Crops of the Araceae Family

Uses

Corms and cormels of aroids are used as a starchy vegetable that is prepared by boiling and consumed after baking, roasting, steaming, or frying. The pasty product made from cooked taro corms is known in Hawaii as poi. In Africa, a similar and very popular product known as fufu is made from tannia corms. Both products are sometimes eaten after fermentation, which makes the product slightly sour. Apparently, fufu from tannia is preferred to that from taro, which partly explains the much greater production of tannia in Africa. Although tannia is of tropical American origin, it has displaced much of the world’s taro production and is the dominant aroid crop in Africa, owing to its ease of cultivation, greater tolerance of low soil fertility, disease resistance, drought tolerance, and edible qualities.

Corms and cormels of aroids are an inexpensive, high-carbohydrate food source, but low in protein and vitamins. Aroid starch is easily digested and does not cause allergies. The composition of these storage organs averages from 65 to 80% water and from 20% to 25% carbohydrates. Starch content in corms or cormels of tannia is usually higher than in taro, and in tannia corms it is higher than in cormels. Dried corms and cormels of both species can be ground into flour. Sugar content in mature corms and cormels is low.

Young tender leaves and petioles are used as greens. A similar species, Xanthosoma braziliense, is grown exclusively for its edible foliage. The foliage of Amorphophallus campanulatus is also widely used in the Philippines, Malaysia, and China. Protein content in leaves is about two to three times higher than in cormels, and vitamin C content is usually good. Blanched taro shoots, which are consumed before leaf expansion, are obtained by forcing corms in darkness at high temperature. Acidic components, although present, are usually at lower levels than in cormels and are generally removed by boiling in water one or more times.

Toxicity

A common feature of aroids is their acrid nature and irritating raphides. Acridity is the presence of a sharp burning or bitter taste associated with unidentified compounds that may be glycosides or proteins, as well as with the presence of raphides. Raphides are tiny needle-like crystals of calcium oxalate produced in specialized cells that, when eaten, pierce and irritate the tissues of the mouth and tongue. Electron microscopic studies show that the crystals are about 50 µm long with sharp barbs that make them difficult to remove after the raphides penetrate the skin. Skin irritation and swelling can also result from handling aroids. Contact with peeled taro roots usually causes more skin irritation than contact with malanga, although people differ in their sensitivity. Food must be prepared before eating to remove the acrid principle. Wild aroid species contain more of the acrid principle and are known to contain cyanogenic glycosides. However, the leaves and roots of most domesticated edible aroids contain negligible amounts of this substance. Roots also contain a trypsin inhibitor, which is not a problem because the inhibitor is inactivated by cooking, and the roots are always cooked before consumption.

History and Origin

The history of edible aroids as food crops is ancient in both Old and New World civilizations. In some areas of the humid tropics and subtropics, edible starchy rhizomes and tubers grown as staple and subsistence crops are an important food source for millions of people. The leaves and petioles of some species and varieties are also used as vegetable greens.

It is widely believed that crops of the Araceae family originated in swampy areas and forests of the tropics with high rainfall. The domestication of low-growing or non-acrid wild aroids was probably achieved by selection in early cultures, which are thought to have begun as long as 7000 years ago, possibly before rice.

With the exception of Xanthosoma species in tropical America, the edible genera of aroids originated in the Old World. Wild forms of Colocasia occur in India, north of the Bay of Bengal. From tropical India, taro spread eastward to China, Japan, and some Pacific islands. Westward spread also occurred to Egypt, East Africa, and the eastern Mediterranean. Further migration occurred through Africa. In the post-Columbian period, taro was introduced to the Caribbean region and tropical America.

The movement of Xanthosoma sagittifolium (malanga) from tropical northeastern South America occurred relatively recently. Introduced to Africa during the slave trade (16th century), malanga quickly became established and now ranks after cassava and yam in importance. Because of its similarity to Colocasia, which is called “cocoyam” in some places, malanga became known as “new cocoyam.” Its resistance to diseases and pests greatly enhanced its rapid adaptation and spread in Africa, Asia, and some Pacific islands. The cultivation of other Xanthosoma species is of limited importance, except in some regions of Southeast Asia, India, and some South Pacific islands.

Production

Most of world aroid production consists of taro and tannia species, widely grown in West Africa, Southeast Asia, and many Pacific and Caribbean islands. Tannia production exceeds taro production, as evidenced by its extensive production in Africa. Several minor aroids, usually grown as subsistence crops, have limited global importance, but regionally they are often very important, especially when adverse weather destroys other food crops.

Aroid production occurs mainly in developing countries with warm climates. African countries produce about 61%, and Asian countries about 32%, of world production. Climate, variety adaptation, diseases, and pests are other factors affecting yield, but yield variability is mainly due to agronomic practice, which ranges from intensive farming to subsistence farming. As a result, yields range from 1 or 2 to more than 40 t/ha. Accurate production statistics are difficult to obtain because of undocumented small-scale and/or subsistence production and generally incomplete reporting.

Taxonomy

The family Araceae (Araceae) is a large family including about 100 genera with about 1500 species. The taxonomy of most edible aroids is very confused because species differences are unclear and many local names are used interchangeably. Most production areas have their own distinctive local varieties or clones, and their names often have no value for precise identification. In the case of Colocasia, the confusion is extreme because hundreds of named clones exist. The broadest distinction is that taro represents Colocasia esculenta, while tannia represents Xanthosoma sagittifolium. Examples of different local names for these species are as follows:

• Colocasia esculenta — taro, cocoyam, eddoe, dasheen, gabi, kel-adi, tari, arvi, kolkas, dalo, sato-imo; botanical name Colocasia esculenta;
• Xanthosoma sagittifolium — tannia, yautia, malanga, tanier, new cocoyam, belembe, maduma, gualusa; botanical name Xanthosoma sagittifolium.

Although the taxonomy of taro is not fully resolved, it is considered one polymorphic species with many botanical varieties. The cytotaxonomic background of cultivated taro varies, and the basic chromosome number is believed to be n = 14.

Most clones have 2n = 28, others have 2n = 42. Ploidy also varies in other edible aroids; tannia species have a chromosome number of 2n = 26.

Several general types are distinguished in cultivated taro. Those that produce one large corm and few cormels are often called dasheen; those that produce a relatively small corm and many somewhat smaller cormels are called eddoe. Some other types are grown mainly for their leaves and petioles.

Another distinction is that different varieties are used for flooding, while others are used for upland cultivation. The separation of varieties is based on plant size, leaf and petiole shape and size, tuber flesh color, flower form, and culinary use. Differences in acidity level are another trait used to distinguish varieties or clonal differences.

Botanical Description

Aroids are perennial herbaceous monocotyledonous plants. With the exception of Amorphophallus species, leaves emerging from the top of the corm form whorls with erect long petioles supporting a large broad peltate or sagittate leaf blade. The long thin petioles are solid but have numerous air spaces, which allows adaptation to flooding conditions. A common characteristic is acrid mucilaginous sap contained in all tissues.

Taro plants usually vary in height from 1 to 2 m and have long (25–50 cm), broad (12–25 cm) peltate leaf blades. The petiole attaches to the leaf blade approximately at the upper middle of the lower leaf surface.

Malanga plants are usually larger than taro and sometimes grow to 2 m or more in height. The large heart-shaped leaf lobe can sometimes reach almost 1 m in length. They have a prominent vein along the edge of the leaf blade that taro lacks. The petiole is similar to that of taro but is attached to the lower surface in a depression between the leaf blade lobes. Leaf alternation occurs continuously, and their number tends to be constant as old leaves are replaced by new ones.

Taro and tannia rarely flower, and some varieties do not flower at all. When flowering does occur, the inflorescence arises in the leaf axils, and depending on the species, one or several may form. Flowers, depending on the species, are unisexual or bisexual. All are sessile and formed on a spadix enclosed in a large bract (spathe). In a unisexual inflorescence, pistillate flowers are at the base of the spadix, staminate flowers at the top, and between them is a group of abortive sterile flowers. Insects pollinate the flowers, but fruits and seeds are rarely formed. The fruits are berries, each containing two to five small (1 mm), oval, hard, difficult-to-germinate seeds.

Each species produces enlarged fleshy corms that store starch. Roots developing from the lower part of the corm are adventitious, fibrous, and relatively shallow. Roots and corms, as well as leaves and petioles, are edible. Corms are the enlarged fleshy parts of the compressed stem base. Cormels are enlarged axillary or lateral buds originating either from the stem or from the mother root. Morphologically, both corms and cormels are stem tissue. They usually form underground but can also develop on the surface.

Corms are usually spherical or cylindrical. Depending on the variety, corm weight ranges from 500 g to several kg, and cormel weight ranges from 30 to 450 g. Large corms can reach 30 cm in length and 15 cm in width. Tannia cormels, being less tasty, are usually fed to animals; the corms are the part intended for human consumption. In taro, both corms and cormels are eaten; small taro cormels are often used for propagation. The flesh color of corms and cormels varies by variety; most varieties are white, but some are cream, yellow, and sometimes pink. The numerous concentric rings visible on the surface of cormels are remnants of leaf scars. The structure of a corm or cormel consists of a relatively thick corky periderm covered by a large volume of starch-containing parenchyma cells, within which vascular bundles are scattered.

Ecological Requirements

Crops of the Araceae family can be grown year-round in tropical and subtropical regions, usually under moist or flooded conditions. The favorable average temperature ranges between 21 and 27 °C. Limited taro production also occurs in some warm temperate regions of Japan and parts of the United States, although the growth of most aroid species is retarded at temperatures below 15 °C. All aroid species are adapted to shade, but they can be grown in shade. All species are adapted to shade, but they can also be grown in full sun. In traditional and home culture, partial shade is often preferred, usually provided by intercropping with plantain, coconut, maize, and other crops. Malanga tolerates shade better than taro and is well suited for intercropping.

Regions where most taro and malanga are grown usually receive high rainfall. Aroid crops evidently prefer moist soil conditions and readily available water during growth. Many varieties of taro and other aroids can be grown in flooded culture. In some production regions, supplemental irrigation is used during dry periods. Although taro tolerates some salinity, yield is reduced.

Taro plants are adapted to a wide range of soils, from the clay used in rice cultivation to well-drained, deep, fertile loams for upland cultivation. Dasheen-type taro grows well on heavy soils with high water-holding capacity and performs well under flooding. For eddoe-type taro, well-drained loamy soil is preferred, but some waterlogging is tolerated. Eddoe types are also able to tolerate dry soils.

Tannia is most productive on deep, fertile, well-drained loams and, unlike taro, does not tolerate waterlogging. However, tannia tolerates low fertility and drought better than taro and can tolerate periods of heavy rain if drainage is provided. Both taro and malanga can be grown satisfactorily in soil with pH from 5 to 8.

Synthetic fertilizers are usually not applied in most tropical areas because of the relatively low market value of these crops. Most aroids respond to additional nitrogen. High soil nitrogen levels stimulate vegetative growth and, if excessive, can delay maturity.

Propagation and Planting

Crops of the Araceae family are propagated vegetatively using corms, cuttings, or stem cuttings consisting of the apical portion of the corm with attached petioles. The latter are called “setts” and are usually obtained during harvest by cutting the 2–3 cm top portion of the corm with attached petioles, which are trimmed to 10–15 cm. The remaining part of the corm can be used for food. In Hawaii, taro shoots are called “hulis.” Setts usually root earlier and generally perform better than propagation by cuttings or corms. In flooded taro cultivation, hulis are often the exclusive propagation material. In upland taro culture, propagation may involve sprouted corm pieces, cormels, or hulis. True seed production is relatively rare, and seeds are unsuitable for propagation because the resulting plants show undesirable genetic variability and low productivity. Tissue culture has some use for cloning virus-free plants and for plant breeding.

Taro has no dormancy period, so corms and cormels are used for propagation when the buds begin to sprout. A corm or portion of a corm with an intact apical meristem is ideal because such propagules root quickly and produce uniform populations. Small, 50–75 g, corms are used intact. A portion of the corm other than the apical part may result in delayed sprouting and uneven rooting, so such planting material should first be planted in nurseries. After rooting, single-shoot seedlings can be used for transplanting. Plantings are made so that each planting site receives one shoot; excess shoots are thinned.

For tannia propagation, head setts are often used. These are larger setts than taro setts. Large corms can be used for tannia propagation because the corms of most species are not valued for consumption. However, small non-marketable tannia corms are also used for planting. Although tannia corms have a dormancy period, it disappears after 1–3 months. Therefore, if necessary, propagation material can be stored briefly. However, storing setts is more difficult than storing intact corms or cormels. Propagation practices for other edible aroid species are similar.

Corms, cormels, or setts are planted at a depth of 10–15 cm. Large propagules are preferred, and about 2 tons of propagation material are used per hectare. The average taro plant population is about 30,000 per hectare, but can range from 12,000 to 100,000 per hectare, with higher density in flooded cultures. For tannia, spacing is wider, and planting density ranges from 14,000 to 20,000 plants per hectare. Plant density and the length of the growing period affect cormel size. At low plant density and with a long growing period, large corms and cormels are formed.

Taro Cultivation

In tropical climates, taro planting time is mainly determined by moisture availability and usually begins at the start of the rainy season. In temperate regions, if water is available, planting can be done at favorable temperatures. However, because propagation material is obtained from previous crops, the time of harvesting the previous crop often affects the planting time of the new crop.

Lowland taro culture begins with fields being plowed and then flooded. A dike is built around the field to retain water. The soil can be puddled as for paddy rice; a deep hard layer in the soil horizon may be useful for water management. Shoots are transplanted, usually by hand, into moist soil and covered. Although less preferred, pre-sprouted corms or cormels are also used. Plantings are made on a ridge or raised bed if harvesting machinery will be used; otherwise the field surface is level.

When new unfolded leaves appear, the field is flooded again. The water level is raised to 10 cm and maintained at this level for a short time to control weeds. Lack of early weed control can lead to significant yield losses. Then the water level is lowered to the optimum value — from 4 to 8 cm, since a high water level reduces yield. During most of the growth period, water is supplied continuously. Interruption of flooding disrupts corm and cormel development and can reduce yield. Fields are not drained except for fertilizer application and harvesting. Considering evaporation of about 0.5 cm per day and water seepage into the soil, the average daily water requirement is about 2–3 cm. Flooded culture usually gives higher yields than upland culture under rainfed or irrigated agriculture, but requires a longer growing period.

For upland taro cultivation, deep, loose, well-drained soil is preferred. Unlike the generally more fertile soils used for flooded culture, soils used for upland culture are often infertile. After land preparation, furrows 30 cm deep and 75 to 90 cm apart are dug for planting pre-sprouted cormels or corms; setts are used less often. Row spacing is about 40 cm. Corms and cormels are initially covered with 5–8 cm of soil; later more soil is added to avoid root exposure and improve daughter corm development. Deep planting is considered to give higher yields than shallow planting. Upland taro culture at low planting density is better suited for intercropping; maize or legumes are often chosen.

Malanga Cultivation

Malanga is grown as an upland crop because it does not tolerate waterlogging, although it grows well at high soil moisture. Good yields require uniformly distributed moisture, whether from rainfall or irrigation.

For propagation, setts or pre-sprouted corms, previously cut into pieces each with one or more sprouts and weighing from 200 to 900 g, are used; they are two to three times larger than taro setts. Plant spacing is most often 90 cm within the row and 90 cm between rows, but equidistant spacings can vary from 60 to 180 cm. Close plant spacing helps plants compete with weeds and usually increases total yield, but reduces corm and cormel size.

As with taro, after planting the soil around plants is repeatedly compacted to avoid root exposure, help cormel development, and control weeds. Malanga responds to high nutrition and is often the first crop planted after land clearing. Being more shade-tolerant than taro, malanga is more often used as an intercrop.

Growth and Development

For taro and tannia, rapid vegetative growth typically occurs in the first 4–6 months after planting, during which maximum leaf canopy is reached. To increase yield, farmers use larger cormels or setts to maximize early leaf growth as much as possible.

Leaves develop at intervals of a few to many days over several months. From unfolding to senescence, the average leaf life in the early growth period is 40–45 days, and 55 to 80 days for later-formed leaves. Initially each successive leaf increases in size until mid-season, then leaf sizes begin to decrease. Photosynthetic activity of an average newly unfolded leaf increases until about day 15, then remains at the highest level for another 10 days, after which gradual decline and senescence occur. Leaf area index increases with additional nitrogen and also with close plant spacing. Shaded plants usually have greater leaf area, which compensates for reduced photosynthetic activity. During most of the growth period, about five to six functional leaves are present. A total of about 20 leaves is usually formed.

Cormel formation begins 3–5 months after planting, and their growth continues until harvest. As the plant grows, older leaves yellow and die until few remain. The upper parts of mature corms are often visible above ground. At this leaf-withering stage, corms and cormels are ready for harvest. If the crop is not harvested, new leaf growth may begin and corm enlargement may continue. Some aroids are deliberately grown for several years before harvest.

Harvest of upland taro usually occurs about 8–9 months after planting, and for malanga a little longer. Flooded taro requires a 12–15-month growth period. Generally, leaf yellowing is a common sign of crop maturity, and physiological maturity of cormels occurs when sugar content reaches a minimum. Harvesting is best done in the dry season. For flooded taro culture, fields are drained several weeks before harvest begins. Upland taro harvest can be delayed if necessary.

Multiple or partial harvesting is not a common practice because it has more disadvantages than advantages. For taro, the entire crop is harvested, while for tannia the corms can be carefully harvested and the mother corm or smaller corms left for further growth.

Most world taro and tannia crops are harvested by hand, which is especially true for lowland taro. For ease of cormel harvesting, the whole plant is most often lifted. The main corm and cormels are separated from roots, and petioles are cut. At this time, setts are selected for propagation. In commercial production, machinery may be used for harvesting and cleaning corms. Yields vary greatly within and between production regions. Taro yields are usually higher than tannia yields; sometimes reaching 30 t/ha for taro and up to 20 t/ha for tannia. In experimental plantings, taro yields have reached 100 t/ha.

Harvesting leaves and petioles of either plant for vegetable use has little effect on corm production if the frequency and amount of harvesting are minimal. Sometimes crops are grown specifically for leaf production.

Postharvest Handling and Storage

Corms and cormels are subject to damage, so careful handling is important to minimize postharvest losses. After harvest, corms and cormels are cleaned, washed, and their petioles and attached fibrous roots are trimmed, but those intended for storage are not washed. Taro corms, unlike tannia corms, are not dormant and may sprout, which is the reason for their short storage life.

Except for short periods, most aroid storage is limited; the facilities that exist are usually located near production sites. In addition to above-ground storage, growers store harvested corms and cormels in underground pits covered with soil. The surface is kept moist and mulched to limit moisture loss and prevent a rise in product temperature. At ambient temperature in ventilated structures, corms can be stored for up to 6 weeks. When stored in a refrigerator at 7 °C and relative humidity of about 80–85%, tannia corms can be kept for 4 months or more. Storage below 5 °C causes chilling injury, symptoms of which appear as internal browning. Taro corms withstand a short storage period at 5 °C.

Diseases and Pests

Some diseases of crops of the Araceae family:

• Xanthomonas campestris (English Bacterial leaf blight);
• Erwinia carotovora (English Bacterial soft rot);
• Ceratocystis fimbriata (English Black rot);
• Cladosporium colocasicola (English Cladosporium (brown) leaf spot);
• Cercospora chevaliegi (English Cercospora leaf spot);
• Fusarium solani (English Fusarium dry rot);
• Phyllosticta colocasiophila (English Phyllosticta leaf spot);
• Pythium spp. (English Pythium root and corm);
• Rhizopus stolonifer (English Rhizopus rot);
• Sclerotium rolfsii (English Sclerotium tuber rot);
• Phytophthora colocasiae (English Taro leaf blight).

Leaf diseases are more common in flooded taro plots than in upland ones. In general, Xanthosoma plants have greater resistance to diseases and pests than Colocasia plants. Storage-related diseases are usually less important because corms and cormels are generally not stored for long periods.

Vegetative propagation as currently practiced facilitates the transmission of diseases, including viruses. Dasheen mosaic virus (Dasheen Mosaic Virus, DMV), transmitted by aphids, causes significant damage. In the Solomon Islands, two serious viruses are alome and bobone. Clonal propagation of meristem tissue to eliminate viruses is not yet practical for widespread use by growers.

High populations of root-knot nematodes, Meloidogyne spp., attack both feeding roots and corms, thereby reducing yield and making corms unmarketable. Loliloli is a physiological disease of taro in which starch loss leads to corm softening.

Insect pests, although troublesome, usually do not limit production because many are specific to one aroid species. The taro leafhopper, Tarophagus proserpina, is one of the most important pests and also a virus vector. The hawk moth caterpillar, Hypotion celerio, the caterpillar Spodoptera litura, and some other Lepidoptera pests damage plants. Beetles of the genera Araeocerus, Papuana, and Ligyrus, spider mites (Tetranychus spp.), thrips (Heliothrips indicus), scale insects (Aspidiotus destructor), mealybug (Dysmicoccus brevipes), aphids, whiteflies, ants, and snails are additional pests.

Minor Aroid Crops

Crop Improvement

Improvement of crops of the Araceae family through breeding is likely to be slow and limited, especially because of difficulties in sexual reproduction and the lack of research interest and funding. Most genetic gain has been achieved through mutants and selection of superior clones, which is likely to continue. Labor costs for planting and harvesting are obstacles to expanding production. Generally, subsistence production and small farms preclude investment in mechanization and other improvements. Demand for edible aroids remains high, and in many areas they are a preferred food. However, vegetative propagation, which requires using a significant portion of the crop for propagation, high water requirement, diseases and pests, and inefficient storage are obstacles to production.

Because of their unique adaptation to swamps or soils with high moisture and the reliability of their production, edible aroids will continue to make a significant contribution to meeting the food needs of many tropical peoples. Many people rely on the production of these edible aroids to meet their calorie needs, and consumption of the easily digested product can be expanded through the use of its many processed forms.

References

1. Rubatzky, Vincent E., Yamaguchi, Mas. (1997). World vegetables: principles, production, and nutritive values. 2nd ed.