Relationship between vitamin D_2 content in Shiitake (Lentinus edodes) and quantum of UV light in the environment of cultivation was investigated and the results suggested that vitamin D_2 (including pre-D_2) was photochemically produced from ergosterol by solar radiation during farming because of the following reasons: (1) The assayed values of vitamin D_2 (including pre-D_2) in various kinds of Shiitake were correlated with the quanta of UV light in solar radiation at the cultivation places. (2) Existence of pre-D_2 in raw Shiitake was identified by HPLC and GC-MS. (3) The weight ratio of pre-D_2 to vitamin D_2 in raw Shiitake was about 7:1, whereas that in dried Shiitake was about 1:10. The result suggested that pre-D_2 was photochemically produced from ergosterol during cultivation was thermally isomerized into vitamin D_2 during heat drying. (4) The assayed value of vitamin D_2 (including pre-D_2) in a sample of Shiitake increased about 2.3 times when exposed to direct sun light for 3 hrs. (5) The assayed values of vitamin D_2 (including pre-D_2) in the Shiitake cultured in abundant solar radiation were higher than those cultured in poor radiation.
