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Monocotyledons / Monocotiledóneas
Dioscorea alata L.
EOL Text
introduced; Fla., Ga.; Southeast Asia; cultivated worldwide.
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Rights holder/Author | eFloras.org Copyright © Missouri Botanical Garden |
Source | http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200028084 |
More info for the terms: fire frequency, fire regime, frequency, ladder fuels, severity, shrubs, tree, vines
Yams occur in a variety of habitats (see Habitat Types and Plant Communities). Altered fire frequency, severity, or behavior in habitats invaded by yams was not described in the available literature (2009). However, yams typically grow into tree canopies ([30], reviews by [51,66,78]). In areas where surface fires would have been common in the absence of yam vines, yam ladder fuels could encourage crown fires. Many field observations indicate that the weight of yams can break stems of supporting vegetation and cause mortality of trees and shrubs (reviews by [51,66,78]). Increased dead material in areas where yams have killed associated vegetation could increase fire frequency, intensity, or severity. Although these changes in FIRE REGIMES and behavior are speculative, they highlight the need for more information about how yams may affect the fire ecology of invaded habitats. For more information on the aggressive growth of yams and its effect on associated vegetation, see Impacts. See the Fire Regime Table for more information on FIRE REGIMES in vegetation communities in which yams may occur.
(Pinus elliottii or P. serotina) south of Tallahassee, FL.
Photo © Chris Evans, River to River CWMA, Bugwood.org
Cultivated throughout moist tropics.
License | http://creativecommons.org/licenses/by-nc-sa/3.0/ |
Rights holder/Author | eFloras.org Copyright © Missouri Botanical Garden |
Source | http://www.efloras.org/florataxon.aspx?flora_id=110&taxon_id=200028084 |
More info for the terms: adventitious, bulbil, density, fitness, litter, natural, phase, vine, vines
Sexual reproduction by yams has not been observed in the United States, where regeneration occurs exclusively through sprouting from tubers and bulbils [21,69]. Tubers provide for perennial regeneration within established populations, and aerial bulbils allow for dissemination of propagules that can establish new populations [44]. Flowers are rare or restricted to a single sex in most US areas. Only male Chinese yam flowers were observed in the field in Illinois [8]. Only female flowers are reported for air yams, water yams, and fiveleaf yams in North America [21,70]. Only 3 flowering air yam specimens occur in the University of Florida herbarium (review by [66]). During 10 years of observations made near the University of Singapore, Zanzibar yam fruits were rare [62].
Vegetative regeneration: Yam populations increase and persist through asexual regeneration. Although bulbil production is the primary means of regeneration, stems touching the ground are capable of producing adventitious roots and tubers [46,51]. Sprouting is possible even from small, fragmented bulbils and can occur in nearly any environment. Sprouts can develop from 2-week old bulbils (review by [78]), and small pea-sized bulbils or bulbil fragments are capable of producing new vines (Line 1992 personal communication cited in [66], review by [78]). Bulbil sprouts have been observed in environments without light, water, or soil (review by [3], Brinkley and Gann-Matzen 1992 personal communications cited in [66]). Sprouts have also been reported from discarded tuber or bulbil scraps or peels (review by [13]). A 2-pound (1 kg) water yam tuber can be divided into 112 pieces, all of which are capable of sprouting [60].
Bulbil production: Based on information from Chinese yam and air yam populations, bulbil production is little affected by light availability but may be affected by plant age and predation. Observations and experiments in Senai, Japan, indicate that Chinese yam produces bulbils with and without light [56]. Bulbil production can be substantial from mature plants. A review reports that Chinese yam vines produce an average of 20 bulbils/year [78], but a study in southern Illinois reported much greater production [8]. Along a Drury Creek tributary in Illinois' Giant City State Park, 18 of 50 Chinese yam vines produced bulbils, and production averaged 14.8 bulbils for each 3 feet (1 m) of stem length [77]. In Miami's Kendall Indian Hammock Park, volunteers collected 1,500 lbs (680 kg) of air yam bulbils in just 3 hours [30]. The number of volunteers was not reported.
Studies as of 2009 highlighted only plant age and predation as limits to yam bulbil production, although other factors likely affect production. In southern Illinois, 2 Chinese yam populations were described. Type 1 populations were the least productive and averaged 19.69 bulbils/plant; type 2 populations produced an average of 33.4 bulbils for each 3 feet (1 m) of stem length. The researcher suggested that type 2 populations were likely older and established much earlier than type 1 populations. Type 2 populations often occurred at old homesteads and may have been planted in the early 1900s, whereas type 1 populations likely represented newer, satellite populations [8].
In southern Illinois, squirrels that consumed portions of Chinese yam bulbils rarely killed the bulbils [77]. Those bulbils retaining 33% of their original size were capable of sprouting [8], but vines from fragmented bulbils produced fewer bulbils in the next growing season than vines from entire bulbils. Although production differences were not significant, bulbils with 66% of their mass removed produced vines that averaged 11.6 bulbils/plant, and vines from unfragmented bulbils averaged 23.7 bulbils/plant [8].
Bulbil dispersal: Although yam bulbils are easily dislodged from the parent [45], primarily dispersed by gravity, and often fall near the parent plant, secondary dispersal by water, animals, or humans can increase dispersal distance. During field studies in southern Illinois, most Chinese yam bulbils fell within 33 feet (10 m) of the source population. In one population an average of 152 bulbils/m² was recovered within 33 feet (10 m) of the center of the source population, and just 3 bulbils/m² occurred 36 to 98 feet (11-30 m) from the population center. Among all populations visited, the furthest bulbil occurred 72 feet (22 m) from the center of the source population [8].
Reviews report that mature yam bulbils float in water [13,51], but studies conducted in Florida found that most air yam bulbils sank in water (Pemberton and Overholt unpublished data cited in [3]). A review by Coursey [13] notes that immature yam bulbils are denser than water. Bulbil maturity was not reported for the research conducted in Florida [3]. In Great Smoky Mountains National Park, 1-year-old Chinese yam bulbils did not float but did move downstream through rolling, traction, and saltation actions [54]. Researchers in southern Illinois found that secondary dispersal by water occurred along streams in the Drury Creek Watershed. From December 2001 to February 2003, about 300 bulbils/m³ were collected from traps near the downstream boundaries of 3 Chinese yam populations, where stand density ranged from 73 to 200 vines/ha. Most recovered bulbils were viable. Just 19 of 180 marked bulbils were recovered. The maximum dispersal distance was 677 feet (206 m), measured in February following a major rainstorm. Researchers suggested that reported dispersal distances and bulbil densities likely represented minimum values due to poor downstream trapping and poor recovery of marked bulbils; they speculated that Chinese yam bulbils could disperse great distances in water [76].
Evidence of secondary dispersal of yam bulbils by rodents is somewhat speculative, and rodents may reduce bulbil viability. A review reports that some Chinese yam bulbils are dispersed when rodents drop bulbil pieces or fail to retrieve caches [78]. During field experiments conducted in central Japan's Kyoto University Kamigamo Experimental Forest, researchers found that rodents transported Japanese yam (Dioscorea japonica) bulbils 0.7 to 24 feet (0.2-7.4 m) from their initial location. Transport distance averaged 3.6 feet (1.1 m), and just 3% of the transported bulbils sprouted [49]. Although this species does not occur in the United States, similar dispersal is possible in North America.
In Great Smoky Mountains National Park, humans have dispersed Chinese yam bulbils. Previous Park residents recalled playing with and throwing Chinese yam bulbils (Beck and Ownby 1995 personal communications cited in [54]). Secondary dispersal likely also occurred through human collection of the odd-looking bulbils and from inadvertent kicking of the bulbils downhill [54].
Bulbil banking: Studies designed to determine the maximum amount of time a yam bulbil can remain viable in the soil, beneath litter, or in water were generally lacking (as of 2009). Reviews report that bulbils remain viable after 1 to "several" years on the vine or in the soil [51,66,78]. In a field study conducted in Illinois, a much larger percentage of Chinese yam bulbils remained viable when buried beneath leaf litter (76% sprouting after 1 year) than when buried in soil (21.2%), submerged in water (21.6%), or lying on top of leaves (5.2%) [77].
Sprouting and establishment from bulbils: Sprouting of yam bulbils often occurs with warm temperatures and moisture, but internal dormancy and time of sprouting are variable. There are reports of sprouts from bulbils without light, moisture, or soil contact ([3], Brinkley and Gann-Matzen 1992 personal communications cited in [66]). When bulbils were collected in the fall in Japan, nearly 100% of air yam and fiveleaf yam bulbils sprouted at 77 °F (25 °C), and almost all water yam bulbils sprouted at 86 °F (30 °C) [57].
Air yam: Warm temperatures and moisture are conducive to sprouting of entire and fragmented air yam bulbils (review by [45]). Although moisture maintained in leaf litter is considered beneficial for sprouting [77], one study found that humidity had little effect on sprouting [59]. Bulbils may sprout within a few weeks of detachment from the vine or may remain seasonally dormant. Cool, dry conditions discourage sprouting. Optimal planting depths may be 3 to 4.7 inches (8-12 cm) [45]. In controlled experiments, researchers found that, while temperature and bulbil weight influenced the time required for air yam to produce sprouts, humidity, day length, and bulbil origin did not. Sprout development took longer at lower temperatures and for light-weight bulbils (<20 g) that were collected in November from Gainesville and Fort Pierce, Florida. Collected bulbils weighed 0.5 to 107 g and averaged 16 g. At the end of the experiment, which lasted 39 weeks, 100% of bulbils sprouted at 80 °F (27 °C), and 85% of bulbils sprouted at 60 °F (16 °C). The findings from the study were used together with weather data to predict the dates at which half of all air yam bulbils would have sprouted (see Seasonal Development) [59].
Chinese yam: During controlled studies and field observations on Chinese yam in the United States, a high percentage of bulbils sprouted. Sprouts have developed from 2-week-old bulbils [15], and burial was not required for sprouting [54]. In southern Illinois, all marked bulbils remaining in the study area sprouted and survived to the end of the growing season. In the greenhouse, 59 of 60 bulbils sprouted, and 58 of 59 sprouting bulbils produced tubers that averaged 3.9 g (dry weight) by 4 months of age. Fifty-five of the sprouts produced bulbils, and production averaged nearly 16 bulbils/vine. Generally, plant fitness was not significantly affected by the size of the bulbil or bulbil fragment that was planted (see Plant growth) [8].
Experiments conducted on Chinese yam bulbils collected in Sendai, Japan, showed that immature bulbils demonstrated "summer dormancy" and required light to sprout. Mature bulbils exhibited "winter dormancy" and required chilling to sprout. Immature bulbils kept in the dark failed to sprout and died at 41 °F (5 °C), but mature bulbils chilled at the same temperature for 84 or 127 days produced sprouts when moved to a dark, warm environment. The optimum temperature for breaking "winter dormancy" was 41 °F (5 °C) [56].
Water yam: Water yam bulbils generally sprout when temperatures are warm and humidity is high [46]. A 2-pound (1 kg) tuber can be divided into 112 pieces, all of which are capable of sprouting [60]. Moderate temperatures 77 to 86 °F (25-30 °C) are considered optimal for sprouting of water yam bulbils. Moist conditions or short-duration submersion may encourage rapid sprouting and growth (reviews by [14,46]). Cool, dry conditions can increase the time to sprout, and high temperatures (100 to 140 °F (40-60 °C)) can decrease the time to sprout by up to 2 weeks [46]. Submerging water yam bulbils in water for 4 hours reduced the time to sprouting by 21 days when compared to unsubmerged bulbils (review by [14]). Based on studies conducted on multiple water yam varieties, Martin [46] found that dormancy in bulbils may be internally controlled. Even when sprouting conditions are optimal, water yam bulbils may not sprout immediately.
Plant growth: Yam growth can be rapid (Copeland 1916 cited in [13]), and production of bulbils has been reported on 3-month-old air yam vines [45]. Growth rates and reproductive output can differ between newly planted bulbils and established plants growing from tubers [44].
Rate of growth: When yam vines begin to grow, the rate can be as high as 5.9 inches (15 cm)/day (Copeland 1916 cited in [13]). Growth rates generally decrease as the growing season progresses [8,52]. Air yam vines from bulbils planted in outdoor pots grew 1 inch (2.5 cm)/day in their first 81 days. After being cut to 2 inches (5 cm) tall, vines grew 0.9 inch (2.3 cm)/day [19]. The growth rate of Chinese yam populations in southern Illinois decreased exponentially as the growing season progressed [8]. In Great Smoky Mountains National Park, Chinese yam growth was rapid but decreased from April to August. From May to June, vines grew an average of 1.5 inches (3.9 cm)/day and from June to August grew 1.4 inches (3.5 cm)/day. Tuber growth averaged 0.2 cm/day [52]. In field experiments conducted in India, water yam grown from tuber pieces or bulbils initially grew 0.02 to 0.03 g/g/day. During the phase of initial tuber development, growth was a little more than 0.04 g/g/day. During the tuber maturation phase, growth was about 0.01 g/g/day [73].
Underground tubers were generally small in the 1st year of growth for air yams and water yams (review by [13]). Large tubers may not be produced until yams reach 3 years old (Bois 1927 cited in [13]).
Growth from bulbils and tubers compared: Vegetative and reproductive potential increased dramatically once Chinese yam vines were established. Growth and reproductive output of 1st-year Chinese yam vines from bulbils was much less than that of 2-year-old vines from tubers near Knoxville, Tennessee. Bulbil production on vines from tubers was about 3.5 times that of vines from bulbils. Bulbils produced 1 thin vine, while tubers produced an average of 4 robust vines. There were no flowers produced on vines from bulbils, but vines from tubers did flower [44].
Comparison of Chinese yam growth from bulbils and tubers [44] | |||
Chinese yam vine source and age | Average maximum vine length (cm) |
Average bulbil production (no./m of vine length) |
Average bulbil dry weight (g/bulbil) |
From bulbils (1st year) |
213a | 23a | 0.28a |
From tubers (2nd year) |
479b | 84b | 0.43b |
Differences between means within a column followed by different letters are significant (P<0.05). |
Sexual reproduction: Because synchronous flowering, fruit production, and successful seed set are extremely rare for yams, information about sexual reproduction is limited and generally involves aritificial conditions. Information on flower pollination, seed dispersal, seed banking, and germination is sparse.
Pollination: Air yam and water yam pollen is sticky and not wind dispersed ([87], Coursey 1967 cited in [3]). Although air yam flowers produce a pleasant odor thought to attract bees and other insects [45], documentation of insect pollination is lacking (review by [3]). It is also possible that male water yam flowers may fail to open or open so little that pollinators cannot access the flower [46].
Seed production: In studies conducted in India, either hand pollination or staggered planting to encourage synchronous flowering were required for seed set in water yams. Water yam fruit and seed set is rare in India, and water yams are generally considered a "sexually degenerate species". When hand pollinated, however, 2 water yam vines had 45% and 54% fruit set. Although fruits can produce up to 6 seeds, the average number of seeds/fruit was 2.7 and 3.2. When seedlings produced from these seeds were artificially pollinated, none developed seed. Researchers suggested that dry conditions may have affected production [2]. During other artificial pollination experiments, fruit set by water yams could be increased by increasing relative humidity and decreasing air temperatures [1]. In a field study, male and female water yams were planted at staggered dates to encourage synchronous flowering and possibly natural fruit set. Just one female clone set fruit and seed (2.02% and 0.92%, respectively). Researchers suggested that a lack of pollinators was the reason for poor fruit set. During careful observation, just 1 small unidentified crawling insect was observed [87]. The growth of water yam seedlings produced from seed in these experimental plants is discussed in Seedling growth below.
Seed dispersal: Yam seeds are winged and likely wind-dispersed [21,45,93].
Germination: A little over half (52.6%) of the water yam seeds collected from hand-pollinated plants germinated within 40 days of sowing in a greenhouse [87].
Seedling growth: Growth of water yam seedlings from seed produced on artificially pollinated plants was variable. When researchers transplanted water yam seedlings to a field site in India, seedlings had few or no branches, few leaves, and reduced height compared to clones of the same age that had "luxuriant vegetative growth". Seedlings produced few flowers and had low tuber yields [2]. In another study in India, however, seedlings from seed collected from experimental seed-producing water yams were considered "healthy and vigorous". Small tubers developed on seedlings that were only 3 months old, and by 8 months old, seedlings produced up to 1.1-pound (500 g) tubers [87].
Yams are well known as an edible crop and as an attractive ornamental ([21,45], review by [51]), but yams also have many medicinal and other uses (reviews by [3,5,78]).
Dietary and social importance: Not all nonnative yam tubers are edible or palatable, yet some are of such dietary importance that they have achieved social and religious importance as well. Unpalatable or toxic forms of yams are used as fish poison and are sometimes planted within edible yam crops to discourage thieving [3,13,45].
Tubers and bulbils produced by Zanzibar yam are toxic [21]. According to the Center for Aquatic and Invasive Plants, nonnative yams are very toxic and should not be consumed. However, other sources indicate that air yam bulbils are edible when toxins are removed through proper preparation [13,17,21]. In Hawaii, air yam tubers are only eaten in times of famine [89]. Water yams are one of the most commonly cultivated yam crops. Both water yam tubers and bulbils are edible, but tubers are considered more palatable [21,45]. In Papua New Guinea, water yam cultivation holds religious and psychological importance for some. Water yam gardens are tended strictly by males. Gardens that produce giant plants and tubers are a source of male pride [46].
Medicinal importance: Yam leaves and tubers are used to treat a variety of ailments. The leaves of air yam, Chinese yam, and water yam are used as a poultice for pimples and tumors and in bath water to soothe skin irritations and stings (reviews by [5,78]). Air yam is also used to treat sores, swelling, hemorrhoids, sore throats, and diabetes. In northern Bangladesh, air yams are used to treat leprosy and tumors. Researchers have identified antitumor properties in air yams [3]. Chinese yams are used to treat poor appetite, diarrhea, asthma, coughs, frequent urination, diabetes, and emotional instability. Chinese yam tubers contain allantoin, which speeds the healing process. Diosgenin, which is used to manufacture progesterone and other steroids, is also produced in Chinese yam tubers (review by [78]).
More info for the terms: adventitious, bulbil, dioecious, ecotype, root crown, selection, vine, vines
This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [21,72,89]).
Yams are herbaceous, climbing, twining, perennial monocots [13,72,89]. Vines are without tendrils and use dead stems from the previous year's growth to climb into other vegetation [51,89]. Most nonnative yams occurring in the United States grow belowground tubers annually; perennial tubers are generally restricted to the Testudinaria section of the Dioscorea genus [13,46]. None of the 5 nonnative yams belong to the Testudinaria section (see Taxonomy). Most yams also produce aerial tubers or bulbils [89]. Both belowground tubers and bulbils are comprised of stem tissue [45,46]. Regeneration of yams in the United States has been exclusively asexual from tubers or bulbils [51,66,78]. Additional information on asexual regeneration by yams is presented in Regeneration Processes and Vegetative regeneration. Yams are dioecious [89] and produce very small flowers, if any [21,72]. Sexually produced fruits are 3-winged capsules [72], but yam fruits are extremely rare in the United States and, if produced, are often sterile [21,24,61]. Even when male and female water yams grew in close proximity, fruits were rare and seeds were typically aborted before reaching maturity [46]. Yam seeds are winged but have not been observed in the United States [78].
For thousands of years, yams have been a staple food for humans. They have been widely cultivated and domesticated throughout the world. Selection and adaptation have occurred in native and nonnative habitats, producing an abundance of ecotypes. Variable yam forms are possible in the United States, depending on the number of types introduced and escaped. Great variation in the growth and forms of vines, leaves, bulbils, and tubers are reported for air yams and water yams [45,46] and are also likely in other species.
Aboveground descriptions:
Air yam (D. bulbifera): Air yam vines twine counterclockwise and may grow to 100 feet (30 m) long [21,89]. Air yam stems are not angled and do not have prickles [89]. Leaves are simple, heart-shaped, and arranged alternately along the stem. Leaves may reach 10 inches (26 cm) wide and long; leaf petioles are generally shorter than the leaf blade [21,51,89,93]. Air yam size and appearance can be variable. Variability in bulbil form and size may be partly due to the different air yam types that exist in Asia and Africa. The Asian type produces relatively smooth, spherical bulbils that may weigh 2 pounds (1 kg). African types produce sharply angled bulbils [45]. Most or all of the air yams in Florida are considered to be the African type [58]. In US floras and other references, air yam bulbils are often described as more than 2 inches (5 cm) in diameter and up to 5 inches (13 cm) long [3,12,21,47]. Flowers, if produced, are widely spaced or reduced to a single flower in simple staminate spikes 4 to 28 inches (11-70 cm) long. Pistillate spikes are generally stiff, up to 9 inches (23 cm) long, and occur in clusters of 2 or more [21,66,89,93].
Chinese yam (D. polystachya): Chinese yam vines are slender, twine clockwise, and may reach 16 feet (5 m) or more in length [24,72]. Chinese yam leaves may grow to 4.3 inches (11 cm) long and wide [21,24]. Leaves are deeply lobed at the base, and upper leaves may have 3-lobed margins [21,78]. Like water yam, the arrangement of Chinese yam leaves is variable. A review by Tu [78] reports that leaves may be alternate near the top of stems or occassionally found in whorls of 3, and others report that leaves are generally alternate near stem bases and opposite near the end of stems [21]. Chinese yam produces small, rounded, warty bulbils. Bulbils are typically less than 1 inch (3 cm) long and less than 0.8 inch (2 cm) in diameter [21,47,61,69]. Flowers are rare and smell like cinnamon. Male flowers occur in bundles, spikes, or panicles at branch ends [81], and female inflorescences are few-flowered and generally less than 2 inches (5 cm) long [21].
Fiveleaf yam (D. pentaphylla): Fiveleaf yam vines are prickly, twine counterclockwise, and may grow to 30 feet (10 m) long [70,89,93]. Leaves are alternate and compound with 3 to 5 leaflets. Leaflets measure 2 to 4 inches (5-10 cm) long and 0.8 to 1.5 inches (2-4 cm) wide. Bulbils are horseshoe shaped and about 1 cm in diameter [89]. Bulbils were rare in Hawaii according to St John [70]. Male flowers, if produced, occur in spikes up to 12 inches (30 cm) long at the branch ends; female flowers also occur in spikes, but these are shorter, 2 to 10 inches (5-25 cm) long [89].
Water yam (D. alata): Water yam vines twine clockwise and may reach 100 feet (30 m) long [21,46,94]. Vines have ridged stems and are prickly at the base [93]. Water yam leaves are large, elongate, and heart-shaped [46,51]. Leaf blades typically measure 2 to 6 inches (6-16 cm) long, 2 to 5 inches (4-13 cm) wide, and have entire margins. Leaf petioles are generally as long as the leaf blade [21] Leaf arrangement appears inconsistent. Some report that water yam leaves are alternate at the stem base and opposite near the stem end [21], while others report that leaves are primarily opposite, but can appear alternate due to leaf abortion [51]. Bulbils are oblong and rough with fleshy protrusions. Bulbils may reach 4 inches (10 cm) long and 1.5 inches (4 cm) in diameter [12,21]. Staminate flowers occur in a zig-zag pattern along a rachis up to 10 inches (25 cm) long; pistillate flowers occur in 4- to 20- flowered inflorescences that may reach 14 inches (35 cm) long [21]. Water yam fruits are capsules that could produce 2 seeds per each of 3 locules, but often if 1 seed is produced it is aborted before maturity. Capsules are rare even when male and female plants are in close proximity [46].
Zanzibar yam (D. sansibarensis): Zanzibar yam vines twine counterclockwise. Vine stems are about 1 inch (3 cm) thick and may grow to 10 inches (25 cm) or more. Stems can reach support trees prior to producing leaves [21,51,82]. Zanzibar yam leaves and petioles are long. Leaf blades are heart-shaped and can be up to 18 inches (46 cm) long and 23 inches (58 cm) wide. Petioles may be 10 inches (26 cm) long, and leaves often have a tail-like projection at the tip [21,51,82]. Leaves are generally opposite, and the margins of juvenile leaves may be irregularly lobed [21,82,94]. Bulbils are small, smooth, often a purplish color, and can measure 2 inches (6 cm) in diameter [21,51,82]. Male inflorescences typically have 2 to 4 flowers and may be up to 20 inches (50 cm) long; female inflorescences are slightly shorter [21,82].
Belowground descriptions: Most yams produce underground tubers. Yams of the Enantiophyllum section (Chinese yam and water yam) generally produce 1 tuber per season but may produce up to 3 tubers in a single season [13]. Yams generally have "weak rooting systems". Although referred to as roots in the literature and in this review, tubers and belowground structures attached to the root crown are adventitious stem tissue [45,46]. Early in the growing season, the previous year's tubers and sprouting bulbils produce thick, unbranched roots; later in the growing season, thinner, branching, fibrous roots develop (reviews by [3,13]). True roots are only produced by seedlings, and these are short-lived.
Air yam: Most references indicate that air yam generally produces tubers, although they may be small and solitary [3,66,89,93]. However, some suggest that air yam may lack underground tubers [3,21]. Tubers are commonly 2 to 4 inches (5-10 cm) in diameter and weigh less than 2.2 pounds (1 kg) [21,66,89,93]. Tubers from air yams in Florida were usually less than 4 inches (10 cm) in diameter but were up to 8 inches (20 cm) in diameter on vines growing in sandy soil (Gann-Matzen and Line 1992 personal communications cited in [66]). A particularly large tuber was dug from Alachua County, Florida; it was 9.8 inches (25 cm) in diameter and weighed 11 pounds (5 kg) (personal observation 1992 cited in [66]). Flora of North America describes air yam tubers as globose and occurring just below the soil surface [21]. However, tuber shape is likely related to ecotype. Air yams from Asia produce spherical tubers, while air yams from Africa produce irregular branching tubers [45].
Chinese yam: Chinese yams produce 1 to many large, cylindrical tubers [21,93]. Tubers grow vertically from long stalks and typically as deep as 3 feet (1 m) below ground [24,55]. Mature tubers can weigh 8 to 10 pounds (3.6-4.5 kg) [55].
Fiveleaf yam: Fiveleaf yam typically produces single, irregular to elongated, egg-shaped tubers [70,89,93]. Tubers may occur near the soil surface or more than 3 feet (1 m) under ground [70,89]. Fiveleaf yam tubers dug in Hawaii ranged from 1 to 6.7 inches (2.6-17 cm) long and were about equally wide. The largest tuber weighed 3 pounds (1,370 g) [70].
Water yam: Tubers produced by water yams are described as massive [21,41]. Water yam may begin producing tubers 14 to 40 days after planting (review by [14]), and tubers grow rapidly near the end of the growing season [46]. Although single tubers are most common, several are possible [21,46]. Tubers are branched, grow vertically, and may be deeply buried [21,46,93]. Water yam tubers weigh 22 pounds (10 kg) to more than 110 pounds (50 kg) [3,51]. A 180-pound (81 kg) water yam tuber was harvested from a garden in Trinidad [46].
Zanzibar yam: Irregular, rounded lobes are common on the generally globose tubers produced by Zanzibar yams. Generally tubers are shallowly buried (up to 6 inches (15 cm) deep) and may reach 16 inches (40 cm) in diameter [21,82].
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More info for the terms: geophyte, hemicryptophyte
Raunkiaer [63] life form:
Hemicryptophyte
Geophyte
Several sources report evidence of small mammals feeding on yam bulbils ([8,77], review by [78]), but little to no predation of air yam bulbils was reported in Florida (Brinkley and Putz 1992 personal communications cited in [66]).
Palatability and/or nutritional value: Chemical composition and nutritional value of yams is reported in the following references: [13,45,45]. Although air yams have moderate protein levels, protein quality is generally low [45].