Gravitropism

What is Gravitropism?

Gravitropism, also called geotropism, is a way plants and some fungi grow in response to gravity. This can be the Earth's natural pull or any other kind of gravity. Most plants show this behavior.

Early Studies

Charles Darwin was one of the first scientists to study gravitropism. He found that roots grow downward, which is called positive gravitropism. Stems grow upward, known as negative gravitropism. You can see this with a potted plant. If you lay the plant on its side, the stems will slowly turn and grow upward again. This process is explained by the Cholodny–Went model, which scientists have studied and updated.

In roots

Root growth happens when cells at the tip divide and stretch out. The root tip feels gravity and sends this message to the stretching cells to keep the root growing straight down.

A plant hormone called auxin helps with this. It moves to the lower side of the root when the plant feels gravity. This makes that side grow slower and bends the root downward. Scientists N. Cholodny and Frits Went first described this idea in the 1920s.

In shoots

Gravitropism helps plants grow in the right direction. It makes roots grow down and shoots grow up.

This happens because of a plant hormone called auxin. Auxin changes how plant cells grow.

As plants grow bigger, gravitropism keeps guiding them. It works with phototropism, which helps plants grow toward light. Young plants can still grow upward until they find light.

In roots, auxin makes the lower side grow slower, so roots bend down. In shoots, auxin makes the lower side grow faster, so stems bend up.

For gravitropism to work in shoots, the plant needs to be tilted a lot, not just a small pull of gravity.

In fruit

Some fruits, like bananas, show a special growth pattern called negative geotropism. When the leaves covering bananas dry up, the bananas start to bend upwards toward the sunlight. This upward bend is called phototropism.

This change happens because of a plant hormone called auxin. When sunlight hits the banana, auxin moves from the sunny side to the shady side. This makes the cells on the shady side grow more, causing the banana to curve upward.

Gravity-sensing mechanisms

Plants can sense gravity in different ways. One way is through special parts called statoliths. These statoliths are found in certain cells and help the plant know which way is down. When the plant feels gravity, these statoliths move and send a signal. This signal tells a special plant chemical, called auxin, to move to one side. The auxin then makes the plant grow more on one side than the other, helping the plant bend and grow toward or away from gravity.

Light can also affect how a plant grows. Special parts in the plant called phytochromes sense red and far-red light. This light can change how the plant grows and sometimes stop it from bending too much in one direction, letting it grow in different ways instead.

Compensation

Mushroom stems bend in special ways when they change from standing up to lying flat. After turning flat, the top part of the stem starts to straighten up again. This is called compensation.

Scientists have two ideas about why this happens. One idea suggests that mushrooms find the best angle to grow. The other idea thinks that when the stem bends too much, it gets a signal to straighten again. Both ideas help explain what we see, but more work is needed to fully understand this behavior.

Gravitropic mutants

Scientists study special plants called mutants that do not grow the same way as normal plants when responding to gravity. These mutants are found in many plants, including a common research plant called Arabidopsis thaliana. Some mutants do not grow upwards like normal stems, while others do not grow downwards like normal roots. By studying these mutants, scientists learn more about how gravity affects plant growth.

Some of these special plants have changes in how they store a material called starch, making their cells less sensitive to gravity. Others have changes in how they use a plant hormone called auxin, which helps control growth. These studies also help scientists understand how auxin moves and works inside plant cells. There are also some garden plants and crops, like certain trees, corn, rice, barley, and tomatoes, that grow in unusual ways compared to most plants, either drooping downwards or growing flat along the ground.