The ginseng plant is a very slow growing perennial propagated from seed. Seeds are obtained from the ripe fruits of healthy 5-year old plants in September or October before the roots are harvested. The small, flat, white disc-shaped seeds are kept moist and carefully planted out in prepared beds (Figs 3 and 4).
Although ginseng plants grow in relatively rough climatic conditions, the air temperature must vary only within the range 0-25° C throughout the year and the plants require protection from direct sunlight. Therefore cultivated plants are normally grown in the shade of tall, forest trees or under specially constructed shelters with thatched roofs that permit about 25 per cent of the available rain to seep through. Plantations are best sited on cool north or northeast facing slopes, never on south facing slopes and preferably in natural woodland. In northern and north-eastern China ginseng grows in areas of the lime trees Tilia mandshurica Rupr. and T. mongolica Maxim. In North America ginseng favours areas where basswood (T. americana), oak (Quercus spp.), hickory (Carya spp.), beech (Fagus spp.) and maple (Acer spp.) grow. Such trees indicate suitable areas for ginseng cultivation, areas where the soil is rich and damp but not wet or muddy. The growing beds are allowed to remain fallow for one year before planting and are treated with a mulch of ginseng leaves although other applications such as leaf mould and horse manure can be used. Rice hull mulch and peat can be used to maintain the soil bed water content. Artificial fertilisers
are not favoured. The soil should be dug to a depth of 45 cm several times before planting (Harriman, 1973; Hu, 1977).
Seeds planted in the autumn and kept moist in the ground will produce seedlings in about 18 months. Nevertheless a problem with ginseng seeds is the slow emergence of the embryo from the dormant state. Indigenous ginseng farmers in China and Korea had discovered the traditional methods of pre-sowing treatment of seeds by trial and error but their methods have proved unsatisfactory for modern horticulture. Typically with American ginseng (P. quinquefolium) approximately two years are required for normal seedling growth. The ripe seed at the time of fruit maturation contains abundant endosperm but the embryo is poorly developed. During the first year the stored food permits slow development of the embryo in the seed and the amylase, catalase and peroxidase enzyme activities are low. Normally about 60 per cent of seeds will germinate in the spring of the second year and then, as development is more rapid, there is a steady increase in enzyme activity. Seeds can continue to germinate up to 5 years after sowing. For ginseng seeds there are three after-ripening phases. The first phase is the formation of the morphologically entire embryo from the rudimentary embryo observed in the ripe seed. The second phase comprises the elongation of the embryo from about 0.2 to 0.3 mm length to up to about 5 mm in August or September of the year following fruit ripening. It is at this stage that the leathery endocarp splits along the sutures of the seedcoat; this stage is referred to as "cracking".
Despite the development of the embryo the endocarp may not dehisce and the third phase, germination, is therefore incomplete due to an apparent physiological block. Choi and Takahashi (1977) noted that this physiological block can be overcome by moist chilling of seeds at 2° to 5° C (equally applicable to P. ginseng and P. quinquefolium). Stratification, the method of treating moist seeds at low temperatures (0° to 10° C) for a period of time, stimulates more rapid embryo development. Under natural conditions the cold period of winter ensures that the seed germinates in the spring but for cultivation purposes the moist seeds are usually packed in horticultural flats and stacked either in the open during winter months or in cold-storage rooms. Seeds stratified immediately upon ripening sprouted after about 8 months but those stored for 4 months in dry conditions before stratification required 19 months for similar development (Baranov, 1966). In Korea, Kwon and Lee (1997) examined P. ginseng seeds undergoing stratification at 4° C for 16 weeks. Free cytokinins, compounds stimulating cell division, were found in highest concentration (5.06-8.89 ¡igjg seed in butanol extracts) in seeds chilled for 2-4 weeks but concentrations gradually decreased during subsequent weeks. Total gibberellin-like substances, also extracted in butanol and responsible for cell enlargement and elongation, increased slowly during the stratification process but, after emergence of the radicle, gradually decreased. The Hong Kong group of Ren et al. (1997) similarly investigated the hormone changes in P. quinquefolium seeds during the after ripening period under controlled stratification conditions. The seeds were stratified in sequence at 18-20° C for 80 days, at 8-13° C for 54 days and at 05° C for 88 days, the water content of the stratification medium (sand) being maintained at about 10 per cent. Initially the gibberellic acid (GA3) and indole acetic acid (IAA) levels in the endosperm were low, slowly increasing to small peaks in 164 and 144 days respectively. However both increased remarkably by the 198th day. Similar changes occurred in the embryo. The naturally occurring cytokinin zeatin (6-(4-hydroxy-3-methylbut-2-enyl)aminopurine) probably stimulated cell division in the developing embryos and shewed a pattern of content low-high-low paralleling the embryo growth rate. Abscisic acid, a natural inhibitor of gibberellins and cytokinins, occurred in high concentration in the embryo and endosperm before chilling at 0-5° C, but thereafter decreased to a very low concentration by the end of the stratification period thus permitting increased giberellin and cytokinin activity. Further work by the Chinese group Huang et al. (1998) described the presence of the germination inhibitors acetic acid, butanoic (butyric) acid, isobutanoic acid and 1-phenyl-ethanone in the fruit pulp of P. quinquefolium and suggested that this could explain why the embryo of the seed was still immature when the seed itself attained maturity. Recent work by the Korean team of Kwon et al. (1998), investigating P. ginseng seeds, confirmed that the cytokinin content increased significantly during stratification reaching a maximum by the end of stratification but was very low when the radicle emerged. The major endogenous cytokinin was dihydrozeatin (36-60 per cent of total cytokinins); dihydrozeatin riboside and trans-zeatin riboside increased steadily during stratification and chilling, trans-zeatin riboside becoming the most abundant cytokinin in the germinating seeds. Cis-zeatin and cis-zeatin riboside were similarly detected. It was concluded that these cytokinins were closely involved in the germination of the seeds and that dihydrozeatin riboside and possibly trans-zeatin riboside were the principal cytokinins involved.
Germination proper occurs as the radicle emerges through the membraneous testa and the split endocarp. The radicle grows downwards as the plumule with its epicotyl protected between the cotyledons elongates upwards prompted by the enlargement of these structures. In late April or early May the seedling develops a single shoot a few cms high with a composite spray of three small, oval leaves and the cotyledons, having completed their task of nourishing and protecting the rising epicotyl hook as it pushes its way upwards through the soil, gradually die away. The petiole varies in length dependent on the nature and thickness of the surface leaf mould or mulch but is usually 4 to 10 cms. A solitary bud at the base of the leaf petiole differentiates into shoot and bud primordia for the next year's vegetative growth. The growth of the aerial shoot is determinant, that is, it is ultimately limited by the cessation of meristematic activity. Therefore the aerial growth dies down in the autumn and the roots, rhizomes and developing buds for the next season remain dormant until the following spring. The preformed primordium that produces the aerial growth is well enough developed by late summer to reveal the number of leaves and leaflets and, if present, the inflorescence that will emerge in the following spring. All subsequent growth is limited to the development of the preformed primordia in each year of the life of the plant (Baranov, 1966; Thompson, 1987).
Seedlings may be transplanted when they are one or two years old. It is usual to transplant in the autumn or spring when the young plants are leafless. Lateral roots are removed and the main roots positioned vertically to a depth of about 15 cm, the roots being surrounded with sphagnum moss to maintain a moist environment. Seedlings are transplanted usually about 20-25 cm apart.
In the second year two or three compound sprays or prongs of five leaflets arise, extending like parasols from the rootstock. This growth also dies down in the fall or autumn and in the following year a stem some 20-25 cm high develops with characteristically three compound leaves each subdivided into five leaflets and the plant bears flowers for the first time. Thus the plants are dormant during the winter period; the stimulus that initiates growth in the next spring is not fully understood but low temperature or chilling is apparently important. Working with one-year-old American ginseng plants Konsler (1984) noted that 100 per cent emergence could be obtained if the roots were maintained at 0° to 9° C for 75 to 90 days. In his experiments the total dormancy averaged 126.5 days, roughly four months, the days to aerial growth emergence being inversely proportional to the number of days of cold treatment. This agreed with the subsequent work of Lee et al. (1985) who suggested that the dormancy requirement was a temperature range of 0° to 10° C for 100 days, the preferred temperature being 5° C for 100 days for 3-year-old roots that were subsequently grown at 15° C.
Flowers and fruits are often removed in order to stimulate vegetative growth. In successive years the plant grows to a height of about 60 cm with a crown of dark green, verticillate leaves. Under cultivation conditions the root mass grows in a linear manner and there is often a direct correspondence between the number of prongs and the age of the plant e.g. 2 prongs at 2 years, 3 prongs at 3 years, although 5 prongs is rarely exceeded even after prolonged cultivation. Lewis and Zenger (1982) noted that for the slower growing wild P. quinquefolium plants there was also a linear relationship between age and the number of prongs but it was not annual.
The flowers are inconspicuous, small and green and normally arise in the third and subsequent years. The single peduncle or flower stalk arises at the junction of the whorl of compound leaves and elongates to lift the multipedicelled umbel of flowers above the leaflets. The umbel is similar to that of the related Araliaceous plant Hedera helix L., common ivy. Konsler (1984) observed that P. quinquefolium immature inflorescences could be clearly discerned in northern plants as the aerial growth emerged in the spring but in southern plants the immature inflorescence remained obscure until the peduncle elongated. Flowering usually occurs from late May to early August for American P. quinquefolium and during June and July for Oriental P. ginseng.
The inflorescence produces a cluster of bright red fruits commonly referred to as berries, individual berries being about 1 cm in diameter (Fig. 5). True berries are succulent fruits in which the mesocarp and inner endocarp remain succulent but ginseng fruits are drupes because the innermost endocarp is hard and leathery. The drupes contain 1-3 flat, white disc-shaped seeds. The plants are usually grown for about 5 or 6 years before harvesting roots and seeds. Although it is believed that the fruits falling to the ground naturally are the best
source of seeds, fruits can be collected in the autumn and then spread for several days in the shade to permit the skin and pulp to blacken. The seeds can then be removed by rubbing the fruits and must be stored moist, usually in sand. Dry seeds will not germinate and it is essential to protect all seeds from predators such as birds and rodents.
The roots are described botanically as contractile roots; such roots possess a mechanism for the annual repositioning of the regenerative buds essential for growth in the following season. The short vertical rhizome, an underground stem or rootstock, is sometimes known as "the neck" and this rhizome grows upwards during each growing season. As the regenerative buds arise at the tips of the rhizomes, it is necessary that the roots contract, pulling the rhizome and buds downwards and thereby keeping the buds at soil level; the rate of contraction balances the rate of upward growth (Baranov, 1966).
The P. ginseng roots are harvested in September or early October; this time is ideal as the roots are firm thus permitting careful cleaning by hand (Fig. 6). Earlier, in spring or summer, the roots are soft and less easy to process. The size of the roots is age-dependent; 5-year old commercial roots are about 10 cm in length and 2.5 cm in diameter whereas 10-year old examples would be about 25 cm long. Lateral roots are up to 20 cm in length and 5 to 10 mm diameter. It has been claimed that ginseng (P. ginseng) will grow for hundreds of years and a substantiated example of a 400-year old root was reported by Grushvitzky (1959). Nevertheless for commercial purposes 5 or 6 years is practicable, the plant being large enough and the yield of desirable chemical constituents optimal.
Careful analysis of ginseng roots by high performance liquid chromatography (HPLC) indicated an increase in total saponins in the main root up to the 5th year of growth, an increase related to marked weight development; in the 5th year there was a slight decrease before a significant increase in the 6th year. Lateral roots shewed a marked increase in the 3rd year followed by a decrease in subsequent years. The highest yield of ginsenoside-Re was found in the lateral roots. In rhizomes the yield was highest in the 2nd and 3rd years and the content of ginsenoside-Ro was greater than elsewhere in the plant (Yamaguchi et al, 1988). Subsequent work by Samukawa et al. (1995) also using HPLC shewed that the saponin content of the roots increased for 3 years but decreased in the fourth year and increased again in the fifth and sixth years, justifying the commercial decision (Samukawa et al., 1995).
Ginseng plants are sensitive to temperature changes, freezing occurring at from -3.5° to -9.6° C for P. quinquefolium seeds, dependent on water content, and roots can withstand -5° C for 24 hr without damage but -10° C for only 5 hr will cause serious damage. Varying low temperature was more damaging than constant low temperature and therefore the chances of such damage are greater during the thawing period in early spring than in winter (Lee and Proctor, 1996).
The brownish-white harvested roots are carefully cleaned to remove adherent soil. Without further treatment the roots will not remain in good condition for more than 10 days under normal autumnal conditions; therefore the roots are dried slowly in the sun to yield air-dried white ginseng. Alternatively roots can be dried at 15.5°-27° C in airy, heated sheds, the temperature being gradually raised in a few days to 32° C as the roots dry. Drying times vary according to root diameters and the roots lose up to two thirds of their weight in moisture. After drying the roots are sorted according to size and quality, and, in good laboratories, checked for the absence of bacteria, yeasts and mould fungi, tested for the absence of harmful pesticide residues and examined for possible aflatoxin presence. Further quality control should, and often does, include suitable identity tests for the presence of major chemical constituents (e.g. the ginsenosides) and some quantitative assessment before sale. White ginseng can also be obtained commercially with the surface "skin" removed by careful peeling. Dried white ginseng, if stored carefully, can retain saleable quality for about 12-15 months.
Red ginseng indicates an alternative method of preservation of ginseng. The cleaned roots are sterilised by steam treatment at a temperature of 120° to 130° C for 2 to 4 hours. Sugars present in the roots partially caramelise causing the characteristic red-brown colouration, hence the name "Red Ginseng". The saponin content of red ginseng is increased during processing and Samukawa et al. (1995) further observed that in terms of total saponin content the best 6-year old red ginseng came from Korean sources followed by Japanese and Chinese products. The heat treatment increases the hardness of red ginseng roots and also produces chemical artefacts. Carefully stored red ginseng will retain its quality for 2-3 years.
White and red lateral roots are also marketed and all ginseng products require careful storage to prevent rodent damage, bacterial and mould growth and beetle infestation. Commercial pressure has stimulated research into better methods and techniques for the cultivation of the large quantities of good quality ginseng needed to satisfy the increasing worldwide market.
From the 1960's onwards Japanese and Russian scientists have investigated the use of the gibberellins as growth promoting agents for ginseng. Gibberellins comprise a series of closely related substances obtained from fungi of the genus Gibberella. In 1926 the Japanese researcher Kurosawa studied the sterile cellfree filtrates obtained from the fungus Gibberella fujikuroi (Sawada) Wollenweber, a fungus causing the pale, spindly growth of rice, and noted that application of the filtrate to rice seedlings caused marked growth stimulation. More than 25 such growth hormones or gibberellins are now known and are all closely related to the commercially available gibberellic acid GA3. Gibberellins normally occur widespread in plants and operate in conjunction with plant auxins (e.g. indole-3-acetic acid) in growth processes such as cell elongation and enlargement and, in some cases, stimulate the production of hydrolytic and proteolytic enzymes essential to seed germination.
The Japanese horticulturists Ohsumi and Miyazawa (1960) experimented with ginseng seeds soaked in various concentrations of gibberellic acid and incubated in sandbeds and they noted that there was improved growth of the embryos and better germination rates. The number of seeds germinating improved from 50-70 per cent to 90-100 per cent and optimum results were obtained after 25 hour treatment of seeds with 0.05 per cent to 0.1 per cent gibberellic acid solution. The Russian botanists Grushvitskii and Limari (1965) also studied the effect of gibberellic acid on ginseng seeds and observed that the first stage of after-ripening, the dormancy time before germination controlled by cold conditions, could be reduced from 4 months to 2 months and therefore the period of time required for preparation of seeds before sowing could be reduced from 8 to 6 months.
Although other plant growth substances such as the cell enlargement promoting auxins I.A.A. (indole-3-acetic acid), naphthal epiacetic acid and 2,4-D (2,4-dichloro-phenoxyacetic acid) and the cell division stimulating cytokinin kinetin (6-furfuryl adenine) have been investigated, the general conclusion is that gibberellic acid is the best ginseng growth stimulator so far discovered. Certainly Kuribayashi et al. (1971) advocated use of 24 hours immersion of seeds in a 100 p.p.m. aqueous solution of gibberellic acid coupled with temperature lowering to 2°-15° C for about 10 days, the optimum germination temperature being 10° C. In a subsequent work Kuribayashi and Ohashi (1975) used kinetin solution to stimulate germination, the minimum concentration required was 25 p.p.m. for 24 hr treatment or 50 p.p.m. for 12 hr and germination was further accelerated when treated seeds were kept at 5°-10° C for 20 days.
Many other plant growth hormone treatments have been studied with the object of accelerating enzyme activity thus increasing decomposition of the stored nutrients in the endosperm with a resultant breakdown of starch and non-reducing sugars in the endosperm and an increase in reducing sugars. Consequently earlier germination occurred and enzymic activity was increased especially in the second year when rapid growth took place (Li et al., 1994a). In particular, gibberellin treatment (soaking in 100 p.p.m. solution for 24 hr prior to cultivation) caused higher activities of catalase, peroxidase and acid phosphatase in 8-week-old plantlets and accelerated the plant development (Fang et al., 1992).
Sruamsiri et al. (1995) investigated the growing of ginseng in Japan and Northern Thailand. For successful cultivation it was realised that photoperiod control, low temperature treatment, growth regulator addition and light intensity limitation were essential. At the Nong Hoi Experimental Station, situated 1000 m above sea level in Northern Thailand, the Chiang Mai University agriculturists submitted ginseng seeds to 24 hours soaking in either 100 p.p.m. gibberellic acid solution or water. The soaked seeds were sown in moist sand layered between clay in pots before placing in a shaded house with an ambient temperature of about 10°-30° C. Stratification for 2 months resulted in opening of the seed coat in 24.0 per cent of the gibberellic acid treated seeds; by 3 months this had increased to 36.8 per cent and in 4 months to 54.4 per cent. The water-soaked seeds revealed no opening of the seed coats after 3 months and a mere 4 per cent after 4 months. Complete germination was not achieved in these experiments so the team next investigated the effect of low temperature storage. Healthy seeds without opened seed coats from the first experiment were again soaked in 100 p.p.m. gibberellic acid solution or water, mixed with moist sand and stored in a refrigerator at 5° C. On monthly examination of the seeds it was apparent that seeds soaked in water and gibberellic acid solution responded similarly; significantly by the end of 2 months stratification 57-59.4 per cent of the seeds shewed open coats, a percentage that increased to 64-77.8 per cent in the 3rd month and in more than half of such seeds the radicle was already protruding. Therefore it was concluded that germination was more efficiently prompted by cold treatment than by gibberellic acid treatment.
Once established the growth of the young ginseng plants is also controlled by other factors including soil nature and pH, trace metals in the soil, light intensity, light colour, etc.
Ginseng grows best in good, natural woodland soil which is preferably well drained, rich, sandy loam. Wei et al. (1985) demonstrated that humic soils produced only slightly better yields of total and individual glycosides e.g. in P. ginseng grown in farmland soil 3.44-5.50 per cent total saponins were obtained and in humic soil 4.69-6.81 per cent were found. In a 6-year field growth survey the Korean group of Lee et al. (1989) reported that the yield of 6-year old roots was 2.4, 2.13 and 1.44 kg/3.3 m2 in clay loam, loam and sandy loam respectively. They also noted that clay loam produced an overall plant loss rate of 33.6 per cent over 6 years and a greater stem length and diameter whilst sandy loam yielded a higher plant loss rate of 51.6 per cent and smaller stems. Soil aggregation and porosity were slightly greater in 6 year old plots when compared with 2 year old plots and plant survival and yield was significantly correlated with soil clay contents and porosity.
Ginseng growers have noted that ginseng plants do not favour chemical fertilisers or mixtures rich in nitrates and in Korea rice straw, barley straw and corn stem mulches have been shewn to be equally efficient in producing 6-year-old roots and to be better than green manure (Lee et al., 1990). Pine tree leaves have also been recommended as a mulch (Kim and Kim, 1991) and there was no apparent difference in the growth of the plants but the flavour components were enhanced. Some growers prefer to use unleached wood ashes and other fertilisers used include decomposing leaves mixed with soybean, cotton seed or peanut pressing residues and horse, chicken and even human manures (Hu, 1977). Normally winter mulches are 10-12.5 cm deep in order to protect crowns from cold, frosty conditions and thinner layers are employed to keep soil moist during dry spells. There is a positive correlation of the root yield of sugars and saponins with the organic matter content of the soil but a decrease in available phosphorus and exchangeable cations in the soil is desirable (Kim et al., 1995). Russian workers have suggested the use of zeolites as fertilisers. Zeolites are hydrous aluminosilicate minerals containing barium, calcium, potassium and sodium ions and derived from feldspars obtained worldwide from natural rocks. The open framework structure permits easy diffusion of gases, ions and molecules and zeolites are important catalysts in organic chemical reactions. Applied to ginseng seeds zeolites stimulated germination and increased stratification and, spread on soils with peat, protected seedlings from rot infestations, encouraging growth, development and yields of roots, flowers and seeds (Pushkina et al., 1996).
Trace element studies have indicated that essential elements in soil include aluminium, calcium, magnesium, nitrogen and sulphur in a greater proportion than usual, corresponding with the composition of the greyish-brown forest soil together with the organic matter normally found in such soil. The inorganic nitrogen content increased in 2nd and 3rd year ginseng plots (up to 100-120 p.p.m.) but decreased to 75, 34 and 25 p.p.m. in the 4th, 5th and 6th years respectively and was more variable in sandy soils. The soil phosphorus (P2O5), potassium, calcium and magnesium contents varied little with plant age (Lee et al., 1989) but calcium and magnesium intake is reduced by application of nitrogen/phosphorus/potassium fertilisers (Li et al., 1994b). Nevertheless Ma et al. (1990), growing P. quinquefolium, had concluded that the highest plant mass was obtained when a solution containing nitrogen 100 p.p.m., phosphorus 25 p.p.m. and potassium 250 p.p.m. was applied to the soil. Later work also involving P. quinquefolium (Chen et al., 1996) recommended ammonium hydrogen phosphate ((NH4)2HPO4) fertiliser to increase the yield of roots by up to 30 per cent, the fertiliser being applied annually when the leaves were fully expanded at the early fruiting stage. Urea foliar spray also increased plant yield and ginsenoside content. The Canadian group Proctor et al. (1996) demonstrated the morphoregulatory value of thidiazuron (TDZ or N-phenyl-N1-1,2,3-thiadiazol-5-yl-urea) used as a soil drench (2.20 p.p.m.) or a foliar spray (0.22 p.p.m.). The treatment, which was appropriately applied to P. quinquefolium greenhouse grown seedlings and 3-year old plants, had economic potential. For greenhouse-grown seedlings foliar sprays or soil drenches of TDZ (0.22 or 2.20 p.p.m.) increased stem length and diameter and shoot and root weight and a single foliar application of 62.5 or 125 p.p.m. TDZ to 3-year old field-grown plants 3 months before harvesting increased the root biomass by 19-23 per cent. Thickened secondary roots developed on the upper part of the tap root and adventitious buds formed on the shoulders of 3-year old roots.
Such buds produced shoots after a period of dormancy and subsequently multi-stemmed plants. In addition, dormancy could be reduced by gibberellic acid GA3 treatment prior to planting out.
The most recent field trials with P. ginseng and nitrogen top dressing (Gao et al., 1997) confirmed that excess nitrogen retarded plant growth and overall ginseng yield because the nitrate reductase level in ginseng is low and consequently the absorbed nitrate, not being promptly reduced, accumulates in the tissues.
Studying 1-year-old American ginseng plants in water culture Ren et al. (1993) noted that zinc concentrations were important and critical. Thus below 0.05 p.p.m. zinc deficiency caused inhibition of root growth with sparse fibrous root development and abnormally small leaves. Above 0.5 p.p.m. zinc toxicity is demonstrated by absence of fibrous roots, yellowing of the roots and leaves that are very small and chlorotic or totally absent. In the optimal 0.1 to 0.3 p.p.m. range saponin production was normal but zinc deficiency or excess resulted in reduced synthesis and and therefore lowered deposition of saponins.
Experiments with variable pH levels in soil have clearly established that ginseng plants prefer acid conditions. In strongly alkaline soils and even at pH 7 or pH 8, few plants survive and further studies at pH 3-pH 8 confirmed that ginseng prospered at pH 4-pH 6 (Hou, 1978).
Ginsengs are shade-loving plants. Significantly the leaves of shadier rows of ginseng plants contain more chlorophyll, carotenes and xanthophyll than leaves of plants grown in sunnier situations. Therefore trials were undertaken to discover the optimum light requirements for satisfactory growth (Kuribayashi et al., 1971). It was concluded that for P. ginseng no plants would survive 4 months exposure to 50 per cent or 100 per cent sunlight. However at 5 per cent or 10 per cent transmittance (3000 to 6000 lux) most plants survived. Later work by Lee (1988b), also using P. ginseng, confirmed that for 3-year-old plants 5, 10 and 20 per cent transmittance shade did not produce significant differences in root length, stem length, stem diameter and leaf area although root diameter increased markedly in shade of 10 and 20 per cent transmittance. For 6-year-old plants, the largest root diameters and heaviest roots were obtained under 20 per cent transmittance although root length was not affected.
Other effects of strong light are the reduction of leaf area, increase in the transpiration rate, reduction in the size but not the number of stomata and reduction of the total amount of chlorophyll present, factors collectively leading to poorer roots as the light increases (Lee, 1988a).
Although thatch shade is commonly used (Fig. 7) and often preferred especially for white ginseng roots (Kim et al., 1990), the protection of ginseng plants from strong sunlight can be effected with coloured polyethylene net; red and blue fourfold polyethylene net provided good light intensity for ginseng growth but the red net increased air temperature and prompted early defoliation despite increasing the photosynthetic rate. Blue and black shade also produced an increased photosynthetic rate but root production and saponin yield were significantly increased with the red and blue nets, blue being the overall best option (Mok et al., 1994).
Ginseng plants are cultivated using various mulches including ricestraw mulch. Unfortunately such mulch is a suitable habitat for the field slug Deroceras varians A.Adams. In the Korean ginseng fields the slugs normally lay eggs from April to June with a few being deposited as late as September. During the summer months the young slugs mature prior to wintering below the moist soil surface and emerging to repeat the cycle in the following April. Ginseng plants, especially 3 to 5-year old plants, are particularly damaged during the egg-laying period. Adult slugs can be eliminated using 5 per cent ethoprop granules or 6 per cent metaldehyde bait, and Bordeaux mixture (copper sulphate 600 mg/mL with quick lime 1200 mg/mL) spray is particularly effective after infestation (Kim et al, 1990). Coincidentally and after 6 years experience Bordeaux mixture had also been preferred and recommended for spraying American ginseng foliage infested with Alternaria panax Whetzel blight (Wilson and Runnels, 1944). It was suggested that the maximum interval between sprays should not exceed 14 days and addition of calcium arsenate would give increased protection against chewing insects. Another pest on Korean ginseng is the snail Acusta despecta siebddiana which also flourishes in rice straw mulch and, particularly in the period May to July, can cause up to 10 per cent damage to the total number of plants. Metaldehyde bait is a useful control agent (Kim, 1992).
The soft roots of ginseng are particularly attractive to some nematodes, the eelworms or round worms characterised by slender, unsegmented bodies. Ahn et al. (1981) observed that the organic pesticides Terbufos and Mocap offered better control of root-knot nematode than Carbofuran alone or mixtures of these pesticides. None of these compounds transferred off-flavours to the ginseng plants. Another pest damaging the roots is the potato rot nematode Ditylenchus destructor. Treatment with non-fumigant nematocides e.g. ethoprop or aldicarb were effective, ensuring a high survival rate in 4-year-old plants (Ohh et al., 1986). Meloidogyne hapla root-knot nematodes have also been reported attacking Panax notoginseng roots in Yunnan Province in China (Hu et al., 1997).
Fungus infected soil bearing the spores of Rhizoctonia solani can cause damping-off of emerging seedlings of P. ginseng. Dipping of the seeds in Toclofos-Me solution (1000 p.p.m. for 3 hr) before sowing coupled with soil drenching at 300 g/ha in mid-April will protect the emerging seedlings and the treatment remained effective in the soil for about 32 days (Yu et al., 1989).
Powdery mildew caused by Erysiphe spp. can infect Asian ginseng and has recently been reported occurring on American ginseng plants, the infection being characterised by extensive, superficial, white mycelia. Damaged leaves turn yellow and fall prematurely (Sholberg et al., 1996). The species E. panacis, a new species and a teleomorph of the powdery mildews found on other Araliaceous species, has been discovered on P. ginseng collected in Changchun in the Jilin Province of China and has been described in detail by Bai and Liu (1998).
American ginseng (P. quinquefolium) plants occurring in ginseng-growing gardens in British Columbia were carefully monitored for incidence of fungal pathogens. During 4 years 235 samples of roots and 25 of leaves were found to be infested with Pythium altimum (35.6 per cent of total isolates), Fusarium spp. (30.5 per cent), Rhizoctonia solani AG4 (19.6 per cent) and Cylindrocarpon destructans (9.8 per cent). Diseased leaf and petiole tissues were attacked by Phytophthora cactorum, Alternaria panax, A. alternata and Botrytis cinerea and seedling roots were diseased in vitro by P. ultimum, F. solani, R. solani and P. cactorum in that order. On detached leaves the most common pathogenic fungi were P. cactorum, B. cinerea and A. alternata (Punja, 1997). It was also suggested that chemical control by metalaxyl and biological control with Bacillus cereus-complex (GB10) could counter Phytophthora leaf blight and root rot under field conditions (Li et al., 1997).
Keeping roots free from disease is imperative. Zhao et al. (1997) analysed ginseng roots with a red coating disease, comparing normal healthy and diseased roots. In diseased roots the content of ginsenosides, starch, other carbohydrates and amino acids was reduced although the levels of reducing sugars and pectin were increased. Levels of the metals aluminium, iron and manganese were higher in the diseased roots. As the diseased roots were deficient in essential phytochemicals the commercial market value was likewise reduced.
Careful observation of standing and stored crops ensures detection of problems of mould, slug, worm and nematode infestation and suitable counter-measures can be applied. Nevertheless further problems can be caused by larger natural predators such as gnawing rodents, moles and chipmunk ground squirrels attacking the roots and birds and chipmunks consuming the ripe fruits. Vigilance and appropriate counter measures are therefore essential.
Commercial cultivation of ginseng species has been clearly shewn to be practicable and desirable and now provides the bulk of the ginseng roots available in worldwide markets. Ongoing research will undoubtedly provide further pointers to improved methods and techniques yielding better pharmacological agents for the future.
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