Aestivation - Synonym of warm stratification

Autumn sowing - This method of sowing allows vernalization under natural conditions, but the necessarily lengthy period during which the seed must remain in the earth can lead to risks deriving from unearthing by animals, frosts in springtime etc. In the case of small seeds (alder etc.), which are not sown deeply, any changes in temperature may dry out the topsoil thereby causing damage and preventing sprouting.

Dormancy - State of rest, due to physical and / or intrinsic physiological causes, which prevents germination, even in favourable environmental conditions. It is a genetically controlled characteristic, which interacts in various ways with environmental conditions. A number of authors have worked on this subject, we advise reference to the below mentioned definition of dormancy in seeds of broadleaved trees and shrubs by Niclaeva (1977) modified by Gordon & Rowe (1982).

Germinability - Percentage of pure seeds able to germinate under particular conditions, within a certain period, according to the specifications indicated in the Official Methods for Seed Analysis (Ministry of Agriculture and Forestry, 1987). When this is assessed using seed viability determination tests, (generally by cutting or colorimetric analysis) the abbreviation ‘(va)’ is found alongside the value.

Germination energy - Speed with which the seeds germinate, sometimes expressed as a percentage of the seeds germinated within the first week of analysis with respect to overall germination.

Mulching - Protection of the nursery beds using a variety of materials chosen with an eye to cost and availability (mature sawdust, straw, reed matting, leaves, sheets of fabric non fabric, etc.) with the sole aim of protecting the seed from the cold or from other adversities.

Number of seeds per Kg. - Number of seeds present in one kilogram of material suitable for the conservation of the seeds, with an adequate water content and 100% pure. Where possible, minimum and maximum values have been shown, and, in brackets, the most frequent values. Sometimes, the number of fruits (stone fruits, berries etc. ) per kilogram are indicated.

Orthodox seed - Seed which maintains its germinability for long periods if its water content is reduced and it is kept at low temperatures in air-tight containers.

Alterations which may occur during storage, with regards the water content, can be summed up as follows (Harrington 1973):

Pelleting - Process during which the seed is covered with inert substances, which may be excipients for pesticides, and water soluble adhesives, in order to obtain a product which looks like a sugar-coated pill (pellet). This dissolves or is broken when it comes into contact with water, freeing the seed.

Pregerminated seed - Seed in the initial stages of germination, usually following some treatment. There is generally the radicle, and/or the seed integuments are broken.

Pretreatment - A whole range of preparative, pre-sowing treatments, handling, processing with the aim of maximalizing the extent, the speed and the uniformity of germination. In the text, the term is generally applied to treatment which acts directly on the physiology of the seed and on the evolution of the germinating processes (e.g. vernalization), although, by definition, ‘pretreatment ‘ also includes intervention which influences germination in a positive, but indirect way (eg. seed dressing).

Purity - Percentage, in weight, of pure seeds present in the lot. Other types of seed and inert material are considered as impurities.

Recalcitrant seed - Seed which quickly loses its germinability if the water content falls below critical levels. It cannot therefore be kept for long periods of time and is characterized by very high water levels when dissemination occurs (Araucaria araucana, Aesculus hippocastanum, Quercus sp., etc.)The weight of the seed is usually high, thanks to the high water content, which varies from 30 to 70%, and it is usually big.

It is assumed that in this type of seed germination begins the moment dissemination takes place, and this would explain damage caused by any reductions in their moisture level, it is also believed that germination may be either very slow (oak), moderately fast (cocoa) or very fast (mangrove).

In the latter case, the chances of keeping the seeds are very limited.

Since in some cases, the embryos are able to stand a greater water loss than the whole seed, it is thought that controlled dehydration followed by cryopreservation in liquid nitrogen may be a promising technique for the storage of germoplasma with recalcitrant seeds.

Scarification - Erasure of the external integuments of the seeds, using mechanical or chemical methods, in order to encourage water absorption and gas exchange.

Seed - Any material used for sowing, regardless of the correct botanical definition. For example, in ash, samare are defined as seeds, even though they are indehiscent winged fruit.

Seed stratification without a substrate - The seed is layered upon itself, usually after being soaked in water for 24-48 hours and then drained.

In order to acheive this, the seed is generally placed in plastic bags which are not air-tight, to allow gas exchange, in temperature controlled environments (refrigerators). It is advisable to put no more than 10-12 Kg. of absorbed seed per sack and to mix periodically. If the seeds give out an alcoholic odour after a period of vernalization, this is an indication of anaerobic respiration due to limited aeration.

Many species (Pseudotsuga menziesii, Alnus cordata,etc.) respond well to this type of treatment, with no health problems. It is obvious that stratification of the seeds with no substrate is a very space-saving method and simplifies manual care, so whenever it is effective, it is to be preferred over traditional methods.

Stratification - A procedure which consists in the layering of seeds in a soft damp substrate, usually made up of peat, sand or vermiculite, used either individually or mixed in various proportions, with the basic aim of encouraging the seed to come out of dormancy. The volume seed/substrate ratio may vary from 1:1 to 1:3. in certain cses, it may be more practical to mix the seed and substrate directly. small seeds, or those with a similar colour to the substrate should be placed between sheets or other permeable material so that it is easier to gather them at the end of treatment.

Stratification carried out at low temperatures (from +2°c to +6°c), in a controlled environment (refrigerators, cold storage rooms etc.) or outdoors (frames, holes dug in the earth etc.) is known as cold stratification or vernalization; in both cases, sufficient humidity in the substrate, avoidance of perched water and constant uniform temperatures in all of the material are of the utmost importance. In treatment outdoors, where changes in temperature and humidity are more likely, it is a good idea to water the seeds when necessary, making sure that the water can drain away, and to insulate the sowing matter by.placing it in sufficiently deep holes, or in a position out of direct sunlight, underneath a layer of insulating material (earth, sand, sheets of jute, leaves etc.) Usually, for a question of space, 4 large seeds (walnuts, nuts, acorns etc) are layered in this way, and must also be protected against rodents with the use of nets, poisoned baits and repellants.

A variation on vernalization which is only mentioned in passing in this text, is pre-chill priming, which consists in cold stratification followed by dehydration, the extent of which depends on the species, which allow the seed to be kept in a non-dormant state, for a variable length of time. This technique is very promising, but not yet widespread in the field of forest nursery.

In order to inhibit the growth of funghi which are present in the external integuments of seeds, and which thrive on the conditions afforded by stratification, the seeds may be soaked in a solution of sodium hypochlorine with 2% of active chlorine, for 10 minutes.

Since cold stratification is much more widespread, when the term ‘stratification’ is employed without specifying whether it is ‘warm’ or ‘cold’ it goes without saying that this is vernalization.

The beneficial effect of thermal treatments on germination (warm-damp, cold-damp, or an alternating combination), is seen through five main effects:

1. interruption of different types of dormancy.

2.increase in the rate and uniformity of germination and overall germinability.

3. widening of the temperature range within which germination is possible.

4. reduction in the light requirement for species in which germination is favoured by this factor.

5. minimization of the differences in seed quality caused by the diversity in harvesting, preparation and conservation techniques. In general, conserved seeds require longer periods of stratification than recently harvested seeds. On the other hand, samples with a typically low germination power should undergo shorter thermal treatments than those referred in literature.

Treatment - In the text this is a synonym for pretreatment.

Vernalization - This is a synonym for cold stratification

Warm stratification - Stratification carried out at around +20°c, also known as aestivation.

Water content - This is the weight of the water contained in the seeds, expressed as a percentage with respect to the fresh weight of the specimen [(Fresh weight-dried weight) divided by fresh weight x100]