Which plant hormone is mainly responsible for cell elongation and phototropism?

• A. Cytokinin
• B. Ethylene
• C. Abscisic acid
• D. Auxin

Answer: (D)

Auxin controls how plant cells elongate and how shoots bend toward light. It is produced in the growing tip and moves downward through the stem, creating higher concentrations where growth is needed. In elongating cells, auxin stimulates proton pumps that acidify the cell wall, activating enzymes that loosen the wall structure. This makes the cell wall more extensible, allowing water uptake and rapid cell expansion.

For phototropism, light-absorbing receptors detect light direction and cause auxin to accumulate on the shaded side of the stem. The higher auxin concentration there promotes more elongation on that side, bending the stem toward the light.

Other hormones have different primary roles—cytokinins mainly promote cell division, ethylene influences fruit ripening and leaf drop, and abscisic acid helps with stress responses and seed dormancy—but auxin uniquely explains both rapid cell elongation and directional growth in response to light.