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Seed Viability & Germination

Are seeds alive and what do they need to germinate?

Although they might seem like the same thing, when we are talking about seed viability and germination, we are referring to two different things. Viability means whether or not a seed is alive, while germination refers to whether or not a seed can germinate. The number of seeds viable in a collection doesn’t necessarily equal the number of seeds that can germinate, the difference being that some seeds are immature, or dormant (‘asleep’).

Viability

Whenever we make seed collections we try to ensure that seeds are viable and mature (ripe). This is very important because if seeds are not viable, they will not germinate anyhow! If they are immature they will not survive as well in storage as they would if they were mature. Sometimes we can help seeds to mature after collection by the way they are stored. Also, the way seeds are handled during collection and transport will influence the proportion of seeds that remain viable.

When it comes to testing seed viability, there are a number of different techniques that can be used, some of which actually result in the death of the seed! This means we have to choose a test carefully and ensure there are plenty of seeds left for other tests and storage. As such, viability is assessed on a portion of seeds (perhaps 20 seeds for small collections, 100 seeds for large collections), and this is taken to represent the viability of the entire collection.

Viability tests

Texture
Whenever a new collection of seed is made, a ‘cut-test’ is performed, whereby a seed is cut in half, so that the contents inside can be assessed. A viable seeds will usually fill all the space inside of the seed coat (shell), be white in colour, feel firm, and the embryo of the seeds usually identifiable from the endosperm or starch supply. 
Structure
Another way to assess viability is to x-ray the seeds to look inside. Any immature, empty or damaged seeds can be detected without opening the seed – this means viable seeds can still be germinated. 
Colour
Other tests use different stains (dyes) to assess seed viability, and sometimes we can estimate the viability by the appearance of fruit and seed. Plump, coloured fruit are usually viable, which is easier to tell if there are immature or over-ripe and dying fruits available for comparison. 
Germination
One of the main techniques that can be used to assess viability is to simply germinate some seeds! Seeds that do not germinate are not necessarily dead, but possibly dormant and this needs to be considered in the results. Prior to storage, we also need to understand what is required for successful seed germination to occur, which may include breaking seed dormancy.

Dormancy

In the field

Seeds use dormancy to hold off germination until the environment becomes suitable, that is, when the surrounding conditions are going to give the seed the best chance at life. This might mean, for example, that there is water present, that a sunlight gap has formed in the canopy, that nutrients are suddenly available such as after a fire or when the seed is defecated after being eaten by an animal. Sometimes dormancy occurs to prevent all the seeds from germinating at once!  Dormancy may reduce the proportion of viable seeds that germinate at any one time, increase how long germination takes to begin, or prevent the seeds from germinating entirely. We need to understand what type of dormancy a seed has so that when we want to germinate seeds we know how to break it. 

In the laboratory

In the laboratory, we use a number of different techniques to break seed dormancy which include scarifying (cutting/scratching) the seed coat, temporary cold or warm stratification (storage), repeatedly rinsing seeds, soaking the seed in hot water or concentrated acid, soaking the seeds or germinating them in the presence of the plant hormone gibberellic acid or smoke water. One technique alone may be suitable, or two or three may be used together, but multiple things are tried until the highest proportion of germination is found. Below is a graph of germination results from an experiment in which groups of seeds of Hibbertia glabrescences subsp puberula were each treated differently. 

Germination of Hibbertia glabrescences subsp puberula following different pre-treatments and incubation conditions. Seeds were germinated on either water agar (WA) or gibberellic acid agar (GA) and then incubated at either a constant temperature of 20°C, or alternating day and night temperatures of 25 then 10°C. Some groups also had a 2% solution of smoke water added to the agar. Seeds were inspected every few days with cumulative germination (germination %) recorded. 
 

Germination

Our viability and germination testing hopefully results in the production of new rainforest seedlings. As seeds germinate they are removed from experiments, potted up and grown on in the nursery. These plants can then serve new roles, forming the basis for tissue culture material or living collections. Material can be planted into our Gardens or even donated to other organisations. 

Seedlings generated during the Rainforest Conservation Project donated to Illawarra Landcare.