From the Super Mario Wiki
“Hello! Doesn't the sunshine feel lovely?”
The Great Sunflower is a giant sunflower located outside Pinna Park on the beach of Pinna Island only in Super Mario Sunshine. The Great Sunflower watches over the Sunflower Kids, but unlike them, the Great Sunflower is much larger and is brown in the middle instead of yellow. It also speaks differently when talked to or sprayed with water.
In Episode 4 of Pinna Park, Snooze-A-Koopas started chewing up the park's resident sunflowers' roots. Mario then had to defeat the Snooze-A-Koopas to rescue them from being wilted. The Great Sunflower rewarded him with a Shine Sprite after his task was finally done.