How does the auxin promote phototropism?

Tropism is a growth response between a plant and an external stimulus. The stimulus could be weather, touch, time, gravity or light. A positive response is indicated by growth toward a stimulus and a negative response is indicated by growth away from the stimulus.

Light is a stimulus that plants respond to. This is called phototropism (photo= light). Plants usually display a positive phototropic response to light, which means they grow toward a light source. Plant hormones called auxins play a part in phototropism. Auxin is a plant growth hormone.

So it is a chemical substance involved in regulating the action of certain cells within an organism. Phototropism is the process by which plant grow in response to light. Tropisms are responses to directional external stimuli and here Photo- indicates it is a response to light.

Auxin increases the flexibility of plant cell walls which are the main factor limiting plant cell growth. As a consequence, plant cells tend to grow faster in the presence of auxin. When a plant senses a directional light stimulus it redistributes its auxin molecules to the side that is further away from the light. As a consequence, the plant cells that are further away from the light experience a greater auxin concentration and grow faster, causing the shoot to bend towards the light. This mechanism allows plants to optimize the amount of light they receive in order to increase photosynthesis rates.