Edible nuts are used by mankind for food, edible oils, spices, condiments or beverages. They have been an important food source from prehistoric times and are among the most nutritionally concentrated of human foods, high in protein, oil, energy, minerals and vitamins. Nuts that are only rarely used as famine food have been excluded from this present study partly because of the paucity information available but mainly because they are not normally considered edible. Nuts used solely for spices or condiments have also been largely excluded since they are used sparingly, to flavour food and not as a food; traditionally they are considered separately from edible nuts. Nuts that are largely used as commercial sources of edible oil are not discussed in any great detail since they are already adequately dealt with in the literature, e.g., Hartley (1988) regarding the oil palms Elaeis guienensis. However, their local uses are briefly mentioned in Appendix A.
Melville (1947) has, perhaps not unexpectedly, noted a correlation in nutritive values between species within a genus or family. There are also similarities to be found between related species in the harvesting, storage and processing techniques, etc. The arrangement of edible nut species in Appendix A is, therefore, by families in the expectation that users can compare related species and possibly improve management techniques or deduce possible techniques where none are known.
Botanical terminology has been used where known but, in view of the uncritical use in the literature of layman terms, it has not always been possible to identify the true nature of the organs without further research and this may have led to the wrong deduction being made in this paper. As far as research time has permitted, the currently accepted botanical name is used within the text and these, together with their synonyms used in the consulted references, are given in Appendix C.
The term "forest", as used in the title of this
report, embraces all the natural ecosystems where trees and shrubs form
a significant component. As such, "forests" range from evergreen rainforest
to desert, although in the latter habitat the trees and shrubs are confined
primarily to oases and waterways (Wickens, 1991). In certain areas, trees
and shrubs bearing edible nuts are preserved on farmlands and homesteads
after land clearance and constitute the chief supplies available for household
consumption or sale; as such they can be considered as relics of the natural
ecosystems. There is also a rather grey area where forestry, horticulture
and agricultural interests overlap. While nut plantations are included
in this report, certain recognized nut crops, such as peanuts, sunflower
seeds, pumpkin seeds and soya beans are definitely agricultural crops and
are not considered in any great detail here.
The botanical use of the term "nut" refers to an indehiscent fruit that is usually shed as a 1–seeded unit. It is formed from more than one carpel but only one seed develops, the rest abort. The pericarp is usually lignified and is often partially or completely surrounded by a "cupule". True nuts include the hazelnuts, Corylus spp. (Corylaceae), and from the Fagaceae, beechnuts, Fagus spp. and acorns, Quercus spp. The term is also loosely applied to any woody fruit or seed, such as the walnut (drupe of Juglans spp., Juglandaceae), Brazil nut (seed of Bertholletia excelsa, Lecythidaceae) or peanut (indehiscent legume of Arachis hypogea, Leguminosae subfamily Papilionoideae). It is even incorrectly applied in the vernacular names to non-fruiting bodies, such as the root tubers of the pignut or earthnut Conopodium majus (Umbelliferae) and Cyperus esculentus (Cyperaceae), although in the latter case the root tubers are eaten as a form of dessert nut. A further complication is that the seeds of several members of the Cucurbitaceae and the sunflower, Helianthus annuus (Compositae) are eaten as dessert nuts and are included in the literature on edible nuts yet they are always referred to both botanically and in the vernacular as seeds.
For the purpose of this work nuts are hard-shelled fruits, or the edible kernels of fleshy drupes or berries, or seeds that are traditionally referred to as nuts. The first two categories of nuts are the fruits of trees or shrubs, the third include some herbs. Since the value of some nuts as food is relative to what better alternatives are available, there are obvious grey areas regarding whether the nut is to be regarded as edible or not and an arbitrary decision has been made as to whether a species should or should not be included. Some 542 species of edible "nuts" have been listed in Appendix A. More detailed information is given in the text on the major, minor and potential nut species. However, researchers requiring even more information should bear in mind that the information given is a summary of information obtained from a selection of the published literature, the primary sources of which have not been consulted.
While a species may conform to the general description of "edible nut" it does not follow that an allied species also bears acceptable "edible nuts", as for example with the almond, Prunus dulcis, and plum, P. domestica; although children may very occasionally eat the plum kernels it has never been considered other than as a fruit crop.
It is possible that a number of species have been omitted because the information is not readily available or is too vague while others should be eliminated because the information obtained has been misleading.
Previous authors have also had problems in defining
an edible "nut" and have provided their own apparently arbitrary limits.
Menninger (1977) provides the widest definition where a nut is defined
"as any hard-shelled fruit or seed of which the kernel is eaten by mankind".
This definition is so broad that even grasses and a number of herb species
have been included, the former are generally regarded as cereals while
the latter would be more appropriately listed under edible seeds. However,
all the authors consulted, with the exception of Verheij and Coronel (1991),
have accepted the popular or everyday use of the term "nut", including
its conservative use by Howes (1948) to include "any seed or fruit consisting
of an edible, usually oleaginous kernel, surrounded by a hard or brittle
shell". Interestingly, the major authors consulted, Howes (1948), Menninger
(1977) and Rosengarten (1984), have, like the present author, all made
their own interpretation as what to include or exclude as an edible nut,
while Verheij and Coronel (1991) declare "it is not possible to define
the edible fruits and nuts in such a way that clearly sets them apart from
species in other commodity groups".
Most edible "nuts" contain concentrated food reserves for future generations of plants and provide valuable sources of energy, protein, oils, minerals and vitamins suitable for human consumption. Others have their food reserves in the form of starch instead of protein, including chestnuts - Castanea spp., acorns - Quercus spp., water chestnuts - Trapa spp., ye-eb - Cordeauxia edulis, Bambara groundnuts - Vigna subterranea and lotus seeds Nelumbo spp. While Pinus spp. are protein rich, some members of the Gymnospermae such as Ginkgo biloba and Araucaria spp. also have starchy food reserves (Melville, 1947).
Nuts have a reputation for being indigestible, especially if eaten in large quantities or poorly masticated. They are generally a highly concentrated food but low in water and fibre content and consequently require thorough mastication if they are to be properly digested, and are preferably eaten with other foods. They have the additional value in that their fats are, in the main, highly unsaturated and are consequently beneficial in that they do not raise blood cholesterol. Peanuts (Arachis hypogea), sunflower seeds (Helianthus annuus), souari nuts (Caryocar nuciferum) and soynut (Glycine max) are specifically mentioned in this context (Howes, 1948; Rosengarten, 1984).
The nutritional value of a number of nuts are
given in Appendix B. It is difficult to draw any firm conclusions from
these analyses because the majority appear to be based on a single analysis
which have been passed down through a number of publications. Ecotypic
and seasonal variations, the effects of fertilizers, drought, etc., have
not been investigated.
The harvesting of nuts from arborescent species is often of fallen nuts and as such is generally a prolonged, laborious and wasteful process since fruits do not necessarily all ripen within a short period. Picking fallen fruits is time consuming, especially in rough terrain and, depending on the time lapse between consecutive harvestings, can lead to rapid deterioration as well as losses to predators. Plantations can improve efficiency by providing better accessibility and easier harvesting conditions and, provided they are large enough, lead to mechanized harvesting by the use of ground harvesters and/or tree shakers. Mechanization is being increasingly important in North America where labour costs are particularly high.
This discussion is limited to the major nuts, where appropriate techniques and equipment have been developed, about which more detailed information can be obtained from Woodroof (1979). By dealing with processing and storage in general terms it is hoped that readers will obtain ideas for dealing with similar problems in the less-known and little researched species.
Many dessert nuts loose their palatability or otherwise deteriorate if not properly dried or cured after collection or are badly stored, especially those with a high oil content. Among the major nuts walnuts and chestnuts are among the more perishable while pistachios and almonds are among the better keepers.
In-shell nuts are best stored at low temperatures.
While rancidity is the main problem with shelled nuts, the kernels also
have the ability to acquire off-flavours from the environment, such as
tobacco and paint. Apart from rancidity, deterioration during long storage
can be difficult to determine and aflatoxins can be present even when there
are no obvious presence of moulds (Matz, 1984).
Modern packaging and marketing is increasingly demanding kernels ready for immediate use, both in the home and confectionery trade. Hand processing is being replaced more and more by mechanization for providing a marketable product, especially where large-scale production is concerned.
Most tree nuts are enveloped in a fleshy or fibrous outer covering or husk which may or may not remain attached to the nut as it ripens and falls to the ground, as is the case for such important commercial nuts as the walnut, pecan, almond and macadamia. In some varieties of walnut and almond the ripe nut readily separates from the husk of their own accord; a few nuts, termed "sticktights" remain attached to their husks, the proportion depending on the cultivar. A poor or dry season may increase the number of sticktights.
The husk has to be removed either mechanically
or by hand before nuts can be dried, shelled or otherwise processed. Walnuts
especially must be dehusked as soon as they are collected, otherwise the
colour and quality of the kernel will be progressively adversely affected
the longer the husks remain (Howes, 1948; Menninger, 1977; Rosengarten,
Nuts gathered from the ground are frequently soiled or dirty, especially after rain, so that washing is necessary in order to make the product more attractive for the market. Again, hand washing in tubs or machine washing in cylindrical drums may be carried out. Any staining or discoloration is not affected by the washing and is removed at a later stage during bleaching (Howes, 1948; Menninger, 1977; Rosengarten, 1984).
Artificially produced heat and forced draughts are now increasingly replacing sun-drying for the drying or "curing" of nuts, especially by large-scale commercial producers. Outputs range from 1-7 tonnes for a drying time of 12-24 hours at, in the case of walnuts, not more than 43° C. Such mechanical dryers are not only independent of the weather but also give a more thorough and uniform drying of the nuts and, in the case of walnuts, result in less splitting. In America mechanical drying has been found to increase packing-house efficiency by stabilizing the delivery rate from the growers and to aid marketing by providing an earlier product. The investment costs did not exceed the high labour cost of sun-drying.
Sun-drying is still largely practised by small
producers, particularly with almonds in the producing countries of Europe.
It requires a high labour input to spread the nuts out on sheets or in
trays, stirring frequently to ensure uniform drying, and also to provide
protection at night or in the event of rain. Drying is completed when the
kernels can be heard to rattle in the shells or can be broken rather than
bent with the fingers. Drying may require only 2-3 days with continuous
hot sun or take 2-3 weeks in inclement weather. Similarly, chestnuts in
southern Europe were traditionally sun-dried or dried over a wood fire
in a specially constructed kiln. A slow process, it may take several weeks
and a heavy fuel input to achieve, although the smoke is reputed to have
a beneficial effect in inhibiting subsequent fungal infestation but the
process results in the loss of the fresh chestnut flavour and the chestnuts
may even acquire a disagreeable one. The curing is complete when the shells
may be easily separated from the kernels. Modern, mobile mechanical dryers
now provide a faster and more uniform drying with a greater economy in
fuel (Howes, 1948; Menninger, 1977; Rosengarten, 1984).
Some nuts intended for marketing in-shell may have their appearance enhanced by bleaching to remove stains due to sun scorch and disease; walnuts are frequently thus treated by immersion for 1-2 minutes in a bleaching solution.
Sulphur dioxide or burning sulphur may be used, particularly for almonds that have been blemished during harvesting or drying, but this is practised less so now than in the past. Good quality almonds that have been quickly sun dried are already of an attractive appearance and require no bleaching. Over bleaching can give the shells a sickly white appearance and is liable to soften them and flavour the kernels; too little causes irregular bleaching.
Pecans can be considerably improved by removing
the outer rough layer and polishing, a process which, if required, makes
the nuts more responsive to bleaching and drying. The outer layer may be
removed by steel brushes or by the use of revolving drums containing coarse
sand. Bleaching is effected by dipping the nuts for 4 minutes in, for example,
sodium hypochlorite containing 2% active chlorine. Various dyes may be
used to colour the polished and bleached nuts an attractive brown or reddish
brown (Howes, 1948; Menninger, 1977; Rosengarten, 1984).
Mechanical graders, such as a perforated revolving cylinder, are commonly used to sort nuts into various sizes, the smaller nuts being the first to fall. Pecan nuts, due to their ovoid or oblong shape, require special graders consisting of variously spaced rollers.
Imperfect, faulty or broken nuts may be removed
by hand as the nuts pass along a continuous belt. A suction machine may
also be used to lift blank or imperfectly filled nuts over a trap while
the heavier nuts pass on. The low specific gravity of Grade 1 macadamia
kernels allows them to be removed by floatation in ordinary water. Grade
2 may be removed using 30 g of salt in 1 litre of water, with Grade 3 sinking
to the bottom. The nuts are then thoroughly dried (Howes, 1948; Menninger,
1977; Rosengarten, 1984).
Most nuts can be satisfactorily stored for several months provided they are kept cool and dry. Many nuts are stored in the shell for longer periods but there is now a trend towards the cold storage of shelled nuts or kernels. Although requiring more space, the best results are obtained by storing in moisture proof containers after thorough dehydration. Cold storage in open receptacles can result in an uptake of odours such as ammonia, a loss of colour, flavour and texture and the possible development of rancidity. Cold storage in vacuumized containers or the use of inert gases appear to offer little advantage over cold storage for a similar period.
Commercial cold storage is general practice with the pecan. Better prices can be obtained by holding the nuts in store for some time after harvesting and the higher prices more than offset the cost of storage. It may also be profitable to hold a portion of a heavy crop in store in the expectation that the following crop will be smaller and the price higher.
Freshly gathered chestnuts have a high moisture content and consequently prone to fungal attack. Stored in small heaps or shallow layers in an airy store walled on three sides and periodically turned over, they will keep perfectly for up to 3 months or longer. If stored too dry or in a single layer the kernels will shrivel and become hard. The onset of mould, as well as harmful grubs and insects, during storage may be prevented by immersing the fresh chestnuts in a solution of a fungicide for 48 hours. Gas treatment in autoclaves has been effectively used in France; chestnuts have also been successfully held in cold storage. Chestnuts placed in layers of sand and kept cool keep well and retain their germinability but may become less palatable.
Cob nuts are sometimes stored in sawdust, which
helps to prevent shrivelling (Howes, 1948; Menninger, 1977; Rosengarten,
The mechanical cracking of nuts is becoming increasingly important, nevertheless, large quantities of some kernels are still extracted laboriously by hand in some countries, such as the cashew in India and the almond in the Middle East. Mechanical cracking is faster but usually results in a higher percentage of broken kernels which, fortunately, usually finds a ready outlet in the confectionery trade.
Power-driven, self-feeding nut-cracking machines require grading for size before being fed to the cracking jaws or rollers with special shaking devices to separate the kernels from the broken shells. The kernels may still have to pass along a continuous belt for broken kernels and other debris to be removed by hand.
Individual pecan nuts are commonly picked up in cups on an endless chain passing through a hoppere of graded nuts. Each nut passes to a slot where a piston-like rod exerts pressure at the end of the nut and cracks it, the shells and kernels then being released to a receptacle below. Very dry pecans often shatter badly with this treatment; prior wetting can reduce this problem, the kernels being redried after shelling.
The varying differences in the thickness and texture of the macadamia shell has created problems in the development of a suitable machine for cracking the shell without damaging the brittle kernel as there is less than 0.4 mm clearance between the two. The current vice-like design employs the principle of the nut being held between two blunt movable wedges.
Special cracking machines are also used for pine nuts where the demand is almost entirely for kernels.
The tough leathery shell of the cashew is overcome by preliminary roasting, thus making the shell brittle and responsive to cracking, as well as lessening the danger of blistering from the caustic oils in the fruit. The roasting is usually done by hand, placing small quantities of kernels in an open iron pan over a small circular earthenware furnace. Care is required to avoid overcooking or charring of the kernels, while with undercooking, the shell remains tough. Attempts at using roasting ovens have been unsuccessful, resulting in discoloured kernels. Shelling is by hand; in southwestern India a wooden mallet and a flat stone is used to crack the shell, after which the kernel is sometimes removed with a wire prong. The kernels are then spread out on wire gauze trays in a hot-air room under controlled temperature in order to loosen the pink or reddish-brown skin before removal and also to remove any excess moisture.
The oyster nut cannot be cracked in the ordinary way due to its tough, leathery, fibrous shell. However, the shell is easy to cut and a slit is made around the edge of the disc-shaped nut with a knife.
A Chinese method of cracking hard-shelled nuts is to heat them in burning straw and then cool rapidly with cold water.
The shells, which were formerly regarded as a
waste product for fuel, now have a number of industrial usages. Cleaned
and ground to a fine powder, the shells of walnuts, coconuts, Brazil nuts
and almonds have been used as an extender or filler in the manufacture
of certain plastics and synthetic adhesives and is often considered superior
to the softwood flours commonly used. The coarser grades of walnut shell
powder have been utilized as cleaners and finishers in the fur, metal,
tinplate and other industries; other uses are in linoleum, jointless flooring,
and as a carrier for horticultural insecticides (Howes, 1948; Menninger,
1977; Rosengarten, 1984).
Some nuts are traditionally eaten cooked and salted. Peanuts and chestnuts are normally cooked before eating, while almonds are preferred roasted or toasted and salted, likewise the macadamia nut. Cooking definitely reduces the terebinthine or turpentine flavour in pine nuts.
The large, plump grades are used for roasted or toasted almonds; roasting or cooking in oil is said to produce the best flavour. A common method of salting is by immersion in a strong brine solution before allowing the almonds to drain and dry.
In Hawaii macadamia nuts are preferred cooked and salted, either by roasting in an oven or immersion in a vat of hot oil, the latter method is the preferred commercial method as it gives the nut an attractive gold-brown colour. Refined coconut oil is generally used and, because the hot oil tends to corrode many of the commoner metals, the vats are made of monel metal, stainless steel or glass. Excess oil is removed centrifugally and the nuts laid out to cool on wire mesh trays, being salted while they are still luke warm, using a 15% solution of gum arabic or a special oil to enable the salt grains to adhere more readily to the nut. As the cooked nuts tend to be hygroscopic, they are vacuum packed as soon as they are cool.
Cashew nuts are always marketed cooked as the
roasting procedure to render the shell more easily removable automatically
cooks the kernel. The risk of contact with the caustic sap in the fresh
fruit is also removed (Howes, 1948; Menninger, 1977; Rosengarten, 1984).
Successful marketing may be defined as identifying what the consumer requires or can be educated to require and to supply that requirement at a profit. The marketing process, however, is customer orientated in as much as production is designed to meet the customer's needs. Like other food crops, nuts have a limited "marketing life". The production to marketing chain can be extremely complex, ranging through harvesting techniques, grading and sorting, packaging, transport, storage, processing, curing, distribution and selling. Furthermore, as markets develop, from rural markets to urban chain stores, the consumer become more and more demanding in their requirements and, as a consequence, increase pressures upon the various sectors in the marketing chain.
The local market in a rural community may initially represent the surplus in production to household requirements. Population growth and urbanization accompanied by a proportionate decrease in agrarian activities encourages increased production that has to be met either through cultivation or more intensive harvesting from the wild. The marketing process develops further through regional and national markets to the international. The further along the chain the more stringent are the consumer's requirements, with increasing emphasis on quality and constancy of supply. Local and regional markets are less valuable and may be expected to handle cheaper and lower quality nuts than would be acceptable by international markets. The majority of tropical tree nuts are not yet well represented on the international market. Of the ten major edible tree nuts it is noticeable that only the Brazil nut, macadamia and coconut are from the tropics, while only one out of six minor tree nuts and all 22 potential tree nuts are from the tropics or subtropics.
Nuts are largely tree crops and as such the production schedule will be long term, requiring careful selection of cultivars and, in some cases, root stocks. Initial investment may be high, with no return until the trees come into bearing. Labour requirements may be high for short periods in the year for pruning, harvesting and immediate post-harvest treatments and low for the rest of the year. High labour costs may be offset by increased mechanization, as in the North American nut plantations.
Quality is obviously of prime importance and good quality nuts can readily be spoilt, especially during harvesting and immediate post-harvest operations. The initial processing can result in broken or discoloured nuts unsuitable for the dessert nut trade. It is important, therefore, that producers should have ready access to other outlets, such as the food, oil and other industries.
Regularity and continuity of supply is important, especially for the international markets. Failure to comply will undoubtedly favour the importation of alternatives. A buoyant and profitable market can quickly change through oversupply or a sub-standard produce. Expansion in production must balance the market if prices are not to be depressed. However, nuts are a good candidate for marketing as "added value products" through processing, roasting, salting, etc., and have an excellent potential in the West for the health food market (Meadley, 1989; Honess, 1993).
The disposable world production of dessert nuts is shown in Table 1. These figures obviously do not show internal consumption. The general trend is towards increased production. In the league marketing tables almonds are the major product, pistachios have the fastest expansion rate while the trend for the Brazil nut (although not clearly shown from Table 1 but is discussed more fully in the section dealing with that nut) shows a steadily declining production due to deforestation and a chaotic marketing system.
The world disposable production of nuts primarily grown as a commercial source of edible oils is shown in Table 2. The proportion used directly for food is not known.
While the utilization of a few selected nuts by
the developed world is reasonably well documented, there are no figures
available for the remainder of the 520 edible nut-producing species listed.
Their value within the rural community is largely based on anecdotal evidence.
Table 1: Disposable World Production of In-Shell Dessert and Table Nuts
1 000 457
3 648 000
1 232 222
4 675 000
1 281 355
4 924 000
1 194 497
1 006 547
4 579 000
Source: FAO, 1994
a Figures from LaFleur (1992)
b These include Brazil
nuts, macadamia nuts, pili nuts and sapucaia nuts
Table 2: Disposable World Production of Oil Nuts and their Products
Groundnuts (in shell)
Source: FAO, 1994
a Palm kernel oil excluding
The most obvious value of edible nuts is their contribution as a concentrated food to the diet and, in some cases, as a source of edible oils. The major and minor nuts discussed in the following sections also have an important commercial value as dessert nuts.
In many of the developing countries, especially in the tropics and subtropics, nuts are often available during the dry season or winter months when fresh vegetables are not always readily available. They can also have the additional advantage in that they are not bulky and can usually be readily stored. The arborescent nut producers may have other advantages too, as providers of fuel, timber, fibre, medicines, browse, etc., plus their role in conserving the soil against wind and water erosion, providing shade and shelter, etc., as well as being aesthetically pleasing. The nut-producing herbs, i.e., groundnuts, etc., take their place in the agricultural rotation and, in the case of legumes, contribute to soil fertility.
The large-scale commercial production of nuts for the dessert and edible oil industries is well documented. However, the role of edible nuts in the rural economy is less clear. This is partly due to the confusion in the literature in defining what is an edible nut and as a consequence classing them with edible fruits, plus a paucity of information regarding their utilization. Like other economic plants, the value of edible nuts, whether they are used for food or edible oil, is relative to what other alternatives are available. This, in turn, creates a problem in ordering the potential for development. A well-written account of a single species by an enthusiastic author tends to mask the value of other species, including related taxa. The broader the field experience of an author the greater the likelihood of achieving an acceptable evaluation.
The work by Okafor and co-workers in the Nigerian forests on the utilization and improvement of indigenous woody food plants may be taken as an example of how to maximize this forest resource within the forest, homesteads and farmland (Okafor, 1977, 1980; Okafor and Lamb, 1994). Elite trees favoured by local inhabitants were selected and various vegetative propagation techniques investigated, including budding. Budded trees of Irvingia gabonensis and Treculia africana produced viable fruits in 3.5 and 4 years respectively. An additional benefit in the case of the latter species, a tree up to 35 m tall, was that the reduction in fruiting height from budding eliminated the serious danger from the globular fruits falling and injuring passers-by; the fruits can be up to 50 cm in diameter and weigh up to 15 kg! Perhaps rather surprisingly, the reduced risk resulted in an increase in popularity rating.
Taxonomic investigations into the range of variation
within a species can also be rewarding. The recognition of two variants
within Irvingia gabonensis with different flowering seasons, var.
gabonensis fruiting in the rainy season from April to September, and
var. excelsa in the dry season from December to March has encouraged
rational selections to be made for extending seasonal availability.
The requirements for the development, improvement and conservation of edible nut-producing species are similar to the requirements for other species. Information is required on distribution and reproductive biology, including flowering and fruiting patterns, pollination mechanisms and pollinators, seed dispersal, viability, predation, nature of the breeding system, breeding and propagation techniques, pests and diseases, etc. (Okafor, 1977).
There are two conflicting interests involved. Efficient production resulting in a low cost product is mainly obtainable from plantations where the emphasis is on selection of genetic uniformity for ease of management and productivity. Small-scale, usually peasant production, mainly makes use of wild resources or less rigorously selected cultivars and may even prefer an absence of uniformity in, for example, ripening in order to obtain a better spread in availability of mature fruit. Furthermore, peasant farming systems often protect useful trees during any land clearance operation while commercial farming systems tend to favour total land clearance with consequential loss of any genetic variability.
Wild resources often have the disadvantage in that the required species occurs sporadically in the surrounding vegetation. Whereas tree crops provide an excellent method of soil conservation, small-scale production can often have the additional advantage of making efficient use of difficult terrain. However, the demand for reliable quantities of a cheap, high quality product is placing increasing pressure on the small-scale producer and favouring the development of large-scale plantations, especially of Old World temperate species introduced to North America. The often sporadic occurrence of a desirable edible nut-producing species in, for example, the rain forest, means that it can have a disproportionately low value as an economic species in any land clearance or reafforestation schemes. The Brazil nut population is an example of such a threatened species. Problems in maintaining an environment outside the rain forest that is suitable for its pollinator makes the establishment of ex situ conservation and commercial plantations difficult.
Although not strictly the result of selection, the canker-forming bark disease caused by the fungus Cryphonectria parasitica syn. Endothia parasitica which first appeared in the New York in the 1890s was probably introduced from Orient with some Asiatic chestnut planting stock. The American chestnut, Castanea dentata, was subsequently almost completely destroyed over its entire natural range in the Appalachian mountains (Rosengarten, 1984; Holliday, 1989). The moral of the above example is that there is always the possibility of a devastating fungal or virus infestation affecting plantation crops through poor crop sanitation coupled with uniform genetic susceptibility, and places increasing emphasis on the preservation of genetic diversity.
Plant conservation may be in situ or ex situ, the latter in gene or seed banks or in vitro. The simplest method is in situ, by maintaining breeding populations within their natural environment through the establishment of reserves. Several reserves will obviously be required for widely distributed species. Pressures on land use, especially if only a few conserved species are involved within a reserve can make such reserves politically undesirable. Ex situ conservation in gene banks, involving either botanical gardens or research establishments, depending on the land available, enables small numbers of a species to be grown under cultivation. They are expensive to maintain, do not offer safe long-term storage and there can be problems in preventing cross fertilization and maintaining genetic purity. Several fruits, including Prunus are normally conserved vegetatively in the form of clones, thereby conserving gene combinations that would be lost through sexual reproduction.
Seed banks are widely used for the long-term storage
of orthodox seeds. However, many woody species that produce large seeds,
e.g., Castanea spp., Theobroma spp., Artocarpus spp.,
lappaceum, etc., are recalcitrant and any drying increases their rate
of deterioration. Furthermore, most tropical recalcitrant species will
suffer from chilling injury if the temperature of the seed bank falls below
10-15° C. Even
under optimum conditions, longevity of recalcitrant seeds seldom exceed
a few weeks or months. Unsuitable for storage in seed banks, they require
vitro conservation. The latter primarily involves the use of slow growth
and cryopreservation in liquid nitrogen to store germplasm (Hawkes, 1991;
Roberts, 1991; Withers, 1991; Tompsett, 1994).