Principles of Asexual Propagation

• asexual propagation = vegetative propagation = clonal propagation
• Asexual propagation naturally occurs as layers, rhizomes, runners, and pups/suckers
• Goal of asexual propagation: reproduce plants with identical genotypes from a source plant with desirable characteristics (phenotype)
• clonal selection: process of selecting an individual plant or plant part to create a clone
• cuttings:

  • stem
  • leaf
  • root
• scions:

  • buds
  • grafts
• geophytes:

  • bulbs
  • corms
  • rhizomes
  • tubers
• layers:

  • tip
  • mound
  • air/marcott
  • compound
• other structures:

  • stolons/runners
  • leaf embryos
  • slips/crowns
  • suckers/pups
• advantages of clones:

  • genetic improvement
  • uniformity
  • control of phases of plant development
  • combine more than one genotype into a single plant
  • greater commercial value
  • sometimes the only means to propagate
• disadvantages of clones:

  • monoculture
  • slow reproduction rate
  • lack of genetic variation for breeding
  • potential propagating systemic pathogens from clonal sources
  • insufficient genetic diversity for pest/disease resistance
  • potential for latent genetic mutation
• adventitious organs: new roots and buds that are formed from cells and tissues of previously developed shoots and roots
• Formation of adventitious buds, roots, and shoots is important
• adventitious shoots: shoots that arise from any location on the plant except at stem tips or leaf axils
• An adventitious bud is an embryonic shoot
• adventitious roots: roots that arise from an organ/portion of the plant other than the original root system
• Formation of adventitious roots and shoots is dependent on plant cells to dedifferentiate and develop into either a root or shoot system
• dedifferentiation: capability of previously differentiated cells to initiate cell division and form a new meristematic growing point
• types of adventitious roots:

  • preformed
  • wound-induced
• preformed roots

  • develop naturally on aerial portions that are attached to mother plant
  • may or may not be visible
  • contain organized root meristem, but remain dormant until internal or extrenal stimulus provided
• wound-induced roots:

  • de novo adventitious roots: roots that are formed "anew" from stem or leaf cells that experience a stimulus, such as wounding, to dedifferentiate into roots
  • outer injured cells die, a necrotic plate forms, and the wound is sealed with a corky material and the xylem may plug with gum
  • living cells behind the plate begin to divide after a few days
  • certain cells in the vicinity of the vascular cambium and phloem begin to divide and initiate de novo adventitious roots
• de novo adventitious root formation

  1. Dedifferentiation of specific cells
  2. Formation of root initials
  3. Organization into root primordia
  4. Development and emergence of root primordia
• Adventitious roots are located on the outside and between vascular bundles in herbaceous plants
• Adventitious roots are located on the cambium or young phloem in woody perennials
• Direct root formation comes from vascular tissue
• Indirect root formation comes from callus
• callus: irregular mass of parenchyma cells
• Callus is not essential for rooting
• Leaf cuttings must generate adventitious roots and bud
• preformed primary meristems: groups of cells directly descended from embryonic cells that have never ceased to be involved in meristematic activity
• wound-induced secondary meristems: groups of cells that have differentiated and functioned in some previously differentiated tissue system and then dedifferentiate into new meristematic zones resulting in the regeneration of new plant organs
• Not all plants can develop new root and shoot meristems
• Root cuttings must generate adventitious shoots and roots
• Generally, new shoots are developed before new roots
• polarity: the quality or condition inherent in a root or shoot cutting that exhibit different properties in opposite parts
• stem cuttings

  • distal end (end near the shoot tip) -> shoots
  • proximal end (end near the crown - shoot/root junction) -> roots
• root cuttings:

  • distal end -> roots
  • proximal end -> shoots
• Changing the position of a stem cutting with respect to gravity does not alter polarity
• Polarity is regulated by auxin movement
• cone of juvenility: the juvenile to mature gradient in seedling trees