The goal of crop breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit or pod properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower of a different plant variety.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selfing and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One crop species which has been subject to such breeding programs and is of particular value is garden bean (Phaseolus vulgaris (snap)). Beans are annual, warm-season legumes. Garden beans, also known as green beans, snap beans, or pole beans, are grown primarily for their pods, which are harvested for consumption in their succulent form, whereas dry beans (Phaseolus vulgaris (dry)), lima beans (Phaseolus limensis), and soybeans (Glycine max) are usually grown for the seed itself. In addition, the bean leaf is occasionally used as a leaf vegetable, and the straw is used for fodder.
Garden beans are available in bush and pole varieties. Bush varieties form erect bushes 20-60 centimeters tall, while pole varieties form vines 2-3 meters long. The pods are typically 8-20 centimeters long, 1-1.5 centimeters wide, and either green, yellow, black or purple in color. Each pod generally contains 4-6 beans.