PGD Unit 3 Seed Germination

Seed formation is the process of going from a ______ to ___ ___________. It proceeds in an orderly process such that it can be reversed.

Fully hydrated; dehydrated plant

Seed germination is a composite of many events usually defined by what is observable to the eye. Germination starts almost from the point of _________. It might be thought of as a triggering process in which ________, _______, and ______ take place.

Rehydration of the seed (imbibition); cell division, elongation, and differentiation

Two general phases of germination

1. Emergence of the radicle

2. Subsequent growth leading to an established seedling

Purpose of the seed

1. Survival of the species

2. Means by which a plant species can spread to new areas

3. Evolution (through genetic recombination)

Seed Viability

The extent to which the tissue within the seed is alive and the seed capable of of germinating when subjected to the correct sequence of conditions

Respiring

The seed is always ______

Tetrozolium

Used to test excised embryos for viability

Red; NADPH

Tetrazolium stains actively respiring cells ________ by staining for ____ activity.

Seed dormancy

When a viable seed is placed in a favorable environment of light, moisture, temperature , and gaseous environment fails to germinate

Levels of seed dormancy

• Primary

• Secondary

• Relative

Primary or innate dormancy

Dormancy that occurs at the time the seed is shed from the mother plant

Secondary or induced dormancy

Dormancy induced upon exposure to unfavorable environmental conditions

Relative dormancy

Dormancy in which seeds are dormant upon dispersion but slightly higher temperatures or brief exposures to light promotes germination

Suppression of precocious germination; vivipary (born alive)

Thought to be a form of dormancy. Seeds germinate precociously are examples of ______ (= _____)

Embryo dormancy

Occurs even when a viable embryo fails to germinate after it is isolated. However, dormancy breaking treatments will lead to germination

Seed coat dromancy

Dormancy imposed by the structure surrounding the embryo. Included are the pericarp, testa, and endosperm. These structures present a physical barrier

ABA

Suppresses germination

GA

Promotes germination

Aleurone; embryo; embryo; GA; aleurone; ABA

A signal is sent from the _____ to the _______ signaling the ____ to release to the ___. This signal can be blocked by ___

GA; embryos; aleurone; alpha-amylase

from the ___ moves to the ________ layer where it induces synthesis of _____________ and a number of other enzymes.

Alpha-amylase; aleurone; starch

________ in the seed ______ layer stimulates the release of ______ during germination.

Cytokinin

There is a requirement of GA for germination. IF there is an inhibitor present; there is no germination without a _______ also present

Alpha amylase

Primary enzyme in seed germination

1. Imbibition of water

2. Formation or activation of enzyme systems

3. Metabolism of storage products, subsequent transport, and synthesis of new materials

4. Emergence of the radicle and growth of the seedling

4 stages of germination

1. water uptake

2. Changes in respiration

2 things happen during stage 1 of seed germination (imbibition)

Triphasic

Water uptake occurs in a ______ pattern

Water uptake; matabolic

Dry seeds have a low water potential. This promotes rapid ________ in phase I and is independent of _______ activity. IT is dependent on the germination medium, degree of soil packing, and proximity of soil to the seed.

Negative osmotic potential; pressure potential

Phase II water uptake reaches a plateau because the matric potential has been saturated. Further absorption is based on _______ (solutes concentration) and __________ (turgor).

enzyme synthesis and activation

Phase two there is low absorption of water but is the period of

Radicle

Phase III - a second burst of water uptake occurs along with _______ emergence and elongation as a result of cell elongation.

• Cell division is not yet active in growth. During this stage fats, proteins, carbs, phosphorous containing compounds, and other storage products are broken down to simpler compounds and transferred to the growing points

• Phase 1 - Activation and hydration of mitochondrial enzymes

• Phase 2 - respiration plateaus due to low O2. (seed coat restricts gas exchange)

• Phase 3 - increase O2 from new mitochondrial enzymes causing aerobic respiration of carbs

• Phase 4 - respiration decreases as the seedling emerges and starts photosynthesis

4 phases of respiration changes

Activation of enzyme systems

Stage 2 of seed germination

Formation of enzyme systems

Stage 3 of seed germination

Emergence of the radicle

Stage 4 of seed germination

Radicle; radicle; water; nutrient

Stage 4:The second burst of water uptake leads to ______ emergence and the ______ becomes the primary site of _____ and _______ uptake.

Cell division; expansion

Stage 4: ________ and ______ begins

Photosynthesize

Stage 4: Respiration levels drop as the seedling begins to _______

Hypocotyl

Area of seedling from cotyledons down to radicle

Epicotyl

Cotyledons up

Water

The most important factor for germination and seedling survival

Limiting factors of water

• Salts in soil solution

• Lack of water (dry soil)

• Excess water

Temeperature

• Regulates rate of germination

• Regulates germination percentage

• Regulates seedling growth

• All seeds have a minimum, optimum, and maximum

Cool temperature tolerant

Germinate at temps as low as 4.5 C° and up. Will germinate at high temps as well

Cool temperature requiring

Will not germinate at temps above 25 C°

Warm temperature requiring

Fail to germinate or are susceptible to chilling injury at temps between 10 - 15 C°

Alternating temperature requiring

Either fluctuating day to night (10 C°) difference or alternating seasonal (warm summer, cold winter)

Phytochrome

The pigment involved in photosensitivity

Phytochrome far red (PFR)

Promotes germination

Phytochrome red (PR)

Inhibits germination

Red light

More during the day

Far red light

More at night

Means more red light shining. Means more PR is converted to PFR. PFR is important as it promotes germination

Longer day length