Barley

Abstract

In 2009 we published the first edition of Crop Physiology: Applications for Genetic Improvement and Agronomy, with a second edition in 2015. The premise of these books was that farmers rely primarily on two technological outputs: varieties and practices. Plant breeding and agronomy are, therefore, the two technologies with immediate application. Hence crop physiology is most valuable when it engages meaningfully with breeding and agronomy. This remains the premise of this book. Our previous books were structured around processes?development, the carbon, water, and nitrogen economies of crops, and the formation of yield in the context of cropping systems. In Hall (2018) updated his crop physiology book with a focus on processes in a timely context of climate change. With at least three process-based books between 2009 and 2018, we felt it was time for an alternative, albeit not new, perspective.In this book, we follow Evans (1975) scheme of revisiting physiological processes for individual crops. Chapters open with a crop-specific agronomic context and deal with development, the carbon, water, and nitrogen economies of individual crops, the formation of yield, and aspects of quality.The first group of chapters deals with cereals?maize, rice, wheat, barley, sorghum, and oat. Maize, rice, and wheat provide most of the energy and a good part of protein to human diets worldwide, and improvements in the yield of these crops are the core of food security for the foreseeable future. Barley and sorghum are mostly feed grains supporting the increasing demand for animal products in increasingly wealthy segments of some societies. Wheat, barley, and oat have a similar critical period when grain number, and in turn yield, is more vulnerable to stress (Fig. 1). Oat has become a niche crop after a peak in the early 20th century when mechanical engines replaced horses in transport and industries, including agriculture (Fig. 2); this highlights the disruptive nature of any meaningful technology. In common with quinoa, there is a potential for larger acreages of oat motivated by health and nutrition drivers, but the future of both crops is uncertain beyond their present niche. Physiologically, maize and sorghum share a C4 metabolism and are particularly adapted to warmer environments. However, maize and sorghum differ in the reproductive biology; owing to its lower apical dominance, tillering makes sorghum more comparable to rice and wheat.The second group of chapters deals with grain legumes. Soybean stands out as the world?s largest source of vegetal protein (Fig. 3a) and is critical to food security together with the three big cereals and potato. Common aspects of crop physiology in grain legumes include a typical critical period for yield response to stress (Fig. 1). In comparison to cereals, the critical window is delayed towards pod set and grain fill in legumes. Two main reasons underlie the difference in the critical period between small-grain cereals and grain legumes. Firstly, flowering is a convergent processes of development across stems from different categories in cereals. In comparison, flowering in pulses refers to the first flower on the main branch. The second reason, linked with the previous, is the contrasting flowering biology between determinate cereals and the extended flowering period of semi-determinate or indeterminate crops like grain legumes and canola. The resulting approximate 40 days difference in the timing of maximum grain yield sensitivity to stress supports a genuine physiological contrast between grain legumes and cereals with agronomic implications.Other common aspects are nitrogen fixation and implications for the role of legumes in cropping systems and qualitative (determinate vs. indeterminate nodules) and quantitative idiosyncratic features (e.g. degree of inhibition of nitrogen fixation by soil mineral nitrogen). The pulses are increasingly important but remain minor crops (Fig. 3b). As expected from the magnitude of the crop production, research efforts lag in other grain legumes relative to soybean but with no apparent proportionality, whereas current production of soybean is 8 times larger than the production of peanut and 20 times larger than the production of chickpea; a coarse measure of research output is only 5 times larger (Fig. 3c). Quantifying the gap between actual production and research aimed at improved production warrants investigation. Peanut sets pods underground. Because pods do not transpire, they do not receive xylem-transported Ca from the roots but absorb Ca directly from the surrounding soil through mass flow. These physiological traits have unique implications for calcium management in peanut.Sunflower, canola, and cotton feature oil-reach seeds with physiological implications (e.g. lower radiation use efficiency than seed crops with starchy grain), some similarities (e.g. influence of temperature on fatty acid composition in seed reserves), and contrasting seasonality and roles in cropping systems. Sunflower acreage and research effort is decreasing in contrast to the worldwide expansion of canola. Potato, cassava, sugar beet, and sugar cane are grown for carbohydrate-rich vegetative organs. They share a lack of (or tenuous) critical period in comparison to a marked sensitive stage in seed crops (Fig. 1). The potential decoupling of growth and reproduction is therefore not an issue in these species, but some physiologically interesting and agronomically important trade-offs require attention, e.g. between expansive growth and storage of sucrose in sugar cane. In contrast to many other plant species, which exclude sodium, sugar beet is a halophyte that absorbs and utilises it and can respond positivelyto sodium fertilisation.