2.1 Classification and Genetics
2.2 Origin and Distribution
2.3 Morphology and Anatomy
2.4 Growth Cycle
2.5 Ecology and Physiology
Taro belongs to the genus Colocasia, within the sub-family Colocasioideae of the monocotyledonous family Araceae. Because of a long history of vegetative propagation, there is considerable confusion in the taxonomy of the genus Colocasia. Cultivated taro is classified as Colocasia esculenta, but the species is considered to be polymorphic. There are at least two botanical varieties (Purseglove, 1972):
i) Colocasia esculenta (L.) Schott var. esculenta;C. esculenta var. esculenta is characterised by the possession of a large cylindrical central corm, and very few cormels. It is referred at agronomically as the dasheen type of taro. C. esculenta var. antiquorum, on the other hand, has a small globular central corm, with several relatively large cormels arising from the corm. This variety is referred to agronomically as the eddoe type of taro. Most of the taro grown in the Asia/Pacific region is of the dasheen type.
ii) Colocasia esculenta (L.) Schott var. antiquorum (Schott) Hubbard & Rehder which is synonymous with C. esculenta var. globulifera Engl. & Krause.
Chromosome numbers reported for taro include 2n = 22, 26, 28, 38, and 42. The disparity in numbers may be due to the fact that taro chromosomes are liable to unpredictable behaviour during cell divisions. The most commonly reported results are 2n = 28 or 42. Germplasm collections of taro exist at various scientific institutions world-wide. These include the International Institute of Tropical Agriculture (IITA) Ibadan, Nigeria; the Philippine Root Crop Research and Training Center, Beybey, Philippines; the Koronivia Research Station, Fiji; the Bubia Agricultural Research Centre in Papua New Guinea, and numerous other locations in Oceania.
There are hundreds of agronomic cultivars of taro grown throughout the world. These are distinguished on the basis of corm, cormel, or shoot characteristics, or on the basis of agronomic or culinary behaviour. Examples of taro cultivars in various places are given in subsequent chapters that deal with taro cultivation in various countries.
Various lines of ethno-botanical evidence suggest that taro originated in South Central Asia, probably in India or the Malay Peninsula. Wild forms occur in various parts of South Eastern Asia (Purseglove, 1972). From its centre of origin, taro spread eastward to the rest of South East Asia, and to China, Japan and the Pacific Islands (some authors have suggested that the island of New Guinea may have been another centre of origin for taro, quite distinct from the Asian centre). From Asia, taro spread westward to Arabia and the Mediterranean region. By 100 B.C., it was being grown in China and in Egypt. It arrived on the east coast of Africa over 2,000 years ago; it was taken by voyagers, first across the continent to West Africa, and later on slave ships to the Caribbean. Today, taro is pan-tropical in its distribution and cultivation. The greatest intensity of its cultivation, and its highest percentage contribution to the diet, occurs in the Pacific Islands. However, the largest area of cultivation is in West Africa, which therefore accounts for the greatest quantity of production. Significant quantities of taro are also grown in the Caribbean, and virtually all humid or sub-humid parts of Asia.
It has been suggested that the eddoe type of taro was developed and selected from cultivated taro in China and Japan several centuries ago, and it was later introduced to the West Indies and other parts of the world (Purseglove, 1972).
Taro is a herbaceous plant which grows to a height of 1-2m. The plant consists of a central corm (lying just below the soil surface) from which leaves grow upwards, roots grown downwards, while cormels, daughter corms and runners (stolons) grow laterally. The root system is fibrous and lies mainly in the top one meter of soil.
In the dasheen types of taro, the corm is cylindrical and large. It is up to 30cm long and 15cm in diameter, and constitutes the main edible part of the plant. In eddoe types, the corm is small, globoid, and surrounded by several cormels (stem tubers) and daughter corms. The cormels and the daughter corms together constitute a significant proportion of the edible harvest in eddoe taro. Daughter corms usually give rise to subsidiary shoots even while the main plant is still growing, but cormels tend to remain dormant and will only give rise to new shoots if left in the ground after the death of the main plant. Each cormel or each daughter corm has a terminal bud at its tip, and axillary buds in the axils of the numerous scale leaves all over its body.
Corms, cormels and daughter corms are quite similar in their internal structure. The outmost layer is a thick brownish periderm. Within this lies the starch-filled ground parenchyma. Vascular bundles and laticifers ramify throughout the ground parenchyma. Idioblasts (cells which contain raphides or bundles of calcium oxalate crystals) also occur in the ground tissue, and in nearly all other parts of the taro plant. The raphides are associated with acridity or itchiness of taro, a factor which will be taken up in greater detail when the utilisation of taro is discussed. The density and woodiness of the corm increase with age.
Occasionally in the field, some taro plants are observed to produce runners. These structures grow horizontally along the surface of the soil for some distance, rooting down at intervals to give rise to new erect plants.
In both eddoe and dasheen types of taro, the central corm represents the main stem structure of the plant. The surface of each corm is marked with rings showing the points of attachment of scale leaves or senesced leaves. Axillary buds are present at the nodal positions on the corm. The apex of the corm represents the plants growing point, and is usually located close to the ground level. The actively growing leaves arise in a whorl from the corm apex. These leaves effectively constitute the only part of the plant that is visible above ground. They determine the plants height in the field.
Each leaf is made up of an erect petiole and a large lamina. The petiole is 0.5-2m long and is flared out at its base where it attaches to the corm, so that it effectively clasps around the apex of the corm. The petiole is thickest at its base, and thinner towards its attachment to the lamina. Internally, the petiole is spongy in texture, and has numerous air spaces which presumably facilitate gaseous exchange when the plant is grown in swampy or flooded conditions. For most taro types, the attachment of the petiole to the lamina is peltate, meaning that the petiole is attached, not at the edge of the lamina, but at some point in the middle. This peltate leaf attachment generally distinguishes taro from tannia which has a hastate leaf i.e. the petiole is attached at the edge of the lamina. An important exception to this rule are the piko group of taro found in Hawaii; quite uncharacteristically, they have hastate leaves.
The lamina of taro is 20-50cm long, oblong-ovate, with the basal lobes rounded. It is entire (not serrated), glabrous, and thick. Three main veins radiate from the point of attachment of the petiole, one going to the apex, and one to each of the two basal lamina lobes. Some prominent veins arise from the three main veins, but the overall leaf venation is reticulate (net-veined).
Natural flowering occurs only occasionally in taro, but flowering can be artificially promoted by application of gibberellic acid (see later). The inflorescence arises from the leaf axils, or from the centre of the cluster of unexpanded leaves. Each plant may bear more than one inflorescence. The inflorescence is made up of a short peduncle, a spadix, and spathe. The spadix is botanically a spike, with a fleshy central axis to which the small sessile flowers are attached. The spadix is 6-14cm long, with female flowers at the base, male flowers towards the tip, and sterile flowers in between, in the region compressed by the neck of the spathe. The extreme tip of the spadix has no flowers at all, and is called the sterile appendage. The sterile appendage is a distinguishing taxonomic characteristic between dasheen and eddoe types of taro. In eddoe types, the sterile appendage is longer than the male section of the spadix; in dasheen types, the appendage is shorter than the male section.
The spathe is a large yellowish bract, about 20 cm long, which sheathes the spadix. The lower part of the spathe wraps tightly around the spadix and completely occludes the female flowers from view. The top portion of the spadix is rolled inward at the apex, but is open on one side to reveal the male flowers on the spadix. The top and bottom portions of the spadix are separated by a narrow neck region, corresponding to the region of the sterile flowers on the spadix.
Pollination in taro is probably accomplished by flies. Fruit set and seed production occur only occasionally under natural conditions. Fruits, when produced, occur at the lower part of the spadix. Each fruit is a berry measuring 3-5mm in diameter and containing numerous seeds. Each seed has a hard testa, and contains endosperm in addition to the embryo.
Taro is herbaceous, but survives from year to year by means of the corms and cormels. Root formation and rapid root growth take place immediately after planting, followed by rapid growth of the shoot. Shoot growth and total shoot dry weight show a rapid decline at about six months after planting. At this time, there is a reduction in the number of active leaves, decrease in the mean petiole length, a decrease in the total leaf area per plant, and a decrease in the mean plant height on the field. All through the season, there is a rapid turnover of leaves; new ones are continually unfurling from the centre of the whorl of leaves, as the oldest ones below die off. Such a high rate of leaf obsolescence is physiologically wasteful.
Corm formation commences at about three months after planting; cormel formation follows soon afterwards in cultivars that produce appreciable cormels. By the sixth month when shoot growth declines, the corm and cormels become the main sink and grow very rapidly. As the adverse (dry) season sets in, the decline of the shoot accelerates, until the shoot finally dies back. The corm and cormels permit the plant to survive through the adverse season. If they are not harvested, they will sprout and give rise to new plants at the onset of the next favourable season. Where there is no adverse season, the shoot may fail to die back, and instead persist and continue growth for several years.
Flowering, in the few instances where it happens naturally, occurs in the early part of the season.
Partly because of their large transpiring surfaces, taro plants have a high requirement for moisture for their production. Normally, rainfall or irrigation of 1,500-2,000mm is required for optimum yields. Taro thrives best under very wet or flooded conditions. Dry conditions result in reduced corm yields. Corms produced under dry conditions also tend to have a dumb-bell shape; the constrictions reflect periods of reduced growth during drought.
Taro requires an average daily temperature above 21ºC for normal production. It cannot tolerate frosty conditions. Partly because of its temperature sensitivity, taro is essentially a lowland crop. Yields at high altitudes tend to be poor. In Papua New Guinea, for example, the maximum elevation for taro cultivation is 2,700m.
The highest yields for taro are obtained under full intensity sunlight. However, they appear to be more shade-tolerant than most other crops. This means that reasonable yields can be obtained even in shade conditions where other crops might fail completely. This is a particularly important characteristic which enables taro to fit into unique intercropping systems with tree crops and other crops. Daylight also affects the growth and development of taro. The formation of corms/cormels is promoted by short-day conditions, while flowering is promoted by long-day conditions.
Taro is able to tolerate heavy soils on which flooding and waterlogging can occur. Indeed, the dasheen type of taro does best when grown in such soils. It seems that under flooded or reducing soil conditions, taro plants are able to transport oxygen (through their spongy petioles) from the aerial parts down to the roots. This enables the roots to respire and grow normally even if the surrounding soil is flooded and deficient in oxygen. In practice, however, flooded taro fields must be aired periodically in order to avoid iron and manganese toxicity under the reducing soil conditions. Poor soils, such as the red soils in certain parts of Fiji, tend to give low yields of taro.
Taro does best in soil of pH 5.5-6.5. It is able to form beneficial associations with vesicular-arbuscular mycorrhizae, which therefore facilitate nutrient absorption. One particularly useful characteristic of taro is that some cultivars are able to tolerate salinity. Indeed, in Japan and Egypt, taro has been used satisfactorily as a first crop in the reclamation of saline soils (Kay, 1973). This definitely opens up the possibility for the use of taro to exploit some difficult ecologies where other crops might fail.
Flowering and seed set in taro are relatively rare under natural conditions. Most plants complete their field life without flowering at all, and some cultivars have never been known to flower. For many years, this characteristic was a great hindrance to taro improvement through cross pollination. However, the problem was solved when it was discovered that gibberellic acid (GA) could promote flowing in taro (Wilson, 1979).
Essentially, plants are grown from corms or cormels to the 3-5 leaf stage in the field, and then treated with 15,000 ppm GA, a process known as pro-gibbing (Alvarez & Hahn, 1986). Alternatively, the plants could be multiplied in a seedbed, and pro-gibbed at the 1-2 leaf stage with 1,000 ppm GA. A third method involves leaving taro in the field at the end of the growing season and then pro-gibbing the first leaves that emerge at the onset of the next rainy season. Whichever method is used, pro-gibbed plants produce normal flowers 2-4 months after treatment.
Today, researchers are able routinely to induce flowering of both taro and tannia by the application of GA. Controlled pollination can then be carried out on the flowers that are produced. The resulting seeds, thousands per spadix, are first germinated in nutrient media in petri dishes. The plantlets are later transplanted to humid chambers in the greenhouse. When the seedlings have reached a height of 15-20cm, they can be transplanted to the field. The large genotypic and phenotypic variability resulting from this process affords the plant breeder ample scope for selection.
Another propagative technique which has recently been used for taro is the production of plantlets through meristem tissue culture. Essentially, the technique involves excising the tip meristem of taro, sterilising it, and culturing it in sterile nutrient medium in petri dishes. The cultured meristem first proliferates a mass of callus tissue, from which bits can be taken for subculturing to produce plants with roots and shoots. If desired, the plantlets can later be transferred to pots in the greenhouse, and eventually to the field. The multiplication of taro by tissue culture has several distinct advantages:
a) It provides an extremely rapid means for multiplying elite clones. Starting from one plant, it is possible to produce a million or more plantlets in a year;
b) It affords a phytosanitary method for producing disease-free material. This factor relies partly on the fact that the culture starts with the extreme meristem tip which is as yet free from various disease organisms. This method, for example, has been used successfully to eliminate dasheen mosaic, alomae, and bobone virus diseases from taro;
c) The tissue culture technique provides a handy yet phytosanitary method for international and inter-regional transfer of germplasm;
d) The technique provides an economical, space-saving, and labour-saving method for the preservation of germplasm over long periods of time. Rather than repeatedly growing germplasm collections in the field, they can be stored as tissue culture in nutrient media. Only occasionally (once in several months) does the material need to be re-cultured; and even then, the space, time and labour consumption for the exercise are minimal.