water chestnut vs tuber

water chestnut and tuber both are nouns.

Word	Noun	Adjective	Verb	Adverb
water chestnut	Yes	No	No	No
tuber	Yes	No	No	No

As nouns, tuber is a hypernym of water chestnut; that is, tuber is a word with a broader meaning than water chestnut:

water chestnut: edible bulbous tuber of a Chinese marsh plant
tuber: a fleshy underground stem or root serving for reproductive and food storage

water chestnut (noun)	tuber (noun)
edible bulbous tuber of a Chinese marsh plant	a fleshy underground stem or root serving for reproductive and food storage
Chinese sedge yielding edible bulb-shaped tubers
a plant of the genus Trapa bearing spiny four-pronged edible nutlike fruits