Correct Response: B. The triple response occurs when the sprouting seedling encounters an obstacle such as a rock on its way to breaking through the soil's surface. The seedling then produces ethylene, which causes stem elongation to slow, the stem to thicken to be stronger and the stem to have horizontal growth to find an easier path to the surface. Once the path is found the seedling continues to move upward. Although ethylene does play a role in the triple response, phototropism and programmed cell deaths (A) do not affect the triple response. Changes in turgor pressure, phytochromes, and blue-light receptors (C) do not affect the triple responses of the seedling because the seedling has not yet broken through the soil. Auxin concentration, the production of growth inhibitors and the abscission layers (D) may occur in a plant, but they do not play a role in the triple response of a seedling that is still underground.
