pigeon-pea plant vs shrub

pigeon-pea plant and shrub both are nouns.

Word	Noun	Adjective	Verb	Adverb
pigeon-pea plant	Yes	No	No	No
shrub	Yes	No	No	No

As nouns, shrub is a hypernym of pigeon-pea plant; that is, shrub is a word with a broader meaning than pigeon-pea plant:

pigeon-pea plant: tropical woody herb with showy yellow flowers and flat pods; much cultivated in the tropics
shrub: a low woody perennial plant usually having several major stems

Other hypernyms of pigeon-pea plant include bush.

pigeon-pea plant (noun)	shrub (noun)
tropical woody herb with showy yellow flowers and flat pods; much cultivated in the tropics	a low woody perennial plant usually having several major stems