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Reynolds, B.D., W.J. Blackmon, E. Wickremesinhe, M.H. Wells, and R.J. Constantin. 1990. Domestication of Apios americana. p. 436-442. In: J. Janick and J.E. Simon (eds.), Advances in new crops. Timber Press, Portland, OR.

Domestication of Apios americana

B.D. Reynolds, W.J. Blackmon, E. Wickremesinhe, M.H. Wells, and R.J. Constantin

  1. INTRODUCTION
    1. History
    2. Description
  2. GERMPLASM MANIPULATION
    1. Selection Pressure and Techniques
    2. Germplasm Acquisition
    3. Field Evaluation and Selections
    4. Open Pollination
    5. Controlled Crosses
    6. In vitro Techniques
  3. HORTICULTURE
    1. Cultural Techniques
    2. Pest Control
  4. NUTRITION AND PRODUCT DEVELOPMENT
    1. Nutrient Content
    2. Product Development
  5. CONCLUSIONS
  6. REFERENCES
  7. Fig. 1
  8. Fig. 2
  9. Fig. 3
  10. Fig. 4
  11. Fig. 5

INTRODUCTION

History

When European explorers first visited the New World they found the naives eating the seeds and tubers of Apios americana Medikus (apios, groundnut). Ethnohistoric records for eastern North American Indians probably cite this species for its use as food more often than any other kind of tuber. Despite its wide use, most Indians gathered apios from the wild, and although some tribes transplanted them near their campsites, apios probably was never cultivated (Beardsley 1939).

Apios may have reached Europe as early as 1597 (Seabrook 1973). It was evaluated in 1845 during the potato famine in Ireland (National Academy of Sciences 1979) and was listed as a garden crop in 1885 (Vilmorin-Andrieux 1885). These early lines usually took two years to give acceptable yields (apios is a perennial) and the impetus to further develop apios probably waned with the discovery of disease-resistant lines of potato (Solanum tuberosum). Considering the probability that under the conditions in which it was grown in Northern Europe, apios did not produce viable seed (Vilmorin-Andrieux 1885, Seabrook 1973), it seems likely that early attempts to improve apios only involved evaluations of tubers selected from the wild for performance under different cultural conditions. In 1985 experiments were initiated by the Louisiana Agricultural Experiment Station to evaluate the food-crop potential of apios (Blackmon and Reynolds 1986).

Description

Apios is a nitrogen-fixing legume that is native to and distributed throughout eastern North America (Fig. 1) from Canada to southern Florida (Seabrook and Dionne 1976, Woods 1988). Its counter-clockwise-twining vine may vary from 1-6 m in length. The succulent vine is killed by freezing temperatures and will deteriorate during the winter. Leaves are alternate, odd-pinnately compound, and most often exhibit five to seven leaflets (Fig. 2). Flowers have a complex structure typical of many highly specialized papilionoid legumes. They are usually pink to purplish or brownish-red, about 12 mm long and occur in compact racemes 75 to 130 mm long (Fig. 2). Apios flowers are distinct in having a relatively large concave standard with a small hood at its apex into which the narrow sickle-shaped keel is hooked. The keel contains the style and the diadelphous stamens. The intricate floral structure involves an explosive tripping mechanism that usually requires insect activation. Bean pods are 50 to 130 mm long (Fig. 3). Apios produces numerous white-fleshed tubers along its rhizomes (Figs. 2 and 5). It is most often found in moist areas near streams or bodies of water where it can get full sunlight at least part of the day. Plants of apios have been observed to be extremely variable both within and among progenies from different locations (Blackmon and Reynolds 1986, Woods 1988). Ploidy is usually diploid (2n = 22); however, naturally occurring triploids have been observed (Seabrook and Dionne 1976, Bruneau and Anderson 1988).

GERMPLASM MANIPULATION

Selection Pressure and Techniques

Selection emphasis has been to obtain lines that produce large, closely-spaced tubers (short-rhizome types) during one season, and that yield well untrellised. Other traits such as the degree of twining (lack of twining is desirable when grown untrellised), and disease resistance to aerial web blight caused by Rhizoctonia solani Kuhn (M.C. Rush, pers. commun.) have also been given attention. Apios is also being studied for traits which might be beneficial to future breeding projects. For example, the seed pods of some lines are much more prone to shatter after they dry (Fig. 3). Since the current focus is on tuber production, this problem is currently circumvented by harvesting the seeds before pods are thoroughly dry.

A schema for the enhancement of apios germplasm which illustrates the techniques currently being utilized or under development is presented in Fig. 4. Gene Pool I represents a starting point. Conventional and in vitro techniques might be applied so that an enhanced germplasm is obtained. The sequence of events may be repeated for subsequent generations with perhaps a different emphasis allotted to the various techniques.

Germplasm Acquisition

When research on apios was initiated in 1985, the only germplasm available was that from its native habitat (Fig. 1). As a result of the wide native distribution of apios and the relatively small sample that can be evaluated it would be beneficial to develop a plan to locate superior germplasm.

Apios flourishes along streams and rivers and its tubers are readily distributed downstream as stream banks are eroded. Hence, the various river drainage areas represent natural habitats for apios with the foci for genetic diversity occurring near the river mouths. In addition, old Indian campsites might harbor remnants of selected germplasm if the Indians who lived there and ate apios exerted any selection pressure for its improvement.

Those states from which germplasm has been collected are indicated in Fig. 1. A large majority of the germplasm tested was collected from the lower Mississippi valley, followed by Florida and the Outer Banks of North Carolina. An association of blooming dates with latitude of collection site has been observed. When planted in Louisiana, the more northern lines tend to bloom earlier than southern lines.

Where possible, seeds instead of tubers are collected from wild sites. Seeds are easier to harvest and store, and the variability of a population is more likely to be expressed from plants coming from seeds than from tuber-derived plants. Visual evaluation of tubers randomly harvested from wild sites has been unreliable for predicting performance under field conditions.

After drying at room temperature seeds are sealed in glass jars and stored in a freezer at -10°C. After harvest, tubers are washed, allowed to air dry until surface moisture has evaporated, and stored in plastic bags at 5°C in the dark until used. Tubers left in a low humidity environment will desiccate and die. Tubers have dormancy properties that appear to be associated with genotype and possibly the relative positions along the rhizome. Storage at 5deg.C for two to three months is generally sufficient to break or reduce dormancy, although germination after planting is seldom uniform. No dormancy for seeds has been observed.

Field Evaluation and Selections

Regardless of origin germplasm must be evaluated under anticipated cultural conditions. It is the phenotypic expression of the plant at this level which determines usefulness of a plant as a crop. Early attempts at evaluation with hopes that useful markers could be identified were made by trellising individual plants, but this is expensive and laborious. No useful markers have been found. At present plants are evaluated untrellised with primary emphasis on yield and short rhizomes with closely spaced tubers (Fig. 5). Plants with long rhizomes are difficult to cultivate and harvest. When grown untrellised, a high degree of twining is also undesirable, as plants may twine on themselves and each other reducing leaf surface area available and creating an environment in the dense canopy that may encourage diseases. Plants in field evaluations originate from several sources, tubers and seeds collected from feral plants, tubers selected after field evaluations, seeds produced in field nurseries, a few seeds obtained after hand crosses, and a few tissue-culture-derived plants. Observations for date of blooming, vine vigor, and special traits are made during the growing season, and critical selection is made at tuber harvest. Unfortunately plants grown from tubers harvested from seed-grown plants often do not have as closely spaced tubers as the original selection.

Open Pollination

Four factors appear important in obtaining seeds: compatible lines so that crossing may occur; pollinator insects; control of harmful insects, particularly stink bugs (Acrosterum hilare), from flowering to seed maturity, and adequate moisture during seed maturation. Trellising is also advantageous because plants often grow better and seeds are not in contact with the ground. A reduced tuber yield appears to be associated with heavy seed production. The largest proportion of plants from seeds tested during the 1988 season were obtained from open pollinations in breeding nurseries. Semi-controlled crosses can be facilitated by pruning to achieve simultaneous flowering of clones that would not cross otherwise because of asynchronous bloom dates.

Controlled Crosses

Success with hand pollinations has been limited. Apios produces numerous flowers in a raceme, however, its complex floral structure associated with an explosive tripping mechanism renders hand pollinations laborious (Fig. 2). Most hand pollinations do not result in pod set and many pods that do set later abort or do not contain viable seeds. Given the number of unknowns involved and the fact that the pod set per flower in open-pollinated nurseries is usually less than 10%, it is not surprising that hand pollination results in low seed yield. Occasionally for open pollinations by insects seven or eight pods will be set sequentially along a raceme. These observations imply a potential for increased seed set from hand pollination if conditions favoring fertilization can be identified. A first requirement is starting with mutually compatible lines (Bruneau and Anderson 1988).

Virtually nothing has been reported about the genetics of apios. Genetic studies have been made difficult by the inability to readily make specific crosses and by the rapid turnover in the gene pool resulting from the selection process.

In vitro Techniques

Micropropagation systems of apios have been developed (Reynolds and Blackmon 1986; Wickremesinhe et al 1988a, b). Soma-clonal variation has not been studied because variability inherent in wild and nursery-grown seeds exceeds the current evaluation capacity.

HORTICULTURE

Cultural Techniques

Most of the research involving cultural practices has been directed towards developing techniques to screen large numbers of plants. Direct-seeding has presented problems. Seeds may take 10 to 30 days to germinate. Seedlings are small and early seedling growth is not vigorous. Seedling death, presumably from insects or diseases, has plagued this technique for starting apios. The most satisfactory method has been to start plants in peat pellets. After germination, when the shoots begin elongation, the plants are pinched back to the first leaves. This prevents the plants in a flat from twining on each other, allows for better root development prior to planting, and permits plants from slower germinating seed to reach sufficient size to transplant. However, pinching back carries a potential risk of spreading disease among the seedlings. Weak seedlings can be discarded at this stage.

Tubers are planted intact. The buds that give rise to the shoots and rhizomes occur at the distal end of the tubers. The potential of dividing tubers into sections prior to planting needs evaluation. Generally the larger the tuber, the more rapid the early growth.

Seeds may be harvested from the time the pods first begin to dry. If left on the vine too long some pods will shatter (Fig. 3). Tubers are harvested after frost. Since most of the plants are different (originating from seeds), the tubers are harvested with a shovel to insure that genotypes can be evaluated individually. Fortunately, tubers can remain in the soil for extended periods without rotting even under water-logged conditions, thus allowing an extended harvest period.

Although apios in its native habitat is found growing on water-logged and acidic soils (Reed and Blackmon 1985), observations under field conditions indicate that apios grows best on well-drained soils. A pH less than 5 or as high as 8 may also be detrimental to growth. Adequate moisture is important, but excess moisture encourages longer rhizomes.

Pest Control

Weed control is the most labor-limiting step affecting the number of plants that may be evaluated. Prior to planting, beds are fumigated under black plastic with methyl bromide. The black plastic is removed before planting. Using black plastic as a mulch has proven unsatisfactory as the tender vines of untrellised plants may be severely damaged from contact with the hot plastic on sunny days. In addition to its usefulness for weed control fumigation with methyl bromide is beneficial where a heavy nematode infestation exists. Because methyl bromide is used to fumigate beds, planting material is routinely inoculated with cowpea inoculant. Seeds are inoculated prior to planting into peat pellets. Putnam et al. (pers. commun.) are studying the efficiency of various Rhizobium species for inducing nodulation.

No herbicides are presently labeled for use in apios. Wells et al. (1989) have identified several preemergence herbicides which have reduced the hand cultivation required. Bentazon (Basagran), sethoxydim (Poast), and fluazifop (Fusilade) have been used satisfactorily as postemergence treatments.

NUTRITION AND PRODUCT DEVELOPMENT

Nutrient Content

A crude protein content (16.5% of dry weight), about three times that of potato, has been reported for tubers (Yanosky and Kingsbury 1938, Walter et al. 1986). Studies by Cornelio (1987) and Wilson et al. (1987) found the crude protein to be 25-30% for seeds and 11-14% for tubers on dry defatted basis. Aspartic and glutamic acids were the predominant amino acids in both seeds and tubers. The relative balance of essential amino acids was found to be excellent except for cystein and methionine, which are usually low in legumes. Tryptophan levels were not measured.

An analysis of apios by Wilson et al (1986) found seeds contained 12-18% and tubers approximately 4% lipid on a dry weight basis. Fatty acid profiles of seeds and tubers differed; however, linoleic acid predominated. There appears to be variability of nutritional components among lines which we hope to exploit as a selection parameter.

In rat-feeding studies, cooked tubers consistently performed better than raw tubers, suggesting the necessity of cooking tubers when using as a food source (Johnson 1988). Pancreatic hypertrophy of the rats consuming raw apios indicates the presence in the tubers of protease inhibitors which can be destroyed by heat.

Product Development

Product development has received only limited attention. Some effort has been given to recipe development. When boiled, the taste of the tubers resembles an imaginary cross between a boiled peanut (Arachis hypogae) and potato with a texture mealier than potato. The taste appears to appeal to a significant number of people. First utilization by consumers is envisioned to center on dishes prepared from tubers. The tubers make excellent chips and the low levels of reducing sugars (D. Picha, personal communication) essentially eliminate a tendency to brown from cooking. Preliminary results also indicate excellent baking properties when apios is combined with corn meal or wheat flour. This combination potentially provides a better amino acid balance than either food item used alone.

CONCLUSIONS

Although more than 20,000 edible plants are known, most of the world's food supply comes from about 20 species (Vietmeyer 1986). Last century, America's leading botanist, Asa Gray (1874), stated his opinion that if advanced civilization had begun in America, apios would have been the first developed edible tuber crop and would have held its place in competition with potato and sweet potato.

In trying to appreciate the nature of the research with apios it must first be recognized that this is not a scenario in which an established crop is translocated into a different area and thus becomes a "new crop." Instead a "wild" plant has been selected for domestication. Hence, desirable plant types, cultural techniques, products, and markets must all be developed. This is an intimidating challenge. However, progress since 1985 has been very promising and has reinforced our conviction that domestication of apios could be successful.

Several accessions have been identified with traits that make them potentially useful as breeding lines. Line LA85-034 has been a very consistent producer under different growing conditions and is being considered for release for use in home gardens. The total number of plants screened is less than 20,000. This represents only a minuscule sample of the overall germplasm. Future research will continue to concentrate on the selection and interbreeding of superior lines. Efforts will also be placed on developing techniques for controlled crosses and the generation of polyploids with colchicine. In the area of product development, in addition to efforts with recipe development, research is planned to evaluate uses of flour made from apios seeds and tubers.

REFERENCES

Fig. 1. Approximate natural distribution of Apios americana (Woods, 1988). Numbers indicate the germplasm samples tested in Louisiana from various states and Canada as of 1988.

Fig. 2. Composite drawing of Apios americana Under low light intensity the opposite leaflets are nearly in a plane with the petiole. The phototoxic response in bright sunlight creates an acute angle between opposite leaflets. Two views of an individual flower are inset. Bar represents 5 mm. For ranges in variability see Blackmon and Reynolds (1986) and Woods (1988).

Fig. 3. Pods and seeds of Apios americana. Pods to the right side have shattered. Ruler in cm.

Fig. 4. Schema for enhancement of germplasm of Apios americana.

Fig. 5. Tubers produced from a plant of Apios americana grown from a seed. Note the close spacing of tubers along the rhizome to the left and the fused tubers to the right. These are considered desirable traits. The lighter areas on the tubers coincide with the locations of lenticels. Roots form at these locations when tubers sprout. Ruler is in cm.

Last update September 4, 1997 by aw