A hybrid plant is a cross between two or more stable in-bred varieties. A successful hybrid combines the most desirable characteristics of the parent plants, such as color, hardiness, disease resistance, earliness or productivity.
In plant breeding, the most common type of hybrid involves a cross between populations, varieties, or cultivars within a single species. The first result of a single cross between two parent plants is known as a F1 hybrid, which stands for Familial 1.
Seed from later generations of hybrid plants will not reliably produce a plant with the same characteristics. That seed will grow into a plant with an unpredictable mix of qualities and features of the parents.
F1 hybrid plants are often stronger and more vigorous than either of the parent plants. This is known as heterosis, or hybrid vigor. Another term less often used is outbreeding enhancement, since heterosis is the opposite of inbreeding depression (the steadily declining vigor resulting from repeated inbreeding).
Botanists have developed two competing theories to explain heterosis, which is not predictable. Neither hypothesis has yet been proven, so the reasons for hybrid vigor remain unknown.
Hybrid food plants can dramatically increase yield. The best-known example of this is corn (maize). This important food plant has been hybridized repeatedly over many years. More recently, many vegetable varieties have been hybridized to produce large quantities to satisfy grower demand for higher per-acre yields. Some hybrids produce crops earlier in the season.
Heirloom and other open-pollinated food plant varieties are valued for their taste and other unique characteristics, but fruit size and yield sometimes vary unpredictably. Hybrids have been created that produce uniform-size fruit or leafy heads, which are commercially desirable traits. Similarly, hybrid ornamental plants grow to predictable heights and shapes, which is of value to the landscaping industry.
The ornamental plant industry is heavily dependent on annual introductions of new hybrid annuals and perennials. More showy flowers or foliage, along with creation of new flower colors, is the result of ongoing research into hybrid technology.
Both food and ornamental hybrids have been produced that are capable of surviving extremes of heat and cold, thus increasing the range of areas in which they can be grown. This is becoming important in developing countries where people are increasing the demand for fresh produce of varieties not traditionally grown there.
Equally important in agriculture is the creation of hybrids that resist or at least tolerate plant diseases. Many hybrid vegetables, for example, have resistance to a long list of diseases. Whether diseases are spread in soil or by insects and other organisms, hybrids have been created that can survive these attacks.