PLANT PROPAGATION

Choice of Methods

Sexual– Propagation by seed

Asexual (vegetative)– Cuttings– Grafting– Layering– Division

Propagation Method Distinctions

Sexual– “True” to seed implies . . .

No characteristics changed Cultivar termed a Line

– Line is homozygous Self-pollinated gives progeny like parent Cereals and vegetables are examples

Other Seed-propagated Cultivars

Inbred lines– Pure lines, self-pollinated and selected– Used to produce hybrid cultivars

Hybrids– Example: hybrid corn

Propagation Method Distinctions

Asexual (vegetative)– Necessary when plant is heterozygous– Heterozygous implies:

Many dissimilar genes Meiosis segregates/recombines genes Seed propagation can’t maintain characteristics of

parent

Propagation Method Distinctions

Asexual (cont)– Used with heterozygous plants

Piece of vegetative tissue Suitable environment “missing parts” develop Whole plant genetically identical to original Flower not involved in asexual propagation

Asexual Propagation Facts

No genetic change (barring mutations) Heterozygous cultivars carried generations Cultivars are clones Numerous methods (see text, Table 5-1)

SEXUAL PROPAGATION

Seed produced in flower Meiosis involved Reduction division yields haploid gametes Gametes combine in fertilization Zygote develops into embryo

SEED PRODUCTION

Cultivar preservation

– Control of seed source essential If homozygous, self-pollinated . . .

– purity assured If homozygous but cross pollinating . . .

– Must separate plants– Prevent pollen contamination

SEED CERTIFICATION PROGRAM

Government standards– Isolation– Culling– Inspection– Final seed testing– Harvesting equipment cleaning

CERTIFICATION PROGRAMS (cont)

Four classes of seeds (agronomic crops):– Breeder seed

White tag; plant breeders

– Foundation seed White tag; public/private foundation stock

– Registered seed Purple tag; progeny of breeder/foundation

– Certified seed Blue tag; sold to farmers; known genetics and purity

Vegetable and Flower seeds

Regulated by seed companies

Seed purity continually tested

Special test gardens

SEED FORMATION

Seed essential parts:– Embryo

Develops into new plant

– Food storage material Nourishes embryo; endosperm/cotyledon(s)

– Seed coverings Seed coats/other parts ovary wall

SEED FORMATION (cont)

Development

– Ovary to Fruit– Ovule to Seed– Integuments to Seed coats– Nucellus to Perisperm– 2 polar nuclei/1 sperm to Endosperm (3n)– Egg nucleus/1 sperm to Zygote to Embryo (2n)

SEED STORAGE AND VIABILITY

Some seeds short-lived– Willow, maple, elm

Others may live many years– Hard-seeded legumes

Many seeds range between extremes Often dependent on storage conditions

SEED VIABILITY TESTS

Cut test Float X-ray

Only tell you there is an embryo! Still don’t tell you the viability!

GERMINATION TEST

% seedlings developing from seeds planted– Use on seeds with no dormancy problems– e.g. flower, vegetable, grain

Several methods– Moist paper towel (simple)– Plant in seed flats (greenhouse)– Germination chambers (seed-testing labs)

CHEMICAL TEST

Tetrazolium Test– Living tissue test– Chemical reacts with enzymes in tissue– Color change– Interpretation variable

EXCISED EMBRYO TEST

Used on wood plant species with dormancy– Don’t respond in direct germination tests

Embryo cut from seed– Seed laboratory technique– Moist paper tested in covered dish– Viable embryos show activity (greening)– Non-viable embryos remain white and die

SEED DORMANCY

Dormancy may allow a seed to resist germination even though conditions would be favorable to germinate

Survival mechanism May require specific techniques to overcome

TYPES OF SEED DORMANCY

Seed coat dormancy– Impermeable to water and gases (oxygen)– Associated with hard seed coats– Legumes, pine, birch, ash– Natural weathering softens seed coat– Artificial methods:

Scarification Heat treatment Acid scarification

TYPES OF SEED DORMANCY

Embryo dormancy– Common in woody perennials– Physiological conditions– Germination blocks in embryo– Break by stratification:

Chilling temperatures Moisture Oxygen Time

ADDITIONAL DORMANCIES

Double dormancy; e.g. Redbud Rudimentary embryos; e.g. Magnolia Chemical inhibitors:

– Coumarin– Caffeic acid– e.g. tomatoes, lemons, strawberries

Secondary dormancy; e.g. some woodies

GERMINATION REQUIREMENTS

Adequate moisture (varies with species) Proper temperature (varies with species) Good aeration Light (some cases) Free from pathogens Free from toxic salts

VEGETATIVE PROPAGATION

Asexual – Not involving flowers or fusion of egg and sperm

Accomplished through mitosis:– Nucleus contains genetics for entire plant – Cells genetically identical– Cells can still differentiate– Capable of becoming any kind of cell

Due to:

VEGETATIVE PROPAGATION

Totipotency – ability of mature cell to return to embryonic state and produce whole new individual- Plant cells easy- Many plants use totipotency to self-propagate

Importance – yields genetically identical plant– Not possible with seed (sexual) reproduction– Meiosis combines genes at random

VEGETATIVE PROPAGATION

Mitosis produces:– Adventitious roots– Adventitious shoots– Callus

VEGETATIVE PROPAGATION

Used primarily for woody perennials– Highly heterozygous– Don’t breed true from seed

Desirable characteristics lost

Produces clones– Fruit, nut, ornamental cultivars– Many are ancient e.g. ‘Thompson Seedless’ grape

Cultivated Clones

Two processes:– Vegetative propagation of superior seedlings

Typical method e.g. ‘Golden Delicious’ apple

– Mutations Bud sports; e.g. ‘Ruby’ from ‘Thompson Pink’ Chimeras; e.g. variegated pink lemon (fig. 5-6) Range from slight to serious

– Depends on where in mitosis and where in plant

Apomixis

Interesting phenomenon– Asexual production of seedling from seed

formation in the usual sexual structures of the flower but without the mingling and segregation of chromosomes

– no union of male and female gametes

– Seedling characteristics same as maternal parent

Propagation by Cuttings

Classified according to part of plant obtained– Stem cuttings

Hardwood Semi-hardwood Softwood herbaceous

– Leaf cuttings– Leaf-bud cuttings– Root cuttings

Grafting

Joining of two separate plant structures Used on difficult to root plants Make use of particular rootstock

characteristics

Budding

Grafting and Budding Notes

Cambial layers of stock and scion must meet Parts must be held securely Keep air out! Union heals by callus production from

parenchyma cells Adequate temperature for cell division There are limitations!

Layering

Additional Layering Techniques

Simple layering (like tip layering) Mound layering Air layering

Other Plant Structures

Runners (stolons); e.g. strawberries Suckers (adventitious shoots); e.g. blackberry Crowns (used in division) Specialized stems and roots

– Bulbs - Tuberous roots– Corms - Rhizomes– Tubers

Tissue Culture

Micropropagation– Utilizes small ‘explants’– Callus formation– Cell differentiation– First used on ferns, orchids and carnations