Sunflower

Sunflowers demonstrate remarkable phototropic behavior. A sunflower seedling exposed to light from the east will develop 40% higher auxin concentration on its west side, causing west-side cells to elongate 40% faster. Within 6 hours, the stem visibly bends east. By the next morning, the plant faces the sun directly.

Young sunflowers exhibit circadian-modulated phototropism - they track the sun throughout the day and reorient overnight to face east before sunrise, anticipating where light will be. This represents Tier 1 (hours) responsiveness in the tropism hierarchy.