Adzuki bean

The wild ancestor of cultivated adzuki bean is probably Vigna angularis var. nipponensis,[2] which is distributed across Japan, Korea, China, Nepal and Bhutan.[3] Speciation between Vigna angularis var. nipponensis and Vigna angularis var. angularis occurred around 50,000 years ago.[4] Archaeologists estimate it was domesticated around 3000 BC.[5] However, adzuki beans (as well as soybeans) dating from 3000 BC to 2000 BC are indicated to still be largely within the wild size range. Enlarged seeds occurred during the later Bronze Age or Iron Age, periods with plough use.[6] Domestication of adzuki beans resulted in a trade-off between yield and seed size. Cultivated adzuki beans have fewer but longer pods, fewer but larger seeds, a shorter stature, and also a smaller overall seed yield than wild forms.[3] The exact place of domestication is not known;[2] multiple domestication origins in East Asia have been suggested.[5]

Adzuki beans, illustration from the Japanese agricultural encyclopedia Seikei Zusetsu (1804)

In Japan, the adzuki bean was one of the first crops subjected to scientific plant breeding.[3] Important breeding traits are yield, pureness of the bean colour, and the maturing time.[7] Separate cultivars with smaller seeds and higher biomass are bred for fodder production and as green manure.[7] Locally adapted cultivars are available in China, Japan, Korea, and Taiwan.[8] More than 300 cultivars/landraces/breeding lines are registered in Japan.[8] Moreover, China (Institute of Crop Germplasm Resources (CAAS), Beijing, more than 3700 accessions) and Japan (Tokachi Agricultural Experiment Station, Hokkaido, about 2500 accessions) accommodate large germplasm collections of adzuki bean.[8]

Weed forms of adzuki bean frequently occur in Japan. The wide spread of weed forms is due to adaptation to human-disturbed habitats, escapes of old cultivars, and natural establishment from derivatives of hybrids between cultivars and wild forms.[2] In contrast to wild forms, the weed forms of adzuki bean are used as a substitute for the cultivated form and consumed as sweet beans, especially if cultivated adzuki beans are attacked by pests. However, in cultivated gardens the weed form is recognized as contamination and lowers the seed quality of adzuki cultivars.[2]

The adzuki bean is mainly cultivated in China (670,000 ha), Japan (60,000 ha), South Korea (25,000 ha), and Taiwan (15,000 ha) (data published 2006).[8] The bean is also grown commercially in the US, South America, and India,[10] as well as New Zealand, Congo, and Angola.[7] In Japan, the adzuki bean is the second most important legume after the soy bean; its 1998 annual yield of this crop was around 100,000 tons.[7] With a consumption of about 140,000 t/year (data published 2006), Japan is also the most important importer of adzuki beans.[8] The imports are received from China, Korea, Colombia, Taiwan, US, Thailand, and Canada.[7][8]

The bean yields per area spread over a broad range due to differing cultivation intensity. Amounts of 4 to 8 dt/ha are reported, but in Japan and China yields between 20 and 30 dt/ha are reached.[7]

Optimal temperature range for adzuki bean growth is between 15 °C and 30 °C. The crop is not frost-hardy and needs soil temperatures above 6–10 °C (30–34 °C optimal) for germination. Hot temperatures stimulate vegetative growth and are therefore less favorable for pea production.[7][8][10] The adzuki bean is usually not irrigated. Annual rainfall ranges from 500 to 1750 mm in areas where the bean is grown. The plant can withstand drought but severe reduction in yield is expected.[7][8] The cultivation of the adzuki bean is possible on preferably well drained soils with pH 5–7.5.[8][10] Fertilizer application differs widely depending on expected yield but is generally similar to soybean. Due to nodulation with rhizobia, nitrogen fixation of up to 100 kg/ha is possible.[8][10]

The sowing of the peas is in 2–3 cm depth in rows 30–90 cm apart and 10–45 cm within the row. Rarely seeds are sown by broadcast. The amount of seeds ranges between 8–70 kg/ha. Growth of the crop is slow, therefore weed control is crucial mainly between germination and flowering. Cultivation systems differ largely among different countries. In China adzuki bean is often grown in intercrops with maize, sorghum and millet while in Japan the bean is grown in crop rotations. Harvest of the peas should not be done as long as moisture content of the seed is higher than 16%.[8]

Fungal and bacterial diseases of the adzuki bean are powdery mildew, brown stem rot, and bacterial blight. Furthermore, pests such as the adzuki pod worm, Japanese butterbur borer, and cutworm attack the crop. The bean weevil is an important storage pest.[8]

The adzuki bean has a taproot type of root system that can reach a depth of 40–50 cm from the point of seed germination.[12][13]

The leaves of the adzuki bean are trifoliate, pinnate and arranged alternately along the stem on a long petiole.[7][8][12] Leaflets are ovate and about 5–10 cm long and 5–8 cm wide.[7][12][13]

Adzuki flowers are papilionaceous and bright yellow.[8][12][13] The inflorescence is an axillary false raceme[12] consisting of six[11] to ten[7] (two to twenty[8]) flowers.[7][11][12]

Adzuki pods are smooth, cylindrical and thin-walled.[12][13] The colour of the pods is green turning white to grey as they mature.[12][13] The size is between 5–13 cm × 0.5 cm with 2 to 14 seeds per pod.[8][13] Pod shatter during seed ripening and harvesting might be a difficulty under certain conditions.[12][13]

The seeds are smooth and subcylindric with a length of 5.0-9.1 mm, width of 4.0-6.3 mm, thickness of 4.1-6.0 mm.[12][13] The thousand kernel weight is between 50 and 200 g.[7] There are many different seed colours from maroon to blue-black mottled with straw.[6]

The emergence of the seedlings is hypogeal[12][13] and takes 7–20 days.[8] Compared to other pulses the growth of the plant is slow.[8] Normally the adzuki plant reaches maturity between 80 and 120 days depending on the cultivar and the environmental conditions.[12] Flowering lasts 30–40 days.[8] Commonly the plant self-pollinates but cross-pollination also exists.[8]