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Understanding Plant Propagation: Sexual and Asexual Methods for Horticultural Success

Plant propagation is the process of multiplying plants to preserve their unique traits, influenced by plant biology. There are two main types: sexual (seed) and asexual (vegetative) propagation. Seed propagation relies on controlled genetic variability exemplified by the uniformity of Phaseolus vulgaris 'Greencrop', achieved through homozygosity via self-pollination. Asexual propagation, like that of Solanum tuberosum 'Russet Burbank', produces clones for genetic uniformity. The selection between methods often depends on predictability, cost, and desired traits, impacting the horticulture industry significantly.

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Understanding Plant Propagation: Sexual and Asexual Methods for Horticultural Success

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  1. Plant propagation • Definition: multiplication of a plant so as to preserve it’s unique trait(s) • Plant biology influences the propagation process in a number of ways

  2. Types of plant propagation • Sexual (seed) propagation • Asexual (vegetative) propagation

  3. Seed Propagation • Basic category: line - a population of seed-propagated plants in which genetic variability is controlled • Example: Phaseolus vulgaris 'Greencrop' green bean is uniform because it is homozygous • Homozygosity is achieved by self-pollination each generation for 5 to 6 generations

  4. Heterozygosity vs. Homozygosity Garden pea: a diploid, where D = tall and d = short, and D is dominant to d DD Dd dd DD (all tall) 1/4 DD (tall) 1/2 Dd (tall) 1/4 dd (short All dd (short) DD and dd (homozygous) individuals “breed true”; Dd (heterozygous) individuals segregate tall and short progeny

  5. How inbreeding “fixes” a trait Dd F1 (1/2 homozygous, 1/2 heterozygous) F2 (3/4 homozygous, 1/4 heterozygous) F6 (~96% homozygous seedlings)

  6. Vegetative Propagation • Basic category: clone - genetically uniform group of individuals derived originally from a single individual by asexual propagation • Example: Solanum tuberosum 'Russet Burbank' potato is uniform because each plant is a clone of the original (heterozygous) seedling • Uniformity is maintained by cutting tubers into pieces and growing new plants from each piece

  7. Why some plants don’t “breed true” • In most cases, a particular combination of genes are required • The probability is low of recreating that combination • For example: • AaBbCcDcEe (5 genes influencing a trait) • If this genotype is self pollinated, the chances of recreating this gene combination in the next generation is: (1/2)5 = 1/32 (1 seedling in 32 will be AaBbCcDdEe) • Individuals required for a 95% probability = 94 • Individuals required for a 99% probability = 145

  8. Types of horticultural crops • Tree and small fruits • Vegetables (annuals and perennials) • Turfgrasses • Landscape woody plants • Flowering plants (annuals and perennials)

  9. Why are some horticultural plants propagated vegetatively and some by seed? • Predictability - how much variation will be present in the seedling progeny? • Cost - seeds are always cheaper, but they may not provide enough uniformity

  10. Types of crops that are seed-propagated • Vegetable crops, bedding plants, turfgrass species • Characteristics • short-season (1-2 generations per year) • diploids • natural selfing species or crossers that are easily inbred • seedling progeny are uniform for some trait(s)

  11. Types of crops that are vegetatively propagated • Small-fruit and tree-fruit crops, landscape woody plants, foliage plants, flowering perennials, cut-flower crops and some flowering potted plants • Characteristics • Are almost always heterozygous (causing segregation in seedling progeny) • Are often long-season crops • May be sterile • Individual plants are often highly valued

  12. Other biological terms/concepts relating to plant propagation • Competency and determinism • The 5 major plant hormones • Plant life cycles and phase changes • Species and cultivar concepts

  13. Competency and determinism • Competency - potential for specific development in cells (bud, flower, root) • Determinism - the degree of commitment to a certain pathway of development (i. e., to make a flower, root, or veg. bud) • Relevance to propagation • Development of flowers for seed production • Development of roots for vegetative propagation

  14. The 5 major plant hormones, relative to propagation • Auxins - important in rooting processes • Cytokinins - important in shoot induction • Gibberellins - seed development • Abscisic acid - seed maturation, dormancy • Ethylene - fruit ripening, rooting (minor effect)

  15. Plant life cycles and phase change • Generalization: annuals, biennials are usually easier (cheaper and/or more practical) to propagate by seed, perennials by vegetative means • Phase change • Juvenile - incapable of flowering, fruiting • Mature - perennials propagated vegetatively will usually flower/fruit precociously

  16. Concepts of species, cultivar, and propagation • True species can (usu.) be propagated by seed • The cultivar (cultivated variety) name is added at the end of a scientific name - Lycopersicon esculentum ‘Rutgers’ • The scientific name (with or without the cultivar designation) does not describe how the named plant is propagated (sexually or asexually)

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