20/07/2013 - Research on Seed Introduction

Seed  Introduction

Advantages of using seed:-

• cheap and efficient for large quantities
• simpler and cheaper facilities needed
• seedlings often grow faster than cuttings
• plants produced by seed are genetically variable (important for revegetation work)

Disadvantages of using seeds:-

• Genetic variation where consisitency is required
• some seeds can take years to flower
• plant grown from seed may take longer to reach flowering age

Variability - Propagation by seed is called sexual propagation. The genes from two parents have combined to create an offspring (some plants are self fertile and can pollinate their self). The creates variability from one plant to the next, especially to wild populations.

Viability - Seeds may remain for commercial production usually state their percentage. This allows the grower to accurately calculate how much seed is needed to produce the required crop numbers. The percentages can also be an indicator of teh quality of teh seed, although some plants consistently produce large numbers of none viable seed.

Seed Provenance - the place that a particular plant originated from produces a range of genetic characteristics that are very specific to that batch of seed. This is called seed provenance. It can be very important for revegetation plantings and seeds should ideally be collected within 50km radius of the planting site.

Seed Structure

Most seeds contain a built in food supply. Structurally the seed is a matured ovule. Various parts of ovary may be incorporated in the seed coat.

There are 3 basic parts of seed:

• testa-seed coat from various parts of the ovary
• embryo-differentiaties into plumule and radicle
• Endosperm-stored food source

By the time the seed is mature, the embryo is differentiated into a shoot (plumule) and  a root (radicle) and one or two specialised seed leaves (cotyledons).

The area between the young root and the cotyledons is known as the hypocotyl.

Stored food is present in seeds as carbohydrates, fats and proteins. This stored food may come from tissue called endosperm which is formed as a result of the fertilisation process.

Two types of germinating seeds:-

1] Epigeous germination - the cotyledons come above the ground. Eg : bean
2] Hypogeous germination - the cotyledons stay below the ground.  Eg : onion

Dormancy

Some seeds may fail to germinate  even after care has been taken to provide the correct germination environment. This can be due to seed dormancy. Seed dormancy can have several different causes, the most common of which are:-

• Physical Dormancy - the seed coat is too hard.
• Mechanical Dormancy - the seeds are inside a hard outer covering
• Physiological Dormancy - Several causes including incompletely developed embryos

Breaking Dormancy

There are several methods that can be used to break seed dormancy depending on its cause. These include :

Dry storage -        Simple store the seeds dry for the specified period of time.

Light sensitivity - Ensure that the correct light conditions are provided.

Hot water -           Soak the seeds in hot water for 12 -24 hours, then mix with moist media and leave 

                              for 24 hours to encourage water uptake. Sow immediately.

Scarifying -          Breaking the seed coat or reducing its thickness.This can be achieved by either 

                              abrading (sanding) the seed coat, or chipping to remove a small section.

Stratification -     Warm moist which soak the seeds in water for 24 hours,  then mix with moist 

                              media and store in a warm environment for between 1 - 2 weeks.
                              Keep moist. Sow immediately. 

                              Cool moist which soak the seeds in water for 24 hours, then mix with media
                              in a bag and leave to sit for 3-4 days to allow maximum water uptake.
                              Place the bag in a fridge and leave for between 4 - 12 weeks.
                              Sow immediately on removal.

Seed Germination 

 

For seeds to germinate the following environmental factors have to be provided.

• Water - seeds need to take up water to start germination process. Once germination starts the seed must not to dry out again or the embryo may die. Constant moisture during germination is vital to success.
• Air - in order to carry out high rate of respiration which accompanies germination, the seed must have access to lots of oxygen. Therefore, media must be well aerated.
• Temperature - the ideal temperature for germination of a wide range of plants is 15 - 25 degrees.
• Light - most of the seeds germinate in light or low light conditions. A few species will not germinate in the dark, while some will not germinate in light. 

Disease Control during Germination 

Damping off can occur as can occur as the seed germinates even before it emerges through the soil, marking it seem that the seed has not germinated. Damping off can also attack older seedlings that seem to have been healthy. A stem rot develops at the base of the stem and the seedlings fall over and wilt. Often occurs in patches in a tray and infection can spread rapidly.

Disease Control and Prevention :-

Good hygiene. Use only clean pots and trays and a good quality, well drained seed sowing mix.
Over watering is often a cause of damping off, but lack of water can also weaken seedlings, so that they are susceptible to infection.
Use fungicides to control infection. Regularly check health of seedlings. Dust seed with fungicide before sowing. After seeds have germinated, drenching occasionally  with a fungicide will help avoid the risk of damping off.

Temperature 12 degrees Celsius.
Rainfall : 0.00%
Humidity : 85%
Light wind.