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Germination

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Dormancy SEED Germination

Lecture 2

Seed Structure Brief Review

Structure of the mature seed

Dicot • Key structures

– Hypocotyl - under cotyledons

– Epicotyl - over cotyledons

– Plumule - embryonic shoot

– Radicle - embryonic root

– 2 Cotyledons - transfer food from endosperm to embryo when seed germinates

Seedling Morphology

(Dicot)

Structure of the mature seed

Monocot

• Key structures - monocots – Scutellum, radicle and plumule

– Single cotyledon - thin with large surface area

– Embryo enclosed in a sheath

• Coleorhiza, coleoptile

Seedling Morphology

(Monocot)

Germination

• 3 important part of seeds

1. embryo- develop and create plumul, radicle, coleoptile, coleorhiza, scutellum. Seed without embryo can not produce plant

2. Spores – is a food storage material. For supply food to embryo. Can be found in endosperm tissue and cotelydon (apart of embryo)

3. Testa –seed covering • From flower – ovule become seed and ovary

become fruit.

SEED GERMINATION Def: morphology – transformation from embryo to seedlings

Def: physiology – a sequence of events transforming a ganiescent embryo into metabolically active and synthesis structure.

Def: biochemical – process that involves oxidation, degradation and synthesis.

Agronomist – process started when its in the media and end with seedlings appear and become autotropic

-with enough requirement (water,light, temperature) – seeds can vival, or else, called dead seeds

Some of the seeds can germinate even still attach with the mother plant. Eg. Peas bean still can germinate even in the pod, corn/paddy can germinate even still with the mother plant, citrus/jackfruit – seed can germinate even in the fruit itself.

Some other seeds cannot germinate even the requirement was fulfill- and take time to germinate, eg. Week, month or year =� dorminancy

• Germination can be categorized in 2 methods:

1. Cotelydon will appear on soil surface after

germination. Eg. Long bean � epigeal

2. Cotelydon left in the soil and plumule appear on the surface after germination. Eg. Kacang

tanah � hypogeal

Seeds component

• 1. starch – high molecular with polymer of d-glucosa.

CHO reserved.

– Amilose – glucose polymer (1-4) with glucosidic linkage.

Water soluble

– Amilopektin – 70-90% from the starch. Branched

polysacaride at (1-6) glucosidic linkage

2. Protein – in cotelydon, endosperma, aleuron. Most protein

storage contain large of amida- nitrogen-> incorporated of a

glucose. 4 types of protein were stored in seeds- albumin,

prolamin, globulin and glutelin.

• 3. fat – in cotelydon, scutellum, endosperm. Eg. Triglyceride – fatty acid

• 4. mineral – macro and micro elements

SEED PHYSIOLOGY

Germination

• Germination process:

1. Water imbibition

2. Enzyme activation

3. Hydrolisis and catabolism food storage

4. Embyro growth and development

5. Seed coat swelling

6. Seedling appear

Water imbibition

• Water- as an activating agent

• Function- media transportation, solvent in which reaction occur, turgor pressure for increasing cell volume

• Water intake stage: – 1. imbibition – physical process

- seeds vival or dead

- depends on water potential

- very fast

Water imbibition (cont…)

– 2. lag stage - absorb less water

- with active metabolism

- function of osmotic potential

- seeds survive

– 3. second period of time intake

- relate with embryo growth

- physical appearance – germinate part

- transportation of food storage actively

- function of osmotic potential

Water imbibition (continue…) • Factor that influence the rate of water absorbtion

1. Colloidal constituent: protein (main product), selulose, starch and fat (nil) � seed swelling & total amount of water utilized reflect the seed storage constituent.

Protein – ‘zwitter-ions’ eg. -ve & +ve charges – these highly charged polar water molecule, attract to each other

eg: soya bean – 2-5x DW zeamays – 1.5 – 2x DW cereal embryo – 2x endosperm 2. Seed coat permeability – depends on type of fat contain in the testa. - control permeability and entry of water eg. In

legume (hard seed) – penetrate through microphylla (thinned seed coat)

2. Water availability – surrounded by water - water stress – less water - need minimum MC for germination 2. temperature – less sensitive

Enzyme activation

• GA – growth and development promoting substrate and a

dominant activator

- effect of embryo activities eg. Membran permeability and

ATP synthesis

- substrate mobilisation eg. Stimulate enzyme hydrolysis in

aleuron cell

starch ------� maltose, by e. amilase

protein hydrolysis eg. Ribonukleas & protease

release of 1,3 glukanase – cause cell wall of aleuron

weak � increase food storage from endosperm to embryo

Enzyme activation cont…

• Cytokinin – membran permeability

- control translation of genetic code to a

new protein form

- to overcome inhibator effect

• ABA- inhibit germination

- inhibit DNA synthesis and translation by RNA

- for enzyme synthesis

- to overcome problems – supply cytokinin & GA

Hydrolysis of food storage • To gain ATP and cell development in early stage • Hydrolysis starch/carbohydrates, fat and protein to glucose, free fatty acid, amino acid

– Starch � glucosa

by amilase e. By maltase e.

Amilose ---------------------� maltose --------------------------�glucose ---� ATP + sucrose

(disacharide)

by amilase e. By maltase e.

Amilopectin -----------------� maltose --------------------------� glucose -� ATP + sucrose

– Fat � fatty acid

by lipase e.

Fat --------------� fatty acid ----------� TCA cycle ----� acetyl CoA --� glucose ----� sucrose + glucose

– Protein � amino acid

by protease e. By peptidase e.

Protein ------------------� dissolve protein ------------------------� amino acid -� synthesized new protein + nucleic acid and membrane

Growth and development of embryo

• Embryo � cell division and development

• 1st part appear - radicle � develop to the root

• - plumule/terminal --� shoot

• Transclocation food storage to develop part

• Potential to survive – depends on food storage

Growth and development of embryo

cont..

• Metabolic event – respiration

• Increase almost immediately upon imbibition and continue to increase

– � indepent to protein synthesis but dependent on substrate stored

• Initial ATP product – glycolysis (anaerobic) & TCA cycle (aerobic)

• Free sugars & amino acids in embryo

Lag period in oxygen consumption

1. Triggering agent stimulus activation & synthesis of germinating agent. Eg. Hormones

2. Action of terminating agents on the genetic materials - new RNA & enzymes

3. Embryo exhausts the available respiratory products & await more hexose from hydrolysis.

Developmental to Germination Mode

• The Mechanisms

There are some regulatory factors/mechanisms that can explain why seed will undergo the transition from developmental to germination mode.

1. Seed Water Content Control

2. Hormonal Control

The Seed Water Content Control

1. Desiccation as switch

When seed moisture content (m.c) is high, usually >60%, seed is still considered in developmental mode.

At high seed m.c, seed is still accumulating storage reserves due to no changes in physiological activities.

Desiccation as switch (cont.)

Decline in seed m.c, normally <60%, is considered as a switch, leads to changes in metabolic activities.

However, seed must first accumulate minimum dry weight. Than the decline in seed m.c to <60% will act as a switch.

Once a seed has accumulated a min. dry weight and contains <60% moisture, seed is now ready to use storage reserve materials for germination.

The Seed Water Content Control

(cont.)

2. Osmotic Environment

Developing embryo is surrounded with

negative osmotic potential environment, thus prevent embryo from germinating.

When osmotic potential become less negative with increase seed maturity, germination will increase (Refer Fig. below, on muskmelon)

Insert Graph, muskmelon

DAA = Day After Anthesis

The Hormonal Control

ABA

ABA is the most important hormone which control developmental process in the developing seeds.

During mid-developmental seed growth stage, ABA in seed is high in amount.

Role of ABA in Developing Seed

1. Promotive roles

• Promotes storage reserves accumulation such as protein.

• Promotes enzyme activities during late embryogenesis

• Regulate production of mRNA during late embryogenesis to produce a particular set of proteins called `Late Embryogenesis Abundant' or LEA. Important in seed to withstand desiccation.

• Promotes desiccation tolerance in the embryo

Role of ABA in Developing Seed

(cont.)

2. Inhibitory roles

• Prevents precocious germination

(germination on mother plant)

• Prevent water imbibition by the embryo.

• Prevent synthesis of specific protein which

is responsible for germination.

• ABA and GA work oppositely to regulate

dormancy and germination

**Role of GA in Germination

Cereal Seed • In cereals such as rice, corn or barley, upon

inhibition GA is produced in embryo and than released into the endosperm.

• GA diffuses into the aleurone layer to produce α-amylase enzyme. GA involves with transcription.

• This enzyme is than release into the endosperm to breakdown starch into smaller molecules

• These small molecules are absorbed by scutellum and transported to growing embryo.

Role of GA in Developing Seed

• Endogenous GA has two different mechanisms in the control of germination;

i. The induction of the expression of hydrolyzing enzymes of endosperm tissues.

ii. Direct simulating effect on the growth potential of the embryo which this growth potential is restricted by ABA

Factors that influence seed

germination 1. Dormancy

2. Water – MC field capacity and must meet min requirement in the seed

3. Temperature – min, opt, max ���� varies 1. Eg. 1st phase in imbibition – low sensitivity to temperature

2. 2nd and 3rd phases – temperature dependent ���� involve biochemical process

Imbibed seeds failure to germinate:

a. Below freezing – ice crystal formation & membrane discruption

b. High temperature – denaturation of nucleic acids, proteins and membrane

* extreme limits - affects final percentage of emergence

eg. Cool-season crops 0 -10C (best)

warm-season crops 15 – 20C

Factors that influence seed

germination 4. Aeration (oxygen)

-an energy – consuming process – release energy from storage materials (aerobic) except plants (rice)

- excess water -���� barrier to oxygen diffusion

* critical when MC is near saturation (pores spaces filled with water – limits oxygen availability)

eg. Corn, soyabean oxygen < 9-10% - affect germination

5. Light

- essential to some species – eg. Lettuce, celery and most grasses & many herbaceous garden flower

- classes --- a. Promoted by light

b. Inhibited

c. Germinate both light & dark (netural)

6. Organisma pathogen & salinity

- salinity – salt deposites on seed as water evaporate. Eg. Saline water & shallow planted seed

Factors that influence seed

germination (cont…) 7. Phytochrome

- photoreceptor pigment responsible for light – stimulated promotion/inhibition of seed germination.

- seed coat or pericarp

- cotyledon

- radicle

- 2 forms --- Pfr and Pr

- biologically active in red light

far red -���� convert to an inactive form -���� prevent germination

Pr (660nm) stable ---------------���� Pfr (730nm) unstable

End of Germination Process

SEED

SEEDLING