Principles of Plant Genetics and Breeding. George Acquaah
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from mutation breeding, not being transgenic, are more acceptable to consumers who are unfavorably disposed to GM crops.
2.9.5 Changes in identifying and evaluating genetic variability
Identifying and measuring quantitative variability continues to be challenging, even though some progress has been made (e.g. QTLs analysis and mapping). This has been possible because of the new kinds of molecular markers that have been developed and the accompanying throughput technologies. QTLs are more precisely mapped, in addition to the increased precision of linkage maps (marker‐dense). The abundance of molecular markers and availability of more accessible genomic tools has made it easier for researchers to readily characterize biodiversity.
2.9.6 Selecting and evaluating superior genotypes
Selection schemes have remained relatively the same for a long time. Here, too, the most significant change over the last half century has been driven by molecular technology. The use of molecular markers in selection (MAS) gained significant attention over the period. Most traits of interest to breeders are quantitatively inherited. The continuing challenge with this approach is the lack of precision (need for more high‐resolution QTL maps) and higher throughput marker technology, among others. Selected genotypes are evaluated across time and space in the same old fashioned way.
2.10 Achievements of modern plant breeders
The achievements of plant breeders are numerous, but may be grouped into several major areas of impact – yield increase, enhancement of compositional traits, crop adaptation, and the impact on crop production systems.
Yield increaseYield increase in crops has been accomplished in a variety of ways including targeting yield per se or its components, or making plants resistant to economic diseases and insect pests, and breeding for plants that are responsive to the production environment. Yields of major crops (e.g. corn, rice, sorghum, wheat, and soybean) have significantly increased in the USA over the years (Figure 1.1). For example, the yield of corn rose from about 2000 kg ha−1 in the 1940s to about 7000 kg ha−1 in the 1990s. In England, it took only 40 years for wheat yields to rise from 2 metric tons ha−1 to 6 metric tons ha−1. Food and Agriculture Organization (FAO) data comparing crop yield increases between 1961 and 2000 show dramatic changes for different crops in different regions of the of the world. For example, wheat yield increased by 681% in China, 301% in India, 299% in Europe, 235% in Africa, 209% in South America, and 175% in the USA. These yield increases are not totally due to the genetic potential of the new crop cultivars (about 50% is attributed to plant breeding) but also due to the improved agronomic practices (e.g. application of fertilizer, irrigation). Crops have been armed with disease resistance to reduce yield loss. Lodging resistance also reduces yield loss resulting from harvest losses.
Enhancement of compositional traitsBreeding for plant compositional traits to enhance nutritional quality or meet an industrial need are major plant breeding goals. High‐protein crop varieties (e.g. high lysine or quality protein maize) have been produced for use in various parts of the world. Different kinds of wheat are needed for different kinds of products (e.g. bread, pasta, cookies, semolina). Breeders have identified the quality traits associated with these uses and have produced cultivars with enhanced expression of these traits. Genetic engineering technology has been used to produce high oleic sunflower for industrial use, while it is being used to enhance the nutritional value of crops (e.g. pro‐vitamin A golden rice). The shelf life of fruits (e.g. tomato) has been extended through the use of genetic engineering techniques to reduce the expression of compounds associated with fruit deterioration.
Crop adaptationCrop plants are being produced in regions to which they are not native, because breeders have developed cultivars with modified physiology to cope with variations in the duration of day length (photoperiod). Photoperiod insensitive cultivars will flower and produce seed under any day length conditions. The duration of the growing period varies from one region of the world to another. Early maturing cultivars of crop plants enable growers to produce a crop during a short window of opportunity, or even to produce two crops in one season. Furthermore, early maturing cultivars can be used to produce a full season crop in areas where adverse conditions are prevalent toward the end of the normal growing season. Soils formed under arid conditions tend to accumulate large amounts of salts. In order to use these lands for crop production, salt‐tolerant (saline and aluminum tolerance) crop cultivars have been developed for certain species. In crops such as barley and tomato, there are commercial cultivars in use with drought, cold, and frost tolerance.
Impact on crop production systemsCrop productivity is a function of the genotype (genetic potential of the cultivar) and the cultural environment. The Green Revolution is an example of an outstanding outcome of the combination of plant breeding efforts and production technology to increase food productivity. A chemically intensive production system (use of agrochemicals like fertilizers) calls for crop cultivars that are responsive to such high‐input growing conditions. Plant breeders have developed cultivars with the architecture for such environments. Through the use of genetic engineering technology, breeders have reduced the need for pesticides in production of major crops (e.g. corn, tobacco, soybean) with the development of GM pest resistant cultivars, thereby reducing environmental damage from agriculture. Cultivars have been developed for mechanized production systems.
Key references and suggested reading
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