tuber vs potato bean

tuber and potato bean both are nouns.

Word	Noun	Adjective	Verb	Adverb
tuber	Yes	No	No	No
potato bean	Yes	No	No	No

As nouns, potato bean is a hyponym of tuber; that is, potato bean is a word with a more specific, narrower meaning than tuber:

tuber: a fleshy underground stem or root serving for reproductive and food storage
potato bean: nutlike tuber; important food of Native Americans

Other hyponyms of tuber include water chestnut, groundnut, wild bean, Jerusalem artichoke, yam.

tuber (noun)	potato bean (noun)
a fleshy underground stem or root serving for reproductive and food storage	nutlike tuber; important food of Native Americans
	a North American vine with fragrant blossoms and edible tubers; important food crop of Native Americans
	twining plant of Amazon basin having large edible roots