Table of Contents
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,
Domestication of Apios americana
B.D. Reynolds, W.J. Blackmon, E. Wickremesinhe, M.H. Wells, and R.J. Constantin
- GERMPLASM MANIPULATION
- Selection Pressure and Techniques
- Germplasm Acquisition
- Field Evaluation and Selections
- Open Pollination
- Controlled Crosses
- In vitro Techniques
- Cultural Techniques
- Pest Control
- NUTRITION AND PRODUCT DEVELOPMENT
- Nutrient Content
- Product Development
- Fig. 1
- Fig. 2
- Fig. 3
- Fig. 4
- Fig. 5
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).
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
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.
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.
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.
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
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
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.
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
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.
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.
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 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.
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
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
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.
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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
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