Crop Responses to Environment

Adapting to Global Climate Change, Second Edition

Anthony E. Hall

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9781351377584
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Following in the tradition of its predecessor, Crop Responses to Environment, this fully updated and more comprehensive second edition describes aspects of crop responses to environment that are particularly relevant to the development of improved crop cultivars and management methods on a global scale. It includes an extensive discussion of the difficulties in developing agricultural systems that accommodate increasing human needs for agricultural products during the twenty-first century in a sustainable manner. The book features new sections on adaptation to global climate change including adapting to global warming, elevated atmospheric carbon dioxide concentration, and increased flooding and salinity through plant breeding and changes in crop management. Warming effects include stressful effects of heat on pollen development and reduced winter chilling effects on fruit and nut trees.

The book examines principles, theories, mathematical models, and experimental observations concerning plant responses to environment that are relevant to the development of improved crop cultivars and management methods. It illustrates the importance of considering emergent plant properties as well as reductionist approaches to understanding plant function and adaptation. Plant physiological and developmental responses to light and temperature, and plant water relations are considered in detail.

Dr. Hall also describes climatic zone definitions based on temperature, rainfall, and evaporative demand in relation to plant adaptation and the prediction of crop water use. Irrigation management and crop responses to salinity, flooding and toxic levels of boron and aluminum are considered. Crop responses to pests and diseases as they interact with crop responses to physical and chemical aspects of the environment are examined. The book concludes with analyses illustrating the relevance of crop responses to environment to plant breeding.

1	Introduction

Plant responses to environment determine the adaptation of plants and influence the improvement of cropping systems that can be achieved through changes in management practices and plant breeding. The importance of this discipline is that, as will be shown later in this chapter, substantial increases in the efficiency of crop production will be required during the twenty-first century. My initial definition of efficiency of crop production is production per unit land area, that is, yield. Subsequently, I will describe some other types of efficiency, which in general terms is a ratio of output to input. An understanding of crop responses to environment will provide the fundamental basis for developing improved varieties and complementary management methods that result in increases in yield.

In many parts of the world increases in yield will be needed because there will be greater demands for agricultural products. Increased demands will occur due to increasing human populations and changes in consumption patterns, such as increased consumption of milk and other dairy products and meat from livestock reared on feed grains. There also may be increased demands for crop products to produce biofuels, such as the use of grains of maize or stalks of sugar cane to produce ethanol.

Increases in crop yield are particularly important for developing countries because this is where the greatest increases in demand for food will occur, and because improvement in agriculture can stimulate rural and urban development. This development is essential for decreasing the income gap between the rural poor and the rich people in this world. Increases in crop production efficiency are required in all countries to maintain profitability and enhance sustainability of agricultural enterprises, and to contribute to environmental health.

The alternative to increasing yield—increasing production through expansion of arable lands by reclaiming marshes and other wet lands, cutting down forests, and plowing up grass lands as was practiced in the past—should be discouraged. These wild lands should be preserved because they provide important services to the biosphere on which we all depend. Biosphere is a term used to describe all of Earth’s living organisms interacting with the physical and chemical environment as a whole. At this time, China is planning the preservation of large areas of land for use as national parks.

For developing countries increasing crop production through expansion of arable lands could result in another problem: further increases in the human population. Evans (1998) examined relationships between human populations and agriculture from ancient times to the late 1900s. He describes two contrasting hypotheses: (1) that increases in human population have stimulated attempts to increase agricultural production, or (2) that opportunities for increasing food production have encouraged or permitted increases in human population. Distinguishing between these two possibilities is important for communities of people whose populations have a tendency to increase rapidly.

New technologies and other developments that make possible the exploitation of new lands by agriculture could tend to stimulate even greater increases in human population, especially if the new lands provide only poor living conditions for farm families. Note that, typically, the best lands already are being used for agriculture, so expanding the amount of land that is cultivated usually involves moving into areas where the soils have major problems such as acidity, alkalinity, or salinity. In general, the very rapid increases in human populations that could occur due to expanding farming into marginal soil areas would cause major problems.

New technologies that enable poor farmers to make more efficient use of their current arable land in general are more desirable. Although they lead to increased profits and improved living conditions for farm families, the new technologies can indirectly result in decreases in human birth rates. As was pointed out by Murdock (1990), “Poor parents have many children because the economic benefits of the children outweigh their economic costs. The benefits come in the form of labor, income, and security for parents in their old age. As parents’ incomes rise, and inevitably also their level of education, and as the economic structure of society changes, the benefit/cost ratio of children declines. As income increases, the balance will favor smaller families.” Note that improved education and emancipation of women will enhance this trend toward smaller families. Consequently, increases in crop yield can give three important benefits: provide more food for hungry people, increase profits, and encourage decreases in the growth rate of the human population.

By 2050 the human population is predicted to increase to 9.3 billion from the current level of 7.4 billion. A major analysis has been made by Fischer et al. (2014) of whether increases in crop yield will continue to feed the world up to 2050. Their book, which is available at no cost on the Internet (http://aciar.gov.au/publication/mn158), covers many topics that are relevant to crop responses to environment. The authors analyze data for the world’s most important food crops: rice, wheat, maize, and soybeans. These crops either indirectly (as feed grains) or directly (as food for people) provide two thirds of the calories and protein consumed by humans. They point out that global food demand for these crops is predicted to increase by 60% from 2010 to 2050. The authors conclude that the minimum yield increases of these major staple food crops needed to feed the world in 2050, while preventing price increases, is 1.1%–1.3% per year (relative to 2010 yields). They point out that the current increases in yield in 2014 of wheat, rice and soybean were 1.0% relative to 2010 yields. Consequently, a 10%–30% increase is needed in the rate of increase in yield of these very important crops. I consider that these increases in the rate of increase in yield of wheat, rice, and soybean will be very difficult to achieve because in many cases the easiest ways for increasing yield already have been taken. Also, changes are occurring in the abiotic and biotic environment that will tend to decrease yield. In 2014 the global increase in yield of maize was 1.5% per year relative to 2010 yields. If maintained, this increase could be enough to meet future demands for maize. But much of the grains of maize are being diverted to either feed livestock or produce ethanol, and maize contributes only moderately to the food needs of poor people.

Simply maintaining yields at current levels also often requires the development of new cultivars and new management methods, since pests and diseases continue to evolve, and aspects of the chemical, physical, and social environment can change over several decades (Dobermann et al., 2000). About 50% of the current effort by rice and wheat breeders is devoted to maintenance breeding to incorporate resistances to diseases and pests and does not result in increases in yield potential (Fischer et al., 2014).

In the 1960s, many people considered pesticides to be mainly beneficial to mankind. Developing new, broadly effective, and persistent pesticides often was considered to be the best way to control pests on crop plants. Since that time, it has become apparent that broadly effective pesticides can have detrimental effects on beneficial insects, which can negate the overall effects of the pesticide in controlling pests. In addition, persistent pesticides can damage non-target organisms in the ecosystem, such as birds and people. Also, it has become difficult for companies to develop new pesticides, even those that can have major beneficial effects and few negative effects. Very high costs are involved in following all of the procedures needed to gain government approval for new pesticides. Consequently, more consideration is being given to other ways to manage pests, such as incorporating greater resistance to pests into cultivars by breeding and using other biological control methods.

Global climate change is occurring. Due to the burning of fossil fuels the carbon dioxide concentration in the atmosphere has been increasing and this impacts the photosynthesis of different crop plants in different ways, as is discussed in Chapter 4. The increases in carbon dioxide concentration and other greenhouse gases raise the heat load on the Earth, as is discussed in Chapter 7, and are causing rises in temperature. In some circumstances, the increases in temperature are reducing crop yields. For example, yields of rice crops grown under optimal management on the experimental fields of the International Rice Research Institute in the Philippines decreased during a 12-year period from 1992 to 2003 (Peng et al., 2004). From 1979 to 2003, day temperatures had increased 0.35°C while night temperatures had increased 1.13°C. One might expect high day temperatures to be most stressful, however, grain yield exhibited a 10% decrease in yield per degree C increase in night temperature with no correlation with day temperature.

Correlations do not necessarily indicate a causal relation. However, experiments where nighttime temperatures were raised in the field resulted in 4% decreases in grain yield of cowpea per degree C increase in night temperature (Nielsen and Hall, 1985b). In some hot regions there will be a tendency for grain yields of some crops to decrease with global warming if heat-tolerant varieties are not developed. In contrast, in some cold regions there will be a tendency for yields of some crops to be increased by global warming.

Chapter 5 discusses crop physiological responses to temperature, while Chapter 6 discusses crop developmental responses to temperature such as the chilling requirements of crops. Whether chilling requirements are met is being influenced by global warming—for example, winter chilling hours are decreasing in California and jeopardizing the stone fruit and nut tree industries (Baldocchi and Wong, 2008). Global warming also is raising seawater levels and could result in greater flooding of delta and low-lying coastal areas that will impact crop production, as is discussed in Chapter 11. The adaptations of different crops to global climate change in different parts of the world are discussed in a book edited by S. S. Yadav et al. (2011).

When considering methods for increasing crop yields it is useful to distinguish among the average yields obtained by farmers in an area (FY), the potential yield that can be obtained in that area by using the best variety grown with the most effective management methods (PY), and the yield gap, that is, the difference between FY and PY (Fischer et al., 2014). The yield gap can be described as a % of FY.

For maize in much of sub-Saharan Africa, the biggest opportunity for increasing FY is to decrease the yield gap by using improved management methods. For example, the yield gap for maize in East Africa is about 400% (i.e., with a FY of 2 ton ha⁻¹, the yield gap would be 8 ton ha⁻¹ and PY would be 10 ton ha⁻¹). The main limitation for yield is the infertile soil. FY of maize in sub-Saharan Africa can be substantially increased by increasing the supplies of nitrogenous fertilizer together with using the best hybrid varieties. However, the ratio of the cost of a kg of nitrogen (N) in nitrogenous fertilizer in relation to the price the farmer receives for a kg of maize grain is high in East Africa illustrating why it can be uneconomic for these farmers to apply much fertilizer to their maize crop. Farmers also often can have difficulty obtaining credit to buy fertilizer or seed of improved varieties. Encouraging farmers to apply more nitrogenous (and phosphate) fertilizer to their maize crops and use hybrid varieties that are responsive to fertilizer applications will require improving infrastructure so that they can obtain credit at a reasonable price, buy fertilizer and hybrid seed at cheaper prices, and obtain higher prices when they sell their crops. Also, more extension agents are needed to advise African farmers concerning the most effective varieties and management methods for their region.

Increasing crop yields by applying more fertilizer can be complex. For example, the yield gap of other cereals grown in Africa can be high, as with the cases of pearl millet in the Sahelian zone of West Africa and sorghum in northern Sudan. The yields of these crops can be substantially increased by small applications of fertilizer (i.e., 20 kg ha⁻¹ of N plus 9 kg ha⁻¹ of P) even though these zones suffer from droughts, providing the crops are grown at a denser spacing (1 × 1 meter) than is the common practice of placing one plant every 2 × 2 meters. This farming practice of using very wide spacing probably was an adaptation to the very infertile soils and the need to plant a large area in a short period of time. Note that increasing the plant density without applying fertilizer can result in a decrease in yield because the plants may suffer a severe deficiency of nitrogen and turn yellow. Also, applying fertilizer but retaining the wide spacing may only result in a small increase in yield that may not be economic. Both new practices must be used together—fertilizer application and denser plant spacing.

In the U.S. state of Iowa, the yield gap for maize is about 36% (i.e., with a FY of 10 ton ha⁻¹, the yield gap would be 3.6 ton ha⁻¹ and PY would be 13.6 ton ha⁻¹). FY is close to the attainable yield and there is little opportunity for closing the yield gap by extending improved management methods and varieties. In this case, the major opportunity for increasing FY is by breeding improved maize hybrids and developing complementary improved management methods that result in greater yields, thereby increasing PY. Opportunities exist for increasing PY of maize because in 2014 the average rate of increase of PY was 1.1% per year relative to PY in 2010.

For wheat and rice, attempts should be made to increase FY by both decreasing the yield gap, through improved extension, and increasing PY by breeding improved varieties and developing complementary improved management methods. The average rates of increase per year in PY in 2014 were 0.6% for wheat and 0.8% for rice relative to PY values in 2010. Consequently, increasing PY of wheat and rice will not be easy.

In addition to improved cultivars, increasing yields of cereals usually will require enhanced soil nitrogen supplies. For example, cereals with PY of 6–9 ton ha⁻¹ must take up 200–300 kg ha^–1 of nitrogen (N) if they are to achieve these yields. Deficiencies in soil N are common in the tropics and subtropics. The major available additional source of soil N for cereal crops is from the application of nitrogenous fertilizers. On a global basis, increased applications of nitrogenous and phosphate fertilizers will be needed, but injudicious use can have costs in terms of nitrate pollution of groundwater, phosphate pollution of surface waters, and pollution of the atmosphere with gaseous nitrogen oxides (NO_x).

Another source of soil N for cereal crops is the symbiotic fixation of atmospheric nitrogen by previous leguminous crops. As Graham and Vance (2000) point out, however, there has been a worldwide decline in agricultural use of leguminous crops and inoculation with rhizobia. For example, expansion in land area devoted to cereal production has been associated, in some cases, with a decrease in area devoted to grain legumes. Graham and Vance (2000) have reviewed the advantages and constraints on increasing nitrogen supplies to cropping systems by increasing nitrogen fixation. From this review, it is clear that the main opportunity for enhancing contributions to agriculture from nitrogen fixation may be with the more extensive systems such as pastures that contain legumes, and that intensive agricultural systems will continue to need large applications of nitrogenous fertilizer or manure.

In addition to increasing food quantity, there is a need to enhance the nutritional quality of the food that people eat (Welch and Graham, 1999). For example, in South Asia, where cereal production increased fourfold between 1965 and 1995, grain legume production declined about 20%. Yet, grain legumes provide certain essential amino acids, vitamins, and minerals that are not provided in sufficient quantities by cereal grains.

I directed a project that increased yields of a grain legume, cowpea, on fields of poor farmers in a very dry part of sub-Saharan Africa (Hall, 2017). Cowpea is a cheap source of protein that complements the protein in cereals grown in the Sahel, such as pearl millet, sorghum, and rice. In addition, through nitrogen fixation and uptake of phosphate, and supply of hay to animals, cowpeas enrich the soil, such that cereals grown in rotation produce greater yields.

Information from several international centers that are working to enhance yields and qualities of the major cereals and grain legumes and other food crops can be obtained from the Consultative Group on International Agricultural Research website (www.cgiar.org). The U.S. Department of Agriculture’s science magazine provides information on a wide range of agricultural topics and can be found online (www.ars.usda.gov/is/AR/).

Future needs for agricultural products will be influenced by the size of the human population. There are some parts of developing countries where human populations are increasing at rates as fast as 3% per year which, if maintained, will result in a doubling of these populations within the short period of 23 years. The doubling time in years can be calculated from the following equati...