Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea longa), Japanese yam (Dioscorea tokoro), and bitter yam (Dioscorea japonica) all belong to the genus Dioscorea in the family Dioscoreaceae. They produce edible tubers and bulbils and are very common climbing perennial plants often seen in urban areas. The genus Dioscorea, also known as yam, is a vast group containing many species, but these four species are the most common in Japan. However, they are often confused, especially Japanese yam (Dioscorea japonica ) and Chinese yam (Dioscorea longa), which are used to make "tororo" (grated yam), and this confusion has even occurred at the government level. While there are other species besides these four, these four can be distinguished by the shape of their leaves and flowers, and the presence or absence of tubers (rhizomes) and bulbils. Their reproductive strategies are extremely diverse; tubers, bulbils, flowers, and fruits are all involved in reproduction, but how they are used differs greatly from species to species. Therefore, although they are often confused, each species has a strong biological individuality. This article will explain the classification, morphology, and ecology of the genus Dioscorea.
- What are Japanese yam, Chinese yam, Japanese nightshade, and bitter yam?
- What are the differences between Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea tokoro), and bitter yam (Dioscorea japonica)?
- What are the differences in how to use Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea tokoro), and bitter yam (Dioscorea japonica)?
- Are there any other similar species?
- How do they reproduce? Why do they reproduce through bulbils and pollination?
- How do they pollinate? The insects that visit them vary completely depending on the species!
- What are the seed dispersal methods?
- References
What are Japanese yam, Chinese yam, Japanese nightshade, and bitter yam?
Japanese yam (Dioscorea japonica), also known as yamaimo or jinenjo, is a climbing perennial herb distributed in the temperate to subtropical regions of Honshu, Shikoku, Kyushu, and the Ryukyu Islands in Japan, as well as the Korean Peninsula, China, and Taiwan. It commonly grows in alluvial plains and mountainous areas, along forest edges, roadsides, field edges, vacant lots, and park planting areas. However, since only the underground rhizome (edible part) is annual, the leaves above ground wither in winter.
Nagaimo (Japanese yam), also known as Dioscorea polystachya, is a climbing perennial herb distributed in the temperate to subtropical regions of Honshu, Shikoku, Kyushu, and the Ryukyu Islands in Japan; the Korean Peninsula; China; and Taiwan. It grows in alluvial plains and hillsides of the Castanopsis and Quercus zones, along forest edges, fields, vacant lots, and park planting areas. However, since only the underground rhizome (edible part) is annual, the leaves above ground wither in winter. There are various theories regarding the distribution of nagaimo in Japan; one theory suggests it may not be a native species, but rather was introduced from China and may have been cultivated in the late Jomon period, preceding grains and rice (Yoshida, 2019). Numerous cultivated varieties exist.
Dioscorea tokoro is a climbing perennial herb that is distributed in Hokkaido, Honshu, Shikoku, and Kyushu in Japan; as well as in the temperate to warm zones of the Korean Peninsula and China. It is very common to find it growing in alluvial plains and mountainous areas from the Castanopsis and Quercus zones to the Beech zone, including forest edges, grasslands, vacant lots, and planted areas in parks.
Dioscorea bulbifera, also known as space potato or air potato, is a climbing perennial herb widely distributed in Honshu, Shikoku, Kyushu, and the Ryukyu Islands of Japan; and in the subtropical to tropical regions of Asia and Oceania. It grows in forest edges and grasslands along rivers in alluvial plains and hilly areas of the Castanopsis and Quercus zones.
いずれもヤマノイモ科ヤマノイモ属に含まれ、町中でもよく見かける非常に身近なつる性多年草です。ヤマノイモ属は非常に膨大な種類を含むグループですが、4種はその中でも日本で最も多く遭遇します。
Its most distinctive feature is that it can form roots or bulbils, commonly referred to as "potatoes." Of the four species, only Japanese yam (Dioscorea japonica) and Chinese yam (Dioscorea japonica) are edible in Japan.
The root of the nagaimo yam is eaten as "tororo" when grated, and it's an indispensable topping when you're eating out and paying a little extra for it, especially with soba noodles or rice bowls.
However, Japanese yam (Dioscorea japonica) and Chinese yam (Dioscorea japonica) are extremely often confused. They are so confused that, according to statistics from the Ministry of Agriculture, Forestry and Fisheries, they weren't even distinguished from each other until 2008. Few websites clearly explain the difference between them.
Furthermore, in the wild, Japanese yam (Dioscorea japonica) and Dioscorea tokoro often grow together, so it is necessary to distinguish between the two.
さらにニガカシュウの日本の個体は苦く、通常食用されないため、これも見分ける必要があります。
このように非常に似たグループですが、区別する必要に迫られることが多いです。
What are the differences between Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea tokoro), and bitter yam (Dioscorea japonica)?
ヤマノイモ属は非常に多数の種類が知られるグループで、ここで全ての違いを書くのは難しいですが、比較的一般的な4種に絞って考えています。
Regarding the leaves, there is a difference in that the leaves of Japanese yam (Dioscorea japonica) and Chinese yam (Dioscorea japonica) are triangular-ovate to triangular-lanceolate, while those of Japanese yam (Dioscorea tokoro) and bitter yam (Dioscorea japonica) are orbicular-cordate (heart-shaped).
However, since yams exhibit considerable variation, with some having leaves that are nearly heart-shaped, it is important to examine multiple leaves.
Regarding Japanese yam (Dioscorea japonica) and Chinese yam (Dioscorea japonica), the difference lies in the fact that in Japanese yam, the lateral base does not protrude in an ear-like shape and the base of the leaf blade is green, while in Chinese yam, the lateral base protrudes in an ear-like shape and the base of the leaf blade is reddish-purple.
Regarding Dioscorea tokoro and Dioscorea japonica, the difference is that in Dioscorea tokoro, the base of the petiole is smooth, while in Dioscorea japonica, the base of the petiole protrudes in a stipule-like manner and clasps almost half of the stem.
There are even greater differences in the presence or absence of rhizomes and bulbils, as well as in the shape of the flowers. Please check the details below.
What are the differences in how to use Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea tokoro), and bitter yam (Dioscorea japonica)?
The four uses vary greatly depending on the developmental stage of the organs and the taste of each of the four types.
When used, yams are usually for food, and there are typically two types: the wild yam (Dioscorea japonica), which is common in the wild, and the cultivated Chinese yam (Dioscorea longa). There are various theories about the distribution of Chinese yams in Japan, and one theory suggests that it is questionable whether it is a native species, and that it may have been introduced from China and cultivated in the late Jomon period, preceding grains and rice (Yoshida, 2019). It already appears in the dictionary "Wamyō Ruijushō," which was compiled in the mid-Heian period, suggesting that it was already being used.
For a long time, the two species were rarely distinguished for consumption, and even in the Ministry of Agriculture, Forestry and Fisheries' crop survey statistics, they were not differentiated until 2008. However, since 2007, it has been found that the majority of what is sold as "yamaimo" (Japanese yam) is actually nagaimo (Chinese yam).
Both Japanese yam (Dioscorea japonica) and Chinese yam (Dioscorea japonica) have a single cylindrical, fleshy "root" (also known as a tuber, or "potato") underground. In Japan, the roots of both species are collectively called "tororo imo," and in the case of Japanese yam, they are also called "jinenjo." Chinese yam has a thicker shape.
However, cultivated varieties of Japanese yam, such as Tsukuneimo and Yamatoimo (Yamatoimo), are round in shape.
This part is cut into sticks and mixed with okra, or grated and mixed with white soy sauce and dashi to make the sticky "tororo," which is commonly eaten as "tororo soba," "tororo udon," or "mugitoro gohan." The stickiness is due to mucilage (or mucin, according to some papers), and it can be eaten raw.
It is said that grated yam (Yamaimo) is far stickier than grated Chinese yam (Nagaimo).
While cultivated nagaimo (Chinese yam) is typically used in restaurants, wild yam (Yamaimo) was likely also used before the Edo period, and even today, wild yam is sometimes used intentionally due to its different texture.
Furthermore, Japanese yam, Chinese yam, and bitter yam form "bulbils" in the leaf axils.
ムカゴとは「珠芽」とも呼ばれ、芽の部分が繁殖用の栄養を含むように発達したものです。
Because the bulbils of both Japanese yam and Chinese yam have a potato-like texture, they are eaten. They are cooked with rice to make "mukago rice," or served as a snack with drinks at izakayas (Japanese pubs), sometimes stir-fried with salt, or as "deep-fried mukago" or "mukago tempura." However, it is usually the Chinese yam that is eaten.
In addition, it was sometimes called wild yam or soil yam and used as a herbal medicine.
On the other hand, while bitter corn has rhizomes and bulbils, both are very bitter and are not usually eaten. This is as its Japanese name suggests. However, cultivated varieties are commonly eaten worldwide because the bitterness can be removed by boiling, and it is popular in West Africa. In Japan, it is sold under the names "space potato" or "air potato."
オニドコロに至っては担根体とムカゴがどちらも存在しませんので、普通は食用にしません。ただし、担根体の代わりに根茎（rhizome）を形成します。
Are there any other similar species?
The genus Dioscorea is a very broad group, so it's impossible to list them all here. However, some species that resemble Dioscorea tokoro found in the wild in Japan include Dioscorea japonica, Dioscorea japonica, Dioscorea tokoro, Dioscorea japonica, and Dioscorea japonica, so caution is advised.
Globally, edible yams of the genus Dioscorea are collectively called "yam," and include species such as Dioscorea alata and Dioscorea elephantipes (also known as Dioscorea elephantipes).
Yams are rich in dietary fiber, starch, sugars and other carbohydrates, as well as protein, lipids, vitamins, and minerals, providing approximately 200 calories per person per day to 300 million people in tropical regions, making them one of the world's most important staple root vegetables and tubers (Obidiegwu et al., 2020).
How do they reproduce? Why do they reproduce through bulbils and pollination?
Reproductive methods vary greatly depending on the species.
Japanese yam (Dioscorea japonica), Chinese yam (Dioscorea japonica), and bitter yam (Dioscorea japonica) reproduce asexually through clonal propagation via bulbils and sexually through insect-pollinated flowers, incorporating genes from other plants.
In the reproduction of bulbils, they are either dropped to the ground by gravity, washed away by water, or, like acorns, carried by mice and stored underground (stored), and then germinate when forgotten by the mice (Mizuki & Takahashi, 2009).
Compared to asexual reproduction via rhizomes, stolons, and stolons, bulbils are considered to have advantages in that they can move to spatially distant, suitable environments for growth and are resilient to disturbed areas (such as aquatic environments and roadsides where the environment changes drastically) (Inoue, 2007).
On the other hand, *Dioscorea tokoro* does not produce bulbils and reproduces only sexually through insect-pollinated flowers.
Why do Japanese yam, Chinese yam, and bitter yam reproduce in two different ways, including by adding bulbils?
Generally, two advantages are explained (Takahashi & Inoue, 2005; Mizuki & Takahashi, 2009).
1つ目は、ムカゴは乾燥重量にして種子の100倍程度あり、暗環境下でも発芽・成長できるので、子個体の生存率は種子よりムカゴの方が高いというメリットです。
2つ目は、生物は可能なら自分の遺伝子を全て伝達したいので、クローンであるムカゴを作れば雌親の遺伝子が種子の2倍伝えられるというメリットです。
However, this might lead you to wonder why they reproduce sexually through insect pollination.
There are two potential advantages to this as well.
1つ目は、有性生殖を行った方が、様々な遺伝子の効果で、子供が病害虫や病原菌や自然環境の変化に強くなるというメリットです。
2つ目は、ヤマノイモ属に特有のもので、ヤマノイモ属は果実が風を受けやすい「翼」を持つ蒴果で、種子に関しても円形で周りに薄い翼があります。これらで効率的に種子散布（風散布）を行っているので、ムカゴよりも遠くに子供を飛ばすことによって、子供同士の競争（栄養の奪い合いなど）のリスクを減らすというメリットです。
While it is true that bulbils can be transported far from the mother plant by mice, it has been found that their range of movement is shorter than that of seeds.
From the above, it can be said that Japanese yam, Chinese yam, and bitter ginger combine the best aspects of asexual and sexual reproduction, thereby increasing their efficiency.
However, this still leaves a question: If that's the case, why doesn't *Dioscorea tokoro* reproduce asexually?
This has not yet been investigated. Dioscorea tokoro is a species that reproduces just as well as Dioscorea japonica, and it does not seem to suffer significant losses from losing the advantages mentioned above.
One possibility is that the African species D. sansibarensis has highly buoyant bulbils that spread by water currents, resulting in a wide distribution in valleys (Chen et al., 2022). Furthermore, D. alata is known to form bulbils when the soil becomes moist during the rainy season (Hamaoka et al., 2023).
Unlike seeds, bulbils are expected to be a more efficient way for plants to reproduce even in aquatic environments, and since *Dioscorea tokoro* is a species adapted to land, it may not need to produce bulbils.
実際、水域に多いニガカシュウはムカゴを持っています。しかし、これは推測の域を出ません。
How do they pollinate? The insects that visit them vary completely depending on the species!
花は4種いずれも雌雄異株なので、雄花だけを咲かせる雄株、雌花だけを咲かせる雌株が存在します。共通で6枚の花被片があり、雄花では6本の雄しべを持ちます。
As mentioned above, it is an insect-pollinated flower, relying on insects for pollination. However, it gives the impression of being very inconspicuous compared to other insect-pollinated flowers. What kind of insects visit it?
It turns out that the flowers are actually visited by a special, very small insect that is perfectly sized for them.
In Japanese yam (Dioscorea japonica), the male inflorescences are erect from the leaf axils, while the female inflorescences droop, and the perianth segments are white and do not open very much (Hayashi et al., 2013). When 427 insects that visited these flowers were collected, 97% were found to be thrips, which are very small pollinating insects (Mizuki et al., 2005).
ナガイモでも雄花序が垂れ下がる以外ヤマノイモとほぼ同じ特徴で、中国での研究ですが Ernothrips lobatus というアザミウマの一種のみが訪れることが分かっています（Li et al., 2014）。このアザミウマは花内で産卵・幼虫の発育も行っており、生活史レベルで密接に関係しています。ただし、アザミウマは短距離移動しかできず、雌雄が近接していないと受粉できません。
On the other hand, in Dioscorea tokoro, the male inflorescences are erect from the leaf axils, while the female inflorescences are drooping, and the perianth segments are pale green and spread flat (Kudo et al., 2021). When 389 insects that visited this flower were collected, 57% were gall midges and 11.6% were biting midges, but it was mostly biting midges that were actually carrying the pollen (Mochizuki et al., 2025).
Furthermore, despite the fact that both the male and female inflorescences of *Rhododendron parvifolium* droop and the perianth segments change from white to purple, a study in China found no records of insects visiting the flowers even once in two years, suggesting that it relies almost entirely on asexual reproduction via bulbils, with only a small portion being wind-pollinated (Li et al., 2014). It also rarely produces fruit in Japan (Kanagawa Prefecture Flora Survey Association, 2018).
Thus, we have obtained contrasting results: thrips visit Japanese yam and Chinese yam, small flies visit Japanese yam, and no insects visit bitter ginger. Although they are very similar in appearance, it can be said that their reproductive strategies are completely different.
What are the seed dispersal methods?
The fruits of all four species are capsules, possessing wings that make them more susceptible to wind, and the seeds also have wings (Kanagawa Prefecture Flora Survey Association, 2018; Chen et al., 2022).
上述のように、これは風散布という種子散布を行うためで、これによりムカゴ以上に遠くの距離に子個体を飛ばすことに成功しています。
References
Chen, M., Sun, X., Xue, JY, Zhou, Y., & Hang, Y. (2022). Evolution of Reproductive Traits and Implications for Adaptation and Diversification in the Yam Genus Dioscorea L. Diversity, 14 (5), 349. https://doi.org/10.3390/d14050349
Hamaoka, N., Moriyama, T., Taniguchi, T., Suriyasak, C., & Ishibashi, Y. (2023). Bulbil formation on water yam (Dioscorea alata L.) is promoted by waterlogged soil. Agronomy, 13 (2), 484. https://doi.org/10.3390/agronomy13020484
Hayashi, Yasaka, Kadota, Yuichi, and Hirano, Takahisa. (2013). Yamakei Handy Illustrated Guide 1: Wildflowers (Revised and Expanded New Edition). Yama-kei Publishers. ISBN: 9784635070195
Inoue, Mizuki. (2007). Dispersion and spatial structure in dispersing clonal growth plants (bulbils, rhizomes, etc.)—comparison with non-dispersing clonal growth plants (rhizomes, stolons, projectiles). Journal of the Ecological Society of Japan, 57 (2), 238-244. https://doi.org/10.18960/seitai.57.2_238
Kanagawa Prefecture Flora Survey Association. (2018). Kanagawa Prefecture Flora 2018 Electronic Edition. Kanagawa Prefecture Flora Survey Association. ISBN: 9784991053726
Kudo, A., Sugihara, Y., Ota, A., & Terauchi, R. (2021). Upright males and drooping females—sexual differences in flower traits and visitor numbers in the dioecious plant Dioscorea tokoro. Abstracts of the Annual Meeting of the Ecological Society of Japan, 68, P1-061. https://esj.ne.jp/meeting/abst/68/P1-061.html
Mizuki, I., Ishida, K., Tani, N., & Tsumura, Y. (2010). Fine-scale spatial structure of genes and sexes in the dioecious plant Dioscorea japonica, which disperses by both bulbils and seeds. Evolutionary Ecology, 24 (6), 1399-1415. https://doi.org/10.1007/s10682-010-9396-z
Mizuki, I., Osawa, N., & Tsutsumi, T. (2005). Thrips (Thysanoptera: Thripidae) on the flowers of a dioecious plant, Dioscorea japonica (Dioscoreaceae). The Canadian Entomologist, 137 (6), 712-715. https://doi.org/10.4039/n05-003
Mizuki, I., & Takahashi, A. (2009). Secondary dispersal of Dioscorea japonica (Dioscoreaceae) bulbils by rodents. Journal of Forest Research, 14 (2), 95-100. https://doi.org/10.1007/s10310-008-0106-4
Mochizuki, K., Elsayed, AK, & Kawakita, A. (2025). Pollination by biting midges in Dioscorea tokoro and Vincetoxicum aristolochioides with a secondary contribution of gall midges: Pollination by biting midges in Dioscorea tokoro and Vincetoxicum aristolochioides with a secondary contribution of gall midges. Arthropod-Plant Interactions, 19 (3), 41. https://doi.org/10.1007/s11829-025-10142-4
Li, MM, Yan, QQ, Sun, XQ, Zhao, YM, Zhou, YF, & Hang, YY (2014). A preliminary study on pollination biology of three species in Dioscorea (Dioscoreaceae). Life Science Journal, 11 (2), 436-444. https://www.lifesciencesite.com/lsj/life1102/060_B00049life110214_436_444.pdf
Obidiegwu, JE, Lyons, JB, & Chilaka, CA (2020). The Dioscorea Genus (Yam): An appraisal of nutritional and therapeutic potentials. Foods, 9 (9), 1304. https://doi.org/10.3390/foods9091304
Takahashi, Akiko & Inoue, Mizuki. (2005). Comparison of dispersal distances of seeds and bulbils of *Dioscorea japonica*: Wind, gravity, and mice. Abstracts of the Annual Meeting of the Ecological Society of Japan, 52, 528. https://doi.org/10.14848/esj.ESJ52.0.528.0
Yoshida, M. (2019). Japanese people and sweet potatoes. Journal of Food and Nutrition Research, 39 (5), 235-248. ISSN: 0288-0806, https://ku-food-lab.com/wp/wp-content/uploads/2019/07/2774f33ed7e947fb03c1d40880e92a23.pdf
