To provide an in vitro flavonoid-rich rhizome tissue of Neomarica gracilis, which is obtained by tissue culture preparation using an N. gracilis tissue having proliferating capacity (e.g. a root, a leaf, a basal portion of a leaf, and/or a rhizome), and further, to provide a method for culturing the in vitro flavonoid-rich rhizome tissue, a method for extracting tectorigenin from the flavonoid-rich rhizome tissue, and a method for quantifying tectorigenin in the in vitro flavonoid-rich rhizome tissue, respectively.

The in vitro flavonoid-rich rhizome tissue of N. gracilis, which is obtained from a tissue culture-prepared product variable in the flavonoid content of Neomarica gracilis, contains tectorigenin. This flavonoid-rich rhizome tissue of N. gracilis, although containing tectorigenin, is distinctively different from the naturally grown rhizome of N. gracilis which contains no tectorigenin.