(75594248) Seed Germination

7/29/2019 (75594248) Seed Germination

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Seed germinationrequires:

SeedGermination

1. Viable seed: Seed must be alive(embryo) 2. Correct environmental conditionsincluding:

a. Waterb. Temperature

c. Oxygend. Light

3. Lack of dormancy or dormancyreleased


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Viviparyseedsmaynot dryorbecomedormant

resulting in

precociousgermination.Maturationisnotcomplete;often

germinatewhilestillattached toplant.

Postharv

estspro

uting.

Reduced/little

dormancyseeddriesandmaturesbutseed(emb

ryo)dorm

ancy is either absent orreduced and will readilygeminate under properconditions.

Dormant seedrequiressome type of after-ripeningprocess before it cangerminate. Dormancyimposes a restriction onability to readilygerminate.


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Germination begins with the imbibition or uptake of water.

Can be viewed, as the change is fresh weight due mostly to theuptake of water. Occurs in three phases:

Phase Iwater uptake by imbibitionPhase IILag phase little if any increase in FWPhase IIIradicle emergence

Phase

I

Water

upta

k


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e is related to the low water potential of dry seeds (-100mPa).

cell

= m

+

+ p

cell = cell water potential/ chemical potential of water and is relatedto the free

energy, G, required for water to move from a high to low potential.

m

= matric potential and is related to the interaction of water

withmacromolecules of the seed and with surfaces (cell walls,

macromolecules). Important because initial water uptake byseed is driven by this component.

=

osmotic

potential

and

is

related

to

the

dissolved

solutes

in

the

cytoplasm

(organic acids, amino acids, sugars, ions), expressed as a negativevalue and water


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moves to a more negative. Thus as solute concentration increases

becomesmore negativegreater chemical potential with increased soluteconcentration.

p = turgor pressure, force or pressure due to cell wall and membrane

How does this relate to seed germination? Phase I (imbibition) ismostly related to matric potential and is associated with theinteraction of water will surfaces and macromolecules. Phase 1 occurs

in two stages: 1st involves rapid uptake of water associated with

surface interactions (cellulose hydration) and 2nd involve linearincrease in water content associated with hydration of membranesand establishment of membrane function.


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During this stage since membranes are not completely functional,

metabolites can leak from the seedsenvironmental stress thatextend this time interval (low temperature) can reduce seedgermination and viability.


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Reduced ambient water potential during phase 1 results in:--reduced seed water content--extends length of phase II--delays and/or blocks entry into phase III

Phase IILag Phase

Char

acterized

by

lit

tle

o

r

no

wate

r

upt


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ake.therefore increase in FW lags. Highly active metabolically


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Important Phase II events:--mitochondria become active and respiration and ATP synthesisincreases--protein synthesis increases and new proteins are synthesized from

newmRNAs

--metabolism of storage reserves begins--specific enzymes are produced such as ATPase, hydrolases for storagereserves.

Phase IIIradicle emergence

1st visible evidence of germination is emergence of the radicleembryonic root.This results from cell enlargement with little cell division.

Once radicleemerges,

cell

division

begins

in

the radicle tip/meristem

Two types of germination/radicle emergence

One step emergence


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Usually occurs in seed without endosperm (Brassica, pea areexamples) the testa (seed coat) ruptures and initial radicle elongationresult in the completion of germination. Radicle does not rupture endosperm since it does not exist. ABA does not inhibit testa rupture,but inhibits subsequent radicle growth.


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Two Step EmergenceRupture of testa (seed coat) and endosperm are two separateevents. This is wide-spread in the plant kingdom. ABA appears toinhibit endosperm rupture but not testa rupture and appears to becounteracted by GA.


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What is known about the regulation of seed germination and role ofplanthormones?

GA promotes the induction of cell wall hydrolases necessary for endosperm weakening and rupture

ABA inhibits the induction of cell wallhydrolases which inhibits endospermweakening and endosperm rupture

GA promotes and ABA inhibits embryo growth


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Model for the regulation of seed germination


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Key points:

Some factor releases seed dormancy (in above model, light and GAact to release dormancy)

GA promotes -1-3 glucanase accumulation

ABA inhibits endosperm rupture but not testa rupture

Ethylene

stimulated

-

1-


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3 glucanase accumulation but does not affect dormancy

Class I -1-3 glucanase accumulate prior to endosperm rupture andpromote radicle

emergence by weakening of endosperm.


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Environmental Factors that Influence Seed Germination

Main factors include:WaterTemperature

OxygenLight

Waterfundamental to seed germination

Most species have a threshold water potential (wateravailable in a givensubstrate) that results in optimum emergence.


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Temperature

Next to water is most important factor

In dry state seeds are very resistant to temperatures, both high

andlow

Temperature affects both germination rate and percentage

3important

temp

e


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rature points can be used to characterize seed germination

1. minimumlowest temp for germination to occur2. maximumhighest temp above which injury of dormancy occurs

3. optimumhighest % or seed germinate and the highest rate


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Classification of seeds based on temperature requirements

1. Cool-temperature tolerant; usually plants from temperate zonesand can

germinate over a wide temperature range. Optimum usually not

cool(24

-

30C) but can tolerate cool temperature (broccoli, carrot, cabbage)

2. Cool-temperature requiring; require low temperatures forgermination and fail to germinate at warmer temperatures usually above 25C (celery, lettuce, onion).

3. Warm temperature requiring; fail to germinate below about 10-15C and tend to be tropical or subtropical in origin; can beinjured by low temperature and affect plant vigor (tomato, beans,pepper)

4. Alternating temperature; fluctuations in day/nigh temp

canimprove

germination and is usually at 10C difference (many wild species).


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Oxygen or aerationO

2is required for respiration, excess water limits diffusion of O

2.Trade offbetween sufficient water and to much water.

LightBoth Quality (wavelength) and duration (photoperiod) can

influence some specieslettuce and red/far-red light is classicexample. Usually small seeded species will often germinate betterwith light exposure.

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(75594248) Seed Germination