Seeds and Seed Germination

Seeds and seed germination Seeds are normally the product of sexual reproduction Pollination Fertilization Embryo development Mature seed

Seeds are for propagation The biological function of seeds is for propagation of the species How does seeds help in propagation of the species?

Diversity among seeds Seeds are very diverse in term of size: Begonia seed weigh 10-20 micrograms Coconuts weigh more than a kilogram Thousands of Begonia seeds

Diversity among seeds In terms of adaptation to survive various environments until conditions are favorable for germination In terms of method of dispersal

Seeds are for propagation Protect the zygote against physically injury Store foods for seedling-- before it becomes independent Remain dormant to survive harsh environment Disperse the seeds to capitalize on their genetic variability Respond to environment cues and germinate at the right time and place

Seeds are alive! To function in propagation, seeds must be alive Seeds respire, albeit slowly consume O2, produce CO2 and H2O Seeds have a finite lifespan they cannot be stored indefinitely Which kind of seeds can remain viable longer? Seeds of tropical plant or temperate plant?

Seeds are alive! Seeds of many tropical plants remain viable for only a short time, a few days Tropical plants grow in environments that do not have a winter season through which seeds must survive before the favorable growing conditions of spring arrive

General features of seeds Embryo Root and shoot, in a miniature form Food reserves Allow seedling to grow before it is capable of performing photosynthesis Seed coat Provides protection from the environment

Monocots and Dicots Flowering plants (angiosperms): Dicotyledonous plants with two seed leaves Monocotyledonous plants with one seed leaf Dicots 200,000 species includes beans, roses, cacti, melons, citrus Angiosperms flowering plants Monocots 50,000 species includes grasses, lilies, orchids, palms

Seed structure Cotyledon Plumule Radicle Micropyle Seed coat or testa

Endospermous and non- Endospermous seeds Takes place in the fruit on the parent plant Endospermous seeds: Retain the endosperm tissue, and is surrounded by a layer of living cells, the aleurone layer. Non-endospermous seeds: The endosperm tissue is absorbed by the cotyledons. The cotyledons then become the food reserve for the seed.

Endospermous or non- Endospermous seeds?

A LS of maize grain / fruit Endospermous and non- Endospermous seeds A LS of maize grain / fruit

SEED DORMANCY It is a phenomenon in certain seeds in which they would not germinate if given an optimal condition ( water, oxygen, optimum temperature ). Dormancy can be seen in seeds ( eg: legumes ), buds, spore & food storage organs ( tubers ). Due to many factors include: Lack of oxygen Dryness Presence of substances that inhibit germination 15

GROWTH UNDER EXTREME CONDITION DORMANCY A period in the life cycle of many animals & plants when their metabolic activities become minimum & growth stop. Is a resting stage It can occur in the adult, egg, pupa, spore or seed stage. A way of protecting an organism against unfavourable conditions such as insufficient food, cold ( winter ) & dry ( drought ). It is controlled by hormones that ~ response to physiological in plants & animals ~ affecting the behaviour in animals CHAPTER : GROWTH 16

What physiological changes lead to dormancy Metabolism falls Number of organelles per cell _____ Dehydration – water content _____ Vacuoles in cells _____ Food reserves become _____ _____ _____

What physiological changes lead to dormancy Metabolism falls Number of organelles per cell falls Dehydration – water content falls Vacuoles in cells deflate Food reserves become dense crystalline bodies 18

SEED Germination Dormancy of these seeds may be broken by one or more of the following: light, sunlight being the most effective; low temperatures (1 to 5 degrees Celsius [33.8 to 41 degrees Fahrenheit]) for several weeks; day/night fluctuating temperatures of 1 to 10 degrees Celsius (41 to 50 degrees Fahrenheit); chemicals, such as nitrate in the soil, or applied hormones (gibberellins) in the laboratory; and fire. CHAPTER : GROWTH 20

Dormancy mechanism is related to the seeds’ natural environment is particularly important for small, wind-dispersed weed seeds. Seeds that require light involves a receptor protein, phytochrome the cold winter may cause the parent plant to die, and thus remove competition for space in the spring. Seeds that need a period of low temperature The requirement for alternating temperatures will prevent germination of seeds beneath dense vegetation because the latter dampens the day/night temperature fluctuations; these seeds will germinate only when there is little vegetation cover, again reducing competition with established plants. The requirement for fire CHAPTER : GROWTH 21

Dormancy mechanism is related to the seeds’ natural environment Seeds that require light involves a receptor protein, phytochrome Seeds that need a period of low temperature The requirement for alternating temperatures The requirement for fire CHAPTER : GROWTH 22

Maintaining dormancy Physical barriers The seed coat (testa) is waxy = waterproof and impermeable to oxygen Physical state – dehydrated Chemical inhibitors present e.g. salts, mustard oils, organic acids, alkaloids Growth promoters absent

The breaking of dormancy Break down of barriers Abrasion of seed coat (soil particles) Decomposition of seed coat (soil microbes, gut enzymes) Cracking of seed coat (fire) Change in physical state - rehydration Destruction and dilution of inhibitors Light, temperature, water Production of growth promoters

Seed Germination: To break dormancy seeds need: Water Warm Temperature Emergence of Radicle through Seed Coat To break dormancy seeds need: Water Warm Temperature So if you want to store seeds what are the conditions? Dry Cold Dormant seeds need more than moisture and warmth: Dormancy is caused by: Is overcome by: Example: nick digest scrub fire freeze-thaw cycles Thick Seed Coat Scarification Kentucky Coffee Tree or Thin Seed Coat Light Dark Lettuce Pea Insufficient Development Soil Fungus Association Orchids Inhibitor: Abscisic Acid Stratification > Vernalization Most CT feral plants 6 weeks at 4° C 20° C Inhibitor: Phenolics Leaching by Repeated Rain Cacti

Germination STAGE EVENTS PREGERMINATION Rehydration – imbibition of water. RNA activated & protein synthesis starts. Increased metabolism – increased respiration. Hydrolysis (digestion) of food reserves by enzymes. Induction of cell division & cell growth. GERMINATION Rupture of seed coat. Emergence of seedling, usually radicle first. POST GERMINATION Growth of root and shoot axis. Transport of materials from food stores to growing axis. Senescence (aging) of food storage tissues.

Stages leading to cell division Respiration Initially anaerobic Later aerobic Mitchondria reconstituted Soluble sugars ATP RNA activated http://www.rbgsyd.nsw.gov.au/ Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) Mitosis (70h)

Mobilization of food reserves Control by growth promotors such as gibberellin and growth inhibitors such as abscisic acid These directly affect the genes for enzyme synthesis or the activity of the enzymes themselves The growth substances are affected by environmental factors (e.g. light, temperature, humidity)

The control of food reserve hydrolysis Negative feedback control of enzymes The action of the enzyme also limited by substrate Once all the starch in an amyloplast is hydrolysed the enzyme stops work Therefore the release of the stored food is adjusted to suite the demand Starch + H20 Maltose  - amylase Negative feedback

The mobilisation of food reserves Carbohydrates Starches (amylopectin & amylose) Amylases Maltose and glucose Proteins e.g. Zein Proteases Amino acids Lipids Oils Lipases Fatty acids & glycerol The food reserves are stored as large insoluble macromolecules They are hydrolysed using enzymes to smaller soluble molecules for transport

ABA Triggering factors for germination Light, chilling or water (rain) trigger the inactivation of ABA, which makes dormant seeds able to germinate. ABA ABA ABA GA

After seeds take up water, GA is released from the embryo to signal aleurone.

The aleurone responds by synthesizing and secreting digestive enzymes (a-amylase) to hydrolyze stored nutrients in the endosperm. a-amylase

Nutrients (ex. sugars) absorbed from the endosperm by the scutellum (cotyledon) are consumed by the seedling during germination.

The growth of seedling starting with the roots first, then shoot growth follows.

Barley Seed Germination hydrolysis starch Fruit+Seed Coat maltose Endosperm sugar exocytosis Aleurone Layer cotyledon -amylase translation Storage Protein monocot hydrolysis shoot apex RNA growth transcription Embryo Amino Acids DNA GA water radicle apex imbibition

Capsella Seed: Embryo Seed Coat Shoot Apex Endosperm Cotyledons - dicot Hypocotyl Radicle Root Apex Micropyle

Lettuce Seed Germination shoot apex hydrolysis starch Seed Coat sugar cotyledons -amylase translation dicot growth RNA transcription Embryo DNA photoactivation phytochrome radicle apex photoreversibility red and white light stimulate germination water 660 nm Pfr Pr dark 730 nm imbibition

Germination of seeds 1. Utilization of stored reserves In cotyledons or endosperm tissue During germination, enzymes are made that convert stored reserves (large molecules) into compounds that can be used by the seedling (smaller molecules) starches  sugars lipids, fats  sugars proteins  amino acids

Germination of seeds 2. Transport of compounds into growing seedling through vascular system These compounds have two functions Support respiration in the embryo Provide a source of building blocks (carbon, nitrogen, etc.) for the seedling 3. Expansion and growth of seedling Root radicle elongates down, hypocotyl expands up Establishment of root system and emergence of shoot

Seedling establishment Shoot emerges and is exposed to light Chlorophyll is produced and seedling starts to perform photosynthesis Seedling is no longer dependent on reserves from the seed If stored reserves are consumed before photosynthesis is established, the seedling will die

Seedling establishment Growth of the seedling can be measured in many ways Length Increases after seed imbibes Fresh weight Increases as seedling grows Dry weight Declines initially as stored reserves are consumed by respiration, increases once photosynthesis is established

Conclusions Seeds are alive but dormant Comprise an embryonic plant and stored reserves Germination requires Water - for imbibition Oxygen - for respiration Suitable temperature Outcome of successful germination is a seedling capable of independent growth

TYPES GERMINATION Radicle will emerge first from the seed Next, the shoot tip breaks through the soil surface Types of germination: i. epigeal - cotyledons appear above the ground { hypocotyl forms a hook and pushes aboveground, raising the cotyledons } ii. hypogeal - cotyledons remain underground { epicotyl forms a hook and shoot tip is lifted out of the soil }

…TYPES GERMINATION

…TYPES GERMINATION

Aggregate fruits

Receptacle is the fruit