An effective method for the storage of seeds is required to maintain a high level of viability, at the least, in the short term between harvest and sowing (or the treatments prior to sowing) during the course of a normal season; or may be necessary in the longer term, from year to year, in order to even out the availability of supply to take account of the vagaries of collection, seasonal influences or periodic cropping.

 

The object of such storage is to extend the longevity of a seed lot by maintaining the highest feasible level of viability for as long as possible. This is achieved by the provision of environmental conditions which have the effect of reducing the rate of deterioration within the lot.

 

Perhaps the most obvious consequence of this concept will be evident by attention to the sigmoid curve representing the ‘normal’ rate of deterioration within a lot. It is evident that the highest level of viability occurs at the top of the curve and hence the greatest potential for extending storage life (longevity) will be achieved when the seeds are subjected to suitable storage conditions at this stage ie the earliest possible moment after dispersal.

 

Viability is at its highest level immediately after dispersal. At this stage the seed lot is in prime condition and in normal circumstances there will be little or no deterioration of the structures or processes for a short period, after this viability will decrease in the anticipated pattern at whatever rate is the norm for the particular species. The effects of storage, therefore, in enhancing the longevity of the lot only begin with the viability at the time the process is begun – in other words this provides the base level of the process.

 

Storage of seed, to achieve an extension to the longevity of the seed lot, is achieved by modifying certain environmental conditions surrounding the seeds.

 

The most usual environmental factors which influence the successful enhancement of longevity are those which slow the metabolic processes, especially the rate of respiration of the seed, and which can be applied under practical circumstances. In practice three environmental factors are relevant – temperature, moisture content and the gaseous constitution of the atmosphere.

 

One of the most commonly manipulated parameters in the storage of the seeds of woody plants, within the environment of the seed, is that of temperature. It is a well documented feature of biological processes that the rate of the process is responsive to temperature. Normally the colder the temperature is maintained - the slower will be the rate of reaction. Thus at a low temperature the rate of respiration is lower, food reserves are mobilised and consumed more slowly and the consequent ancillary processes are slower. Any reduction of temperature will enhance longevity but the most effective response is achieved at the lowest temperature that the seed will tolerate without damage.

 

Harrington, fifty years ago, in his critical observations on seed storage proposed a number of ‘Rules of Thumb’ or generalisations for successful storage. In relation to temperature he proposed that storage life could be doubled for every 10ºF reduction in temperature that could be achieved.

 

Storage temperatures well below freezing (using liquid nitrogen and similar agencies) can be practiced but normally this requires a dedicated unit and such a facility is only usually available to large organisations, seed banks etc. It requires particular knowledge, critical technique, specialised equipment and experienced operators to achieve success.

 

Simply subjecting seeds to sub-zero temperatures in a freezer usually causes damage at the cellular level as the intercellular and then the intracellular water freezes; damage is normally caused during the differential movements of cell walls in the thawing process as ice crystals penetrate cell walls in the embryo and in the food reserves – it is most effective when the moisture content of the seed is low and this is often only feasible with seeds in which the food reserve is stored as carbohydrate.

 

In practice, for most woody plants of temperate provenance, a storage temperature of 2-3ºC is adequate to achieve the level of longevity which is useful for the practical operator. At this level a normal household refrigerator can be manipulated to maintain such a temperature.

 

Historically the most favoured method of increasing the longevity of a seed lot is to reduce its moisture content. However, generally, the seeds to which this refers have been the seeds of major agricultural, crop plants in which the food reserve is stored as carbohydrate. Such seeds are more responsive and successful at reconstituting themselves after the drying process. Many species of woody plants store their food reserves in the form of proteins or lipids (as well as carbohydrates) and these materials do not lend themselves easily to rehydration.

 

The reduction in the water content of the seed effectively slows the metabolic processes of the seed by reducing the water, within the seed, which is available for enzymes and other biologically active chemicals to mobilise and activate the various processes within the metabolic system.

 

Drying is probably the most widely practiced technique for increasing the longevity of any particular seed sample. However the practice although used on a large scale is chiefly limited for the storage enhancement of many crop plants and these subjects, more often than not, store their food reserves as carbohydrates. Such seeds when dried can be rehydrated reasonably successfully without significant damage to tissue, allow the normal mobilisation of enzymes and the complete use of the food reserves.

 

This, however, does not necessarily apply to those seeds in which the food reserves of the seed are stored as protein or lipid. In these instances the dehydration of the materials to below a critical level effectively means that the storage materials become unavailable to the embryo and viability is lost rapidly. This reaction is associated with the nature of these particular materials – for once they have been dehydrated it is not possible to reconstitute them by rehydration as the water repellent characteristics of these materials effectively waterproofs them and so prevents their reconstitution back to their original status. This absence of water thus severely restricts the movement of enzymes and metabolic products so isolating the food reserves as the necessary energy resource and for practical purposes causes the loss of viability by embryo starvation.

 

These so-called ‘intractable’ seeds can be successfully stored if critical attention is given to the degree of the drying process. However, unless evidence is available to the contrary, it is probable that storage should be conducted under water conserving conditions and maintaining the moisture status at which the seeds were dispersed from the parent plant. If marginal drying occurs during collection and/or the extraction process then re-wetting to reduce the possibility of moisture being lost from the food reserves to the seed coats provides a useful safety valve when the seeds are stored in an impermeable container.

 

When a minimum moisture content is cited as part of a recommendation for seed storage, care should be taken in ascertaining on what basis the calculation has been made as this parameter could be critical to survival. The difficulty is perhaps obvious but will depend on whether the calculation was determined as a factor (usually as a percentage) related to the fresh weight or dry weight. It will be evident that the mathematics of such an exercise could have quite a dramatic influence on the outcome of the storage process. For practical purposes the most satisfactory and reproducible results will be derived from measurements made on a dry weight basis - as fresh weights are very likely to be much more variable depending on the nature of the particular season – ie the moisture content is expressed as a percentage of the dry weight.

 

When storing seeds at a predetermined moisture content attention should be given to maintaining this status during the storage period. The seeds should be sealed in a water conserving container in order to isolate them from the vagaries of the atmospheric humidity of the storage environment - otherwise the moisture content of the seed may decline as it equilibrates with that of the atmosphere.

 

Harrington’s Rule of Thumb in relation to the moisture content of the seeds during storage (effectively only for carbohydrate storage) is expressed as ‘that for every reduction of 1% in moisture content of the seed, the storage life will be doubled’ – experience however suggests that a figure on 2% is nearer the mark.

 

Intermediate type seeds - which store their food reserves as a mixture of carbohydrates and lipids or proteins –are more tolerant of drying but nevertheless the quality of the subsequent development will undoubtedly be affected.

 

The effects on the success of storage can be enhanced by combining these two parameters and so lengthening longevity.

 

Although rarely practiced, it would be quite possible to slow metabolic processes in the seed by reducing the availability of oxygen however this possibility should be treated with care as a wholesale removal of oxygen could induce an anaerobic response if the other parameters are not also involved.

 

References

Harrington, JF (1960) ‘Thumb Rules for Drying Seeds’; Crops and Soils 13(1):16-17

Harrington, JF and JC Douglas (1971) ‘Seed Storage’; Nat. Seed Corp., New Delhi, India