How do plants grow towards light? Plants grow towards light using a process called phototropism, which is driven by a hormone called auxin.
Plants photosynthesize, which obviously requires sunlight. They need to be able to grow towards sunlight in order to photosynthesize as much as possible. To do this, they grow more on the side that is away from the sun. That stretches the shaded side of the stem and bends the plant towards the light. This is done thanks to the hormone auxin.
As plants grow, they produce auxin in the growing tips of their stems and shoots. It is then passed from cell to cell using transport proteins. Auxin makes cells grow by triggering them to pump hydrogen ions into the cell wall. This makes the wall of the cell more acidic, which activates proteins that loosen the wall. Water inside a plant is kept under pressure, and when the cell wall softens, the cell is able to stretch and expand. This is called the acid growth theory, and it is one of the ways auxin helps plants grow.
The plant also has to know where the light is coming from. It does this using light-sensitive proteins called phototropins, which are especially sensitive to blue light. When light comes from one side, the plant detects the uneven light. This causes auxin to be redistributed, with more of it moving to the shaded side of the stem. The cells on the sunny side receive less auxin, so they don’t stretch as much. The cells on the shaded side receive more auxin, so they stretch more. The extra growth on the shaded side pushes the plant towards the sunlight.
This is one of those things that seems almost intelligent, but it is really chemistry and growth. The plant is not deciding to move towards the sun. It is just growing unevenly. If one side of the stem grows longer than the other side, the stem has no choice but to bend.
Scientists began to understand this in the 19th century. Charles Darwin and his son Francis experimented with young grass shoots called coleoptiles. They found that if the tip of the shoot was covered, the plant no longer bent towards the light. If the lower part of the shoot was covered but the tip was left uncovered, the plant still bent. This showed that the tip of the plant was detecting the light, but the bending happened farther down. Later scientists worked out that the signal moving down from the tip was auxin.
If plants are kept in a dark room, both sides of the stem will grow more equally because the plant doesn’t know where the light is. It will attempt to find light by growing upward and outward as quickly as possible. This results in long, thin, pale stems. The stems are long because the plant is searching for light. They are pale because the plant has not made much chlorophyll, which is the green pigment it needs for photosynthesis. This is why seedlings grown in darkness often look weak and stretched.
Flowers like sunflowers that track the sun across the sky do something similar, but not exactly the same. This is called heliotropism. Young sunflowers follow the sun from east to west during the day and then turn back to the east during the night, ready for sunrise. This movement is controlled by growth on different sides of the stem and also by the plant’s internal clock. As the sun moves across the sky, one side of the stem grows more than the other, and the flower head slowly turns. Overnight, the pattern reverses and the plant resets.
Sunflowers don’t do this only to aid in photosynthesis. When sunflowers are young and their flowers are still growing out of buds, following the sun helps them grow faster because they receive more warmth and light. Once their flowers have fully grown, sunflowers stop following the sun and usually lock facing east. This helps them be pollinated and develop their seeds. Because they face east, they get the morning sun, which warms them up earlier in the day. Many pollinators are active in the morning, and warmer flowers attract more bees and other insects. The warmth also helps the flower develop better pollen and healthier seeds.
Auxin in roots has almost the opposite effect to the one it has in stems. In a stem, extra auxin usually makes cells stretch more. In a root, extra auxin can slow cell stretching. Roots respond strongly to gravity. When a root is lying sideways, auxin gathers on the lower side. In the root, that higher auxin level slows the growth of the lower side, while the upper side grows faster. This bends the root downward. This is called gravitropism because the roots are growing in the direction of gravity. Roots can also respond to light, but gravity is usually the more important signal. The stem grows toward light, and the roots grow down into the soil, where they can find water and minerals. And this is what I learned today.
Sources
https://www.sciencedaily.com/releases/2013/05/130528105946.htm
https://en.wikipedia.org/wiki/Phototropism
https://www.bbc.co.uk/bitesize/guides/zpt4xfr/revision/1
Photo by Edu Raw: https://www.pexels.com/photo/lush-indoor-nursery-with-various-potted-plants-37347948/