Botany 2104.  Plant Form and Function

Asexual Reproduction

Reproduction in Plants
sexual reproduction
    two complementary gametes (egg, sperm) fuse to form a new, genetically distinct individual
asexual reproduction
    reproduction without fusion of two gametes
    naturally occurring in most plant species
    cells proliferate by mitosis: clone
    totipotency of plant cells:  A mature, specialized cell has the ability to de-specialize and divide to produce many more cells.  The new cells can specialize into any plant cell type.

Vegetative Reproduction
    uses vegetative part(s) of the plant
        roots, stems, leaves
    a type of asexual reproduction
    the resulting plants are genetically identical to the starting plant and to each other (clones)
    adventitious = plant organs that develop on different organs.  For example, when a stem cutting forms roots, the roots are adventitious roots.

Some vegetative reproduction occurs naturally, usually via modified vegetative organs.

rhizome = horizontal, underground stem
Adventitious roots form at nodes.
examples:  iris, crabgrass

stolons (runners) = horizontal, above ground stems. 
Adventitious roots form at nodes. 
examples:  strawberry, spider plant

root sprouts (suckers) = adventitious buds form on roots 
examples: aspen, raspberry, blackberry, apple, cherry, redwood

tubers = underground stem with an enlarged (fleshy) tip. 
example:  potato = the eyes are axillary buds; after the shoot system grows from the buds, adventitious roots form

plantlets = some plants form small plants along leaf margins, leaf tips, leaf base, or fleshy leaves of bulbs

Jumping cholla (Opuntia bigelovii)
The detached joints will readily generate new plants by rooting and branching. During the cooler months the terminal joints are detached by a slight touch by a passing animal or even strong winds. The joints that attach to animals may be transported considerable distances before being dislodged.

Some vegetative reproduction is human-assisted.

Cuttings
    leaf cuttings = African violet (Saintpaulia), Peperomia, Begonia
    stem/shoot cuttings = Geranium, Coleus (photo)
    the cutting is placed in water or damp potting soil or vermiculite; adventitious roots form at the base of the cutting
    Leaf and stem cuttings are sometimes treated with rooting powder to encourage adventitious root formation. The rooting powder contains a form of auxin, a plant hormone.

Grafting
    Union of two related plants
    The shoot system forms from the scion; the root system forms from the root stock.
    Match up the vascular cambia of the scion and root stock so that the two plants grow together as secondary growth of the vascular tissues proceeds.
    Often done with fruit trees
        maintain the fruit quality
        disease- and pest-resistant root system
        winter hardiness
        dwarfing of the tree
        example: navel orange scions + bitter orange root stock
    Why you need to cut off suckers from grafted trees: The suckers are growing from the root stock, which generally has lower quality fruit than the scion.
    Examples: whip graft, cleft graft

Micropropagation (Tissue Culture)
    Plants are grown under aseptic conditions in special media.
    Pieces of a plant (explants) are surface sterilized with dilute bleach and placed in culture vessels. 
    The vessels contain a pH-adjusted growth medium that includes mineral nutrients, sugar (explants are grown in dim light to control plant development, but the light isn’t bright enough for sufficient photosynthesis), vitamins, plant hormones (to control plant development), and agar (to solidify the medium). 
    The explants form callus (a clump of parenchyma cells) followed by shoot and root development. 
    Under some hormone mixtures, the plants are transferred to a new medium for each developmental step:
            callus  -->  shoot  -->  root
    Under some conditions, callus will form entire plantlets.  After plants have formed in culture, they are transplanted to sterile soil and hardened off.

Advantages to Micropropagation
    propagate many plants from limited supply of starting material
    several commercially desirable plants can only be propagated easily, if at all, by tissue culture
        examples: orchids; carnivorous plants
    faster than conventional vegetative propagation
    the plants are disease- and pest-free, making it easier to ship them (interstate or international)

Anther Culture

Apomixis
    the production of seed without fertilization
    results in cloning via seed
    the advantages of seed production in the absence of sexual reproduction
        dormancy
        dispersal
    occurs in citrus, orchids, some grasses, and dandelions

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1 October 2013