35.5:
Asexual Reproduction
Unlike most animals, plants can reproduce asexually, producing offspring that are genetically identical to the parent plant. Asexual reproduction in plants, also called vegetative reproduction, takes place without the fusion of sex gametes or meiosis.
Plants may reproduce asexually through natural mechanisms developed by plants or artificial mechanisms developed by breeders.
Vegetative propagation is a form of asexual reproduction in which detached root or stem fragments can develop into an entire new plant. For example, the ‘eye’, or bud, of a potato can be planted to regenerate a whole new plant.
Dandelions use another method of asexual reproduction in which they produce seeds without pollination or fertilization—a process called apomixis. Apomixis allows certain hybrid plants to overcome sterility and pass their intact genome to their offspring.
Breeders use a variety of artificial methods of asexual reproduction, such as grafting, cutting, layering, and micropropagation. These methods are sometimes used by plant breeders to generate novel plant varieties.
Grafting is a technique commonly used to grow grapes that combines favorable traits from different varieties. The upper portion of one plant is combined with the lower portion of another plant, forming a graft. The upper part of the graft is called the scion, while the lower part is called the rootstock. For example, a scion with large fruit may be grafted onto a disease-resistant rootstock, producing disease-resistant progeny with large fruit.
In conditions unfavorable for sexual reproduction, asexual reproduction allows species to survive and propagate. Asexual reproduction is particularly advantageous for plants that are well-adapted to their environments, as their genetically-identical offspring can help establish a flourishing colony.
However, asexual reproduction is sometimes undesirable, as it can severely limit the ability of plants to adapt to changing environmental conditions. Without the introduction of genetic variability, the offspring are as vulnerable to threats and diseases as the parent plant. Genetic uniformity can even put asexually-reproducing plants at risk of extinction.
35.5:
Asexual Reproduction
Asexual reproduction allows plants to reproduce without growing flowers, attracting pollinators, or dispersing seeds. Offspring are genetically identical to the parent and produced without the fusion of male and female gametes.
Vegetative reproduction is a common type of asexual reproduction, in which detached fragments—such as stems, roots, or leaves—from individual plants develop into complete organisms. Dandelions of the Taraxacum genus use a method of asexual reproduction called apomixis, which produces seeds without pollination or fertilization.
Plant breeders also use artificial methods of asexual reproduction—including cutting, grafting, layering, and micropropagation.
Some plants can be propagated by merely placing stem cuttings that contain nodes into moist soil and allowing them to root.
Grafting can be used to combine the desirable traits of different plants. A stem segment (the scion) from one plant is grafted, or attached, to a root section (the stock) from another plant. Over time, the vascular systems of the two plants fuse, forming a graft. The scion grows, producing new shoots and eventually flowers and fruit. Grafting is typically used to produce different varieties of grapes, roses, and citrus trees, among other species.
Layering involves bending a young stem of a plant and covering the stem with soil. Rooting hormones may also be applied. When roots appear, the new plant can be transplanted to a different area.
Micropropagation quickly produces several plants from a single plant using plant tissue culture methods. These techniques are useful for propagating rare or endangered species that are difficult to grow in natural conditions.
While asexual reproduction confers several advantages, strictly asexually-reproducing species are at an increased risk of extinction. Asexual reproduction can reduce genetic variability, limiting an organism’s ability to adapt to changing environmental conditions.
Suggested Reading
Garcês, H. M., Champagne, C. E., Townsley, B. T., Park, S., Malhó, R., Pedroso, M. C., Sinha, N. R. (2007). Evolution of asexual reproduction in leaves of the genus Kalanchoë. Proceedings of the National Academy of Sciences, 104 (39), 15578-15583. [Source]
Schmidt, A., M. W. Schmid, and U. Grossniklaus. 2015. “Plant Germline Formation: Common Concepts and Developmental Flexibility in Sexual and Asexual Reproduction.” Development 142 (2): 229–41. [Source]