When describing the desiccation sensitivity of a seed, we are talking about the ability of a seed to handle being dried down (as you would dry a grape to make a sultana), and still germinate afterward. Understanding this is important because it determines the way seeds are stored.
Categories of desiccation sensitivity
A seed is considered to be desiccation tolerant if it can be successfully dried to approximately 4-7% moisture content (ie, 4-7% of the weight of the seed is water) and still germinate upon rewetting. A desiccation sensitive seed cannot germinate after drying below 15-25%1. However, all seeds do not fit so discreetly into either category, as desiccation sensitivity (or tolerance) is a continuum, in which some seeds can handle a degree of drying, perhaps to 10-21% before loss of viability occurs. These seeds are usually called ‘intermediate’.
Further to the classification of how much drying seeds can handle, we test whether or not dried seeds can be subsequently frozen. If a seed can be dried and stored at -18 or -20°C (the same temperature as your freezer at home), it is classified as 'orthodox'. Conversely, if a seed cannot be dried (and therefore cannot be frozen), this seed is classified as 'recalcitrant'.
|Continuum of seed storage behaviour in which orthodox seeds can be dried (5-7% moisture content/10-15% equilbrium relative humidity) then cold stored, compared to recalcitrant seeds which cannot survive any degree of artificial drying at all.
Why is desiccation sensitivity, orthodoxy and recalcitrance import?
Well, it dictates how we can and can’t store seeds. Orthodox seeds can be easily stored using standard seed banking methods, and we expect them to remain viable in storage for at least 10 years. For recalcitrant and even intermediate seeds we need to find other ways to store the seeds such as cryopreservation or as living collections.
So why can some seeds tolerate being dried and not others?
It is thought that desiccation sensitivity in seeds has occurred as a response to environmental factors2. A high proportion of seeds that come from wet environments (eg rainforest) have desiccation sensitive seeds, even as high as 50%2, while seeds from harsh environments such as those that are cold or arid, tend to be desiccation tolerant. This makes sense as plants need water to grow and if it is not available when seeds are mature in the desert, then seedlings would not survive, so being able to survive drying until water is available ensures seeds (and subsequent plants) live. Conversely, seeds in wet humid environments are not going to survive very long on the forest floor before they get consumed or rot, so it’s important that they germinate quickly. Interestingly, this is in direct contrast to desiccation sensitivity in plants, whereby seeds need to be desiccation tolerant in dry environments but plants struggle, while in rainforests, plants thrive with copious amounts of moisture, yet their seeds don’t necessarily do quite as well.
How do we know the best way to store each rainforest species?
When it comes to determining desiccation sensitivity and suitable storage methods for seeds, the best way to do this is by actually testing representative seed samples from each collection. A portion of seed is germinated fresh, a portion is dried and germinated, and a final portion is dried, frozen then germinated. The difference between total germination for each set of seeds gives an indication of desiccation sensitivity.
Indicators of desiccation sensitivity
There are also other aspects to the seed that give an indication of desiccation sensitivity including the seed size, the ratio of the seed coat to the whole seed, the seed moisture content and metabolic rate at time of maturity, the plant habit and the climatic conditions in which the seed developed. Desiccation sensitive seeds may also have a very limited endosperm, that is they only have a small energy supply and proportionally large cotyledons (seed leaves). If they are going to germinate quickly, they will soon have functioning leaves and can make their own energy from photosynthesis, doing away with the need for a stored energy supply.