What role do plant hormones play in phototropism?
The primary function of the auxin hormones is to elongate plant cells in the stem. For instance, auxins are the hormones responsible for phototropism, the growth of a plant toward the light. Phototropism results from the rapid elongation of cells on the dark side of the plant, which causes the plant to bend in the opposite direction. The acid growth hypothesis explains this occurrence by speculating that auxins trigger proton pumps in cell membranes, lowering the pH in the cell wall to such an extent that the hydrogen bonds holding its cellulose fibers together break apart. These broken bonds give the cell wall greater flex ibility and expandability, so that more water can enter the cell by diffusion, causing the cell itself to elongate.