Gravitropism

Gravitropism, also known as geotropism, is a special way that plants and some fungi grow in response to the pull of gravity. Whether gravity is natural, like the Earth's pull, or artificial, all higher plants and many lower plants show this behavior.

Charles Darwin was one of the first scientists to study this carefully. He discovered that roots grow downward, a behavior called positive gravitropism, while stems grow upward, known as negative gravitropism. You can see this yourself with a potted plant. If you lay the plant on its side, the stems will slowly turn and grow upward again. This happens because of a process described by the Cholodny–Went model, which scientists have studied and updated over time.

In roots

Root growth happens when stem cells in the root tip divide and then stretch out. Gravity is sensed at the root tip and this information is sent to the part where the cells stretch to keep the root growing straight down.

A plant hormone called auxin plays a big role in this. It moves to the lower side of the root when the plant feels gravity, causing that side to grow slower and making the root bend downward. This idea was first described in the 1920s by scientists N. Cholodny and Frits Went.

In shoots

Gravitropism helps plants grow in the right direction. It makes sure roots grow down into the soil and shoots grow up toward the sunlight. This happens because of a plant hormone called auxin, which changes how plant cells grow.

As plants get bigger, gravitropism keeps guiding them, working together with phototropism, which helps plants grow toward light. Even though mature plants might not sense gravity well in the dark, young plants can still grow upward until they find light. In roots, auxin makes the lower side grow slower, causing roots to bend down. In shoots, auxin makes the lower side grow faster, causing stems to bend up.

A recent study found that 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 causes the cells on the shady side to grow more, making the banana 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 understand 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, and the bend ends up near the base of the mushroom. This is called compensation.

Scientists have two ideas about why this happens. One idea suggests that mushrooms have a special way to find the best angle to grow, which changes with time and position. The other idea thinks that when the stem bends too much, a signal tells it to straighten again. Both ideas explain some of what we see, but more work is needed to fully understand this behavior.

Gravitropic mutants

Scientists have studied special plants called mutants that do not respond to gravity in the same way as normal plants. These mutants were found in many types of plants, including a common research plant called Arabidopsis thaliana. Some mutants do not grow upwards as usual, while others do not grow downwards like normal roots. By studying these mutants, scientists can learn more about how gravity affects plant growth.

Some of these special plants have changes in how they store a type of 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.