Why Do Plants Bend Towards Light?

Phototropism is defined as the movement of plants towards light. It was first investigated by Charles Darwin. Using grass and grain seedlings he proved that the cells on the dark side of the plant (the side away from a light source) elongate causing the seedling to bend towards the light source.

A plant hormone called indolacetic acid or auxin is responsible for making plant cells on the side away from light elongate. It is also responsible for making roots grow down into the soil (called gravitropism). Auxin triggers a chemical reaction which weakens cell walls allowing them to elongate and ultimately bending the plant towards light.

Plants detect light with photoreceptors. Blue light absorbed by photoreceptors stimulates auxin to move laterally across the stem and leaves of plants to the side which is away from the light.

Auxin cause H+ ions to move into cells. The H+ ions change the pH in plant cells making them more acidic. The acidity disrupts hydrogen bonds in cell walls making the walls weaker. Enzymes called expansins are activated by the cell's change in pH. Expansins break hydrogen bonds in the cell walls allowing the cell to elongate. The elongation of cells on one side of the plant creates unequal growth which then bends the plant towards the smaller cells on the side receiving light.

Phototropism allows plants to maximize the amount of light energy they receive which in turns maximizes the photosynthesis potential for the entire plant. Increased photosynthesis means greater growth allowing a plant to out compete adjacent plants. This allows the plant more access to nutrients and water in the soil which in turn increases flower and seed production. The most vigorous plants are often the most visited by pollen transferring insects and birds. This increases the odds of a particular plant passing on its genetics to future generations.

If you have ever grown sunflowers, you will have noticed how the flowers seem to follow the sun across the sky. This is an example of phototropism. You can use phototropism to create a floral sundial. Select plants that change their orientation in response to the sun's position in the sky. Sunflowers, marigolds, daylilies, and morning glories are all good choices for a floral sundial. Combine plants or just choose one species. Plant in a circle leaving a space in the middle. You can tell where the sun is by the position of the leaves and flowers.