japonica vs flowering quince

japonica and flowering quince both are nouns.

Word	Noun	Adjective	Verb	Adverb
japonica	Yes	No	No	No
flowering quince	Yes	No	No	No

As nouns, flowering quince is a hypernym of japonica; that is, flowering quince is a word with a broader meaning than japonica:

japonica: deciduous thorny shrub native to Japan having red blossoms
flowering quince: Asiatic ornamental shrub with spiny branches and pink or red blossoms

japonica (noun)	flowering quince (noun)
deciduous thorny shrub native to Japan having red blossoms	Asiatic ornamental shrub with spiny branches and pink or red blossoms
greenhouse shrub with glossy green leaves and showy fragrant rose-like flowers; cultivated in many varieties