have been described, it should be pointed out that they are best considered as complementary rather than independent approaches. Usually, the molecular tools are used to generate variability for selection, or to facilitate the selection process. After genetically modifying plants using molecular tools, it may be used as a parent in subsequent crosses to transfer the desirable genes into adapted and commercially desirable genetic backgrounds, using conventional tools. Whether developed by conventional or molecular approaches, the genotypes are evaluated in the field by conventional methods, and then advanced through the standard seed certification process before the farmer can have access to it for planting a crop. The unconventional approach to breeding tends to receive more attention from funding agencies than the conventional approach, partly because of its novelty and advertised potential, as well as the glamour of the technologies involved.
Regardless of the approach, a breeder follows certain general steps in conducting a breeding project. A breeder should have a comprehensive plan for a breeding project that addresses the following:
ObjectivesThe breeder should first define a clear objective (or set of objectives) for initiating the breeding program. In selecting breeding objectives, breeders need to consider:The producer (grower) from the point of view of growing the cultivar profitably (e.g. need for high yield, disease resistance, early maturity, lodging resistance).The processor (industrial user) as it relates to efficiently and economically using the cultivar as raw materials for producing new product (e.g. canning qualities, fiber strength).The consumer (household user) preference (e.g. taste, high nutritional quality, shelf life).The tomato will be used to show how different breeding objectives can be formulated for a single crop. Tomato is a very popular fruit with a wide array of uses, each calling for certain qualities. For salads, tomato is used whole, and hence the small size is preferred; for hamburgers, tomato is sliced, round large fruits being preferred. Tomato for canning (e.g. puree) requires certain pulp qualities. Being a popular garden species, gardeners prefer a tomato cultivar that ripens over time so harvesting can be spaced. However, for industrial use as in the case of canning, the fruits on the commercial cultivar must ripen together, so the field can be mechanically harvested. Further, whereas appearance of the fruit is not top priority for a processor who will be making tomato juice, the appearance of fruits is critical in marketing the fruit for table use.
GermplasmIt is impossible to improve plants or develop new cultivars without genetic variability. Once the objectives have been determined, the breeder then assembles the germplasm to be used to initiate the breeding program. Sometimes, new variability is created through crossing of selected parents, inducing mutations, or using biotechnological techniques. Whether used as such or recombined through crossing, the base population used to initiate a breeding program must of necessity include the gene(s) of interest. That is, you cannot breed for disease resistance if the gene conferring resistance to the disease of interest does not occur in the base population.
SelectionAfter creating or assembling variability, the next task is to discriminate among the variability to identify and select individuals with the desirable genotype to advance and increase to develop potential new cultivars. This calls for using standard selection or breeding methods suitable for the species and the breeding objective(s).
EvaluationEven though breeders follow basic steps in their work, the product reaches the consumer only after it has been evaluated. Agronomists may participate in this stage of plant breeding. In a way, evaluation is also a selection process, for it entails comparing a set of superior candidate genotypes to select one for release as a cultivar. The potential cultivars are evaluated in the field, sometimes at different locations and over several years, to identify the most promising one for release as a commercial cultivar.
Certification and cultivar releaseBefore a cultivar is released, it is processed through a series of steps, called the seed certification process, to increase the experimental seed, and to obtain approval for release from the designated crop certifying agency in the state or country. These steps in plant breeding are discussed in detail in this book.
1.6 How have plant breeding objectives changed over the years?
In a review of plant breeding over the past 50 years Baenzinger and colleagues in 2006 revealed that while some aspects of how breeders conduct their operations have dramatically changed, others have stubbornly remained the same, being variations on a theme at best. Current plant breeding objectives still emphasize the following general areas:
Higher yields of harvested produce or product.
Improved quality of produce or product.
Resistance to biotic stresses.
Resistance to abiotic stresses.
Wider adaptability of varieties.
A significant point to emphasize is that the focus of breeders within each of these general areas varies from one crop to another, as dictated by consumer preferences and production systems, among other factors.
Breeding objectives in the 1950s and 1960s and before appeared to focus on increasing crop productivity. Breeders concentrated on yield and adapting crops to their production environment. Resistance to diseases and pests was also priority. Quality traits for major field crops, such as improved fiber strength of cotton and milling and baking quality of wheat were important in the early breeding years. Attention was given to resistance to abiotic stresses like winter hardiness, and traits like lodging resistance, uniform ripening, and seed oil content of some species. Crop yield continued to be important throughout the 1990s. However, as analytical instrumentation that allowed high throughput, low cost, ease of analysis, and repeatability of results became more readily available, plant breeders began to include nutritional quality traits into their breeding objectives. These included forage quality traits like digestibility and neutral detergent fiber.
More importantly, with advanced technology, quality traits are becoming more narrowly defined in breeding objectives. Rather than high protein or high oil, breeders are breeding for specifics like low linolenic acid content, to meet consumer preferences of eating healthful foods (low linolenic acid in oil provides it with stability and enhanced flavor, and reduces the need for partial hydrogenation of the oil and production of trans fatty acids). Also, a specific quality trait like low phytate phosphorus in grains (e.g. corn, soybean) would increase feed efficiency and reduce phosphorus in animal waste, a major source of environmental degradation of lakes.
With advances in science and technology, breeding objectives are being achieved much quicker, as breeders are now able to utilize more efficient selection schemes to advance breeding programs. Instead of focusing on single genes, breeders access gene networks and target whole genomes in their research and applications. They are able to address more complex problems that heretofore were challenging. Perhaps no single technology has impacted breeding objectives in recent times more than biotechnology (actually, a collection of biological technologies). The subject of technological advances in breeding is discussed in detail in later chapters. Biotechnology has enabled breeders to develop a new generation of cultivars with genes included from genetically unrelated species (transgenic or GM cultivars). The most successful transgenic input traits to date have been herbicide resistance and insect resistance, which have been incorporated into major crop species like corn, cotton, soybean, and tobacco. According to a 2010 International Service for the Acquisition of Agri‐Biotech Crops (ISAAA), GM is far from being a global industry, with only six countries (US, Brazil, Argentina, India, Canada, and China) growing about 95% of the total global acreage (US leads with about 50%). The trend has changed, except that acreages in these countries in 2019 were significantly higher than in 2010. Some argue that biotechnology has become the tail that wags the plant breeding industry. Improvement in plant genetic manipulation technology has also encouraged the practice of gene stacking in plant breeding. Another significant contribution of biotechnology
