Which growth response causes stems to bend toward a light source?

Which growth response causes stems to bend toward a light source?

• A. Gravitropism
• B. Phototropism
• C. Hydrotropism
• D. Thigmotropism

Answer: (B)

Phototropism is the growth response that makes a stem bend toward a light source. Light is detected by blue-light receptors in the plant, which causes the hormone auxin to accumulate on the shaded side of the stem. With more auxin on that side, cells elongate faster there, so the shaded side lengthens more than the lighted side and the stem curves toward the light. This is positive phototropism in shoots. Other tropisms involve gravity, moisture, or touch—gravitropism responds to gravity, hydrotropism to water, and thigmotropism to touch—none of which explain bending toward light.

Phototropism is the growth response that makes a stem bend toward a light source. Light is detected by blue-light receptors in the plant, which causes the hormone auxin to accumulate on the shaded side of the stem. With more auxin on that side, cells elongate faster there, so the shaded side lengthens more than the lighted side and the stem curves toward the light. This is positive phototropism in shoots. Other tropisms involve gravity, moisture, or touch—gravitropism responds to gravity, hydrotropism to water, and thigmotropism to touch—none of which explain bending toward light.