Principles of Propagation by Seed

Proteins in protein bodies

In cotyledons and endosperm

Enzymes (proteinases) are required to break down proteins into amino acids

Proteinases synthesized during imbibition

1. Water uptake

By imbibition

a physical process in seeds with a permeable seed coat

Occurs whether seed is alive, dead, dormant or non-dormant

First 10 - 30 minutes

rapid uptake

Followed by 1 - 3 hours of slow uptake

Seeds generally do not wet uniformly

Volume of seed increases

2. Lag phase

Volume of seed increases

Proteins are synthesized (enzymes are activated)

Food reserves are metabolized

Enzymes loosen cell walls

Radicle emergence

Result of cell enlargement

Food reserves continue to be used

Enzymes degrade certain cell walls to permit exit of the radicle

GA promotes enzymatic cell wall hydrolysis and radicle emergence

ABA inhibits enzymatic cell wall hydrolysis

Subtopic

Water HAS to be absorbed which causes the seed to swell and rupture the seed coat

Radicle (embryonic root) emerges first from the germination seedlings

Hypocotyl pushes way up through the soil

Protecting fragile shoot apex and large cotyledons

Epicotyl spreads “true leaves” and begin photosynthesis

A seed requires moisture (water) to germinate.

as the embryo grows, the root and shoot break through the seed coat.

Nutrients stored in the cotyledon provide nourishment and support early development

Seed must be viable

embryo alive and capable of germination

Seed must receive:

Water

Proper temperature

Oxygen

Light (depending on species)

Primary dormancy must be overcome

after-ripening. Often removed by environmental conditions

Success depends on fulfilling the conditions detailed in the previous chapters on seeds.

These include

Using seeds of proper genetic characteristics to produce the cultivar, species, or provenance desired.

Using good-quality seeds

Manipulating seed dormancy.

Supplying proper environment to the seeds

Quality Seed Should Have Following Criteria

It should have good germination.

It should be free from infection of seed borne disease and stored grain pests.

It should not contain impurities like other crop seed, trash material beyond permissible limits.

Use of quality seed of improved variety of notified variety of hybrids is a basic input in cultivation of any crops as it ensures high crop stand, vigorous and healthy crop growth and thereby it ensures high productivity of that crop.

Germination percentage (%)

number of seedlings produced in a specified time

Germination rate - T50 value

# of days required to achieve 50% germination of the seed lot

1.) Water - threshold water potential = amount of water needed by the seed for radicle emergence

Rate of water movement in soil depends on

Texture (pore space)

Packing (pore space)

Closeness of seed/soil contact

Water with high salt content can counter-balance the effects of water imbibition (this is a problem in California with subirrigated fields and high water evaporation)

2. Temperature

The MOST important environmental factor that regulates TIMING of germination

Boil-treat seeds to control disease. This won’t damage the seed as long as the seed is DRY

Store seed at low temperatures to prolong viability

Temperature affects germination percentage and germination rate

Germination rate increases with an increase in temperature (up to a point)

Germination percentage is constant in the mid-temperature range and low on either end

Temperature ranges:

Minimum

lowest temperature for germination

Maximum

highest temperature for germination

Optimum

a range where the greatest percentage of seedlings are produced at the highest rate

Temperature categories

Cool-temperature tolerant - native to temperate zones prefer 4 - 30°C

broccoli, cabbage, carrot, peas, alyssum

Cool-temperature requiring - native to a Mediterranean climate. No germination if > 25 °C

celery, lettuce, onion, delphinium

Warm-temperature requiring - native to subtropical and tropical regions

Must be > 10°C for sweet corn & tomato

Must be > 15.5°C for beans, pepper, cucumbers, cotton

Alternating temperatures

Day/night temperature fluxes are better than constant temperatures

Used in seed testing labs

Imbibed weed seeds deep in soil do not germinate since there is little temperature flux, however, they will germinate if the soil is cultivated and seeds are brought to the surface where there is temperature flux

Oxygen uptake is proportional to the amount of metabolic activity

Oxygen diffuses through water slowly therefore waterlogged soils slow/inhibit germination

Involves quality (wavelength) and photoperiod (duration)

SubtLight-sensitive seed are generally small in size or are epiphytes (grow on other plants)
opic

alyssum, begonia, coleus, orchids

A few plants have germination inhibited by light

amaranthus, allium, phlox

Some require dark to germinate

calendula (pot marigold), delphinium, pansy

Some require a specific daylength

birch, hemlock

Dormancy - regulation of germination

Quiescent seeds

only need to be imbibed and incubated @ an appropriate temperature for germination. NO dormancy!

Primary dormancy

a type of dormancy where seeds will not germinate despite adequate environmental conditions

Secondary dormancy

induced under unfavorable environmental conditions

Dormancy is important to propagators because it allows storage, transport and handling of seed

After-ripening

changes in the dry seed during storage that allow the seed to germinate following favorable conditions

Type of primary dormancy

Exogenous dormancy - factors outside the embryo (seed coat or parts of the fruit)

Inhibits water uptake

Physical restriction on embryo expansion or radicle emergence

Controlling gas exchange (O2/CO2)

Preventing leaching of internal inhibitors

Supplies inhibitors to the embryo

Exogenous physical dormancy (“seed coat”)

Exogenous physical dormancy (“seed coat”)

coconut, honey locust, Kentucky coffee tree

In drupes (cherry, peach, etc.). Have a hardened endocarp (“pit” or “stone”)

In nature, hard seed coats are softened by

Microorganisms

Passage through an animal’s digestive tract

Endogenous dormancy

Morphological dormancy

Rudimentary embryo = araliaceae (ginseng), papaveraceae (poppy), ranunculaceae (anemone)

Linear embryo = ericaceae (rhododendron), annonaceae (pawpaw)

Overcome by

Alternating temperatures

Treat with KNO3 or GA

Physiological dormancy

Non-deep = after-ripening. Fresh seeds of herbaceous plants (annuals and many perennials) lose dormancy during standard storage

Photodormancy

Seeds require either light or dark conditions

Involves phytochrome (in most plants) which is photoreversible

There is often an interaction between light and temperature

Light requirement can sometimes be offset by cool temperatures or alternating temperatures

lettuce seed, can germ. in dark if temp. below