Most artificial and natural forest regeneration programmes start with seeds or cuttings. An assured supply of seeds is a pre- requisite to successful tree planting and forest regeneration. The amount of available seeds determines the amount of plantable stock to be produced in the nursery. The quality and quantity of seeds of the desirable species on the forest floor will determine success or failure of natural regeneration. To produce high quality tree requires high quality seeds which can be obtained from professional collectors or from non-professional local sources.
Careful examination of available stands must be made before seed matures to locate areas where fruit production is sufficiently heavy to make collection profitable. In the absence of seed orchards, the genetic purity of trees can best be approached by collecting from pure stands of mother trees which exhibit the desired phenotypically good characteristics such as good form and vigour. Seeds from immature trees which have not yet fully exhibited their phenotypical characteristics and therefore the quality of trees produced from them cannot be predicted with certainty. Attention should also be given to quality of the fruits such as freedom from pest and diseases.
It is undesirable to collect seeds from isolated plants or from single plants or species growing near other plants of related species. Under such conditions, either self-pollination resulting in reduced seed quality or cross pollination with different species may occur or produce variable seedlings that are different from the parent ones. The quality of a seed source can be determined by growing a population of seedlings from it.
Time for Collection
It is the duty of the seed collector to know the best criteria to indicate the optimum time to harvest his seeds so that they accumulate sufficient food resources, can germinate, and are easy to harvest. Some of the criteria used include: moisture content or dryness, colour, specific gravity.
Method of Collection
Tree fruits collection can be divided into direct and indirect methods. Direct methods include climbing, plucking with hand or pole, tree shaking, tree felling or trimming, and stoning or shooting. Indirect methods involve collection of fruits that have either fallen on their own or by other means such as animals.
The method employed in direct fruit collection is determined by the tree height, quantity and location of fruits and the nature of the tree hole. Fallen fruits and seeds can be collected by sweeping and raking them or spreading a sheet of plastic, tarpaulin or mat under the tree or simply clear and sweep under it to collect fallen fruits.
In most cases seeds are collected by hand picking before fruit begin to fall, open and scatter its seeds. This is very important especially wit small seeds which fall from them while they are still attached to the tree. Felling the tree to collect the fruits is destructive but may be useful where the trees are already earmarked for felling and there is no other better alternative method such as climbing and lopping of branches, putting down branches with hook or rope and weight, shooting down fruits and collecting fallen seeds on the ground or floating in water (for large fruits and seeds).
Collected seeds should be labeled before putting into sacks or suitable containers and transported quickly to places where they can be extracted and cured for storage. The label should contain the name of the species, date of collection, location and nature of the mother tree.
Seed Extraction and Cleaning
The length of time that freshly picked fruits can remain in sacks without becoming heated, mouldy or deteriorating, depends on the species and weather conditions. The pre-extraction treatment of dry fruits usually involves direct exposure of most of the species to the sun. Both mechanical (machines) and manual methods of seed extraction are available, but in Nigeria, like in most developing Countries, manual methods such as hand splitting or removing, crushing, pounding, beating, soaking, shaking, drying, and fermenting are most common. The particular method adopted depends on the species as well as the facilities available. Good seeds are sorted from impurities by picking, floating, and sieving/winnowing.
Seed storage is sometimes necessary because they are often collected in greater quantities than immediately required and their time of production may be different from the time required to raise them. They may also need to be transported over long distances from the point of collection to their point of use.
The viability of seed at the end of any storage period is the result of initial viability and the rate at which deterioration takes place. When placed under conditions that slow down respiration and other life processes without injury to the embryo, (like in sealed containers) some seeds can be stored for many years.
Temperature moisture relationship are of most practical significance in seed storage Seeds that have hard seed coats and are impermeable to water and gases generally have long storage viability. Very low moisture content (4-6%) of the storage environment (such as sealed storage containers) is necessary for long storage. The combination of low moisture content, sealed containers and low temperature provides one of the most desirable seed storage conditions. In areas where facilities are lacking, dry storage method which does not involve temperature control and the seeds are stored under room or air temperature may be used, in cold dry storage the temperature is usually between 30- 50% and below zero for long storage. This is carried out in refrigerated rooms with temperature control. Cold moist storage seeds are sealed in containers and mixed with moisture retaining materials which will maintain their moisture contents. The temperature is usually kept above zero (32-50). Common seed storage containers include: jute/fibre bag, tin/can, glass jar/bottle, pot, jerrican (metal/plastic) or drum, sealed plastic and bags. Moisture absorbing materials include: wood ash, dry charcoal, lime, silica gel, pieces of news paper, and rice husks.
Mature seeds of many species of woody plants will germinate immediately if planted under favourable environmental conditions, but seeds of most species may fail to germinate under favourable environmental conditions and are therefore said to exhibit some degree of dormancy. Seed dormancy has both advantages and disadvantages, it provides for the establishment and survival of a species by remaining in soil for many years before germination. It also restricts seed germination in hot and dry regions to the short, wet period of the year.
In contrast, seed dormancy is often a nuisance to nursery operators who wish to have large quantities of seeds germinate promptly in order to produce large and uniform crops of seedlings. The causes of seed dormancy and methods of breaking it are therefore of both physiological and practical importance.
The failure of viable seeds to germinate may be due to external factors such as lack of favourable moisture, temperature and oxygen conditions and internal factors within the embryo such as dormancy and influence of the enclosing seed parts on the embryo which may mechanically inhibit water uptake, restrict gaseous exchange or resist embryo expansion. It may also be due to chemical inhibition by specific substances in some parts of the seed or fruit or may be the combination of two or more of the above.
A viable seed which is unable to germinate under appropriate germination conditions is said to be dormant. Dormancy is a state of reduced activity of the plant or plant part in which readily discernable growth does not occur. The causes may be due to external effects of the environment, the internal conditions within the plant part, and inhibiting influence of adjacent parts. There are 3 types of dormancy namely, chemical dormancy, physical dormancy, and embryo dormancy. Chemical dormancy (inhibitors) is caused by growth inhibiting chemical such as conmarin, phenols, caffeine, and cocaine present in the seed. Physical or seed coat dormancy is often due to thick, hard, bony or waxy covering of the seed coat which prevents entrance of water and air into the seed to facilitate germination. Immature embryo dormancy occurs where embryos are only partially developed at time of fruit ripening. This requires a rest period to complete the development process and therefore disappears during storage.
In nature dormancy prevents germination soon after seed maturation during which time the environmental conditions may be unfavorable for survival of the resulting seedlings. It also prevents immature germination and makes seed dispersal, exchange and importation possible. Variability in the degree of dormancy ensures that not all seeds germinate at the same time thus ensuring the survival of the species. However dormancy has made pre-germination testing and treatment of seeds necessary. While seed testing causes delays, germination may be unsatisfactory if pre-germination treatment of the species is not known.
Knowledge of the causes of seed dormancy often makes it possible to intelligently apply appropriate treatments to overcome the dormant condition of individual seed lots. Seed dormancy results from a number of causes including: immaturity of the embryo, metabolic blocks within the embryo, mechanical resistance of seed coat to growth of the embryo, impermeability of seed coats to water and gaseous exchange, a combination of two or more of the causes, and secondary dormancy.
Sometimes the embryo is immature and requires a period of “after-ripening” (storage under favorable conditions) to reach a certain stage of development before germination occurs. However, the most common type of seed dormancy is one in which morphologically mature embryo are unable to resume growth and germinate. Sometimes the failure of seeds to germinate is traceable to more than one specific type of dormancy. In some species of Rose, seed germination is prevented by the mechanical restriction of a thick pericarp on embryo expansion, as well as by dormancy resulting from growth inhibitors in the achene.
- Hormones and Inhibitors
Presumably all physiological dormancy is controlled by growth regulators. The onset of embryo dormancy is often associated with accumulation of growth inhibitors, and the breaking of dormancy with a shift in balance of growth promoters that overcome the effects of inhibitors.
Various seed germination inhibitors occur in many species of woody plants in all parts of seeds and fruits, including the embryo, nucleolus, testa and pericarp. Abscisic acid (ABA) is perhaps the best known and most inhibitory substance of the inhibitor complex and is prevalent in seeds of peach and apple. In some species, such inhibitor decrease during chilling of seeds, while in others the inhibitor is not removed by chilling but its inhibitory effect may be overcome by an increase in growth promoter such as gibberellins or cytokinins.
Seed Coat Dormancy
A very common cause of dormancy is the impermeability of seed coats to water or oxygen. Seed coat dormancy is especially common in the seeds of leguminosae. Red cedar, Eastern white pine and apple seeds also have seed coat dormancy problems. In apple seeds, the seed coats impede oxygen uptake, thereby making the supply inadequate for the high respiration rate necessary for germination of the embryo to occur. Seed coats of some species are said to be mechanically resistant, preventing the embryo from further development once it becomes fully grown in the seed. However, many of such reported cases are probably caused by other factors such as physiological dormancy of the embryo.