numerous disease‐free plants can be obtained.Heat treatmentThis may be short‐ or long‐duration. Short‐duration heat treatment is administered to the plant material for about 30 minutes to 4 hours at 43–57 °C. This could be in the form of hot air treatment or by soaking the material in hot water. This works well for fungal, bacterial, and nematode infections. For viruses, a longer treatment of about several weeks (two to four weeks) is used. Potted plants are held at 37 °C in a controlled environment for the duration of the treatment. Cuttings from the treated plants may be used as scions in grafts, or rooted into a seedling.Chemical treatmentThis surface sterilization treatment is suitable for elimination of pathogens that are external to the plant material (e.g. in tubers).Use of apomictic seedViral infections are generally not transmitted through seed in cultivars that are capable of apomixis (e.g. citrus).
Clonal selection for cultivar development
This procedure is effective if variability exists in the natural clonal population.
| Year 1 | Assemble clonal population. Plant and expose to diseases of interest. Select resistant clones with other superior traits and harvest individually. |
| Year 2 | Grow progenies of selected clones and evaluate as in year 1. Select superior clones. |
| Year 3 | Conduct preliminary yield trials. Select superior clones. |
| Year 4–6 | Conduct advanced yield trials at multiple locations for cultivar release. |
7.9.3 Crossing with clonal selection
This procedure is applicable to species that are capable of producing seed in appreciable quantities. Because heterosis can be fixed in clonal populations, the breeder may conduct combining ability analysis to determine the best combiners to be used in crosses. The steps below are not applicable to trees in which much longer times are needed.
| Year 1 | Cross selected parents. Harvest F1 seed. |
| Year 2 | Plant and evaluate F1s. Select vigorous and healthy plants. |
| Year 3 | Space plant clonal progeny rows of selected plants. Select about 100 to 200 superior plant progenies. |
| Year 4 | Conduct preliminary yield trials. |
| Year 5–7 | Conduct advanced yield trials for cultivar release. |
Other techniques that are applicable include backcrossing to transfer specific traits and wide crossing. The challenges with backcrossing are several. As previously indicated, clonal species are very heterozygous and prone to inbreeding depression. Backcrossing to one parent (the recurrent parent) provides opportunity for homozygosity and consequently inbreeding depression. To prevent this, breeders may cross the backcross to another clone instead of the recurrent parent, followed by selection to identify superior plants. The process is repeated as needed.
7.9.4 Mutation breeding
The subject is discussed in detail in Chapter 22. Inducing variability via mutagenesis is challenging for two key reasons. Being rare events, a large population of M1V2 is needed to have a good chance of observing desired mutants. Obtaining a large number of vegetative propagules is difficult. Also, mutations occur in individual cells. Without the benefit of meiosis, the mutated clonal material develops chimeras. Using adventitious buds as starting material reduces the chance of chimeras. A mutation in the epidermal cell (usually there is one) would result in an adventitious shoot that originated from a single mutant cell. This technique is not universally applicable.
7.9.5 Breeding implications, advantages, and limitations of clonal propagation
There several advantages and limitations of breeding clonally propagated species:
Advantages
Sterility is not a factor in clonal propagation because seed is not involved.
Because clonal plants are homogeneous, the commercial product is uniform.
Micropropagation can be used to rapidly multiply planting material.
Heterozygosity and heterosis are fixed in clonal populations.
Disadvantages
Clonal propagules are often bulky to handle (e.g. stems, bulbs).
Clones are susceptible to devastation by an epidemic. Because all plants in the clonal population are identical, they are susceptible to the same strain of pathogen.
Clonal propagules are difficult to store for a long time, because they are generally fresh and succulent materials.
7.10 Natural propagation
Some crops rely on clonal propagation: tubers, corms, cuttings, bulbs, stolons, etc. Such crop species may have lost the capacity to flower (leek, some potato cultivars). Their progeny is genetically identical to the plant from which it was derived (except if the primordial cell contained some mutation [chimerism]). Normally, such species may also have sexual reproduction as a natural option. Potato, for example, may form berries with true seeds, and strawberry produces fruits with seeds. Such seeds produce genetically heterogeneous progeny because of segregation, since most clonally reproducing species have a high level of heterozygosity. Plants from natural clonal tissues are usually vigorous and can produce flowers and fruit in the same or next season. Plants derived from true seeds of those same species often have a long juvenile stage, and take long to reach commercially interesting size (orchids, tulips, chrysanthemum, potato). The same is true for species that naturally do not reproduce clonally, but as crops which have been reproduced that way for a long time. Examples are apple, rose, and ornamental trees and shrubs, which are reproduced by grafting or cutting.
7.11 In vitro culture
In vitro culture or tissue culture of cells, tissues, organs, and protoplasts is used as a technique by plant breeders and growers for propagation. It is critical in some modern plant breeding approaches, specifically biotechnology, in which genetic alterations are conducted under aseptic conditions. The cell is the fundamental unit of structure and function of a plant, containing all the genetic information. Tissues and even single cells can be nurtured to develop into full plants. In biotechnology, it is critical to be able to nurture a single cell into a full plant in order to apply some of the sophisticated techniques such as gene transfer or transformation. The technique
