Seed germination

The germination process

If  a seed is given the right conditions and the embryo is alive it will start to  grow. This process is called germination.

This  is how it happens:

The  radish seeds that were hard and dry when you began the seed germination test in  activity 1B now look different. They have germinated. Water has softened and  split the testa. This allows water and oxygen to enter the seed.

Energy is needed before a seed can germinate. The food stored in the seed is used up to produce energy for the growing embryo. This process is called respiration.

Oxygen is required for respiration and it comes from the air. This oxygen combines with the food during respiration, to release stored energy used for growth.

Water enters the seed through a small hole in the testa called the micropyle. The water is then used in chemical reactions inside cells in the seed.

Warmth is needed to speed up the chemical reactions that take place in the seed. Warmth also speeds up the making of new cells when the plant embryo is growing. Therefore low soil temperatures for example, will slow down the rate of germination. Each species of plant will have a particular temperature range that its seeds will germinate in.

The  radicle is the first part of the plant embryo to grow once water and oxygen  enter the seed. Soon it comes out from the seed. Next the plumule emerges. The green leaves develop and function. They will now make the food for the plant using the process of photosynthesis.

Here is a diagram showing the stages of germination.

Viability of seeds

If  a seed is viable it is alive and capable of germinating and growing. Remember  seeds are made up of an embryo plant and a food supply. The embryo must be  alive for germination to occur, so seeds must be stored properly. Seeds vary in  the length of time they will stay viable. Indian lotus seeds, which were buried  in a peat bog for an estimated 1000 years, germinated perfectly. However,  generally, the older the seed the less likely it is to be alive.

Seeds  should be stored correctly to be able to survive and germinate. If they are  stored incorrectly then the food store will be used up during respiration and  the seed may die. Heat will speed up respiration and therefore use up stored  energy. Seeds should be stored in cool, dry conditions away from direct light  (not in a shop window). The viability will decrease over time so seed packets have a ‘use by date’ on them.

When  a number of seeds are sown, they might not all germinate. It is useful to know  how many of a given batch of seeds can be expected to germinate.

Seed packets

Commercial growers of seeds are particularly interested in knowing how many seeds can be expected to germinate from a given batch of seeds sown. At the time of packing, each packet of seed is stamped with an expiry date (or ‘use by date’) and a code number. The expiry date refers to the shelf life of the seed packet.

The ‘use by date’ is August 2010

You will need your results from the germination test you carried out with the radish seed before you can complete activity 3A.

Seed collection, cleaning, grading and storage

Collecting and saving your own seed is a fun activity at the end of summer. It saves money as you may not have to buy seed the following year. You can give away small packets of seed as gifts or you can swap them for seed from friends to add to the plants you have in your garden. You should collect seed from a parent plant with superior qualities. Those superior qualities might include the flavour or size of a vegetable or the colour of a flower.

Most flower and vegetable plant seeds are collected in late summer and autumn. They are, cleaned, graded and dried as the seeds become ripe but before they disperse.

Seeds from fleshy fruit, including fruit we know as vegetables such as pumpkin and tomatoes, are collected once the fruit is fully ripe but before it rots or is eaten.

Cones are collected when ripe but before they open and release the seeds.

Seeds from trees such as kowhai and English beech are collected in autumn once the pod is ripe.

All seeds should only be collected from disease-free healthy plants.

Methods of cleaning seeds

After you collect the seed, you need to clean them before you sow or store them. This will help maintain the viability of the seed.

• Fleshy capsules like those produced by poppies and Renga renga can be laid out on paper and left to dry in a warm, airy place until the seeds drop out.

Poppy flower

Immature poppy pod

Honesty seed heads

Whole flower or seed heads on stems of plants, such as Nigella (love-in-the-mist) and honesty can be bunched together, hung upside down with a paper bag tied over the seed heads. Leave the bag top of the bag slightly open and hang in a warm dry area. This will help to stop the plant material going mouldy and damaging the seeds. The seeds will drop into the bottom of the bag.

Cones can be placed in a paper bag and left in a warm dry place to open. Winged fruits, such as those found in a pine cone, need the wings rubbed off to clean the seed. Cones can be placed in a paper bag and left in a warm dry place to open.

• Seeds from fruit and berries need to have the fleshy part removed before they are dried and stored. To remove fleshy fruit from seeds:
     
  • cut away as much flesh as possible
     
  • squash the fruit to a pulp to press away more flesh. You can put it through a sieve
     
  • swirl the pulp containing the seeds in some water in a shallow bowl. The seeds should move to the middle of the bowl and all the viable seeds should sink to the bottom of the bowl. The flesh should float to the top and move to the outside so it can be skimmed off
     
  • the seeds can then be collected and dried. You can try this with a tomato, tamarillo, cucumber or other similar fruit.
• Seeds from pods such as beans, peas, sweet peas, kowhai and lupin are collected once the outside of pod is brown and dry but before it has opened and dispersed the seed.

Grading seeds

Only the best quality seeds should be kept.

Seeds should be graded for:

• even size
• even shape
• free from disease, insect damage, or damage during harvest
• clean and free from any other plant material including weed seeds
• dryness.

Seed in this condition is more likely to stay viable for a longer period and to germinate evenly when sown. Poor quality seed will lead to poor germination results.

Seed cleaning and grading video

Watch the following video about Seed cleaning and grading.

Seed storage

Only once seeds have been cleaned and graded can they be stored. This is so they remain viable for the longest possible time.

Before they are stored, the seeds need to be placed into a well-labelled paper bag. The paper bag will allow the seed to respire.

The label should include:

• the name of the seed
• the date the seed was collected
• where the seed was collected
• who collected the seed.

This information can be used to let a person know where good seed collecting sites can be found and also if the seed is still likely to be viable when the person comes to sow the seed.

Seeds should be stored in dark, cool conditions at a low moisture level suitable for that species. It is likely to be between 1–5°C. The carbohydrate food store in the seed is slowly used up during storage as the live seed uses the stored food energy to respire. Correct storage will slow down the rate of seed respiration.

If dried and stored correctly many vegetable and flower seeds can be stored for 2–3 years and still have enough carbohydrate food store left to be able to germinate.

Commercially seeds are often stored in foil sachets. The foil keeps out water, and helps to keep the seed at an even temperature.

Fleshy seeds that store food as oils and fats are short-lived and need to be stored in a moist environment in a cool place such as the fridge for a short time only. Some fruit and tree seeds need moist chilling.

What's next?

Go to: 4 Seed dormancy.