Which plant hormone is primarily responsible for cell elongation and tropic responses?

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

Answer: (A)

Auxin drives cell elongation and tropic responses. It promotes smooth, directional growth by loosening the plant cell wall in the region just behind the expanding tip, a process known as acid growth. Auxin is produced mainly in shoot tips and young leaves and moves through the plant in a directional, polar fashion toward other parts of the plant. When light shines from one side, more auxin accumulates on the shaded side of a shoot, causing those cells to elongate more and bending the shoot toward the light. In gravity responses, auxin redistribution to the lower side of a horizontally placed stem or root leads to differential growth and bending, enabling the organ to grow in a direction relative to gravity. In roots, higher auxin concentrations can inhibit elongation, contributing to the turning effect during gravitropism. While other hormones regulate growth processes—cytokinins mainly promote cell division and shoot formation, gibberellins promote elongation in various tissues, and abscisic acid tends toward growth inhibition and stress responses—auxin is primarily responsible for the elongation of cells and the directional bending seen in tropic responses.