Helianthus tuberosus (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Helianthus tuberosus L.

Protologue: Sp. pl.: 905 (1753).

Family: Compositae

Chromosome number: 2n= 102

Vernacular names

• Jerusalem artichoke (En).
• Topinambour, navet de Jérusalem (Fr)
• Thailand: thantawan-hua (Bangkok)
• Vietnam: quỳ dọi.

Origin and geographic distribution

Jerusalem artichoke originates from America. Wild relatives are found in the eastern parts of North America. The cultivated types may have developed in southern Canada, from where they were dispersed to western Europe early in the 17th Century. Soon they were spread over many of the temperate parts of the northern hemisphere. In the tropics, including South-East Asia, it is only occasionally cultivated.

Uses

The most important part is the edible tuber, which may serve in vegetable dishes. Certain parts of France and Germany used to grow H. tuberosus for alcohol production, but this has been superseded by cheaper raw materials. Since the 1980s, there has been a revival of research on the industrial processing of H. tuberosus for fuel and chemicals. Pigs, chickens and rabbits relish the raw tubers. The fresh foliage may serve as a forage, mainly for horses, mules and ruminants. The withered, almost leafless aboveground material of mature plants is hardly nutritious and can be used for litter or fuel. If the foliage is harvested or grazed in an early stage, the nutritive value will be good but the production of tubers will suffer. The green foliage is not especially liked by animals because it is rough-haired and stalky, but it will be rejected only if more tasty feed is available. Early-flowering cultivars are popular as ornamentals.

Production and international trade

Although H. tuberosus has several advantages over Irish potato (Solanum tuberosum L.) such as non-toxic foliage, sturdy erect stems, lower susceptibility to fungal diseases, and frost-tolerant tubers, it has never reached widespread commercial acceptance.

Properties

The boiled tubers, somewhat soggy, have a sweetish taste found pleasant by some, wearisome by most, and even foul by a few people. Thus, no more than a few consumers appreciate the tubers as an occasional vegetable. The main constituent in the dry matter is inulin, a chain-like polysaccharide of fructose units, each chain with one terminal glucose unit. In water it forms a viscous non-particulate "solution", qualities which are useful for providing body to drinks.

The tubers have a nutritive value similar to that of other tubers, although in spite of its high degree of dispersion the inulin is digested very slowly and passes through the small intestine. It is then hydrolysed in the colon by bacterial action, thereby keeping the colon microflora in a healthy state. The similarity with the functioning of fibre has led to the term "dietary fibre". It may explain the wholesomeness experienced by some consumers, notably diabetics, who have used H. tuberosus in their diet. This is certainly not attributable to some content of insulin in the tubers, as has sometimes been thought. The resemblance of both names (inulin and insulin) may have led to this erroneous belief.

Typical composition figures of the tubers per 100 g fresh material are: water 79.6 g, protein 1.5 g, fat 0.2 g, extractable N-free matter 16.9 g, fibre 0.7 g, ash 1.1 g. The starch equivalent is 16.4. With regard to feed, the composition of 100 g fresh foliage is: water 80 g, protein 2.0 g, extractable N-free matter 13.1 g, fibre 2.2 g, ash 2.7 g. The starch equivalent is 16.2.

Description

• Robust, erect, perennial herb, in cultivation usually grown as an annual, up to 3 m tall, scarcely to moderately branched in upper half of stem, hirsute in most aboveground parts. Roots adventitious (in plants not grown from seed), fibrous, spreading deeply.
• Tubers formed by thickening of short and stout or long and slender underground stolons, ellipsoid to globose, 2-8(-15) cm × 3-6 cm, whitish, yellow, red or purple, with small scale leaves and axillary buds.
• Leaves opposite or in whorls of 3 in lower plant part, in upper part alternate, simple; petiole 2-4 cm long, winged above; blade ovate to ovate-lanceolate, 10-20 cm long, base tapering into petiole, margin irregularly serrate, apex acute, veins prominent with 3 main veins.
• Inflorescence a head, 4-8 cm in diameter, few together in a leafy panicle 8-20 cm long; involucral bracts in several rows, lanceolate, long acuminate, subequal, 15-17 mm × 4 mm, ciliate, blackish outside; receptacle flat, 1.5-2 cm in diameter; outer ray florets sterile, with golden-yellow, ligulate corolla, elliptical to oblong, 2.5-4.5 cm × 1 cm; disc florets bisexual, with tubular bright yellow corolla, 6-7 mm long; sterile bracts pale, 8-9 mm long, with greenish-yellow apex; stamens 5; style slender, with 2-lobed stigma.
• Fruit an achene, oblongoid, 5-7 mm long, flattened at the sides, brownish with dark stripes, thinly hairy.

Growth and development

Nearly all documented experience with H. tuberosus is from areas between latitudes 30-50°N. This means that estimation of the potential of H. tuberosus for South-East Asia is based on extrapolation of data from elsewhere. Tuber formation starts with a drastic drop (dormancy) of the elongation rate near the stolon apex. If this rate drops to zero, the tuber will be pear-shaped, tapering backwards. If dormancy is less deep, the top end of the tuber will be oval. If the lateral eyebuds on the developing tuber retain or regain some elongation activity, very irregular tuber shapes will arise. The dormancy behaviour of the different buds and their interaction depend on the cultivar, but are also influenced by soil temperature and texture. Short stolons and early tuber initiation lead to an unwieldy, tightly-packed bunch of tubers. Long stolons and late initiation cause a very loose pattern of tubers wide apart that easily become lost at lifting.

Normally, fruits (seeds) are rarely formed. Under reduced light intensity or cold conditions, no flower heads may appear. Usually, a crop reaches maturity in 3-6 months.

Near the equator, the shorter daylength causes foliage growth to stop earlier and tuber initiation to start sooner, resulting in smaller plants. Moreover, temperatures are more even, so that cool or cold periods in the juvenile and closing phases of the production cycle are absent. Therefore, leaf area reaches full size sooner, but leaves senesce earlier. Also underground functions such as tuber growth, translocation of assimilates from the foliage to the tubers, and mineral uptake are faster near the equator. As a result, two consecutive crops may be produced per year.

Other botanical information

H. tuberosus varies considerably in time to flowering, form and colour of the tubers, leaf form and hairiness. Well-known cultivars include "Fuseau" (tubers purple, smooth, easy to peel), "New White" (tubers white), "Boston" (tubers red), and "Dwarf Sunray" (plants relatively small).

Ecology

Most H. tuberosus cultivars respond to short days with earlier tuber and flowerbud initiation. The growing season needs at least 125 frost-free days, preferably with average temperatures of 18-26°C and an evenly distributed rainfall of up to 1250 mm. It is tolerant of drought and survives short periods of flooding. Tubers are frost resistant, in the soil as well as in storage. In the tropics, H. tuberosus is preferably grown at 300-750 m altitude, but in India it is cultivated up to 3600 m. The soil should preferably not be too heavy, and should be well drained and friable in order to facilitate harvesting of the tubers.

Propagation and planting

A tuber weight class of 50-60 g is suitable for seed tubers. They need a very cool (storage) period to overcome dormancy and uneven sprouting. In the warm areas of South-East Asia too few fully adapted cultivars are available, and therefore imported seed tubers must be used. It may be advisable to grow them in the pilot stage in cooler parts at altitudes from 800-1400 m. If successful, one could extend to lower areas. Optimal spacing is 8-12 plants per m², but closer spacing should be practised where harvesting is envisaged before ripeness or where weed competition is expected to be excessive.

Husbandry

If the growth cycle of H. tuberosus proceeds under a daylength shorter than in the region of origin, one should grow types known to be leafy (low harvest index) and known to have loose, widely separated tubers in the region of origin. These undesirable traits will be undone by the reduced growth and faster development under the prevailing short-day conditions. Hand weeding is advisable in the early growth stage, followed by interrow hoeing lightly on the surface so as not to damage roots and stolons. If unavoidable, a herbicide can be used. Once the plants have formed a closed canopy, further weed growth will be suppressed since H. tuberosus is a strong competitor; intercropping seldom succeeds, for the same reason. It responds favourably to fertilization and irrigation, but it is tolerant of suboptimal supply of nutrients.

Diseases and pests

Expanding the acreage of H. tuberosus in South-East Asia will cause an increase of disease and pest damage to the crop. Most of the parasites of H. tuberosus belong to the types with a wide range of hosts: Pseudomonas syringae, Fusarium, Sclerotinia, Puccinia and Botrytis. These will cause more damage if H. tuberosus remains for years as a perennial in the same field.

Harvesting

Normally, tubers are ready for harvesting when the leaves begin to wither. For home consumption they can be lifted manually with a fork when required, because they store very well in the field. Tubers less than 2 cm in diameter tend to be left on the ground; if hand-harvested, they are easily overlooked and during mechanical harvesting, they are lost by the sieve.

Yield

In temperate climates a normal tuber yield is 30 t/ha per year (tuber dry matter 6 t/ha). Yield expectations for South-East Asia can only be guessed. Assuming 120 days of closed and productive canopy with a daily dry matter gain of 120 kg/ha, total dry matter would amount to 14 t. During this period, tuber growth might proceed during 80 days, taking 75% of the daily gain, resulting in a final 7.2 t/ha of dry matter in the tubers. If weeds, parasites and harvesting losses take 30%, about 5 t/ha of tuber dry matter would remain.

Handling after harvest

The skin of the tubers is tender and moisture is readily lost. Plastic sheets can be used during transport and storage to protect them from drying out. Tubers can be stored for 2-5 months at 0°C and 90-95% relative humidity.

Genetic resources

Germplasm is maintained in a number of research institutions in North America and Europe.

Breeding

Breeding should concentrate on improving some of the weaknesses of the tuber, i.e. aim for better taste, sturdier skin and improved keeping quality. Moreover, leaf longevity should be improved, the harvest index should be raised and damage by local parasites reduced or eliminated. If industrial processing is contemplated, a higher inulin content is advantageous.

Prospects

Prospects for H. tuberosus are not bright. Several attempts in northern countries have petered out. Other plants are preferred for the production of inulin. Yet, a genetic breakthrough or a specific regional set of farming conditions might give it a better place.

Literature

• Ahmed, Z. & Bhatti, M.H., 1985. Jerusalem artichoke: a source of food, feed and fuel. Progressive Farming (Pakistan) 5(2): 24-25.
• Baillargé, E., 1942. Le topinambour. Ses usages, sa culture [Jerusalem artichoke. Its uses and culture]. Flammarion, Paris, France. pp. 128-135.
• Bauer, H.A. & Lasso, R.H., 1974. El cultivo del topinambur [The cultivation of Jerusalem artichoke] (Helianthus tuberosus L.). Informacion tecnica No 58. Estacion Experimental Agropecuaria Manfredi. Instituto Nacional de Technologia Agropecuaria, Cordoba, Argentina. 9 pp.
• Fuchs, A. (Editor), 1993. Inulin and inulin-containing crops. Elsevier, Amsterdam, the Netherlands. 417 pp.
• Gosse, G. & Grassi, G. (Editors), 1991. Topinambour (Jerusalem artichoke). 2nd Workshop, Rennes, France. December 1988. Energic Series No 13405. Office for Official Publications of the European Communities, Luxembourg. 150 pp.
• Messiaen, C.M., 1975. Le potager tropical [The tropical vegetable garden]. Part 3. Presses Universitaires de France. pp. 478-480.

Authors

G.J. Vervelde