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Chromosome Number: 22
Taxonomic Classification
Class
:
Magnoliopsida
Order
:
Fabales
Family
:
Fabaceae
Genus
:
Pachyrrhizus
 
Yam bean, popularly known as ‘potato bean’ and mishrikand, is grown in Orissa, Bihar, eastern Uttar Pradesh, West Bengal and Assam. Its starchy conical or turnip-shaped fleshy tubers are eaten. High sugar content in tubers imparts sweet taste when eaten raw. The fresh tubers are used as salad, whereas over-mature tubers become fibrous, hence unsuitable for consumption. Tender tubers are used as raw or processed in preparation of various sweets.
   
Climate and soil  

Yam bean is adapted well in subtropical to humid, hot, temperate zones. Its main climatic requirement is frost-free condition during vegetative growth. It can be grown up to 1,000m above mean sea-level. Heavy rainfall along with waterlogging condition is unfavourable for its cultivation. But evenly distributed rain throughout the growth period is favourable for good tuber development. Cool temperature at early vegetative growth period also affects tuberization and prolongs vegetative growth period delaying tuberization.

Sandy loam soil of good depth is favourable for its cultivation. The soil pH of 6.0–7.0 is ideal. The clay loam soil with good fertility and drainage is most-suited for its cultivation.

 
Varieties
There are 2 types of cultivars available in market. Mexican types have larger tubers (500–700g) in size, whereas local types have smaller tubers (200–300g). The mexican types are less sweet compared to local ones. The Mexican types have a tendency to develop cracks on their tubers. They are less preferred in the Indian markets. The local types having smaller tubers are more sweet. The flesh is white with less fibre. There is no cracking. Rajendra Mishrikand 1, an improved variety, is very popular in north Bihar. It is recommended for cultivation in Bihar and West Bengal. Its average yield is 40–55 tonnes/ha in 110–140 days. Other promising line, L 19, gives better yield in Bihar, West Bengal and Orissa.
   
Propagation
Yam bean is propagated usually through seeds. The seed pods are generally 7–15cm long with 8–10 brownish-yellow to red seeds. Seeds are dorsiventrally flat. Plants can also be raised from sprouted roots of previous crop but normally it is not practised. The seed rate varies according to spacing of the crop. A seed rate of 20–60kg/ha is generally adopted by the farmers. The mature pods containing seeds are source of a toxic substance ‘rotenone’ and are sometimes harmful for grazing cattles in the field.
   
Cultivation  

Planting

Traditionally yam bean is sown during June–July with the onset of rain in north-eastern India. Deep ploughing of land is done 2–3 times with mould board plough followed by thorough planking to bring the soil to good tilth. This helps conserve moisture level in the field. The seeds should be sown from June–September according to the purpose of the crops. To obtain smaller-sized, tender tubers, sowing is recommended during August–September at a closer spacing of 30 cm× 15cm or even at 15m × 15cm. Normally when the crop is late sown in September and harvested in December/January, it gives comparatively lower yield due to smaller-sized tubers. The later crop due to shorter duration can easily fit into various multiple cropping systems. Usually tubers produced from late-sown crop are free from cracking. Planting on ridges at a spacing of 30cm × 30cm also gives good yield in fertile soil. A spacing of 60cm × 45cm is also practised when thinning of seedlings is not done. Sometime yam bean seeds are sown on hills @ 3–5 seeds/hill. The hills are 15cm high, spaced 0.75–1.00m apart.

Trailing

In north Bihar, yam been is intercropped with maize, where maize plants are utilized for trailing. But in other parts of India, normally trailing is not adopted in growing of yam bean.

Manuring and fertilization

For proper growth, soil rich in humus, having good water-holding capacity is most-suited. Farmyard manure @ 10–20 tonnes/ha should be applied at the time of land preparation. The fertilizer dose of N:P:K @ 80:40:80kg/ha is recommended for north Bihar. Both P and K are applied as basal at the time of planting along with half dose of N. The remaining half dose of N is applied as topdressing 40–45 days after sowing at the time of earthing-up and intercultural operations. In West Bengal, N:P:K dose of 80:60:80kg/ha results in good yield.

Aftercare

Depending on the time of sowing, weed infestation is maximum in June–August sown crop compared with September sown crop. First intercultural operations are done 40 days after sowing followed by another after 30 days first interculture. Straw mulching helps in better moisture conservation and also suppresses weed growth. Normally, yam bean starts flowering 75 days after sowing. It is desirable to remove the flowers without allowing the plants to bear pods for getting better tuber yield. There is a significant negative correlation between tuber yield and pod formation. It is essential to remove the buds before they flower and form pods. Removal of buds by hand is usually followed. Spraying of 2, 4-D (50ppm) at flower-initiation stage induces flower thinning and increases tuber yield.

Irrigation

The rainfed crop sown in June–July does not require irrigation. In September sown crop it is advisable to give supplementary irrigation, so that crop does not face moisture stress during tuber formation.

   
Harvesting & Postharvest management

It can be harvested early or late according to market demand. It is possible to harvest the crop with smaller-sized tubers after 100 days. Otherwise it can be left in the field up to 140 days for better size. Traditionally, the trend is to harvest the crop on the occasion of ‘Saraswati Puja' with the start of spring season because of market demand. Delayed harvest leads to fibrous flesh along with cracks in tubers. This causes deterioration in tuber quality in market. A light irrigation should be given to soften the soil before harvesting. First top vegetative part is removed then tubers are dug manually. Care should be taken to avoid cuts and bruises on tubers.

On an average, local cultivars yield 18–20 tonnes/ha, whereas Rajendra Mishri 1, 36–40 tonnes/ha. For staggering, the tubers are left in the field after removing their tops. The seed crop should be harvested after 240 days during March–April.

After harvesting, tubers are thoroughly washed in water. They are graded according to their size. They can be stored for 3–5 days without any deterioration. During longer storage period, skin colour of the tubers turns brownish with progressive loss of water from tubers. The tubers lose their weight, they become more sweet. Since tubers are firm, no specialized packing is required. They are packed in jute bags and brought to market. For seed crop after harvesting dry seed pods are collected and threshed by beating with sticks to take out seeds.

   
Physiological Disorders
Cracking of tubers is the main problem encountered by the farmers. Availability of proper soil moisture during 45–90 days checks cracking. Application of K reduces cracking of tubers. Delay in harvesting also causes cracking in tubers.
Nutritional Value
 
TOP
 
Chromosome Number: 20 (D. tokoro)
Taxonomic Classification
Class
:
Liliopsida
Order
:
Dioscoreales
Family
:
Dioscoreaceae
Genus
:
Dioscorea
 

Yams ( Dioscorea species)—greater yam, lesser yam and white yam—are considered as under-utilized crops in India. In India, yams are usually grown under multiple cropping systems in homesteads. It is a common practice in tribal belt. Since their tubers are grown underground, they can be harvested at any time. Normally yams are consumed as boiled, baked or fried vegetables. They contain 18–20% starch with mucilagenous substance. Extraction of starch is difficult in yams but on cooking, the sticky character is lost. Yams also contain alkaloids, tanins and steroids which have pharmaceutical value.

Of the 6 important yams, only white yam (Dioscorea rotundata ), water yam or greater yam ( D. alata ), and Chinese yam or lesser yam ( D. esculenta ) are more popular. In India, D. alata and D. esculenta are under cultivation. Madhya Pradesh, north-eastern states, West Bengal, Bihar, Orissa, Uttar Pradesh, Kerala, Tamil Nadu, Gujarat and Maharashtra are major yam-growing states.

Greater yam
Greater yam popularly known as ratalu is extensively cultivated in Madhya Pradesh, Uttar Pradesh, Orissa, West Bengal, Tamil Nadu, Kerala and north-eastern hills region as a subsidiary food crop. It possesses four or more wings on the stem which twines to the right. The petiole also has wings. Leaves are ovate, bigger and opposite in arrangement. Tubers are variable in shape but mostly cylindrical. The skin of the tuber is black or brown, whereas flesh is white, yellowish or purplish. Each plant may produce 1–3 tubers. Its cultivars rarely flower.

Lesser yam
Lesser yam or Asiatic yam or Chinese yam, popularly known as suthan i or ‘ kangar ', is usually thin, often pubescent and spiny which twines to the left. Leaves are simple, cordate and alternately arranged on vine. Small cylindrical tubers are borne in clusters of 15–35 with round ends. Tubers have smooth, yellow-brownish skin with white flesh. After cooking, the taste of tubers is sweeter than other yams.

White yam
It is becoming popular in India. It has wingless stem which is twining to the right. Leaves are simple, cordate and opposite in arrangements. Tuber shape is smooth and cylindrical. Flesh is usually white.

   
Climate and soil  

Yams require temperature of 26°–31°C for better growth. Temperature below 20°C restricts their growth. Day length greater than 12 hr during initial stages and short day condition during tuber initiation favour satisfactory tuber production. Since yams are relatively tolerant to dry conditions, March–April is ideal time of planting. These are sun-loving crops and cannot tolerate shading and frost conditions. A well-distributed rainfall of 1,100mm is enough for their growth. It is essential to maintain good moisture in the field during fourteenth and twentieth week.

Fertile sandy loam soil is ideal for growing yams. Loose, deep soil with high organic-matter content and having pH of 5–7 is most-suited for yams. Yams cannot sustain waterlogging. Since they are long-duration crops, they prefer soils rich in K content. In kitchen and homestead gardens, yams receive lot of ash, which is rich in K content.

 
Varieties
A number of varieties have been released in yams. The improved varieties show a yield potential of 18–25 tonnes/ha in greater yam and 18–30 tonnes/ha in lesser yam, while in white yam it is 33–40 tonnes/ha.
   
Propagation
Yams are commonly propagated vegetatively. Tuber pieces or small whole tubers are used. Propagation by vine-cuttings is not advised because of slow tuber production. A seed tuber weighing 125–150g is optimum size for planting in lesser yam, while in greater yam and white yam, 200–250g tuber pieces or whole tubers are used. Usually big whole tuber is cut into pieces (setts) consisting of top, middle and bottom. Seed tubers and top portions are ideal for planting. Before planting, tuber pieces are dipped in cow dung slurry and dried in shade to protect from damage. Drying of cut pieces gives healing effect, encouraging callus formation. Yam tubers have long dormancy period (2–3 months) depending on species. The degree of dormancy decreases during storage resulting in sprouting and weight loss, a major problem in yam cultivation. In yam, 2–3 months dormant period is necessary for maximum yield. To maintain dormancy and controlling storage sprouting, soaking freshly harvested tubers of lesser yam and greater yam in 0.1% solution of maleic hydrazide for 10 hr is effective. Dormancy can be broken by dipping of tubers quickly in 4–8% solution of ethylene chlorohydrin followed by dry storage.
   
Cultivation  

Yams require loose soil for planting. The soil should be dug deep or ploughed 2–3 times to a depth of 20–25cm to make it friable. Planting methods are different in different types of soils. Yams are planted flat, while in heavy soils they can be planted on mounds or ridges. On mild sloppy land, ridge method is preferred. In homestead farming, usually mounds are made over pits.

Yams require different spacings depending upon their growth habit and purpose of planting. A close spacing of 75cm × 75cm is optimum for lesser yam and white yam with a plant density of 18,000/ha. Since greater yam has luxuriant growth and broad leaves, it requires a spacing of 90cm × 90cm with less plant density. Spacings of 30cm × 30cm and 1m × 1m are also recommended for lesser yam and greater yam in north Bihar and some parts of Kerala respectively. Yams are planted mostly in the later part of the dry season or before the onset of the monsoon, from March–June. Before planting, pits of 45cm × 45cm × 45cm size for greater yam and 25cm × 25cm × 25cm for lesser yam are dug. After planting, pits are filled with mulch immediately. Mulching helps protect the propagating material from excessive soil temperature. It also helps in uniform sprouting and suppressing weeds. In some parts of south India, yams are intercropped with Nendran banana without affecting the yield.

Manuring and fertilization

Yams require fertile soil. Farmyard manure or compost @ 10–12 tonnes/ha should be incorporated in the soil. Application of a dose of N:P:K @ 80:60:80kg/ha for lesser yam is recommended. Since greater yam responds well to higher dose, 120:60:80kg/ha of N:P:K is beneficial. Total quantity of P and half of N and K should be given at the time of first interculturing. Yams take up to 40 days for sprouting. Apply the fertilizer in ring so that it does not touch the sprouts. If there is insufficient moisture, irrigate the field immediately after fertilizer application. The remaining quantity of N and K should be applied a month later along with second interculturing. Application of lower dose of N:P:K @ 40:40:40kg/ha for lesser yam and N:P:K @ 60:40:60kg/ha for greater yam is economical in fertile soils.

Aftercare

Staking in yams is essential for increased tuber production. It exposes the leaves to sunlight resulting in greater photosynthesis. The emerging shoots should be provided artificial supports to avoid any injuries. Stacking should be provided to tender shoots by coir rope attached to artificial support in open to a height of 3m. In homestead, possible trailing can be done on tree. Yams take up to 40 days for sprouting. In initial stage, weeding and earthing-up are to be done to have better growth. First interculturing should be done sufficiently deep after a week of sprouting. At this stage, removal of weeds along with little earthing-up should be done. The operation tallies with application of fertilizers. The second weeding and earthing-up should be done one month after the first one. The second interculture should be normally shallow.

Irrigation

Yams are relatively tolerant to drought. However, yield is affected if moisture stress is faced during initial stages of growth. For uniform sprouting, yams should be irrigated immediately after planting. The second irrigation can be given if the soil is dry before the onset of the monsoon. Once the rain starts, there is no need of irrigation. However, proper moisture is beneficial. Care should be taken to avoid stagnation of water. Earlier to harvest, shallow irrigation helps in digging out the corms.

   
Harvesting & Postharvest management

Yams are harvested 7–9 months after planting. The leaves turn yellow and vines start drying up at maturity. Delaying in harvesting up to 2 months does not affect yield. Lesser yam yields 25–30 tonnes/ha while in Maharashtra the yield is only 18–20 tonnes/ha. Greater yam yields 33–40 tonnes/ha, whereas white yam 18–25 tonnes/ha.

In India, yams are harvested during December–January. For marketing, crop should be harvested in later part and used for next year planting. Yams need proper storage for 2–3 months. The main problems in storage are rotting and sprouting of tubers resulting in storage loss. The common practice of checking rotting and sprouting are to remove the rotten tubers and breaking off the sprouts periodically. Sprouting can also be controlled by soaking tubers in 0.1% maleic hydrazide solution for 10 hr. As the tubers are stored in open barn, the weight loss and rotting in tubers are more. But covered barn structure is more effective in reducing weight loss. Weight loss also can be minimized by heaping tubers inside sand and dry soil treatment
 
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