كتاب يام

An information system of tropical crops in Hawaii
Department of Tropical Plant & Soil Sciences
University of Hawaii at Manoa

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Yam

TOPICS:

• Climate
  
• Culture
  
• Description
  
• Diseases
  
• Fertilizer
  
• Handling
  
• Harvest
  
• Insects
  
• Market/Uses
  
• Planting
  

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Climate

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• Temperature: Prefers growing under warm, sunny conditions between 25-30C. Growth restricted below 20C and no freezing is tolerated.

• Altitude: Sea-level to 900 m.

• Daylength: Short days between 10 to 11 hr promote tuber formation, while days longer than 12 hr promote vine growth.

• Growth: The plants should be supplied with a means of support on which to twine beginning 1 month after emergence.

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White yam (Dioscorea rotundata Poir), a native of West Africa, is grown in greater acreage than any other yam species in the world. White yam is the most favored yam species in W. Africa because it posseses a highly viscous starch. This commends its use in the pounded yam preparation, which is one of themost popular and prestigious foods of West Africans (Mozie and Okoro, 1990).

1. D. rotundata. White yam, white guinea yam. Grown in West Africa, West Indies, and in the Caribbean. Compact spherical or cylindrical tubers are white, cream, and yellow fleshed.

2. D. alata. Water yam, greater yam. Grown in SE Asia and in the Caribbean. Cylindrical, white purplish with loose watery texture.

3. D. esculenta. Lesser yam, potato yam. Grown in China, SE Asia, South Pacific and Caribbean. Forms several tasty flavor-rich tubers with smooth thin yellow-brown skin, and white, sweet-fleshed grained starch.

4. Others: D. Cayensis (yellow guinea yam); D. dumetorum (bitter yam, trifoliate yam); D. bulbifera (aerial yam).

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Culture

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Post Planting Treatments

Irrigation: (water requirements of the plants). Requires 100 cm of water well distribued throughout the growing season.

Mulching. Mulch after planting to protect plants from excessive heat and desiccation. Drastic reduction in yields result if mulching is not performed (results in reduced emergence percentage), especially in areas where hot temperatures and dry weather i s common.

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Description

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Name: Yams, Dioscorea spp.

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Diseases

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Tuber roots (Botryodiplodia theobromae, Rhizopus nodosus, Fusarium oxysporum, F. solani). Control through careful selection of tubers for planting. Cut seed should be dipped in Bordeaux mixture or with a systemic fungicide. Also controlled with crop rotation.

Weeds

* Weed control is important during the first 2 to 3 months after planting.

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Fertilizer

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1. Type of soil. Prefers deep, rich, permeable soil. Loosed textured soil is necessary for tuber development and expansion.

2. Drainage requirements. Heavy waterlogged soils should be avoided. Good drainage may be provided by building the soil into ridges or mounds 30-45 cm high.

3. Nutritional Profile of Soil. Soil should be limed to about pH. 5.5 with essentially no exchangeable Al3+ present.

4. Land Preparation. Soil at planting-time should be loose and well-worked. Planting is thus preceded by ploughing and harrowing. After that the yams are planted in rows. Sub-soiling or very deep ploughing before planting may improve root growth and yields. Alternatively, after ploughing and harrowing, shallow trenches running the lenght of the field are can be dug with machines. The yam setts are put in the trenches, which are then covered up. The opening of the trenches, placing the sett, and closing of the trench can all be done mechanically and in one operation.

   
   

Nutrient needs

* Nutrients (fertilizers used and application quantity and methods). It is best to apply the fertilizer in a continuous band along the row, about 10 cm away from the plants. Fertilizer application should be split to minimize fertilizer leaching: the first application is made about one month after emergence and the second is made about 7-9 weeks later when tuber bulking is in progress. A total application of 400 kg/Ha of 11-11-13 compound fertilizer is recommended.

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Handling

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1. USDA treatments and requirements. Tubers can remain in the ground until needed. Under normal conditions after harvest, tubers remain dormant for a period of 10-15 weeks and then start sprouting. Prestorage curing is necessary. Tubers are usually cured before storage by being spread out evenly in the shade for 2-3 days after harvest. Gibberellic acid (GA3) applied immediately after harvest or before curing starts, extends the dormant period and prolongs tuber shelf life of many yam cultivars. T he response to GA3 is cultivar-dependent. Standard curing practice: Maintain at 29-32C (84-90F) at 90-95 RH for 4-8 days.

   Recent studies show that prior to storage, yams should undergo curing. This is because of the wounds that are inevitably inflicted on tubers during harvesting and subsequent transportation and handling. Curing refers to the healing of these wounds by controlled processes during the first week after harvest. The curing process normally involves exposure to the freshly harvested tubers to a controlled temperature range of about 29-35C and RH 90-95 for 5-7 d before storage in the shade. Curing tends to prolong the life of the tubers during subsequent storage by drastically reducing the spread of pathogenic organisms through the tissue of the tuber. In their study curing (peridem layer formation) improved with yellow (580 nm), green (520 nm) and blue ( 490 nm) tungsten lamps for 7 d at 27C in well-ventilated chambers, but not with red (660 nm) (Mozie and Okoro, 1990).

   Pre-cooling: cold room. Sensitive to cold temperatures below 13C (55F), to decomposition at high humidities, and also sensible to ethylene exposure. Transport through highway and piggyback trailers, and van containers. Life-time during transport and storage= 6-7 months.

2. Packing (no. lbs/box; box type). Full telescoping fiberboard cartons with paper wrapping or excelsior to reduce bruising, loose pack. Boxes of 11 kgs (25 lbs), 23 kgs (50 lbs). Hand loaded or unitized on pallets.

3. Cleaning (methods and requirements). Clean tubers in the field, after harvest.

4. Waxing (type of wax and quantity/methods). Not-applicable

5. Vacuum packing. Not-applicable.

6. Chemical treatments (fungicides etc.). Fumigate with methylene bromide before storage. Dipping with benlate, captan, thiabendazole (250 ppm), or limewash can help to reduce infection and storage rots.

7. Hot water treatments, if necessary. Not applicable, since it damages the roots.

8. Refrigeration requirements (after harvest and during treatment). Store at 15C. Chilling injury occurs below 13C (55F).

9. Humidity requirements. Maintain in storage at 70-80 RH. Decay occurs at higher RHs.

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Harvest

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Expected yield per acre (based upon specific planting density). 40 MT at 10,000 plants per hectare harvested at 8 months after planting.

* Harvesting can be done at any time of the year. Harvesting may be done at any time after large-scale leaf yellowing sets in. Since little or no more tuber material is added during the last month before vine death, the time of harvesting is not very critical. Most species produce only 1-3 tubers per plant. Harvest can be conducted at 6 to 10 months after planting (MAP). An alternative is to harvest first at 4-5 MAP and a second harvest at 8-10 MAP. Harvesting involves digging around the tuber to loosen it from the soil, lifting it, and cutting its attachment to the vine. Roots should preferably be dug on a sunny day to avoid wetting and subsequent decay of yams.

Harvest

Hand harvested. Harvesting large tubers is a time-consuming process because the tubers must be excavated from considerable depths without bruising them.

1. Photos or diagrams of a typical packing plant

2. Flow chart of the packing operation

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Insects

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Yam beetle (Heteroligus meles). Control with Aldrin, endosulfan or grammalin dusting of holes or soil surface.

Chrysomelid beetle (Lilioceris livida). Control by removing larva by hand or by spraying with carbaryl.

Mealybugs (Planococcus citri). Control by using a clean planting stock.

Yam nematode (Scutellonema bradys). Control with rotation and fallowing.

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Market/Uses

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Availability.

Year-round from the Caribbean, and Central America.

* Greater than 90% of world production is concentrated in West Africa. Total world production is estimated at 25 million MT per year. The major producing countries in West Africa include Nigeria, Ivory Coast, Indonesia, Ghana, and Togo.

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Planting

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Planting material. Seed pieces 150-300 g in weight, from mature tubers or bulbils. Around 25-30% of the yams produced are usually reserved for planting. The best planting material, however, are small tubers, resulting in improved germination percentage s and in less tuber rotting.

Preplanting treatment. Cut seeds pieces should be cured for a few days before planting to allow wounds to heal. Dipping in pesticidal solutions of benlate, captan, limewash or bordeaux mixture offers some protection against fungi. Dusting the sett with 2.5% aldrin or "grammalin A' dust protects the planting sett and the subsequent tubers against insect attack.

Diuron (3 kg/Ha), metribuzin, linuron (3 kg/Ha), simazine (3 kg/Ha), TCA, alachlor, and atrazine (3 kg/Ha) can be utilized as pre-emergence herbicides.

Recent experiments with plastic mulch resulted in greater yields, greater moisture and nutrient retention, reduced weed growth, temperature regulation, and it eliminated the need for staking.

Planting of yams on the flat is preceded by ploughing and harrowing. After ploughing and harrowing, shallow trenches running the lenght of the field are dug with machines. The yams setts are put in the trenches, which are then covered up. The opening o f the trenches, placing of the sett and closing of the trench can all be done mechanically and in one operation. Sub-soiling or very deep ploughing may improve yields of flat planting of yams (as compared to the more traditional ridge or mound plantings) .

Depth of Planting. 10 cm.

Timing of planting (crop cycle). Preferable at the start of the rainy season.

Planting density (expected germination). 10,000 pls/Ha. Space at 1 m between plants and 1 m between rows. Germination percentage depends on dormancy which lasts from 1-4 months. Active buds should be present in each seed piece. Emergence may begin ab out 2 weeks after planting and continue for 2-3 months. This characteristic makes it hard to choose and use herbicides efficiently because timing of the applications is difficult.

Harvesting

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2 Post-Production Operations

2.1 Pre-harvest Operations

2.2 Harvesting

2.3 Transport & Packaging

2.4 Curing of Yam Tuber

2.5 Cleaning 

2.6 Storage

2.7 Processing ؤ

2.8 Dormancy in Yams

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2. Post-Production Operations

2.1      Pre-harvest Operations

Maturity assessment is critical to achieving good quality yam. In the field, mature crop is generally distinguishable by cessation of vegetative growth and yellowing of leaves. The period from planting or field emergence to maturity is variable depending on the species (Table 4), and there is no standard reliable and objective index of yam tuber maturity. Some crude indices have been reported based on percentage of tuber length that was whitish at harvest, non-friable after cooking, or bitter after cooking (Onwueme, 1977). The most frequently reported measure is the period from planting to harvest (growing period), but it has been suggested that the time from emergence to maturity provides a better measure of growing period since planted tuber can remain dormant for some time (Onwueme and Charles, 1994).

Table 4:       Time from planting to maturity and yield for different yams species.
Species/Common name	Period from planting to maturity	Yield and size of tubers
D. alata Water yam	220-300 days	20-25 t.ha-1 1-3 tubers per plant 5-10 kg per tuber
D. Bulbifera Potato yam	140-180 days; 90-120 days	Aerial: 2-15 t.ha-1 ; 3-5 t.ha-1 Underground: 2-8 t.ha-1
D. Cayenensis Yellow yam	280-350 days	30 t.ha-1 2 kg per tuber (mean) 7-10 kg per tuber (highest)
D. Dumentorum Bitter yam	240-300 days	> those of most other cultivated West Africa yams
D. esculenta Lesser yam	200-300 days	7-20 t.ha-1 25-35 t.ha-1 (exceptional) 5-20 tubers per plant
D. Opposita Chinese yam	24 weeks	4-6 t.ha-1
D. rotundata White yam	200-330 days	16-20 t.ha-1
D. trifida Cush-cush yam	280-330 days	15-20 t.ha-1
Source: (Opara, 1999).

Most edible yams reach maturity in 8-11 months after planting. Techniques such as using physiologically aged planting material, pre-sprouting of setts, application of sprout-promoting substances (e.g. ethephon and 2-chloroethanol and harvesting before complete shoot senescence can decrease the during of field dormancy and thereby reduce the length period from emergence to maturity (Onwueme, 1977; Gregory, 1968; Martin et al., 1974). In many parts of West African yam zone, mature yams are harvested at the end of the rainy season or early part of the dry season, which coincides with the end of vegetative growth. Yams for long-term storage (for marketing or seed) are usually harvested during the harmattan period (Dec-Jan) in many parts of southeastern Nigeria when the crops has attained maximum growth and maturity. During this period, the soil is generally hard and tuber breakage during harvesting can be an economical problem.

Average yield of tubers is variable amongst the major producing areas, and is influenced by the species, seed piece, and growing environment (Table 4). Yields range between 8-50 Mt.ha^-1 in 6-10 months. Yields of 8-30 Mt.ha^-1 in commercial yam production has also been reported, the exact value depending on the location, variety, and cultivation practices (Onwueme and Charles, 1994. Many yam cultivars produce only a single large tuber, and the approximate multiplication ratio (fresh-weight yield:weight of planting material) for yam is about 5. Between 1975-1990, there were yield increases in all major producing countries except Ghana. During this period, the average world yield increased by nearly 11%.

2.2      Harvesting

Harvesting is done by hand using sticks, spades or diggers. Sticks and spades made of wood are preferred to metallic tools as they are less likely to damage the fragile tubers; however, tools need regular replacement. Yam harvesting is a labour-intensive operation that involves standing, bending, squatting, and sometimes sitting on the ground depending the size of mound, size of tuber or depth of tuber penetration. In rainforest areas, tubers growing into areas where there are roots of trees can pose a problem during harvesting and often receive considerable physical damage. Many also get deformed during growth as a result of the obstacles they encounter. These tubers are usually downgraded. Aerial tubers or bulbils are harvested by manual plucking from the vine.

Although some success in mechanical yam harvesting has been reported, especially for D. composita tubers for pharmaceutical uses (Nystrom et al., 1983), these machines are still limited to research and demonstration purposes. The use of a potato spinner has been suggested for harvesting species which produce a number of small tubers (Onwueme, 1997). Current crop production practices and species used pose considerable hurdles to successful mechanisation of yam production, particularly for small-scale rural farmers. Extensive changes in current traditional cultivation practices, including staking and mixed cropping, and possibly tuber architecture and physical properties will be required.

Yams can be harvested once (single harvesting) or twice (double harvesting) during the season to obtain a first (early) and second (late) harvest. The first harvest has also been referred to by the terms ‘topping’, ‘beheading’, and ‘milking’, all of which have been considered inadequate and obsolete. In single harvesting, each plant is harvested once and this occurs at the end of the season when crop is mature. The harvesting processes involves digging around the tuber to loosen it from the soil, lifting it, and cutting from the vine with the corm attached to the tuber. The time of harvest is critical in terms of tuber maturity, yield and postharvest quality. Depending on the cultivar, the period from planting or emergence to maturity varies from about 6-7 months or even 6-10 months.

Periods of  8-10 months and 4-5 months from planting or emergence to maturity have been recommended for double-harvesting (Martin, 1984; Onwueme, 1977); harvest first at 5-6 months after planting and then 3-4 months later has also been reported (Bencini, 1991). First harvest is carried out by removing the soil around the tuber carefully and cutting the lower portion, leaving the upper part of the tuber or the “head” to heal and continue to grow. The soil is returned and the plant is left to grow to the end of the season for the second harvest. Some yam cultivars produce several small tubers in the second growth following the early harvest. Double harvesting is most applicable to short-term varieties such as D. rotundata, and to lesser extents D. Cayenensis and D. alata. Similar yields have been reported for single and double harvesting; however, single-harvested tubers had better eating quality than the double-harvested tubers (Onwueme and Charles, 1994).

2.3      Transport & Packaging

After harvest, yam tubers are traditionally placed into woven baskets made from parts of the palm tree or coconut fronds. These are ideal for transporting small quantity of tubers over short walking distances. The basket is carried on the head, shoulder, or tied to a bicycle and transported to the market or storage facility. Compression damage is reduced since the basket is able to bend and thereby reduce the amount of force acting on individual tubers. However, when large quantities of tuber are harvested, these baskets are not suitable because of their limited size. Packaging tubers in full telescopic fibreboard cartons with paper wrapping or excelsior reduces bruising and enables large quantity of tuber to be transported over long distances. Tubers can be contained in loose packs, or units of 11 kg and 23 kg (McGregor, 1987). The cartons are hand-loaded or unitised on pallets.

Storing yams in modified atmosphere packaging (MAP) has beneficial effects, particularly using appropriate packaging material with suitable size and number) of holes for gas permeation. Sealing yam tubers in polyethylene film bags reduced storage losses due to weight loss and development of necrotic tissue (Table 5). Coating tubers with Epolene E10 (a commercial vegetable wax improved the appearance quality but there was no effect on levels of fungal infection (Thompson et al., 1977). The effect of this treatment on weight loss of tuber was inconsistent.

Table 5: Effects of packaging material on the quality of D. trifida after 64 days at 20-29°C and 46-62% rh. Fungal score was 0 = no surface fungal growth, 5 = tubers surface entirely covered with fungi. Necrotic tissue was estimated on the total cut surface of lengthway halves.
Type of package	Weight loss (%)	Fungal score	Necrotic tissue (%)
Paper bags	23.6	0.2	5
Polyethylene bags with 0.15% of the area as holes	15.7	0.2	7
Sealed 0.03 mm thick polyethylene bags	5.4	0.4	4
Source: (Thompson et al., 1977).

2.4      Curing of Yam Tuber

Curing of root crops allows suberisation of surface injuries and reduces subsequent weight loss and rotting in root crops. Curing of yams is recommended before storage so as to “heal” any physical injury, which may have occurred during harvesting and handling. This can be accomplished under tropical ambient conditions or in a controlled environment. Traditionally, yams are cured by drying the tubers in the sun for a few days. The optimum conditions for curing are 29°-32°C at 90-96% rh for 4-8 days (McGregor, 1987). Tubers cured at higher temperature (40°C) for 24 hours or treated with gamma radiation at 12.5 krads were free of mold and had least losses during subsequent storage. Storing at 15°C with prompt removal of sprouts was found to improve the eating quality of tubers (Coursey, 1967), presumably due the waterloss associated with curing and the inhibition of the biochemical synthesis that accompany sprouting.

2.5      Cleaning 

Prior to long-term storage and marketing, yams are cleaned (without water) by scrapping off soil and other debris on the surface. A knife or piece of stick is usually used. The root ‘hairs’ are also removed to so that the tuber has a smooth surface. Water must not be used to clean tubers before storage because of increased susceptibility to microbial infection and growth under the ambient humid storage conditions.

2.6      Storage

The three main conditions are necessary for successful yam storage: aeration, reduction of temperature, and regular inspection of produce. Ventilation prevents moisture condensation on the tuber surface and assists in removing the heat of respiration. Low temperature is necessary to reduce losses from respiration, sprouting and rotting; however, cold storage must be maintained around 12-15°C below which physiological deterioration such as chilling injury occurs. Regular inspection of tubers is important to remove sprouts, rotted tubers, and to monitor the presence of rodents and other pests. In general, tubers should be protected from high temperatures and provided with good ventilation during storage. The storage environment must also inhibit the onset of sprouting (breakage of dormancy) which increases the rate of loss of dry matter and subsequent shrivel and rotting of tuber. Both ware yam and seed yam have similar storage requirements.

Notwithstanding cultivar differences, fresh yam tuber can be successfully stored in ambient and refrigerated conditions (Table 6). The recommended storage temperature is in the range 12°-16°C. Optimum conditions of 15°C or 16°C at 70-80% rh or 70% rh have been recommended for cured tubers (Martin, 1984; McGregor, 1987). Transit and storage life of 6-7 months can be achieved under these conditions. The onset of sprouting is enhanced at ambient conditions, especially if ventilation is inadequate. For example, during storage at ambient conditions (20°-29°C, 46-62% rh), D. trifida began to sprout within 3 weeks (Thompson, 1996). Yam tuber decay occurs at higher humidity, and like most tropical crops, they are susceptible to chilling injury (CI) at low storage temperatures. To avoid tuber damage, minimum storage temperatures of 10°C, 12°C and 13°C (Martin, 1984; McGregor, 1987) at or below which CI occurs have therefore been recommended. Storage of D. rotundata tubers at 12.5C resulted in CI (Coursey, 1968), and storage of D. alata at either 3° or 12°C resulted in total physiological breakdown within 3-4 weeks (Czyhrinciw and Jaffe, 1951). Storage of D. alata at 5°C for 6 weeks gave good results but CI symptoms developed rapidly when tubers were subsequently put in ambient (25°C) conditions (Coursey, 1961). There is no reliable data on beneficial effects on CA technology on the commercial storage is important yam cultivars.

Table 6: Recommended storage conditions for yams (Dioscorea spp.).
Cultivar	Temperature (°C)	Relative humidity (%)	Length of storage
D. trifida	3	-	1 month
Elephant yam	10	-	several months
D. alata	12.5	-	8 weeks
D. cayenensis	13	95	< 4 months
D. alata, cured	15-17	70	180
D. alata, non-cured	15-17	70	150
White yam, Guinea yam	16	80	several months
Yellow yam, Twelve month yam	16	80	60 days
Cush cush, Indian yam	16-18	60-65	several months
Lesser yam, Chinese yam	25	-	60 days
Water yam, Greater yam	30	60	several months
Unknown Cultivar
	13.3	85-90	50-115 days
	16	65	4 months
	16	70-80	6-7 months
Source: (Opara, 1999).

There are several traditional storage structures used for yam storage including: (a) leaving the tubers in the ground until required, (b) the yam barn, and (c) Underground structures (Opara, 1999). Leaving the tubers in the ground until required is the simplest storage technique practised by rural small-scale farmers. When carried out on-farm, this type of storage prevents the use of the farmland for further cropping. Harvested yams can also be put in ashes and covered with soil, with or without grass mulch until required.

The yam barn is the principal traditional yam storage structures in the major producing areas. Barns are usually located in a shaded areas and constructed so as to facilitate adequate ventilation while protecting tubers from flooding and insect attack. Barns consist of a vertical wooden framework to which the tubers are individually attached (Fig. 2). Two tubers are tied to a rope at each end hung on horizontal poles 1-2 m high. Barns up to 4 m high are uncommon. Depending on the quantity of tuber to be stored, frames can be 2 m or more in length. The ropes are usually fibrous, but in Southeastern Nigeria, they are made from the raffia obtained from top part of Palm wine tree. Many farmers have permanent barns, which need annual maintenance during the year’s harvest. In these situations, growing trees are used as vertical posts, which are trimmed periodically to remove excessive leaves and branches. Palm fronds and other materials are used to provide shade. The vegetative growth on the vertical trees also shades the tubers from excessive solar heat and rain. The use of open-sided shelves made from live poles, bamboo poles or sawn wood has been recommended to enable careful handling and easy inspection in comparison with tying tubers to poles which can cause physical damage and rotting (Bencini, 1991). In barn storage, yams have a maximum storage life of 6 months and are therefore most suited for long-term varieties. Storage losses can be high and up to 10-15% in 3 months, and 30-50% after 6 months if tubers are not treated for rotting using fungicides such as Benlate, Captan or Thiabendazole.

Fig. 2. Tipical Yam Barn in West Africa.

Yams are also stored in underground structures such as pits, ditches and clamps. These are suitable for limited storage periods, especially the early varieties that are often harvested before the end of the rainy season.  During construction of pits, the earth dug out is used to build a low wall around the edge. The temperature in the storage space can also be moderated by placing cut vegetation over the ditch, clamp or pit. In these structures, ventilation and rodent attack of tubers is a major problem, and it is difficult to inspect the tubers.

Well-ventilated, weatherproof, and stronger shelters can be built as to improve the performance of the traditional shelters described above. New features may also be provided to exclude pests and rodents. A typical improved yam barn has sidewall 1.2 m high and wire mesh to ward off rodents and birds (Akoroda and Hahn, 1995). The roof was double thatch and extended to the eaves with smooth floor of cement or mud, and only one entry door was provided to guard against entry of rodents. Tubers were stored on platforms or shelves. Tubers stored in such improved structures had only 10% spoilage after 5-6 months.

2.7      Processing

Industrial uses of yam includes starch, poultry and livestock feed, and production of yam flour. Readers interested in detailed information on specific yam processing methods, equipment, and packaging techniques can find these information in an FAO technical compendium (Bencini, 1991). Residues from sifting and peels are used as animal feed in many rural areas. One of the major disadvantages of industrial processing of yam for food is that nutrient losses in these products can be high, particularly minerals and vitamins. In products obtained from secondary processing such as biscuits and fufu, the amount of loss depends principally on the amount of edible surface exposed during processing operations. Primary unit operations such as milling affect the thiamine and riboflavin contents of D. rotundata, with average losses of 22% and 37%, respectively. Sun drying results in high losses of B vitamins with little change in mineral content. Pounding yam flour in a traditional wooden mortar or grinding in an electric mixer had similar effects.

2.8      Dormancy in Yams

Dormancy is the temporary suspension of visible growth of any plant structure containing a meristem, and in stored yam tubers, it is the period during which sprouting is inhibited. Knowledge of the potential length of dormancy for stored tuber is important because once dormancy breaks, the tubers also senesce rapidly with loss of the stored food (carbohydrate) (Passam and Noon, 1977). Yam tuber does not sprout during the early part of storage, even under suitable growth conditions. The environmental conditions affecting yam tuber dormancy are photoperiod, white and coloured lights, temperature, relative humidity, and partial oxygen pressure. The length of tuber dormancy is endogenously controlled and conditions such as availability of soil moisture or cool temperature are ineffective triggers of sprouting. Physiological age of tubers affects their readiness to sprout, but by approximately 6 months after harvesting, dormancy disappears completely and budless setts planted after that period will require nearly the same time to sprout (Onwueme, 1975). The length of dormant period is affected by the yam species (Table 7). These data are useful for developing suitable storage and marketing strategies, and also for scheduling the next planting.

Table 7: Dormancy period of tubers of major edible yam species.
Species	Locality	Period of dormancy (weeks)
D. alata	Caribbean	14-16
	Nigeria	14-16
D. bulbifera	Nigeria	19-20
D. cayenensis	Nigeria	4-8
D. dumetorum	Nigeria	14-16
D. esculenta	Caribbean	4-8
	Nigeria	12-18
D. rotundata	Nigeria	12-14
		14-16
D. trifida	Caribbean	2-4
Source: (Opara, 1999).

Number:	5160078
Description:	Foliage; leaves
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	9006015
Description:	Plant(s); in August
Photographer:	Fred Nation, Weeks Bay National Estuarine Research Reserve
	Additional Resolutions and Image Usage

Number:	5163005
Description:	Flower(s);
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5163003
Description:	Flower(s); habit
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5126013
Description:	Feature(s); Aerial tubers (bubils)
Photographer:	Chris Evans, River to River CWMA
	Additional Resolutions and Image Usage

Number:	5163009
Description:	Fruit(s); yamlet
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5126029
Description:	Infestation;
Photographer:	Chris Evans, River to River CWMA
	Additional Resolutions and Image Usage

Number:	5163010
Description:	Plant(s); habit
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5126022
Description:	Infestation;
Photographer:	Chris Evans, River to River CWMA
	Additional Resolutions and Image Usage

Dioscoreales > Dioscoreaceae > Dioscorea alata L.
Synonym(s): water yam
Winged yam is an herbaceous, twining vine that can grow to lengths exceeding 30 ft. (9.1 m). It invades open to shady areas in the extreme southeastern United States. Leaves are opposite, 8 in. (20.3 cm) long and narrowly heart-shaped with long petioles. The stems are square with the corners compressed into wings. The chief means of reproduction is by aerial potato-like tubers (bulbils) located at the leaf axils and by underground tubers. The vine rarely flowers. Winged yam can form dense masses of vines that cover and kill native vegetation including trees. It was introduced from Asia as a possible food source in the early 1800s. In some countries it is presently being cultivated for medicinal uses.

Identification, Biology, Control and Management Resources

• Nonnative Invasive Plants of Southern Forests - USDA Forest Service
  
  http://www.invasive.org/eastern/srs/ECY.html - Sep 7, 2004

• Identification and Biology of Non-Native Plants in Florida's Natural Areas - University of Florida
  
  http://www.fleppc.org/ID_book/Dioscorea%20alata.pdf - Sep 7, 2004

Selected Images

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Number:	5163006
Description:	Foliage; leaves
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5163004
Description:	Foliage; leaves
Photographer:	Forest & Kim Starr, U.S. Geological Survey
	Additional Resolutions and Image Usage

Number:	5126008
Description:	Infestation;
Photographer:	Chris Evans, River to River CWMA
	Additional Resolutions and Image Usage

External Links

• USDA NRCS - PLANTS Database
• Integrated Taxonomic Information System (ITIS)

Invasive Reference(s)

• Alabama - IPC List
• Florida - EPPC list
• Georgia - EPPC list
• Nonnative Invasive Plants of Southern Forests: A Field Guide for Identification and Control

Subject Reference(s)

• USDA, NRCS. 2001. The PLANTS Database, Version

Steroidal saponins and flavan-3-ol glycosides from Dioscorea villosa

M. Sautour^a, T. Miyamoto^b and M.-A. Lacaille-Dubois^{a, ,}

^aLaboratoire de Pharmacognosie, Unité de Molécules d'Intérêt Biologique, UMIB UPRES-EA 3660, Faculté de Pharmacie, Université de Bourgogne, 7, Bd. Jeanne d'Arc, BP 87900, 21079 Dijon Cedex, France ^bGraduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka, Japan

Received 7 January 2005; 

accepted 7 July 2005. 

Available online 8 September 2005.

Keywords: Dioscorea villosa; Dioscoreaceae; Steroidal saponin; Flavan-3-ol glycosides

Article Outline

1. Subject and source

2. Previous work

3. Present study

4. Chemotaxonomic significance

References

1. Subject and source

The underground parts (rhizomes) of Dioscorea villosa L. (Dioscoreaceae) were collected in Mexico in 2002. A voucher specimen (No. 1507) is deposited at the Herbarium of the Laboratory of Pharmacognosy, Dijon, France.

2. Previous work

The wild yam D. villosa was shown to be an important source of diosgenin (Benghuzzi et al., 2003 and Marker et al., 1940), a steroid precursor of progesterone. This yam is used in alternative medicines to minimize post-menopausal symptoms and for the treatment of low progesterone levels (Benghuzzi et al., 2003). The in vitro anticollagenase activity of D. villosa extract (Liviero and Allec, 2002) and the possibility of incorporation of sapogenins in cosmetics for the prevention of skin disorders (Picard, 2002) have also been reported. Nevertheless, no phytochemical investigation has been carried out on this plant.

3. Present study

Dried powdered rhizomes (280 g) of D. villosa were refluxed with MeOH:H₂O (7:3, 3 × 2 l), which after evaporation to dryness yielded 91 g of MeOH:H₂O extract. This was partitioned successively with hexane, CH₂Cl₂ and n-BuOH (each 3 × 200 ml) yielding after evaporation of the solvents the corresponding hexane (0.4 g), CH₂Cl₂ (0.2 g) and n-BuOH (29.7 g) fractions. Of the n-BuOH residue, 5 g was dissolved in MeOH and purified by precipitation with diethyl ether (3 × 300 ml), yielding a crude saponin mixture (3.7 g). This latter was submitted to vacuum liquid chromatography (VLC) on C₁₈ reversed-phase (12 × 3 cm) using as eluents H₂O (100 ml), MeOH:H₂O mixtures (5:5; 4:1, each 100 ml) and finally MeOH (100 ml). After evaporation of the solvents, three fractions were obtained: VLC-F1 (MeOH:H₂O, 4:1), VLC-F2 (MeOH) and VLC-F3 (MeOH:H₂O, 5:5).

VLC-F1 (0.74 g) was submitted to MPLC column chromatography (system a: Si gel (15–40 μm), CHCl₃:MeOH:H₂O (13:7:2, lower phase)) to give nine fractions (1–9). Fraction 8 was rechromatographed by MPLC (system a) yielding the pure compounds 1 (10 mg) and 2 (12 mg).

VLC-F2 (1.23 g) was submitted to MPLC (system a), yielding 10 fractions (1–10). Fraction 5 was rechromatographed by MPLC (system a) to give the pure compound 3 (15 mg). Fractions 4 and 3 afforded the pure compounds 4 (12 mg) and 5 (6 mg), respectively. Fraction 2 was concentrated to dryness yielding the pure compound 6 (8 mg).

VLC-F3 (0.49 g) was submitted to MPLC (system a) to give 10 fractions (1–10). Fractions 2 and 5 were concentrated to dryness to give the pure compounds 7 (25 mg) and 8 (7 mg), respectively.

The identification of all metabolites was achieved by FABMS on a Jeol X-102 spectrometer and by spectroscopic methods including 1D- and 2D-NMR experiments (COSY, TOCSY, HSQC and HMBC) on a Varian Inova instrument equipped with a Sun-4-L-X computer system as well as by comparison with literature values. They were identified as protodioscin (1) (Hu et al., 1996), methylprotodioscin (2) (Hu et al., 1997), parrisaponin (3) (Matsuda et al., 2003 and Sautour et al., 2004a), dioscin (4) (Hu et al., 1996), prosapogenin of dioscin (5) (Hu et al., 1996), progenin II (6) (Agrawal et al., 1985), epiafzelechin 5-O-β-d-glucopyranoside (7) (Sethi et al., 1984) and epiafzelechin 7-O-β-d-glucopyranoside (8) (Jin et al., 1999) (Fig. 1).

Full-size image (64K)

Fig. 1. Chemical structures of compounds 1–8.

View Within Article

The antifungal activity of compound 6 was performed using the broth dilution test in comparison with a positive control of ketoconazole (Quiroga et al., 2001). It presented antifungal activity against Candida albicans (MIC = 12.5 μg/ml), Candida glabrata (MIC = 12.5 μg/ml) and Candida tropicalis (MIC = 25 μg/ml). These results confirm our previous work concerning the antifungal activity of spirostane-type saponins isolated from Dioscorea cayenensis (compounds 3–5) (Sautour et al., 2004a M. Sautour, A.C. Mitaine-Offer, T. Miyamoto, A. Dongmo and M.A. Lacaille-Dubois, Planta Med. 70 (2004), p. 90. View Record in Scopus | Cited By in Scopus (27)Sautour et al., 2004a and Sautour et al., 2004b).

4. Chemotaxonomic significance

The saponins found in D. villosa rhizomes belong to the furostan-type (1–2) and spirostan-type skeletons (3–6), the most common aglycones found in the Dioscoreaceae family (Tang and Eisenbrandt, 1992).

Concerning the phenolic compounds (7–8), the presence of flavan-3-ol such as (+) catechin or (−) epicatechin and procyanidin dimers B-1 and B-3 have been reported in Dioscorea cirrhosa (Hsu et al., 1985), Dioscorea alata, D. cayenensis, Dioscorea dumetorum, Dioscorea rotundata (Ozo et al., 1984) and Dioscorea bulbifera (Ozo et al., 1984 and Gao et al., 2002). Furthermore, procyanidin trimers and tetramers were also found in D. cirrhosa (Hsu et al., 1985) and D. alata. The latter was also reported to contain cyanidin glycosides such as cyanidin-3-glucoside (Ozo et al., 1984), cyanidin-3,5-diglucoside, cyanidin-3-rhamnoglucoside (Rasper and Coursey, 1967) and cyanidin-3-gentiobioside acylated with ferulic acid (Imbert and Seaforth, 1968) but to our knowledge, the present work is the first report of flavan-3-ol glycosides in Dioscorea species.

References

Agrawal et al., 1985 P.K. Agrawal, D.C. Jain, R.K. Gupta and R.S. Thakur, Phytochemistry 24 (1985), p. 2479. Abstract | PDF (1768 K) | View Record in Scopus | Cited By in Scopus (298)

Benghuzzi et al., 2003 H. Benghuzzi, M. Tucci, R. Eckie and J. Hugues, Biomed. Sci. Instrum. 39 (2003), p. 335. View Record in Scopus | Cited By in Scopus (8)