AP Bio Study Guide

Big Idea 2.E

What are phototropism and photoperiodism?

Phototropism and photoperiodism are both physiological events in plants, which involve interactions between environmental stimuli and internal molecular signals.

Phototropism is the growth and orientation of a plant towards a source of light. It occurs when a category of plant hormones called auxins accumulate on the side of the plant away from the light and cause growth, which causes the stem to blend towards the light.

In general, a tropism is the growth of a plant towards or away from a stimulus, such as tough, gravity, or light.

Photoperiodism is a plant’s physiological response to the time of year, depending on the light period at that time. Some plants will flower only when the light period is longer than a certain number of hours, or when the light period is shorter than a certain number of hours.