PLANT PROPAGATION

Choice of Methods Sexual Asexual (vegetative) Propagation by seed 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 14-2)

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 Food storage material 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. 14-10) 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