Principles of Propagation by Seed

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Chapter 7

• Principles of Propagation by Seed

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Principles of Propagation by Seed

• Germination process-
• 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

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Phases of germination
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Principles of Propagation by Seed

• Phases of Germination
• I.) 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

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Principles of Propagation by Seed

• Phases of Germination
• I.) Water uptake (continued)
• Leakage amino acids, sugars, proteins,... Since
  cell membranes are not fully functional yet
• Quantity leaked is proportional to seed quality
• High leakage means the seed is susceptible to
  attack by insects, fungi, and bacteria (can be
  measured by an electrical conductivity meter)

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Imbibition Lag Phase
Fresh Weight
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Principles of Propagation by Seed

• II.) Lag phase
• Mitochondria mature
• Proteins are synthesized (enzymes are activated)
• Food reserves are metabolized
• Enzymes loosen cell walls

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Principles of Propagation by Seed

• III.) 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

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Radicle emergence
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Principles of Propagation by Seed

• Use of storage reserves
• Proteins in protein bodies
• In cotyledons and endosperm
• Enzymes (proteinases) are required to break down
  proteins into amino acids
• Proteinases synthesized during imbibition

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Principles of Propagation by Seed

• Starch
• In endosperm (in grain crops)
• Order of events
• Imbibition
• GA in embryo scutellum (protective sheath
  around cotyledons in monocots) translocated to
  the aleurone layer
• Aleurone layer a secretory cell layer that
  surrounds the endosperm
• Enzymes are synthesized (?-amylase)
• Enzymes convert starch to glucose maltose
  sugars and then transported to the embryo for use
  in development and growth
• Note these sugars are important in beer making!

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aleurone layer
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Principles of Propagation by Seed

• Lipids
• Oil bodies in endosperm cotyledons
• Oils triacylglycerides (glycerol fatty acids)
• Glyoxysomes are organelles found ONLY in seeds!
  They process stored oils.
• Fatty acids are high energy compounds used in the
  glyoxylate cycle to produce sucrose
• Sucrose is then transported to the embryo for use
  in development and growth

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Canola Seed Oil
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Principles of Propagation by Seed

• Measures of germination
• 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

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Germination curve
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Standard seed germination curve
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Seed vigor
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Principles of Propagation by Seed

• Environmental factors influencing germination
• 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)

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Principles of Propagation by Seed

• Seed priming
• Regulates water imbibition of seeds
• Charles Darwin suggested this possibility in
  1855!
• Polyethylene glycol (PEG) used today (aerated)
• Starts metabolic processes without radicle
  emergence
• Seed is re-dried for short-term storage at cool
  temperatures
• Results in uniform germination
• Used on bedding plant plug production (annuals)

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Seeds primed and pregerminated in aerated PEG
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Effects of seed priming on germination
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Principles of Propagation by Seed

• 2.) Temperature
• The MOST important environmental factor that
  regulates TIMING of germination
• Boil-treat seeds to control disease. This wont
  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

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Principles of Propagation by Seed

• 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

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Principles of Propagation by Seed

• Temperature categories
• Cool-temperature tolerant - native to temperate
  zones prefer 40 - 86F
• Ex broccoli, cabbage, carrot, peas, alyssum
• Cool-temperature requiring - native to a
  Mediterranean climate. No germination if gt 77
  F
• Ex celery, lettuce, onion, delphinium

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Principles of Propagation by Seed

• Temperature categories (continued)
• Warm-temperature requiring - native to
  subtropical and tropical regions
• Must be gt 50F for sweet corn tomato
• Must be gt 60F for beans, pepper, cucumbers,
  cotton

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Principles of Propagation by Seed

• Temperature categories (continued)
• Alternating temperatures
• Day/night temperature fluxes are better than
  constant temperatures
• Used in seed testing labs
• 18F (10C) difference often used
• 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

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Principles of Propagation by Seed

• Aeration effects on germination
• Oxygen uptake is proportional to the amount of
  metabolic activity
• Oxygen diffuses through water slowly therefore
  waterlogged soils slow/inhibit germination

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Principles of Propagation by Seed

• Light effects on germination
• Involves quality (wavelength) and photoperiod
  (duration)
• Light-sensitive seed are generally small in size
  or are epiphytes (grow on other plants)
• Ex alyssum, begonia, coleus, orchids

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Principles of Propagation by Seed

• Light effects on germination
• A few plants have germination inhibited by light
• Ex amaranthus, allium, phlox
• Some require dark to germinate
• Ex calendula (pot marigold), delphinium, pansy
• Some require a specific daylength
• Ex birch, hemlock

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Principles of Propagation by Seed

• Disease during germination
• Damping-off
• Pythium ultimum
• Rhizoctonia solani
• Botrytis cinerea
• Phytophthora spp.
• Drying, salts and excess heat at the soil surface
  can also look like damping off

Optimum growth between 68-86F Pb. on
warm-requiring seeds
Secondary pathogens
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Damping-off in tomato soybean
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Principles of Propagation by Seed

• Dormancy - regulation of germination
• Quiescent seeds - only need to be imbibed and
  incubated _at_ 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

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Principles of Propagation by Seed

• 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

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Principles of Propagation by Seed

• Types 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

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Principles of Propagation by Seed

• Exogenous physical dormancy (seed coat)
• Outer integument becomes hard or fibrous during
  dehydration and ripening (Ex coconut,
  honeylocust, Kentucky coffeetree)
• In drupes (cherry, peach, etc.). Have a hardened
  endocarp (pit or stone)
• In nature, hard seed coats are softened by
• Microorganisms
• Passage through an animals digestive tract
• Abrasion -freeze/thaw
• Fire

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Principles of Propagation by Seed

• Exogenous chemical dormancy
• In fleshy fruits
• Contain chemical inhibitors such as ABA
  (Ex citrus, cucumbers, apples, pears,
  grapes, etc.)
• Desert plant fruits have chemical inhibitors that
  must be leached away by rains that then provide
  enough water for germination and seedling
  development

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Principles of Propagation by Seed

• 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

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Principles of Propagation by Seed

• Endogenous dormancy (continued)
• 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
  (Ex lettuce seed, can germ. in
  dark if temp. below 73F)

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Principles of Propagation by Seed

• Endogenous dormancy
• Physiological dormancy
• Photodormancy (continued)
• Seed coat or underlying endosperm act as light
  sensors (if removed, light control disappears)
• Hormones (GA) can overcome a light requirement
• Red gt far-red in natural sunlight (21) therefore
  phytochrome is active (PFr form) and seed
  stimulated to germinate
• Under foliage, far-red light penetrates more than
  red light, therefore phytochrome is inactive (Pr
  form) and seeds fail to germinate
• Red light does not penetrate soil as deeply as
  far-red light, therefore light-sensitive seeds
  stay dormant until they get closer to surface
  (Ex weed seeds)

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Lettuce seed is light sensitive
Dark Light
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Tanada effect
Exposed to red light (phytochrome active)
Exposed to far- red light (phytochrome inactive)
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Principles of Propagation by Seed

• Endogenous dormancy
• Physiological dormancy
• Intermediate/deep physiological dormancy
• Stratification (moist-chilling)
• Must be aerated
• Moisture should be constant
• Temperature should reflect the plants native
  habitat for the winter/spring ( 35 - 45ºF) with
  a minimum of 23ºF
• Time for seed-chilling requirements are related
  to bud-chilling requirements!

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Effects of stratification on
germination
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Seeds hormone levels during
stratification
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Effects of vernalization/stratification on
seedling development
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Principles of Propagation by Seed

• Endogenous dormancy
• Physiological dormancy
• Intermediate/deep physiological dormancy
  (continued)
• For intermediate dormancy, if embryo is removed
  from the seed, it will readily germinate
  (dormancy is mostly seedcoat) and moist-chilling
  time required is short
• For deep dormancy, an excised embryo will not
  germinate readily nor will it form normal plants
  (physiological dwarfs) and these seeds require a
  long moist-chilling period (gt 2mo.)

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Principles of Propagation by Seed

• Double dormancy
• Seed has two kinds of dormancy
• Ex a rudimentary embryo seed coat dormancy
  morphological dormancy physical (exogenous)
  dormancy
• Thermodormancy
• Different from thermal inhibition
• Once exposed to high temperatures, will not
  germinate when temperatures return to the optimal
  germination range
• Ex lettuce in summer, therefore often primed
  (in PEG) at cool temperatures to allow
  germination prior to sowing

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Principles of Propagation by Seed

• Advantages of seed dormancy
• Seedling survival - permits germination only when
  the environmental conditions are favorable
• Creates a seed bank - not all seeds for a
  species germinate in a single year. Spreads
  germination out over time
• Synchronize germination - for a particular time
  of year
• Seed dispersal - especially after being carried
  in the digestive tract of animals

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Summary of types of germination
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Epigenous seed germination
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Epigenous seed germination
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Epigenous seed germination
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Hypogenous seed germination