Germination
and
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