Endogenous Dormancy Control

Seed treatment by chilling

 

Those seeds with an endogenous dormancy control, which is mitigated by a period of cold temperature (chilling), constitute a fair proportion of the species from cold temperate climates.

 

This is an evolved strategy in which the embryo is prevented from germinating, during the autumn and winter, by the presence, within (or adjacent to) the embryo, of an inhibitor which prevents any further physiological activity associated with development and growth. This inhibitor is eliminated and/or neutralised and/or replaced by promoters during this winter exposure of the imbibed seed to a specific period of cold so that germination will occur as soon as environmental conditions are satisfactory for it to proceed.

 

For the process to be successful and to be achieved artificially with exactitude, it is important that all other constraints to germination associated with seed coat conditions and other chemical inhibition have been completely removed from the seeds.

 

The accurate assessment of the period required for chilling to eliminate the dormancy condition is an important step in determining the protocols for a complete blueprint in the seedling production for a particular species.

 

The practicalities revolve around ensuring that the seed receives a suitable and sufficient cold treatment (chill).

 

Prior to the commencement of chilling it is an imperative that the seeds are fully imbibed - as any backlog of moisture required by the seed to complete imbibition, once the chilling process has commenced, will distort the time scales.

 

Initially therefore the seeds will require a warm water soak for several hours to ensure complete imbibition. A warm water, rather than a cold water, soak ensures not only that imbibition occurs effectively and quickly but that certain microstructures within the cells reconstitute more efficiently at higher temperatures. It has been shown that the level of imbibition required to allow the chilling process to achieve is marginally less than the level required for the initiation of germination. Hence it is possible to prime the seeds by chilling at this critical level of imbibition and then holding (ie store) them without further water uptake until they are required for germination. However this process requires much inquiry and measurement and is only feasible for large scale operation and is thus limited to commercial seedsmen who wish to sell a ‘primed’ product.

 

The seeds are then ready for chilling and this treatment should begin without allowing the seed to dry.

 

When the prepared seeds are to be subjected to the cold temperature treatment it will be prudent to determine how this will be achieved. The seeds will need to be able to maintain their water status and have a satisfactory surrounding atmosphere so that there will be a normal and adequate pattern of gaseous exchange for metabolic activity. It is important that oxygen is not limiting as an absence or a low level of availability will induce anaerobic respiration within the seed with the anticipated deleterious results.

 

The seeds are mixed with a suitable inert and more or less sterile, extending medium which will hold sufficient water, will maintain sufficient contact with the seed and will achieve a satisfactory and continuing level of ‘air filled porosity’ - such that the required metabolic processes can proceed normally and without constraint.

 

Conventionally a mixture of equal parts by volume of damp, sieved peat and coarse sand or sharp grit (up to 2-3mm) provide the right conditions, although materials like vermiculite and perlite have been successful – choice is a function of seed size and the materials available.

 

The sieved peat is dampened until, on squeezing a handful, it is just possible to cause a drop of water to be emitted. The components are mixed thoroughly to create an even consistency. The determination of what constitutes a ‘volume‘ of damp peat is a bit of a moveable feast – as peat can be compacted to significant levels. It is therefore necessary to determine this amount (‘volume’) by trial and error (ie what suits the particular materials and process within the operation) and in order to establish the consistency of the working unit for use in any particular circumstances.

 

As long as the physical conditions are satisfied and there is no other chemical or biological problem, the actual constituents are not, in themselves, critical.

 

The seed sample is then extended with four times the volume of this medium.

 

As it may well be that in subsequent operations the seeds will not be separated from the medium it will be prudent to treat a measured unit quantity of seeds – either by weight or by number to facilitate this subsequent procedure. This measured quantity of seeds may well be the quantity needed to sow a container or a particular area of seedbed.

 

For this reason it is advantageous to treat the sample in small units and a container such as a polythene zip-lock type bag is particularly useful. A label should be placed inside the bag carrying relevant details and identification as well as on the outside - to ensure continued individual identification and treatment.

 

The cold temperature is provided by transferring to a refrigerator which has been set to maintain a temperature of 3ºC (+/-1ºC) - as a temperature at this low level is likely to satisfy the requirements of the great proportion of species to be treated.

 

Once the sample has been bagged and placed in a refrigerator, for the cold treatment, it will be important to monitor progress regularly. At least once a week the bags should be opened to check water status and allow gaseous exchange. Shaking will also prevent any uneven compaction of the medium and so eliminate any variation, within the sample, in water uptake or gaseous exchange. Hence it is wise not to stack the bags but to have sufficient shelves and place only one layer per shelf.

 

Such a procedure will also allow a visual assessment of progress - as some species will germinate (at least to the radicle emergence stage) at low temperatures once the condition is mitigated.

 

Once the chilling period has been completed, the seed is ready to enter the germination phase and will need a higher temperature for a speedy, uniform and effective result. Usually the practice has been to sow the seed onto a suitable compost in a relevant container and place on a bottom heat of 18-20ºC. However it is more energy efficient to achieve this stage in an incubator and then sow the seeds a day or so before the anticipated germination date. The same conditions for monitoring and managing the packets of seeds apply as are suggested for the chilling procedure.