Phototropism is a growth response to?

• A. Touch
• B. Light
• C. Gravity
• D. Temperature

Answer: (B)

Phototropism is the growth response of a plant toward light. When a stem is exposed to light, blue-light receptors (phototropins) detect the light and cause the plant to redistribute the growth hormone auxin toward the shaded side. The higher auxin concentration on that side makes cells elongate more there, so that side grows faster and the stem bends toward the light. This helps the plant maximize light capture for photosynthesis. Other directional growth responses exist for different stimuli—gravitropism to gravity, thigmotropism to touch, and various responses to temperature—but phototropism specifically involves light.