Phototropism: what is it?

Phototropism is the movement curvature of plants caused by the orientation of a source of light. It allows plants, which cannot move, to adapt to their light environment. Phototropism is either positive (for the stems) or negative (for the roots) and it is irreversible. Heliotropism is a special form of phototropism related to orientation of the sun during the day but who is he reversible.

Some animals (especially in the larval stage), aquatic organisms (plankton) or fungi are also phototropic. Spherobole in star (Sphaerobolus stellatus) has for example the astonishing ability to eject its peridiole (ball containing the spores) in the direction of the Sun.

Auxin, the hormone responsible for phototropism

Phototropism is induced by a plant hormone called auxin, produced in the end of the stem or root called the apex. Auxin induces plant growth by triggering a chemical reaction which stretches the wall of plant cells. When the plant is illuminated, auxin is inhibited and migrates to the unlit side, resulting in elongation cells on this side. This results in a curvature at the base of the stem and roots. The photosensitive receptors, called phototropins, are mainly located on the apex, which proves that there is auxin transport.

Apart from phototropism, the inclination of the plant is influenced by geotropism (sensitivity to gravity) or chemotropism (sensitivity to the concentration of a chemical substance).