Rare synonym for 向日葵 (ひまわり). Used in botanical or literary contexts; not the everyday word.
See also: ひまわり
Nichirinsō is known as another name for the sunflower.
From 日輪 (sun, solar disk) + 草 (grass, plant), literally 'sun-wheel plant'. The exact origin is uncertain, but it is a descriptive name for the sunflower.