Which plant hormone is primarily responsible for promoting cell elongation?

The plant hormone that is primarily responsible for promoting cell elongation is auxin. Auxins, a class of hormones, play a crucial role in various developmental processes in plants, one of which is the regulation of cell growth. When auxins are concentrated on one side of the plant, they promote cell elongation in that area, leading to the curvature of the plant towards the source of light, a phenomenon known as phototropism.

This hormone works by loosening the cell walls, allowing them to expand as water enters the cells, which ultimately leads to elongation. Auxins are also involved in apical dominance, root formation, and the development of fruits, showcasing their significance in plant growth and development.

Other hormones like gibberellins also play a role in promoting growth and elongation, particularly in stems and fruits, but auxin is primarily responsible for the directional growth responses through cell elongation. Ethylene is mainly involved in the ripening of fruits and the abscission of leaves, while cytokinins primarily stimulate cell division and delay senescence in plants.