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Chapter 5

Plant PropagationPart 1 – Sexual Propagation

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What is Propagation?

• Plant reproduction– Vegetative– Sexual

• Sexual vs. Asexual propagation

Schefflera cutting

Seed of Franklin’s Sandwort

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Seed Formation

• What causes seed formation?– Seeds result from successful pollination and

subsequent fertilization– Cross-pollination– Self-pollination

• What is hybrid seed?– Technically ‘hybrid’ can apply to any cross-pollinated

plant– Term is reserved for plants grown from seed that is the

result of special breeding and generally superior

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Hybrid Seed

• Produces seedlings with ‘hybrid vigor’• Is the result of controlled crossing of a dedicated

‘female line’ and a dedicated ‘male line’– Both lines are genetically pure will have the best

traits of both parents• Seed saved from hybrid plants will not grow into

good plants – Only the original hybrid seed will carry the desirable

traits

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Mendelian Genetics

• Czech Monk Gregor Mendel, the original founder of the genetic science we have today– Original idea that offspring resemble parents– Studied peas – flower color, seed shape, etc.

• Suspected that offspring characteristics are dependent on specific traits that are passed by each parent– These traits are not blended together

• Visible manifestation of traits is called ‘phenotype’

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Mendelian Genetics• This idea indicates that an offspring may look just like one

of the parents, even though it is a product of both parents

• Because it is a product of both parents, the genetic composition will not be the same as either parent.

• Mendel postulated that traits exist in pairs in each parent– Now called ‘genes’– And were classified as recessive or dominant depending on if

they were manifest in the offspring– Now we understand some genes have ‘partial dominance’

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Simplified Mendelian Cross

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Techniques of Plant Breeding

• Plants to be bred must belong to the same genus, and almost always belong to the same species– Cultivars within a species usually cross-pollinate

• In rare instances do plants breed across Genera (intergeneric hybrid)– Fatshedera is one example (xFatshedera lizei). • One parent is Hedera helix and the other is Fatsia

japonica

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Techniques of Plant Breeding

• Cross-pollinated seeds only make up a small percent of seeds produced (estimated around 4%)– The close proximity of the anther and stigma in

most flowers tends to promote self-pollination• Dioecious plants can never self-pollinate• How to control the pollination of flowers to be

bred depends on the type of flower– Perfect or imperfect

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Techniques of Plant Breeding• If exact self-pollination is desired, a small bag can be

placed over the flower just before it opens (perfect flowers)– When the flower opens it will encounter no other pollen

source than from it’s own anthers• For imperfect flowers, pollen must be carefully

transferred from the desired male and put on the desired female parent– Often with a small brush– This can also be done with perfect flowers when pollen from

another parent is desired instead of allowing it to self-pollinate

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Hand pollination of watermelon, a monoecious plant

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Labeling of an eggplant flower after pollination. The flower is taped

shut after pollination to prevent contamination of the cross

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Techniques of Plant Breeding

• After successful pollination, the eggs will be fertilized within several days and the floral parts will fall off the flower leaving the enlarging ovary

• When seed is mature, it must be removed from the fruit and cleaned for most uses

• Storing seeds is best in a sealed bag or jar in a cool, dark environment– Most seeds can be stored for long periods of time– All experience a decrease in germination percentage and

seedling vigor as time progresses

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Wet seed extractor in the field. Used for tomatoes, melons, squash, etc.

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Three-screen seed cleaner separates seed based on size. Can be used on seeds from dry fruit or seeds from wet fruit after is has been dried.

Winnowing or fanning seed with electric fan

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Direct Seeding

• Sowing seeds outdoors is easy and practical• Some basic rules apply to get good germination– Plant in a area suitable for species – light, etc.– Observe proper planting depth (usually a function of seed

size) and spacing• 1 ½ times the seed diameter if no other info is available

– Ensure seed to soil contact– Keep moist– Thin plants as needed– Fertilize after true leaves appear

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Sowing seed indoors

• A popular practice to extend the growing season– Plants should be started 6-8 weeks ahead of

garden planting– Timing is crucial because if started too early they

will grow too long indoors and perform poorly outdoors later

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Sowing Seeds Indoors

• Some basic rules apply– Provide a warm, humid environment– Use a container that will allow water to drain away– Growing medium must be loose, fast-draining, and

free from plant pathogens• Commercial potting soils are usually adequate• Commercial soils contain no actual soil

– Adequate light must be available• Can be natural, artificial, or a combination of both

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Some commercial seed-starting kits

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Commercial seedling production

• Large-scale seedling production is often called bedding plant production

• Specialized seed sowing equipment, trays, and growing environment

• Seedlings can be sold wholesale to other growers for finishing, or transplanted in-house and sold in packs or small pots

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Drum seeder capable of seeding up to 600 trays per hour

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Damping-off

• Whether planted indoors or out, seedlings are susceptible to a number of fungal diseases– When a seedling or group of seedlings is infected with a

fungal disease, it is called ‘damping-off’– It is caused by media that is too wet, excessive humidity,

very close plant spacing, and unclean media or environment– The fungus move fast when plants are in high densities as in

greenhouses and gardens• Prevented with fungicides, lower humidity, more air

circulation, clean and drier media or soil

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Transplanting

• The term transplanting can be applied to moving plants from a container into the ground, or from one container to a larger container

• Should be done when plants crowd each other or roots are excessively crowded

• Seedlings should be handled by leaves and not stem– Can survive a leaf breaking

• If roots are very dense, then cutting or separation of the root ball is appropriate

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Transplanting

• Should be done in evening or on a cloudy day to reduce chance of wilting

• Root ball should be moist and plants not water-stressed

• Shading may be required• Plants should be watered-in immediately after

planting to ensure soil-to-root ball contact

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Hardening-off

• Is the term for acclimating seedlings or adjusting them gradually from the greenhouse or growing area to the real world– Idea is to gradually increase light, and wind, decrease

humidity, and adjust them to outdoor temperature and conditions

– Placing plants outdoors in shade during the day– Placing plants in sunny spot for a few hours a day

• After they are hardened they will not appear as soft or succulent, have a stockier stem, and thicker leaves

• They are much less likely to wilt and die-back at transplanting outdoors

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Spores

• Ferns produce spores instead of seeds• These spores come from small capsules or spore cases

on the bottoms of leaves• Ferns drop their spores on the ground which will grow

into a small, flat plant called the ‘prothallus’• The prothallus then develops the sperm and egg on

the underneath side– The sperm swims in the wetness at the forest floor and

hopefully some will find the eggs– After successful fertilization a new fern will grow under the

prothallus in about a year

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Spore cases of Grape Fern

Split spore cases showing thousands of tiny bead-like

spores

Prothallus – usually not more that a few centimeters or so in size

Pictures from www.hiltonpond.org