Uneven distribution of _______ results in phototropic curvature

The correct option is auxin.

Step-by-step Explanation:

1. Understanding Phototropism:
Phototropism is the growth of a plant in response to a light stimulus. When a plant shoot bends towards a light source, it is called positive phototropism.

2. Role of Auxin:
Auxin is a key plant hormone (phytohormone) synthesized at the coleoptile tip or shoot apex. It is responsible for regulating cell elongation and plant growth. Under uniform light conditions, auxin is distributed evenly throughout the growing tip.

3. Mechanism of Uneven Distribution:
When light shines on a plant from one side (unidirectional light), auxin molecules detect the light and migrate laterally from the illuminated side to the shaded side of the shoot. This results in an uneven distribution of auxin, where the concentration of auxin becomes higher on the shaded side compared to the light-exposed side.

4. Curvature Development:
Since higher concentrations of auxin promote cell elongation in shoots, the cells on the shaded side (with more auxin) elongate much faster than the cells on the lit side. This differential growth rate causes the shoot to bend or curve towards the direction of the light source, resulting in phototropic curvature.