TAXONOMY AND DISTRIBUTION
Wasabi is a member of the Cruciferae or mustard family and is an evergreen crucifer that grows naturally in wet, cool mountain river valleys along stream beds and on river sand bars in Japan. The natural distribution of wasabi ranges from Sakhalin Island (Khokhriakov et al., 1981) north of Hokkaido, the northernmost major Japanese island, to Kyushu, the southernmost major Japanese island.
Two species are found in Japan; Wasabia tenuis the wild species, also known as yuri wasabi, and Wasabia japonica, the cultivated species called swamp wasabi or sawa wasabi (Ohwi, 1984). These two species are distinguished primarily by their rhizome size and color, flowers, and leaf size. W. japonica was also known as Allaria wasabi (Sieb.) Makino.; Cochlearia wasabi (Sieb.); Eutrema wasabi (Sieb.) Maxim.; E. japonica (Miq) Koidz; E. okinosi mensis Takenouchi (Ohwi, 1984); Lunaria japonica Miq.; and W. pungons Malsum. Adachi uses W japonica in his book Wasabi Cultivation (Wasabi Saibai, 1987) as does Oh vi in his Flora of Japan (1984) although some authors still use E. wasabi (Goto et al, 1986ab, 1987; Iwanami et al., 1982).
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Wasabi is a glabrous, perennial herb. Its underground stem, a rhizome, is simple and leafy, without branching, except for suckers, that are, theoretically, underground branches from the main stem. At maturity the rhizome is cylindrical, curved, and pointed at the terminal bud. Cultivated varieties of W. japonica can produce rhizomes 5.0 to 30.0 cm Iong, and 2.0 to 5.0 cm in diameter (Adachi, 1987), though Ohwi (1984) describes W. japonica as having rhizomes only 1.0 to 2.0 cm in diameter (Ohwi, 1984). This species also has leaves up to 15.0 cm across, flowers with styles 2.0 mm long, and petals 8.0 to 9.0 mm long (Ohwi, 1984).
W. tenuis has a short and narrow rhizome, leaves 5.0 cm, across, flowers with styles 0.5 to 0 7 mm long and petals 5.0 to 7.0 mm long (Ohwi, 1984).
Rhizome epidermal cells are dark to light green. The variety Mochidaruma has rectangular epidermal cells at least 50 micrometers thick. Parenchymal cells are either globular or elliptical in shape and 30 to 60 um in diameter. Starch grains, found inside rhizome cells, are 4.0 to 8.0 um in size.
Axillary buds are found between the base and the apex on the rhizome's leaf remnant scars and are divided into three types: dormant buds, which will not grow unless apical dominace is counteracted: thickened axillary buds that form masses at the remnant petiole bases; and simple axillary buds found at the basal end of the rhizome that are not dormant but immature (Adachi 1987, 1988).
Suckers grow near the base of the rhizome in numbers dependent upon the variety of wasabi and climatic conditions (Adachi, 1987, 1988; Yokogi and Ueno, 1975). Up to 20 suckers can emerge from axillary buds located in leaf petiole nodes on the basal part of the rhizome. These appear on 6 month old wasabi plants and enlarge within 15 months but vary with the variety and environment (Adachi, 1987).
Wasabi rhizomes grown under semi-aquatic conditions have about 20 - 25 roots ranging from 30cm to 1m, which are differentiated into thick primary roots and thin root hairs (Adachi, 1987).
Wasabi leaves are long petiolate, simple, cordate-orbicular, undulate, toothed (Ohwi, 1974) and have palmate veins radiating from the leaf petiole protruding on the ventral side (Adachi, 1987).
Mature petioles are vertical to oblique 30 - 50 cm long basally flattened and surround the rhizome. Eighty percent of the petioles are thrust in a right hand orientation. 20% are in a left hand orientation (Adachi, 1987). Petiole epidermal cells are rectangular to oval shaped with dimensions of 20 to 30 um, and sieve cells about 12 to 17 um in diameter (Adachi, 1987, 1988).
W. japonica may have up to 10 long peduncles, averaging 2 to 3. which are 111 to 150 cm long and bear thin small leaves as well as a terminal floral inflorescence (Adachi, 1987, 1988). Flowers are typical Cruciferae. white. 4.0 to 5.0 mm long, bracteates, arranged on racemes, with sepals ascending, petals clawed and obviate, perfect septum, style elongate, and stigma simple (Ohwi, 1984). Blooming starts from the outside moving in towards the center of the raceme (Adachi, 1987). Insects pollinate flowers and fertilization is by either self or cross pollination (Chadwick, per. obs.).
Seeds are born in siliques that are short, linear, oblong, torulose, cylindric with membranous valves and without a midrib. Each silique contains up to 8 seeds and each seed is 2 to 3 mm long and 1 mm wide, pointed at one end and flat on the top (Ohwi, 1984), and highly variable within varieties. Common to the Cruciferae, seeds have little if any endosperm (Swingle, 1946). A thin brown seed coat covers large incumbent cotyledons and a radicle (Ohwi, 1984). Storage outside of a cool moist environment will result in desiccation and death since wasabi seed is temperate and recalcitrant.
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LEAF AND REPRODUCTIVE CYCLES
Leaf development depends on the season. In the summer and winter only 2 or 3 new leaves grow per month since growth generally slows or stops during periods of temperature extremes. From March to June and in August an average of 5 to 6 new leaves per month will appear. Mature plants, up to 15 months old, may have a total of 62 leaves and lose from 2 to 6 leaves per month (Adachi,1987).
In Japan, flowering begins in, January, peaks in April and is finished by May. In January, peduncles may appear on wasabi plants that overwintered as mature plants. Typically, rapid elongation of the peduncles and flowering occurs in April. Individual plants normally flower for two months though individual blossoms only last one month. Siliques and seeds are formed 50 to 60 days after initial flowering, during, early May to Iate June. Immature siliques are soft and green but by late July or early August they become firm, black and ready for harvest.
Seed viability depends on environmental conditions during blossoming. In early March few seeds will mature when temperatures are low. The rate of seed formation increases from late March through early April when there is a rapid rise in the number of flowers producing viable seeds. Peak viable seed production in Japan occurs in mid April (Adachi, 1987).
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Wasabi seeds exhibit very strong dormancy and will not germinate when freshly harvested (Adachi 1987; Tatsuyama et al., 1983).
The depth of dormancy in seeds is a function of the temperature during maturation on the mother plant for both dicot and monocot crops (Boyce et aI., 1976; Kearns et al., 1939; Taylorson et aI., 1979; Van Abrams et al., 1965; Weisner et al., 1972). For example, lettuce dormancy is reduced when seeds mature at high temperatures (Harrington et al., 1952). Ryegrass seeds maturing at lower temperatures, tend to have greater dormancy (Weisner et al., 1972), as does lescue (Boyce et al., 1976).
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PROPERTIES & MEDICAL RESEARCH OF WASABI
The chemicals in Fresh Wasabi have been applied in food uses but may also have medical benefits.
The medicinal values of chemicals extracted from Wasabi were first documented in the Japanese medicinal encyclopedia during the 10th century (Hodges, 1974). Biocidally active chemicals in Wasabi are said to act as an antidote to food poisoning, one factor that might have led to the use of Wasabi with raw fish dishes in Japan.
Hydration of a glucoside (sinigrin) by the enzyme myrosinase results in the production of Wasabi's special flavor component, an allyl isothiocyanate, the major pungent component. We all experience this in the form of that heat rush up our noses when eating Wasabi with our sushi! The components that give Wasabi its flavor are 6-methylthiohexyl isothiocyanate, 7-methylthioheptyl isothiocyanate and 8-methylthioocytl isothiocyanate (Ina et al., 1989). Scientists are now discovering that these Wasabi isothiocyanates may have important medical benefits.
Today research is being conducted both in the United States and in Japan as to the potential medical benefit of Wasabi. Researchers say that the isothiocyanates in Wasabi, not only inhibit microbes, but can also help treat or prevent blood clotting, asthma and even cancer. (J. A. Depree, T.M. Howard & G.P. Savage, Food Research International Vol 31, No5, pp.329-337, 1999). Wasabi has even been known to prevent tooth decay. (Hideki Masuda, Ph.D. 2000).
Depree, Howard & Savage report on the testing done regarding isothiocyanates found in Wasabi on the inhibition of platelet aggregation and for deaggregation. It was found that in the case of a heart attack, where aspirin is commonly prescribed, the isothiocyanates had an immediate effect as opposed to the thirty minutes for aspirin. This anti-inflammatory effect could potentially be used to counter conditions such as asthma or even anaphylaxis.
Research has also been undertaken regarding isothiocyanates in Wasabi and anti-cancer properties. Tests have been done with stomach tumors in rats. It was found that some isothiocyanates have a protective role against breast, stomach and colon cancers (Wattenberg, 1977,1981). It was also found that human stomach cancer cells underwent morphological alterations and many died when cultured in media containing a concentration of an aqueous extract from Wasabi. (J.A. Depree, T.M. Howard & G.P. Savage, 2000).
Released in December 2000 in Hawaii, Hideki Masuda reported that chemical compounds found in Wasabi (isothiocyanates) inhibited the growth of Streptococcus mutants, the bacteria that cause dental cavities during test-tube studies. This is due to the Wasabi's ability to interfere with the sucrose-dependent adherence of cells. (The 2000 International Chemical Congress of Pacific Basis Societies, Hawaii, December 2000)
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