Lecture 6: Asexual Reproduction

Asexual reproduction

• Doesn’t require investment required to produce flower, attract pollinators, or find means of seed dispersal

• Produces genetically identical plants

• Offspring survives well under stable conditions

• Vegetative propagation may be accomplished:

  • Naturally

  • Artificially

• Both methods involve development of plant from mature parts of a single plant

Advantages of asexual reproduction - Mitosis

• Need only one parent

  • Isolated individual can produce offspring

• Rapid reproduction

  • Invading species can dominate quickly

• Less metabolically costly

  • Sexual reproduction involves investments in reproductive tissue (e.g. flowers), even though successful seed formation, dispersal, and germination may not occur that year

• Consistency

  • Less complicated than sexual reproduction, seldom fails

Natural vegetative propogation

• Strategies that plants have developed to self-propagate

• Happens when plants grow and reproduce asexually

• New plants can be produced from vegetative structures such as:

  • Roots

  • Stems

  • Leaves

• Modified stems are most common

  • Plants continue to grow from buds that are present on modified stems, called: SCRBRT

    • Suckers

    • Corms

    • Runners

    • Bulbs

    • Rhizomes

    • Tubers

• In some plants, tubers with buds at the base of the stem give rise to new plants

Runners

• Long stems that grow along soil’s surface, with buds on them

• Adventitious (roots that appear on stems and leaves) roots appear at nodes where buds are

• When nodes touch soil, shoots begin to grow into new plants

• When runner stems with buds are cut off, they become mature plants

• e.g. Strawberries

Rhizomes

• Modified stems that grow horizontally along surface of soil or beneath ground

• Storage sites for growth substances (e.g. proteins, starches)

• As rhizomes extend, roots and shoots may arise from segments of rhizome and develop into new plants

• e.g. Ginger, turmeric

Bulbs

• Shortened underground storage structures

• Stems enclosed with fleshy, concentric, layered leaves

• Bulbs produce smaller buds (lateral buds) that appear between layers of parent bulb

• As mature plant gets to end of its life, lateral bulbs develop into smaller bulbs attached to base of parent bulb

• e.g. Onions, lilies

Corms

• Storage structures

• Swollen underground stems with dry laves covering them

• Different from bulbs since they are solid but not layered

• Grow in size in favorable conditions, remain dormant in adverse conditions

• Pieces of corms can be cut and planted to produce new plants

  • Daughter corms that grow on mature corms are called cormels

• e.g. Gladiolas, crocuses

Stem tubers

• Swollen parts of stem buried in soil, serve as storage

• Have eyes/nodes that produce new shoots

• Eyes begin to sprout into shoots when left for some times

• Each shoot can be cut and planted to produce new plant

• e.g. Potatoes, yams

Root tubers

• Swollen roots of plant, serve as storage

• Develop into genetically identical plants

• Tubers with buds at the base of stems can be separated and produce new plants

• e.g. Sweet potatoes, Dahlias

Suckers

• Start from stem and shoot up to surface

• To propagate, suckers are cut off from main plant with some root still attached

• New suckers grow to become mature plants that produce new suckers

• e.g. Bananas, raspberries, cherries

Artificial vegetative propagation methods

• Involve human intervention

• Most common types: GLDSTC

  • Grafting

  • Layering

  • Division

  • Suckering

  • Tissue culturing

  • Cutting

Cutting

• Some plants root so easily that you can start them in a jar of water

• Majority of species require attention

  • Humid, warm, partially shaded conditions to strike

• Particularly difficult specifies may need cool air above and warm soil

• Rooting hormone Auxin may be used to encourage growth

• Compounds can be applied to cut tip of used as foliar spray to promote formation of roots

Layering

• Involves bending plant branches/stems so that they touch the ground

• Then they are covered in soil

• Young stems that can be bent without injury are preferred

Air layering (marcotting)

• Alternate method of layering roots to grow on stems

• Branch in target region is wounded and bark is removed

• Wound is covered with moisture-retaining medium such as moss/cloth

• Rooting medium is held in place and surrounded by moisture barrier such as plastic film

• When sufficient roots have grown from wound, stem is removed from parent plant is planted

Division

• Plant is broken up into 2+ parts

• Root and crown of each part is kept intact

• Usually applies to mature perennials

• Annual and biennial plants aren’t used since their lifespan is too short

• Most perennials ar best divided and replanted every few years to keep them healthy

• Each section must have both roots and shoots

• Best seasons for division are Spring and Autumn

Grafting

• Combines favorable stem characteristics with favorable root characteristics

• Used for:

  • Roses

  • Grapes

  • Citruses

• Desired cutting called scion is attached to stem of another rooted plant called stock

• Tissue systems of cutting become grafted into tissue systems of base plant

• Can be used to create unusual plants bearing different fruits

• Sharp knife must be used

• Techniques include: BCW

  • Budding

  • Cleft grafting

  • Whip grafting

Chip budding

• Use when mature buds are present

• Can be done in Spring before growth starts and stock and scion are dormant

• Often used when stock and scion are SMALL

Bench, whip, tongue

• Performed indoors (cool area)

• Made when both stock and scion are dormant

• Most successful for grafting pencil-sized material

• Stock and scion should be same diameter

• Grafting tool makes omega, V, or U cut in stock

• Corresponding cut in scion is used to prepare stems

• Pieces are fitted together, wrapped, and treated as before

Rootstock effect on scions

• Cold tolerance

  • Hardy rootstocks have enabled production of perennial crops in severe climates

  • Ability to impart differential cold hardiness on scion is of particular interest

• Disease resistance

  • Ability to control disease is one of the primary historical advantages of rootstocks