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Spring 2022

 

David Wm. Reed

 

Seeds

Germination

Priming

Dormancy

 

 

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Terminology

 

pollination - deposition of pollen on the stigma of the pistil.

fertilization - the union of male and female gamete (nuclei, 1N) to produce zygote (2N).

double fertilization - in higher plants only (angiosperms)

         union of 1 1N male gamete with 1 1N female gamete (the egg) to

         produce a 2N zygote; and union of 1 1N male gamete with 2 1N

         polar nuclei to produce a 3N endosperm.

apomixis - development of an embryo without fertilization; hence, it is not true sexual propagation even though it produces a seed.

parthenocarpy - development of fruit without seeds.

vivipary - germination of seeds inside the fruit while still attached to the parent plant.

 

 

Types of Seed Germination

 

Epigeous Germination Dicot

           hypocotyl elongates & bends

           cotyledons emerge from soil

           apical meristem protected in cotyledons during emergence

 

Hypogeous Germination Dicot

           epicotyl elongates & bends

           cotyledons stay below soil

           apical meristem protected by leaves of plumule during emergence

 

Hypogeous Germination Monocot

(not always called hypogeous)

           epicotyl elongates

           reduced cotyledon & endosperm (stored food) stay below soil

           apical meristem protected inside coleoptile

Images from Pearson Education, Inc., Benjamin Cummings


STAGES OF SEED GERMINATION

 

Stage 1

      imbibition - initial absorption of water to hydrate seed

      leak solutes may occur due to membrane leakiness

      activation of metabolism - increased respiration

      release - stored protein (a-amylase) and mRNA may be released

 

Stage 2

      transcription - to produce new mRNA

      translation - to produce new proteins

      digestion of stored food - for example, starch to sugars in cotyledon or endosperm

      mobilization food reserves towards embryo

      DNA synthesis begins

      cell division begins

      radicle emergence signifies the end of Stage 2 and beginning of Stage 3

       

Stage 3

      continued growth and development: mobilization of food reserves, cell division, seedling emergence, and seedling growth and development

 

 


Seed Treatments to Enhance Germination

 

Seed Priming

Seed priming is a seed treatment that allows imbibition and activation of the some of the metabolic events associated with Stage 1 and Stage 2 of seed germination, but prevents radicle emergence and growth, thus prevents the start of Stage 3 of seed germination.

 

The changes that occur during priming can be:

         RNA synthesis increases, or the seed resume RNA synthesis quicker

         Protein synthesis increased significantly

         Enzymes involve in mobilization of reserves increases, such as a-amylase, malate dehydrogenase or isocitrate lyase.

         Gene expression may occur or change

 

But, DNA synthesis does not increase because the seeds are kept in the Stage 2 lag phase, and cell division usually starts at the end of Stage 2 and throughout Stage 3.

 

Many seeds are tolerant of desiccation. Even though during seed priming imbibition allows water uptake, the tolerance to desiccation upon dehydration is not lost. Thus, after priming the seed can be dried again and stored. If the seeds are primed too long, desiccation tolerance may be lost, and the seeds may loose viability upon re-drying. The secret to successful seed priming is to stop the priming treatment at just the right time to allow re-drying without the loss of seed viability.

 

 

From: Seed priming, Lutts et al, New Challenges in Seed Biology

 


Advantage of Seed Priming

 

1) Germination time is greatly reduced; especially reduced lag phase

 

 

2) Germination is much faster and more uniform.

 

 

 

 

Disadvantages:

 

1) Typically, primed seeds cannot be stored as long as unprimed seeds.

 

2) Some primed seeds may require cool storage temperatures.


Types of seed priming

 

1) Osmopriming (osmoconditioning):

This is the most common technique used. The seeds are soaked in an osmotic solution to allow imbibition and metabolic activation, but the osmotic conditions do not allow expansion and growth of cells. Osmotica used are: mannitol, polyethyleneglycol (PEG) or salts such as KCl.

 

2) Hydropriming:

Imbibition is obtained by:

         partially hydrating seeds using a limited amount of water

         using very humid air

         exposing them for a short time in warm water.

 

3) Matrix priming:

A solid, insoluble matrix is used to obtain a water solution with low water potential. The matrix potential keeps the water potential low. Vermiculite, diatomaceous earth or cross-linked highly water-absorbent polymers are used.

 

 

 

Hormone Treatments to Enhance Germination

Seeds of some species are very difficult or slow to germinate due to primary and secondary dormancy, the need for after ripening periods, immature embryos, etc. Many of these seeds respond to hormones to increase the speed of germination, uniformity of germination and/or percent germination. For example, the seeds of many tropical foliage plants are difficult to geminate, but respond to hormonal treatments. Hormones can also be added to seed priming treatments.

 

1) cytokinin 100 to 200 mg/liter for a 12-24 hour soak.

 

2) gibberellic acid 200 to 1,000 mg/liter for 12-24 hour soak

 

 


Seed Longevity and Storage

 

Seed Longevity

 

Recalcitrant Seeds:

 

Orthodox Seeds:

 

Dormancy: many orthodox seeds have various types of dormancies that must be satisfied before germination can occur. see next section.

 

 

Seed Storage

 

Orthodox seeds can be stored under conditions that maintain vigor, germination and viability.

 

Harrington Rule of Thumb (James Harrington, Professor of Vegetable Crops, UC Davis)

         For every decrease of 1% seed moisture content, the life of the seed doubles.

This rule is applicable when moisture content between 5 and 14%.

         For every decrease of 5C (10F) in storage temperature the life of the seed doubles.

This rule applies between 0C to 50C.

         50/50 rule

o    50 oF / 50 %RH

o    oF + %RH <=100%

o    Good seed storage is achieved when the % of relative humidity in storage environment and the storage temperature in degrees Fahrenheit add up to hundred but the contribution from temperature should not exceed 50F.

Take Home Lesson, in general best is cool temperature and moderate humidity.

 


Categories of Seed Dormancy

Modified from Hartmann, Kester, Davies, Geneve & Wilson, Plant Propagation Principles and Practices, 9th Ed, 2018

 

 

 

 

Categories of Seed Dormancy

Older terms

Dormancy Description:

Advantage or Reason

Method or Practice to Overcome

Primary Dormancy

Dormancy present at end of seed development

Exogenous Dormancy

Quiescence

Ecodormancy

Factor outside the embryo

Dry seeds:

 

Seeds of many plants are dehydrated in the fruit at maturity; 10-15%. This is not a type of dormancy, rather is probably a developmental process to prevent vivipary

Prevents vivipary

sow in moist soil

Physical Dormancy

Hardseededness

Impermeable hard seed coat

Spreads germination of multiple growing seasons.

scarification

   physical abrasion (file, sand paper, rotating drum with sand)

   chemical digestion of the seed coat (sulfuric acid).

Chemical Dormancy

Correlative Inhibition

Paradormancy

Inhibitors in testa or pericarp; especially if fleshy.

Water soluble inhibitors:

in desert plants acts as rain gauge.

 

Inhibitors in flesh: prevents vivipary

   Inhibitors in dry testa or pericarp:
leach/soak in water 24 hours to remove soluble inhibitors

 

   Inhibitors in fleshy testa or pericarp:
extract/remove seeds from fleshy tissue and rinse in water.

Categories of Seed Dormancy

Older terms

Dormancy Description

Advantage or Reason

Method or Practice to Overcome

Primary Dormancy - continued

Endogenous Dormancy

Endodormancy

Factor inside the embryo

Physiological After Ripening

After ripening

Many seeds will not germinate or respond to seed treatments for a period of time after seed maturity

Probably:

      prevents vivipary

      allows time for seed dispersal

store seeds in the appropriate manner for that species for a period of time.

Physiological Dormancy

Rest

Physiological factors inside the embryo, high inhibitors (ABA?) low promoters (auxin, GA?)

Assures germination in the spring.

stratification - cold (35-40 oF) moist storage for 4-12 weeks.

Morphological Dormancy

 

Immature or Underdeveloped embryo at seed maturity

   Prevents vivipary

   Allows time for seed dispersal

   Store seeds in the appropriate manner for that species for a period of time until the embryo is fully developed.

   warm stratification may speed the process

   excise embryo place in tissue culture.

Morphophysiological Dormancy

 

Immature embryo and Physiological factors

   Allows time for seed dispersal

   Spreads germination over multiple springs

store until embryo is developed, then stratification

Combinational Dormancy

 

More than one dormancy exists, such as hardseeded and physiological dormancy

 

 

Use the appropriate methods from above. Always scarify first.

 

 

 

 

 

 

 

 

 

 

Categories of Seed Dormancy

Older terms

Dormancy Description

Advantage or Reason

Method or Practice to Overcome

Secondary Dormancy

Dormancy imposed after all primary dormancy/after ripening has been satisfied.

Thermodormancy

Light requirement

High temperatures inhibit

 

   store normal ambient conditions

   cold stratification

   growth regulators

Photodormancy

Light Requirement

Lack of red light (darkness) or far red light

Usually seeds are very small.

 

Assures the seeds are close to the soil surface

      acts as a depth in the soil gauge.

      sow shallow or on surface

      expose light

o   red light

o   any white light


NOTES