What are seeds?What are seeds?

Parts of a seed – seed coatParts of a seed – seed coat

• Why do seeds have a seed coat? Why do seeds have a seed coat?

Why are seeds so different?Shape Size Colour Texture

Parts of a seed – Inside the seedParts of a seed – Inside the seed

Parts of a seedParts of a seed

First leaves

Epicotyle = stem

What do seeds need to GERMINATE?What do seeds need to GERMINATE?

Oxygen - Growth

Sun Sun - Energy for PHOTOSYNTHESIS

TemperatureTemperature – Cold? Hot? Warm?

WaterWater - Photosynthesis and growth

GERMINATIONGERMINATIONWhat is germination?

Steps:1. Seed coat breaks

2. Radicle becomes ROOT

3. Hypocotyl and epicotyl become the STEM

4. First leaves grow = PHOTOSYNTHESIS

First leaves

Dormancy• Metabolism falls• Number of organelles per cell falls• Dehydration – water content falls• Vacuoles in cells deflate• Food reserves become dense crystalline

bodies

© 2008 Paul Billiet ODWS

Reasons of 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

© 2008 Paul Billiet ODWS

Types of dormancy

• In general, there are two types of seed dormancy:

• Seed coat dormancy: Seeds with seed coat dormancy usually have a seed coat that

• is impermeable to oxygen and/or water.

• Internal dormancy: Occasionally the dormancy is caused by an inhibiting chemical in the epidermis or adjacent interior membranes or by unfavourable environmental conditions

Germination: The breaking of dormancy

The growth of the embryo and its penetration of the seed coat

Break down of barriersAbrasion of seed coat (soil particles)Decomposition of seed coat (soil microbes, gut enzymes)Cracking of seed coat (fire) Change in physical state -

rehydrationDestruction and dilution of

inhibitorsLight, temperature, water

Production of growth promoters© 2008 Paul Billiet ODWS

Characteristics of a quality seed• Minimum of damaged seed:• Damaged (broken, cracked or shrivelled) seed may not germinate

and is more likely to be attacked by insects or micro organisms.

• • Minimal weed seed or inert matter: Good quality seed should be free of weed seeds (particularly noxious types), chaff, stones, dirt and seed of other crops. Almost all these impurities can be discarded during processing/conditioning.

• • Minimum of diseased seed: Discoloured or stained seed are symptoms of seed that may carry micro organisms that already have attacked or will attack the seed when it starts to grow. The plant may live and spread the disease to other plants.

• • Near uniform seed size: Mature medium and large-size seed will generally have higher germination and vigour than small and immature seed.

Physiological charactristics

• • High germination and vigour: The germination percentage is an indicator of the ability of the seed to emerge from the soil to produce a plant in the field under normal conditions. Seed vigour is the capacity of seed to emerge from the soil and survive under potentially stressful field conditions and to grow rapidly under favourable conditions.

Genetic

• • Seed of the same variety:• Adapted to the local conditions• Proper characteristics for use• Pest and disease tolerance• High yielding ability