Rheum australe vs rhubarb

Rheum australe and rhubarb both are nouns.

Word	Noun	Adjective	Verb	Adverb
Rheum australe	Yes	No	No	No
rhubarb	Yes	No	No	No

As nouns, rhubarb is a hypernym of Rheum australe; that is, rhubarb is a word with a broader meaning than Rheum australe:

Rheum australe: Asian herb (Himalayas)
rhubarb: plants having long green or reddish acidic leafstalks growing in basal clumps; stems (and only the stems) are edible when cooked; leaves are poisonous

Other hypernyms of Rheum australe include rhubarb plant.

Rheum australe (noun)	rhubarb (noun)
Asian herb (Himalayas)	long pinkish sour leafstalks usually eaten cooked and sweetened
	plants having long green or reddish acidic leafstalks growing in basal clumps; stems (and only the stems) are edible when cooked; leaves are poisonous