO 2021).
Figure 1.5 The world primary commodity price of wheat (IMF 2021).
1.1.3 Wheat Adaptation
Much of the current success of wheat, as well as the hopes for its future production, relies on the adaptability of different cultivated varieties (cultivars) of wheat to a wide range of growing environments and production systems. Wheat production stretches through temperate areas, extending into the sub‐tropics and the sub‐polar regions, but also includes the tropical highlands of East Africa. Major areas of production (Figure 1.2) include river plains such as the Indo‐Gangetic plain (Figure 1.6h) and the North China plain (Figure 1.6i), where wheat is often irrigated and grown in the dry winters while rice and maize are common summer crops. By contrast, wheat is also grown over large mid‐continental expanses that would have originally supported extensive grasslands and/or forests such as the Eurasian Steppe (Figure 1.6e), the Canadian Prairies (Figure 1.6a), and the plains of the USA (Figure 1.6c). At high latitudes, wheat is frequently grown in the summer months to avoid the harsh winters (Figure 1.6a). In semi‐arid regions, such as some areas in the Pacific Northwest (Figure 1.6b), rainfed wheat is often grown every other year whilst water is replenished in soils left to fallow in intervening years. In contrast, intensive production in Oceanic NW Europe (Figure 1.6e) is made possible by evenly distributed rainfall and moderate temperatures. A wet, mild environment is also experienced in the Humid Pampas of Argentina (Figure 1.6d), where three crops are often grown (e.g. wheat, soybean, maize) in two‐year rotations. The wild wheats still growing in and around the eastern Mediterranean (Zohary et al. 1969), like their modern crop counterparts, are adapted to grow in the mild, wet winters, maturing before the hot dry summers (Figure 1.6f). The Australian wheat belts span numerous climatic zones, including the Mediterranean pattern (Figure 1.6j).
Figure 1.6 Mean temperature (line) and precipitation (bars) for locations in the 10 wheat growing regions labelled in Figure 1.2. Typical timings for sowing, flowering (anthesis), and harvest at location are denoted by s, a, and h, respectively. Wheat types (Section 1.2) are spring wheat (sw), facultative wheat (fw), and winter wheat (ww). K is the Köppen climate classification (Table 1.2), and M is the mega‐environment (Table 1.3) for the location. Note: both climate and mega‐environment can vary significantly within the growing regions. Mega‐environments and wheat types (and hence different cropping seasons) can also overlap at the same location (
Gbegbelegbe et al. 2017).
In the Köppen climate classification system, wheat production spans the semi‐arid, temperate, and continental zones (Table 1.2). For breeding purposes, particularly for the developing world, the International Maize and Wheat Improvement Centre (i.e. CIMMYT: Centro Internacional de Mejoramiento de Maíz y Trigo) has divided wheat‐growing areas into mega‐environments, which again emphasise the wide adaption of the crop (Table 1.3).
It is evident that the different growing environments contribute to a diversity of cropping systems, varying in sowing and harvest dates, rotational contexts, and the use of irrigation. The environment also plays a large role in determining the justification of inputs such as mineral fertilizers and crop protection agrochemicals. For example, 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