Nuphar saikokuensis is a species of rhizomatous aquatic plant endemic to Japan.
Nuphar saikokuensis is an aquatic perennial herb with prostrate, branching rhizomes. The leaves are submerged, floating, or emergent. The petiolate, ovate floating or emergent leaves with a cordate base and an obtuse apex are 10âÂÂ30 cm long, and 7âÂÂ20 cm wide. The adaxial leaf surface is glabrous, and the abaxial leaf surface is pubescent. The membranous, ovate to round submerged leaves with an undulate margin are 7âÂÂ30 cm long, and 5âÂÂ20 cm wide. The petioles are flattened to terete.
The yellow, protogynous, pedunculate, 3âÂÂ4 cm wide flowers are raised above the water surface. The five subcoriaceous, obovate to orbicular sepals are 1.5âÂÂ2.5 cm long. The petals are 5âÂÂ8 mm long. The androecium consists of numerous stamens with 4âÂÂ6 mm long anthers. The gynoecium consists of numerous fused carpels. The yellow, 4âÂÂ11 mm wide stigmatic disk has 5âÂÂ17 stigmatic rays, which are 2.5âÂÂ4 mm long. The ovoid, green, 2.5âÂÂ4 cm long, and 1.5âÂÂ3 cm wide fruit bears many ovoid, 3.5âÂÂ5 mm long, and 3âÂÂ4.5 mm wide seeds.
Flowering occurs from June to October. Despite likely being of hybrid origin, it is fertile.
It was first described by Takashi Shiga and Yasuro Kadono in 2015.
The type specimen was collected by T. Shiga in Oda-cho, Ono-shi, Hyogo prefecture, Japan on the 30th of May 2001.
It is likely a natural hybrid of Nuphar japonica with Nuphar subintegerrima, or Nuphar oguraensis. It is possible, that all three species are involved.
The specific epithet saikokuensis is derived from the Japanese regional name "Saikoku", meaning Western Japan, to which this species is native to.
It is threatened by extinction, and several populations have already gone extinct.
It occurs in ponds, rivers, lakes, and streams.